Move Envoy Common to common/ subdirectory

common/src/main/java/envoy/data/Attachment.java

@@ -0,0 +1,84 @@
+package envoy.data;
+
+import java.io.Serializable;
+
+/**
+ * This interface should be used for any type supposed to be a {@link Message}
+ * attachment (i.e. images or sound).
+ * <p>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Attachment.java</strong><br>
+ * Created: <strong>30 Dec 2019</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class Attachment implements Serializable {
+
+	/**
+	 * Defines the type of the attachment.
+	 * 
+	 * @since Envoy Common v0.1-beta
+	 */
+	public enum AttachmentType {
+
+		/**
+		 * This attachment type denotes a picture.
+		 * 
+		 * @since Envoy Common v0.1-beta
+		 */
+		PICTURE,
+
+
+		/**
+		 * This attachment type denotes a video.
+		 * 
+		 * @since Envoy Common v0.1-beta
+		 */
+		VIDEO,
+		
+		/**
+		 * This attachment type denotes a voice message.
+		 * 
+		 * @since Envoy Common v0.1-beta
+		 */
+		VOICE,
+
+		/**
+		 * This attachment type denotes a document.
+		 *
+		 * @since Envoy Common v0.1-beta
+		 */
+		DOCUMENT
+	}
+
+	private final byte[]			data;
+	private final AttachmentType	type;
+
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * Constructs an attachment.
+	 * 
+	 * @param data the data of the attachment
+	 * @param type the type of the attachment
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Attachment(byte[] data, AttachmentType type) {
+		this.data	= data;
+		this.type	= type;
+	}
+
+	/**
+	 * @return the data of the attachment
+	 * @since Envoy Common v0.1-beta
+	 */
+	public byte[] getData() { return data; }
+
+	/**
+	 * @return the type of the attachment
+	 * @since Envoy Common v0.1-beta
+	 */
+	public AttachmentType getType() { return type; }
+}

common/src/main/java/envoy/data/Config.java

@@ -0,0 +1,86 @@
+package envoy.data;
+
+import java.util.*;
+
+import envoy.exception.EnvoyException;
+
+/**
+ * Manages all application settings that are set during application startup by
+ * either loading them from the {@link Properties} file
+ * {@code client.properties} or parsing them from the command line arguments of
+ * the application.<br>
+ * <br>
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>Config.java</strong><br>
+ * Created: <strong>12 Oct 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.1-beta
+ */
+public class Config {
+
+	protected Map<String, ConfigItem<?>> items = new HashMap<>();
+
+	/**
+	 * Parses config items from a properties object.
+	 *
+	 * @param properties the properties object to parse
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void load(Properties properties) {
+		items.entrySet()
+			.stream()
+			.filter(e -> properties.containsKey(e.getKey()))
+			.forEach(e -> e.getValue().parse(properties.getProperty(e.getKey())));
+	}
+
+	/**
+	 * Parses config items from an array of command line arguments.
+	 *
+	 * @param args the command line arguments to parse
+	 * @throws EnvoyException if the command line arguments contain an unknown token
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void load(String[] args) throws EnvoyException {
+		for (int i = 0; i < args.length; i++)
+			for (ConfigItem<?> item : items.values())
+				if (args[i].startsWith("--")) {
+					if (args[i].length() == 2) throw new EnvoyException("Malformed command line argument at position " + i);
+					final String commandLong = args[i].substring(2);
+					if (item.getCommandLong().equals(commandLong)) {
+						item.parse(args[++i]);
+						break;
+					}
+				} else if (args[i].startsWith("-")) {
+					if (args[i].length() == 1) throw new EnvoyException("Malformed command line argument at position " + i);
+					final String commandShort = args[i].substring(1);
+					if (item.getCommandShort().equals(commandShort)) {
+						item.parse(args[++i]);
+						break;
+					}
+				} else throw new EnvoyException("Malformed command line argument at position " + i);
+	}
+
+	/**
+	 * Initializes config items from a map.
+	 * 
+	 * @param items the items to include in this config
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void load(Map<String, ConfigItem<?>> items) { this.items.putAll(items); }
+
+	/**
+	 * @return {@code true} if all mandatory config items are initialized
+	 * @since Envoy Common v0.1-beta
+	 */
+	public boolean isInitialized() {
+		return items.values().stream().filter(ConfigItem::isMandatory).map(ConfigItem::get).noneMatch(Objects::isNull);
+	}
+
+	/**
+	 * @param name the name of the config item to return
+	 * @return the config item with the specified name
+	 * @since Envoy Common v0.1-beta
+	 */
+	public ConfigItem<?> get(String name) { return items.get(name); }
+}

common/src/main/java/envoy/data/ConfigItem.java

@@ -0,0 +1,92 @@
+package envoy.data;
+
+import java.util.function.Function;
+
+/**
+ * Contains a single {@link Config} value as well as the corresponding command
+ * line arguments and its default value.<br>
+ * <br>
+ * Project: <strong>envoy-clientChess</strong><br>
+ * File: <strong>ConfigItem.javaEvent.java</strong><br>
+ * Created: <strong>21.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @param <T> the type of the config item's value
+ * @since Envoy Common v0.1-beta
+ */
+public class ConfigItem<T> {
+
+	private final String				commandLong, commandShort;
+	private final Function<String, T>	parseFunction;
+	private final boolean				mandatory;
+
+	private T value;
+
+	/**
+	 * Initializes a {@link ConfigItem}.
+	 *
+	 * @param commandLong   the long command line argument to set this value
+	 * @param commandShort  the short command line argument to set this value
+	 * @param parseFunction the {@code Function<String, T>} that parses the value
+	 *                      from a string
+	 * @param defaultValue  the optional default value to set before parsing
+	 * @param mandatory     indicated that this config item must be initialized with
+	 *                      a non-null value
+	 * @since Envoy Common v0.1-beta
+	 */
+	public ConfigItem(String commandLong, String commandShort, Function<String, T> parseFunction, T defaultValue, boolean mandatory) {
+		this.commandLong	= commandLong;
+		this.commandShort	= commandShort;
+		this.parseFunction	= parseFunction;
+		this.mandatory		= mandatory;
+		value				= defaultValue;
+	}
+
+	/**
+	 * Initializes an optional {@link ConfigItem} without a default value.
+	 *
+	 * @param commandLong   the long command line argument to set this value
+	 * @param commandShort  the short command line argument to set this value
+	 * @param parseFunction the {@code Function<String, T>} that parses the value
+	 *                      from a string
+	 * @since Envoy Common v0.1-beta
+	 */
+	public ConfigItem(String commandLong, String commandShort, Function<String, T> parseFunction) {
+		this(commandLong, commandShort, parseFunction, null, false);
+	}
+
+	/**
+	 * Parses this {@ConfigItem}'s value from a string.
+	 *
+	 * @param input the string to parse from
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void parse(String input) { value = parseFunction.apply(input); }
+
+	/**
+	 * @return The long command line argument to set the value of this
+	 *         {@link ConfigItem}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public String getCommandLong() { return commandLong; }
+
+	/**
+	 * @return The short command line argument to set the value of this
+	 *         {@link ConfigItem}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public String getCommandShort() { return commandShort; }
+
+	/**
+	 * @return the value of this {@link ConfigItem}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public T get() { return value; }
+
+	/**
+	 * @return {@code true} if this {@link ConfigItem} is mandatory for successful
+	 *         application initialization
+	 * @since Envoy Common v0.1-beta
+	 */
+	public boolean isMandatory() { return mandatory; }
+}

common/src/main/java/envoy/data/Contact.java

@@ -0,0 +1,91 @@
+package envoy.data;
+
+import java.io.Serializable;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * This class is the superclass for both {@link User} and {@link Group}.<br>
+ * It provides an id and a name for each user and group.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Contact.java</strong><br>
+ * Created: <strong>24 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy v0.1-beta
+ */
+public abstract class Contact implements Serializable {
+
+	private final long								id;
+	private final transient Set<? extends Contact>	contacts;
+	private String									name;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Creates a new instance of a {@link Contact}.
+	 *
+	 * @param id       the ID of this contact
+	 * @param name     the name of this contact
+	 * @param contacts the contacts of this {@link Contact}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Contact(long id, String name, Set<? extends Contact> contacts) {
+		this.id			= id;
+		this.name		= name;
+		this.contacts	= contacts;
+	}
+
+	/**
+	 * @return the ID of this {@link Contact}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getID() { return id; }
+
+	/**
+	 * @return the name of this {@link Contact}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public String getName() { return name; }
+
+	/**
+	 * @param name the new name of this {@link Contact}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void setName(String name) { this.name = name; }
+
+	/**
+	 * {@inheritDoc}
+	 */
+	@Override
+	public String toString() { return String.format("Contact[id=%d,name=%s, contacts=%s]", id, name, contacts); }
+
+	/**
+	 * Provides a hash code based on the ID of this contact.
+	 * 
+	 * @since Envoy Common v0.1-beta
+	 */
+	@Override
+	public int hashCode() { return Objects.hash(id); }
+
+	/**
+	 * Tests equality to another object. If that object is a contact as well,
+	 * equality is determined by the ID.
+	 * 
+	 * @param obj the object to test for equality to this contact
+	 * @return {code true} if both objects are contacts and have identical IDs
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj) return true;
+		if (!(obj instanceof Contact)) return false;
+		return id == ((Contact) obj).id;
+	}
+
+	/**
+	 * @return the contacts of this {@link Contact}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Set<? extends Contact> getContacts() { return contacts; }
+}

common/src/main/java/envoy/data/Group.java

@@ -0,0 +1,55 @@
+package envoy.data;
+
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Group.java</strong><br>
+ * Created: <strong>24 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.1-beta
+ */
+public final class Group extends Contact {
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Creates a new instance of a {@link Group} without any members.
+	 *
+	 * @param id   the ID of this group
+	 * @param name the name of this group
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Group(long id, String name) { this(id, name, new HashSet<User>()); }
+
+	/**
+	 * Creates an instance of a {@link Group}.
+	 *
+	 * @param id      the ID of this group
+	 * @param name    the name of this group
+	 * @param members all members that should be preinitialized
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Group(long id, String name, Set<User> members) { super(id, name, members); }
+
+	private void readObject(ObjectInputStream inputStream) throws Exception {
+		inputStream.defaultReadObject();
+		var contacts = Contact.class.getDeclaredField("contacts");
+		contacts.setAccessible(true);
+		contacts.set(this, inputStream.readObject());
+	}
+
+	private void writeObject(ObjectOutputStream outputStream) throws Exception {
+		outputStream.defaultWriteObject();
+		getContacts().forEach(user -> user.serializeContacts(false));
+		outputStream.writeObject(getContacts());
+	}
+
+	@SuppressWarnings("unchecked")
+	@Override
+	public Set<User> getContacts() { return (Set<User>) super.getContacts(); }
+}

common/src/main/java/envoy/data/GroupMessage.java

@@ -0,0 +1,71 @@
+package envoy.data;
+
+import java.time.LocalDateTime;
+import java.util.Collections;
+import java.util.Map;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>GroupMessage.java</strong><br>
+ * Created: <strong>26.03.2020</strong><br>
+ *
+ * @author Maximilian K&auml;fer
+ * @since Envoy Common v0.1-beta
+ */
+public final class GroupMessage extends Message {
+
+	private final Map<Long, MessageStatus> memberStatuses;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link GroupMessage} with values for all of its properties. The
+	 * use
+	 * of this constructor is only intended for the {@link MessageBuilder} class, as
+	 * this class provides {@code null} checks and default values for all
+	 * properties.
+	 *
+	 * @param id             unique ID
+	 * @param senderID       the ID of the user who sends the message
+	 * @param groupID        the ID of the group which receives the message
+	 * @param creationDate   the creation date of the message
+	 * @param receivedDate   the received date of the message
+	 * @param readDate       the read date of the message
+	 * @param text           the text content of the message
+	 * @param attachment     the attachment of the message, if present
+	 * @param status         the current {@link Message.MessageStatus} of the
+	 *                       message
+	 * @param forwarded      whether this message was forwarded
+	 * @param memberStatuses a map of all members and their status according to this
+	 *                       {@link GroupMessage}
+	 * @since Envoy Common v0.1-beta
+	 */
+	GroupMessage(long id, long senderID, long groupID, LocalDateTime creationDate, LocalDateTime receivedDate, LocalDateTime readDate, String text,
+			Attachment attachment, MessageStatus status, boolean forwarded, Map<Long, MessageStatus> memberStatuses) {
+		super(id, senderID, groupID, creationDate, receivedDate, readDate, text, attachment, status, forwarded);
+		this.memberStatuses = memberStatuses;
+	}
+
+	/**
+	 * Sets the status to be the minimum of all members.
+	 *
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void updateStatus() {
+		setStatus(Collections.min(memberStatuses.values()));
+		switch (getStatus()) {
+			case RECEIVED:
+				setReceivedDate(LocalDateTime.now());
+				break;
+			case READ:
+				setReadDate(LocalDateTime.now());
+				break;
+		}
+	}
+
+	/**
+	 * @return the map of all statuses in this {@link GroupMessage}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Map<Long, MessageStatus> getMemberStatuses() { return memberStatuses; }
+}

common/src/main/java/envoy/data/IDGenerator.java

@@ -0,0 +1,51 @@
+package envoy.data;
+
+import java.io.Serializable;
+
+/**
+ * Generates increasing IDs between two numbers.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>IDGenerator.java</strong><br>
+ * Created: <strong>31.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class IDGenerator implements Serializable {
+
+	private final long	end;
+	private long		current;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Creates an instance of {@link IDGenerator}.
+	 *
+	 * @param begin the first ID
+	 * @param size  the amount of IDs to provide
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public IDGenerator(long begin, long size) {
+		current	= begin;
+		end		= begin + size;
+	}
+
+	@Override
+	public String toString() { return String.format("IDGenerator[current=%d,end=%d]", current, end); }
+
+	/**
+	 * @return {@code true} if there are unused IDs remaining
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public boolean hasNext() { return current < end; }
+
+	/**
+	 * @return the next ID
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long next() {
+		if (!hasNext()) throw new IllegalStateException("All IDs have been used");
+		return current++;
+	}
+}

common/src/main/java/envoy/data/LoginCredentials.java

@@ -0,0 +1,69 @@
+package envoy.data;
+
+import java.io.Serializable;
+
+/**
+ * Contains a {@link User}'s login / registration information as well as the
+ * client version.
+ * <p>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>LoginCredentials.java</strong><br>
+ * Created: <strong>29.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public final class LoginCredentials implements Serializable {
+
+	private final String	identifier, password, clientVersion;
+	private final boolean	registration;
+
+	private static final long serialVersionUID = 2;
+
+	/**
+	 * Initializes login credentials for a handshake.
+	 *
+	 * @param identifier    the identifier of the user
+	 * @param password      the password of the user
+	 * @param registration  signifies that these credentials are used for user
+	 *                      registration instead of user login
+	 * @param clientVersion the version of the client sending these credentials
+	 * @since Envoy Common v0.1-beta
+	 */
+	public LoginCredentials(String identifier, String password, boolean registration, String clientVersion) {
+		this.identifier		= identifier;
+		this.password		= password;
+		this.registration	= registration;
+		this.clientVersion	= clientVersion;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("LoginCredentials[identifier=%s,registration=%b,clientVersion=%s]", identifier, registration, clientVersion);
+	}
+
+	/**
+	 * @return the identifier of the user performing the login
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public String getIdentifier() { return identifier; }
+
+	/**
+	 * @return the password of the user performing the login
+	 * @since Envoy Common v0.1-beta
+	 */
+	public String getPassword() { return password; }
+
+	/**
+	 * @return {@code true} if these credentials are used for user registration
+	 *         instead of user login
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public boolean isRegistration() { return registration; }
+
+	/**
+	 * @return the version of the client sending these credentials
+	 * @since Envoy Common v0.1-beta
+	 */
+	public String getClientVersion() { return clientVersion; }
+}

common/src/main/java/envoy/data/Message.java

@@ -0,0 +1,212 @@
+package envoy.data;
+
+import java.io.Serializable;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+
+/**
+ * Represents a unique message with a unique, numeric ID. Further metadata
+ * includes the sender and recipient {@link User}s, as well as the creation
+ * date and the current {@link MessageStatus}.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Message.java</strong><br>
+ * Created: <strong>28.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.2-alpha
+ */
+public class Message implements Serializable {
+
+	/**
+	 * This enumeration defines all possible statuses a {link Message} can have.
+	 *
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public enum MessageStatus {
+
+		/**
+		 * is selected, if a message was sent but not received by the server yet.
+		 */
+		WAITING,
+
+		/**
+		 * is selected, if a sent message was received by the server.
+		 */
+		SENT,
+
+		/**
+		 * is selected, if a message was delivered from the server to the recipient, but
+		 * has not been read yet.
+		 */
+		RECEIVED,
+
+		/**
+		 * is selected, if a recipient opened the corresponding chat of said message.
+		 */
+		READ
+	}
+
+	private final long			id, senderID, recipientID;
+	private final boolean		forwarded;
+	private final LocalDateTime	creationDate;
+	private final String		text;
+	private final Attachment	attachment;
+
+	private LocalDateTime	receivedDate, readDate;
+	private MessageStatus	status;
+
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * Initializes a {@link Message} with values for all of its properties. The use
+	 * of this constructor is only intended for the {@link MessageBuilder} class, as
+	 * this class provides {@code null} checks and default values for all
+	 * properties.
+	 *
+	 * @param id           unique ID
+	 * @param senderID     the ID of the user who sends the message
+	 * @param recipientID  the ID of the user who receives the message
+	 * @param creationDate the creation date of the message
+	 * @param receivedDate the received date of the message
+	 * @param readDate     the read date of the message
+	 * @param text         the text content of the message
+	 * @param attachment   the attachment of the message, if present
+	 * @param status       the current {@link MessageStatus} of the message
+	 * @param forwarded    whether this message was forwarded
+	 * @since Envoy Common v0.2-alpha
+	 */
+	Message(long id, long senderID, long recipientID, LocalDateTime creationDate, LocalDateTime receivedDate, LocalDateTime readDate, String text,
+			Attachment attachment, MessageStatus status, boolean forwarded) {
+		this.id				= id;
+		this.senderID		= senderID;
+		this.recipientID	= recipientID;
+		this.creationDate	= creationDate;
+		this.receivedDate	= receivedDate;
+		this.readDate		= readDate;
+		this.text			= text;
+		this.attachment		= attachment;
+		this.status			= status;
+		this.forwarded		= forwarded;
+	}
+
+	/**
+	 * Changes the current {@link MessageStatus} to the next logical status.<br>
+	 * <br>
+	 * The underlying order is as follows:
+	 * <ol>
+	 * <li>{@link MessageStatus#WAITING}
+	 * <li>{@link MessageStatus#SENT}
+	 * <li>{@link MessageStatus#RECEIVED}
+	 * <li>{@link MessageStatus#READ}
+	 * </ol>
+	 *
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public void nextStatus() {
+		if (status == MessageStatus.READ) throw new IllegalStateException("Message status READ is already reached");
+		status = MessageStatus.values()[status.ordinal() + 1];
+	}
+
+	@Override
+	public String toString() {
+		return String.format("Message[id=%d,sender=%s,recipient=%s,date=%s,status=%s,text=%s,forwarded=%b,hasAttachment=%b]",
+				id,
+				senderID,
+				recipientID,
+				DateTimeFormatter.ofPattern("dd.MM.yyyy HH:mm:ss").format(creationDate),
+				status,
+				text,
+				forwarded,
+				attachment != null);
+	}
+
+	/**
+	 * @return the ID of this message
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getID() { return id; }
+
+	/**
+	 * @return the sender ID of this message
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getSenderID() { return senderID; }
+
+	/**
+	 * @return the recipient ID of this message
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getRecipientID() { return recipientID; }
+
+	/**
+	 * @return the date at which this message was created
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public LocalDateTime getCreationDate() { return creationDate; }
+
+	/**
+	 * @return the date at which the message has been received by the sender
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public LocalDateTime getReceivedDate() { return receivedDate; }
+
+	/**
+	 * @param receivedDate the date at which the message has been received by the
+	 *                     sender
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public void setReceivedDate(LocalDateTime receivedDate) { this.receivedDate = receivedDate; }
+
+	/**
+	 * @return the date at which the message has been read by the sender
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public LocalDateTime getReadDate() { return readDate; }
+
+	/**
+	 * @param readDate at which the message has been read by the sender
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public void setReadDate(LocalDateTime readDate) { this.readDate = readDate; }
+
+	/**
+	 * @return the text content of this message
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public String getText() { return text; }
+
+	/**
+	 * @return the messageAttachment
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public Attachment getAttachment() { return attachment; }
+
+	/**
+	 * @return {@code true} if an attachment is present
+	 * @since Envoy Common v0.1-beta
+	 */
+	public boolean hasAttachment() { return attachment != null; }
+
+	/**
+	 * @return the current status of this message
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageStatus getStatus() { return status; }
+
+	/**
+	 * @param status the new {@link MessageStatus}, if permitted
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public void setStatus(MessageStatus status) {
+		if (status.ordinal() < this.status.ordinal()) throw new IllegalStateException("This message is moving backwards in time");
+		this.status = status;
+	}
+
+	/**
+	 * @return whether this message was forwarded
+	 * @since Envoy common v0.1-beta
+	 */
+	public boolean isForwarded() { return forwarded; }
+}

common/src/main/java/envoy/data/MessageBuilder.java

@@ -0,0 +1,240 @@
+package envoy.data;
+
+import java.time.LocalDateTime;
+import java.util.HashMap;
+import java.util.Map;
+
+import envoy.data.Message.MessageStatus;
+
+/**
+ * Provides a method of constructing the {@link Message} class.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>MessageBuilder.java</strong><br>
+ * Created: <strong>31.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class MessageBuilder {
+
+	// Mandatory properties without default values
+	private final long senderID, recipientID;
+
+	// Properties with default values
+	private long					id;
+	private LocalDateTime			creationDate, receivedDate, readDate;
+	private String					text;
+	private Attachment				attachment;
+	private Message.MessageStatus	status;
+	private boolean					forwarded;
+
+	/**
+	 * Creates an instance of {@link MessageBuilder} with all mandatory values
+	 * without defaults for the {@link Message} class.
+	 *
+	 * @param senderID    the ID of the user who sends the {@link Message}
+	 * @param recipientID the ID of the user who receives the {@link Message}
+	 * @param idGenerator the ID generator used to generate a unique {@link Message}
+	 *                    id
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder(long senderID, long recipientID, IDGenerator idGenerator) { this(senderID, recipientID, idGenerator.next()); }
+
+	/**
+	 * Creates an instance of {@link MessageBuilder} with all mandatory values
+	 * without defaults for the {@link Message} class.
+	 *
+	 * @param senderID    the ID of the user who sends the {@link Message}
+	 * @param recipientID the ID of the user who receives the {@link Message}
+	 * @param messageId   the ID of the {@link Message}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder(long senderID, long recipientID, long messageId) {
+		this.senderID		= senderID;
+		this.recipientID	= recipientID;
+		id					= messageId;
+	}
+
+	/**
+	 * This constructor transforms a given {@link Message} into a new message for a
+	 * new receiver.
+	 * This makes it especially useful in the case of forwarding messages.
+	 *
+	 * @param msg         the message to copy
+	 * @param recipientID the ID of the user who receives the {@link Message}
+	 * @param iDGenerator the ID generator used to generate a unique {@link Message}
+	 *                    id
+	 * @since Envoy v0.1-beta
+	 */
+	public MessageBuilder(Message msg, long recipientID, IDGenerator iDGenerator) {
+		this(msg.getRecipientID(), recipientID, iDGenerator.next());
+		this.attachment		= msg.getAttachment();
+		this.creationDate	= LocalDateTime.now();
+		this.forwarded		= true;
+		this.text			= msg.getText();
+		this.status			= MessageStatus.WAITING;
+	}
+
+	/**
+	 * Creates an instance of {@link Message} with the previously supplied values.
+	 * If a mandatory value is not set, a default value will be used instead:<br>
+	 * <br>
+	 * <table border="1">
+	 * <tr>
+	 * <td>{@code date}</td>
+	 * <td>{@code LocalDateTime.now()} and {@code null} for {@code receivedDate} and
+	 * {@code readDate}</td>
+	 * <tr>
+	 * <tr>
+	 * <td>{@code text}</td>
+	 * <td>{@code ""}</td>
+	 * <tr>
+	 * <tr>
+	 * <td>{@code status}</td>
+	 * <td>{@code MessageStatus.WAITING}</td>
+	 * <tr>
+	 * </table>
+	 *
+	 * @return a new instance of {@link Message}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public Message build() {
+		supplyDefaults();
+		return new Message(id, senderID, recipientID, creationDate, receivedDate, readDate, text, attachment, status, forwarded);
+	}
+
+	/**
+	 * Creates an instance of {@link GroupMessage} with the previously supplied
+	 * values. <br>
+	 * <b> Sets all member statuses to {@link MessageStatus#WAITING}.</b><br>
+	 * If a mandatory value is not set, a default value will be used
+	 * instead:<br>
+	 * <br>
+	 * <table border="1">
+	 * <tr>
+	 * <td>{@code date}</td>
+	 * <td>{@code new Date()}</td>
+	 * <tr>
+	 * <tr>
+	 * <td>{@code text}</td>
+	 * <td>{@code ""}</td>
+	 * <tr>
+	 * </table>
+	 *
+	 * @param group the {@link Group} that is used to fill the map of member
+	 *              statuses
+	 * @return a new instance of {@link GroupMessage}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public GroupMessage buildGroupMessage(Group group) {
+		var memberStatuses = new HashMap<Long, Message.MessageStatus>();
+		group.getContacts().forEach(user -> memberStatuses.put(user.getID(), MessageStatus.WAITING));
+		return buildGroupMessage(group, memberStatuses);
+	}
+
+	/**
+	 * Creates an instance of {@link GroupMessage} with the previously supplied
+	 * values. If a mandatory value is not set, a default value will be used
+	 * instead:<br>
+	 * <br>
+	 * <table border="1">
+	 * <tr>
+	 * <td>{@code date}</td>
+	 * <td>{@code new Date()}</td>
+	 * <tr>
+	 * <tr>
+	 * <td>{@code text}</td>
+	 * <td>{@code ""}</td>
+	 * <tr>
+	 * </table>
+	 *
+	 * @param group          the {@link Group} that is used to fill the map of
+	 *                       member statuses
+	 * @param memberStatuses the map of all current statuses
+	 * @return a new instance of {@link GroupMessage}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public GroupMessage buildGroupMessage(Group group, Map<Long, MessageStatus> memberStatuses) {
+		if (group == null || memberStatuses == null) throw new NullPointerException();
+		supplyDefaults();
+		return new GroupMessage(id, senderID, recipientID, creationDate, receivedDate, readDate, text, attachment, status, forwarded, memberStatuses);
+	}
+
+	private void supplyDefaults() {
+		if (creationDate == null) creationDate = LocalDateTime.now();
+		if (text == null) text = "";
+		if (status == null) status = MessageStatus.WAITING;
+	}
+
+	/**
+	 * @param creationDate the creation date of the {@link Message} to create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder setCreationDate(LocalDateTime creationDate) {
+		this.creationDate = creationDate;
+		return this;
+	}
+
+	/**
+	 * @param receivedDate the received date of the {@link Message} to create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public MessageBuilder setReceivedDate(LocalDateTime receivedDate) {
+		this.receivedDate = receivedDate;
+		return this;
+	}
+
+	/**
+	 * @param readDate the read date of the {@link Message} to create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public MessageBuilder setReadDate(LocalDateTime readDate) {
+		this.readDate = readDate;
+		return this;
+	}
+
+	/**
+	 * @param text the text of the {@link Message} to create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder setText(String text) {
+		this.text = text;
+		return this;
+	}
+
+	/**
+	 * @param attachment the {@link Attachment} of the {@link Message} to
+	 *                   create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder setAttachment(Attachment attachment) {
+		this.attachment = attachment;
+		return this;
+	}
+
+	/**
+	 * @param status the {@link MessageStatus} of the {@link Message} to create
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageBuilder setStatus(Message.MessageStatus status) {
+		this.status = status;
+		return this;
+	}
+
+	/**
+	 * @param forwarded sets whether this message is a forwarded message or not
+	 * @return this {@link MessageBuilder}
+	 * @since Envoy Common v0.1-beta
+	 */
+	public MessageBuilder setForwarded(boolean forwarded) {
+		this.forwarded = forwarded;
+		return this;
+	}
+}

common/src/main/java/envoy/data/User.java

@@ -0,0 +1,137 @@
+package envoy.data;
+
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * Represents a unique user with a unique, numeric ID, a name and a current
+ * {@link UserStatus}.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>User.java</strong><br>
+ * Created: <strong>28.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public final class User extends Contact {
+
+	private UserStatus status;
+
+	/**
+	 * Used to serialize contact list to a maximum depth of one.
+	 */
+	private transient boolean serializeContacts = true;
+
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * This enumeration defines all possible statuses a user can have.
+	 *
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static enum UserStatus {
+
+		/**
+		 * select this, if a user is online and can be interacted with
+		 */
+		ONLINE,
+
+		/**
+		 * select this, if a user is online but unavailable at the moment (sudden
+		 * interruption)
+		 */
+		AWAY,
+
+		/**
+		 * select this, if a user is online but unavailable at the moment (polite way)
+		 */
+		BUSY,
+
+		/**
+		 * select this, if a user is offline
+		 */
+		OFFLINE;
+	}
+
+	/**
+	 * Initializes a {@link User}. <br>
+	 * The {@link UserStatus} is set to {@link UserStatus#ONLINE}.
+	 * No contacts are initialized.
+	 *
+	 * @param id   unique ID
+	 * @param name user name
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public User(long id, String name) {
+		super(id, name, new HashSet<>());
+		status = UserStatus.ONLINE;
+	}
+
+	/**
+	 * Initializes a {@link User}. <br>
+	 * The {@link UserStatus} is set to {@link UserStatus#ONLINE}.
+	 *
+	 * @param id       unique ID
+	 * @param name     user name
+	 * @param contacts the contacts of this user
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public User(long id, String name, Set<Contact> contacts) {
+		super(id, name, contacts);
+		status = UserStatus.ONLINE;
+	}
+
+	/**
+	 * Initializes a {@link User}.
+	 *
+	 * @param id       unique ID
+	 * @param name     user name
+	 * @param status   the status of this user
+	 * @param contacts the contacts of this user
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public User(long id, String name, UserStatus status, Set<Contact> contacts) {
+		super(id, name, contacts);
+		this.status = status;
+	}
+
+	@Override
+	public String toString() { return String.format("User[id=%d,name=%s,status=%s,contacts=%s]", getID(), getName(), status, getContacts()); }
+
+	/**
+	 * @return the current status of this user
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public UserStatus getStatus() { return status; }
+
+	/**
+	 * @param status the next status of this user
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public void setStatus(UserStatus status) { this.status = status; }
+
+	private void readObject(ObjectInputStream inputStream) throws Exception {
+		inputStream.defaultReadObject();
+		var contacts = Contact.class.getDeclaredField("contacts");
+		contacts.setAccessible(true);
+		contacts.set(this, inputStream.readObject());
+	}
+
+	private void writeObject(ObjectOutputStream outputStream) throws Exception {
+		outputStream.defaultWriteObject();
+		if (serializeContacts) {
+			getContacts().stream().filter(User.class::isInstance).map(User.class::cast).forEach(user -> user.serializeContacts = false);
+			outputStream.writeObject(getContacts());
+		} else outputStream.writeObject(new HashSet<>());
+	}
+
+	/**
+	 * @param serializeContacts whether the contacts of this {@link User} should be
+	 *                          serialized
+	 * @since Envoy Common v0.1-beta
+	 */
+	public void serializeContacts(boolean serializeContacts) { this.serializeContacts = serializeContacts; }
+}

common/src/main/java/envoy/data/package-info.java

@@ -0,0 +1,10 @@
+/**
+ * This package contains all data objects that are used both by Envoy Client and
+ * by Envoy Server Standalone.
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian Käfer
+ * @author Kai S.K. Engelbart
+ * @since Envoy common v0.1-beta
+ */
+package envoy.data;

common/src/main/java/envoy/event/ElementOperation.java

@@ -0,0 +1,28 @@
+package envoy.event;
+
+/**
+ * This enum declares all modification possibilities for a given container.<br>
+ * These can be: {@link ElementOperation#ADD} or
+ * {@link ElementOperation#REMOVE}.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>ElementOperation.java</strong><br>
+ * Created: <strong>25 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.1-beta
+ */
+public enum ElementOperation {
+
+	/**
+	 * Select this element, if the given element should be added to the given
+	 * container.
+	 */
+	ADD,
+
+	/**
+	 * Select this element, if the given element should be removed from the given
+	 * container.
+	 */
+	REMOVE
+}

common/src/main/java/envoy/event/Event.java

@@ -0,0 +1,49 @@
+package envoy.event;
+
+import java.io.Serializable;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Event.java</strong><br>
+ * Created: <strong>04.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @param <T> the type of the Event
+ * @since Envoy v0.2-alpha
+ */
+public abstract class Event<T> implements Serializable {
+
+	protected final T value;
+
+	private static final long serialVersionUID = 0L;
+
+	protected Event(T value) { this.value = value; }
+
+	/**
+	 * @return the data associated with this event
+	 */
+	public T get() { return value; }
+
+	@Override
+	public String toString() { return String.format("%s[value=%s]", this.getClass().getSimpleName(), value); }
+
+	/**
+	 * Serves as a super class for events that do not carry a value.<br>
+	 * <br>
+	 * Project: <strong>envoy-common</strong><br>
+	 * File: <strong>Event.java</strong><br>
+	 * Created: <strong>11 Feb 2020</strong><br>
+	 *
+	 * @author Kai S. K. Engelbart
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static abstract class Valueless extends Event<Void> {
+
+		private static final long serialVersionUID = 0L;
+
+		protected Valueless() { super(null); }
+
+		@Override
+		public String toString() { return this.getClass().getSimpleName(); }
+	}
+}

common/src/main/java/envoy/event/EventBus.java

@@ -0,0 +1,83 @@
+package envoy.event;
+
+import java.util.*;
+import java.util.function.Consumer;
+
+/**
+ * This class handles events by allowing event handlers to register themselves
+ * and then be notified about certain events dispatched by the event bus.<br>
+ * <br>
+ * The event bus is a singleton and can be used across the entire application to
+ * guarantee the propagation of events.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>EventBus.java</strong><br>
+ * Created: <strong>04.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy v0.2-alpha
+ */
+public class EventBus {
+
+	/**
+	 * Contains all event handler instances registered at this event bus as values
+	 * mapped to by their supported event classes.
+	 */
+	private Map<Class<? extends Event<?>>, List<Consumer<Event<?>>>> handlers = new HashMap<>();
+
+	/**
+	 * The singleton instance of this event bus that is used across the
+	 * entire application.
+	 */
+	private static EventBus eventBus = new EventBus();
+
+	/**
+	 * This constructor is not accessible from outside this class because a
+	 * singleton instance of it is provided by the {@link EventBus#getInstance()}
+	 * method.
+	 */
+	private EventBus() {}
+
+	/**
+	 * @return the singleton instance of the event bus
+	 * @since Envoy v0.2-alpha
+	 */
+	public static EventBus getInstance() { return eventBus; }
+
+	/**
+	 * Registers an event handler to be notified when an
+	 * event of a certain type is dispatched.
+	 *
+	 * @param <T>        the type of event values to notify the handler about
+	 * @param eventClass the class which the event handler is subscribing to
+	 * @param handler    the event handler to register
+	 * @since Envoy v0.2-alpha
+	 */
+	@SuppressWarnings("unchecked")
+	public <T extends Event<?>> void register(Class<T> eventClass, Consumer<T> handler) {
+		if (!handlers.containsKey(eventClass)) handlers.put(eventClass, new ArrayList<>());
+		handlers.get(eventClass).add((Consumer<Event<?>>) handler);
+	}
+
+	/**
+	 * Dispatches an event to every event handler subscribed to it.
+	 *
+	 * @param event the {@link Event} to dispatch
+	 * @since Envoy v0.2-alpha
+	 */
+	public void dispatch(Event<?> event) {
+		handlers.keySet()
+			.stream()
+			.filter(event.getClass()::equals)
+			.map(handlers::get)
+			.flatMap(List::stream)
+			.forEach(h -> h.accept(event));
+	}
+
+	/**
+	 * @return a map of all event handler instances currently registered at this
+	 *         event bus with the event classes they are subscribed to as keys
+	 * @since Envoy v0.2-alpha
+	 */
+	public Map<Class<? extends Event<?>>, List<Consumer<Event<?>>>> getHandlers() { return handlers; }
+}

common/src/main/java/envoy/event/GroupCreation.java

@@ -0,0 +1,42 @@
+package envoy.event;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import envoy.data.User;
+
+/**
+ * This event creates a group with the given name.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>GroupCreation.java</strong><br>
+ * Created: <strong>25 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.1-beta
+ */
+public class GroupCreation extends Event<String> {
+
+	private final Set<Long> initialMemberIDs;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * @param value            the name of this group at creation time
+	 * @param initialMemberIDs the IDs of all {@link User}s that should be group
+	 *                         members from the beginning on (excluding the creator
+	 *                         of this group)
+	 * @since Envoy Common v0.1-beta
+	 */
+	public GroupCreation(String value, Set<Long> initialMemberIDs) {
+		super(value);
+		this.initialMemberIDs = (initialMemberIDs != null) ? initialMemberIDs : new HashSet<>();
+	}
+
+	/**
+	 * @return the IDs of all {@link User}s that are members from the beginning
+	 *         (excluding the creator of this group)
+	 * @since Envoy Common v0.1-beta
+	 */
+	public Set<Long> getInitialMemberIDs() { return initialMemberIDs; }
+}

common/src/main/java/envoy/event/GroupMessageStatusChange.java

@@ -0,0 +1,45 @@
+package envoy.event;
+
+import java.time.LocalDateTime;
+
+import envoy.data.GroupMessage;
+import envoy.data.Message.MessageStatus;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>GroupMessageStatusChange.java</strong><br>
+ * Created: <strong>18.04.2020</strong><br>
+ * 
+ * @author Maximilian K&auml;fer
+ * @since Envoy Common v0.1-beta
+ */
+public class GroupMessageStatusChange extends MessageStatusChange {
+
+	private final long memberID;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link GroupMessageStatusChange}.
+	 * 
+	 * @param id       the ID of the {@link GroupMessage} this event is related to
+	 * @param status   the status of this specific members {@link GroupMessage}
+	 * @param date     the date at which the MessageStatus change occurred for
+	 *                 this specific member
+	 * @param memberID the ID of the group member that caused the status change
+	 * @since Envoy Common v0.1-beta
+	 */
+	public GroupMessageStatusChange(long id, MessageStatus status, LocalDateTime date, long memberID) {
+		super(id, status, date);
+		this.memberID = memberID;
+	}
+
+	/**
+	 * @return the memberID which the user who sends this event has
+	 * @since Envoy Common v0.1-beta
+	 */
+	public long getMemberID() { return memberID; }
+
+	@Override
+	public String toString() { return String.format("GroupMessageStatusChange[meta=%s,memberID=%d]", super.toString(), memberID); }
+}

common/src/main/java/envoy/event/GroupResize.java

@@ -0,0 +1,65 @@
+package envoy.event;
+
+import envoy.data.Contact;
+import envoy.data.Group;
+import envoy.data.User;
+
+/**
+ * This event is used to communicate changes in the group size between client
+ * and server.<br>
+ * Possible actions are adding or removing certain {@link User}s to or from a
+ * certain {@link Group}.
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>GroupResize.java</strong><br>
+ * Created: <strong>25 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.1-beta
+ */
+public class GroupResize extends Event<User> {
+
+	private final long				groupID;
+	private final ElementOperation	operation;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link GroupResize} through a Contact where the name has
+	 * already been set.
+	 *
+	 * @param user      the {@link User} who wants to join or leave a group
+	 * @param group     the {@link Group} he wants to join or leave
+	 * @param operation describes what to do with the given user:<br>
+	 *                  add him to this group or remove him from it
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public GroupResize(User user, Group group, ElementOperation operation) {
+		super(user);
+		if (group.getContacts().contains(user)) {
+			if (operation.equals(ElementOperation.ADD)) throw new IllegalArgumentException(
+					"Cannot add " + user + " to group " + group.getID() + " because he is already a member of this group");
+		} else if (operation.equals(ElementOperation.REMOVE))
+			throw new IllegalArgumentException("Cannot remove " + user + " from group " + group.getID() + " because he is no part of this group");
+		groupID			= group.getID();
+		this.operation	= operation;
+	}
+
+	/**
+	 * @return the ID of the {@link Contact} this event is related to
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getGroupID() { return groupID; }
+
+	/**
+	 * @return the operationType
+	 * @since Envoy Common v0.1-beta
+	 */
+	public ElementOperation getOperation() { return operation; }
+
+	/**
+	 * {@inheritDoc}
+	 */
+	@Override
+	public String toString() { return String.format("GroupResize[userid=%d,groupid=%d,operation=%s]", get(), groupID, operation); }
+}

common/src/main/java/envoy/event/HandshakeRejection.java

@@ -0,0 +1,63 @@
+package envoy.event;
+
+/**
+ * Signifies to the client that the handshake failed for the attached
+ * reason.
+ * <p>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>HandshakeRejection.java</strong><br>
+ * Created: <strong>28 Jan 2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.3-alpha
+ */
+public class HandshakeRejection extends Event<String> {
+
+	/**
+	 * Select this value if a given password hash or user name was incorrect.
+	 * 
+	 * @since Envoy Common v0.3-alpha
+	 */
+	public static final String WRONG_PASSWORD_OR_USER = "Incorrect user name or password.";
+
+	/**
+	 * Select this value if a given user name for a registration is already taken.
+	 * 
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static final String USERNAME_TAKEN = "Incorrect user name or password.";
+
+	/**
+	 * Select this value if the version of the client is incompatible with the
+	 * server.
+	 * 
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static final String WRONG_VERSION = "Incompatible client version";
+
+	/**
+	 * Select this value if the handshake could not be completed for some different
+	 * reason.
+	 * 
+	 * @since Envoy Common v0.3-alpha
+	 */
+	public static final String INTERNAL_ERROR = "An internal error occured.";
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Creates an instance of {@link HandshakeRejection} with the generic
+	 * {@link HandshakeRejection#INTERNAL_ERROR} reason.
+	 *
+	 * @since Envoy Common v0.3-alpha
+	 */
+	public HandshakeRejection() { super(INTERNAL_ERROR); }
+
+	/**
+	 * Creates an instance of {@link HandshakeRejection}.
+	 *
+	 * @param reason the reason why the handshake was rejected
+	 * @since Envoy Common v0.3-alpha
+	 */
+	public HandshakeRejection(String reason) { super(reason); }
+}

common/src/main/java/envoy/event/IDGeneratorRequest.java

@@ -0,0 +1,17 @@
+package envoy.event;
+
+/**
+ * Signifies to the server that the client needs a new
+ * {@link envoy.data.IDGenerator} instance.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>IDGeneratorRequest.java</strong><br>
+ * Created: <strong>28 Jan 2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.3-alpha
+ */
+public class IDGeneratorRequest extends Event.Valueless {
+
+	private static final long serialVersionUID = 1431107413883364583L;
+}

common/src/main/java/envoy/event/MessageStatusChange.java

@@ -0,0 +1,59 @@
+package envoy.event;
+
+import java.time.LocalDateTime;
+
+import envoy.data.Message;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>MessageStatusChange.java</strong><br>
+ * Created: <strong>6 Jan 2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class MessageStatusChange extends Event<Message.MessageStatus> {
+
+	private final long	id;
+	private final LocalDateTime	date;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link MessageStatusChange}.
+	 *
+	 * @param id     the ID of the {@link Message} this event is related to
+	 * @param status the status of the {@link Message} this event is related
+	 *               to
+	 * @param date   the date at which the MessageStatus change occurred
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageStatusChange(long id, Message.MessageStatus status, LocalDateTime date) {
+		super(status);
+		this.id		= id;
+		this.date	= date;
+	}
+
+	/**
+	 * Initializes a {@link MessageStatusChange} through a message.
+	 *
+	 * @param message the message from which to build the event
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public MessageStatusChange(Message message) { this(message.getID(), message.getStatus(), LocalDateTime.now()); }
+
+	/**
+	 * @return the ID of the {@link Message} this event is related to
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getID() { return id; }
+
+	/**
+	 * @return the date at which the status change occurred
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public LocalDateTime getDate() { return date; }
+
+	@Override
+	public String toString() { return String.format("MessageStatusChange[id=%d,status=%s,date=%s]", id, value, date); }
+}

common/src/main/java/envoy/event/NameChange.java

@@ -0,0 +1,52 @@
+package envoy.event;
+
+import envoy.data.Contact;
+
+/**
+ * This event informs<br>
+ * a) the server of the name change of a user or a group.
+ * b) another user of this users name change.
+ *
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>NameChange.java</strong><br>
+ * Created: <strong>25 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.1-beta
+ */
+public class NameChange extends Event<String> {
+
+	private final long id;
+
+	private static final long	serialVersionUID	= 0L;
+
+	/**
+	 * Creates a new {@link NameChange} for a user or a group.
+	 *
+	 * @param contactID the id of the {@link Contact} who wishes to change his name
+	 * @param newName   the new name of this contact
+	 * @since Envoy Common v0.1-beta
+	 */
+	public NameChange(long contactID, String newName) {
+		super(newName);
+		id = contactID;
+	}
+
+	/**
+	 * Initializes a {@link NameChange} through a Contact where the name has
+	 * already been set.
+	 *
+	 * @param contact the contact whose name was updated
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public NameChange(Contact contact) { this(contact.getID(), contact.getName()); }
+
+	/**
+	 * @return the ID of the {@link Contact} this event is related to
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getID() { return id; }
+
+	@Override
+	public String toString() { return String.format("NameChange[id=%d,name=%s]", id, value); }
+}

common/src/main/java/envoy/event/UserStatusChange.java

@@ -0,0 +1,49 @@
+package envoy.event;
+
+import envoy.data.User;
+import envoy.data.User.UserStatus;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>UserStatusChange.java</strong><br>
+ * Created: <strong>1 Feb 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @since Envoy Common v0.2-alpha
+ */
+public class UserStatusChange extends Event<UserStatus> {
+
+	private final long id;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link UserStatusChange}.
+	 *
+	 * @param id     the ID of the {@link User} this event is related to
+	 * @param status the status of the {@link User} this event is related
+	 *               to
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public UserStatusChange(long id, User.UserStatus status) {
+		super(status);
+		this.id = id;
+	}
+
+	/**
+	 * Initializes a {@link UserStatusChange} through a User.
+	 *
+	 * @param user the User from which to build the event
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public UserStatusChange(User user) { this(user.getID(), user.getStatus()); }
+
+	/**
+	 * @return the ID of the {@link User} this event is related to
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public long getID() { return id; }
+
+	@Override
+	public String toString() { return String.format("UserStatusChange[id=%d,status=%s]", id, value); }
+}

common/src/main/java/envoy/event/contact/ContactOperation.java

@@ -0,0 +1,40 @@
+package envoy.event.contact;
+
+import envoy.data.Contact;
+import envoy.event.ElementOperation;
+import envoy.event.Event;
+
+/**
+ * Signifies the modification of a contact list.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>ContactOperation.java</strong><br>
+ * Created: <strong>05.02.2020</strong><br>
+ *
+ * @author Maximilian K&auml;fer
+ * @since Envoy Common v0.2-alpha
+ */
+public class ContactOperation extends Event<Contact> {
+
+	private final ElementOperation operationType;
+
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * Initializes a {@link ContactOperation}.
+	 *
+	 * @param contact       the user on which the operation is performed
+	 * @param operationType the type of operation to perform
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public ContactOperation(Contact contact, ElementOperation operationType) {
+		super(contact);
+		this.operationType = operationType;
+	}
+
+	/**
+	 * @return the type of operation to perform
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public ElementOperation getOperationType() { return operationType; }
+}

common/src/main/java/envoy/event/contact/ContactSearchRequest.java

@@ -0,0 +1,26 @@
+package envoy.event.contact;
+
+import envoy.event.Event;
+
+/**
+ * Requests a contact search from the server.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>ContactSearchRequest.java</strong><br>
+ * Created: <strong>05.02.2020</strong><br>
+ *
+ * @author Maximilian K&auml;fer
+ * @since Envoy Common v0.2-alpha
+ */
+public class ContactSearchRequest extends Event<String> {
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link ContactSearchRequest}.
+	 *
+	 * @param searchPhrase the search phrase to use in the contact search
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public ContactSearchRequest(String searchPhrase) { super(searchPhrase); }
+}

common/src/main/java/envoy/event/contact/ContactSearchResult.java

@@ -0,0 +1,29 @@
+package envoy.event.contact;
+
+import java.util.List;
+
+import envoy.data.Contact;
+import envoy.event.Event;
+
+/**
+ * Contains a list of {@link Contact}s for which a search was performed.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>ContactSearchResult.java</strong><br>
+ * Created: <strong>11 Feb 2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class ContactSearchResult extends Event<List<Contact>> {
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Creates an instance of {@link ContactSearchResult}.
+	 *
+	 * @param users the users found during the search
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public ContactSearchResult(List<Contact> users) { super(users); }
+}

common/src/main/java/envoy/event/contact/package-info.java

@@ -0,0 +1,13 @@
+/**
+ * This package contains all contact-related events.<br>
+ * <br>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>package-info.java</strong><br>
+ * Created: <strong>28 Mar 2020</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian K&auml;fer
+ * @author Kai S.K. Engelbart
+ * @since Envoy Common v0.1-beta
+ */
+package envoy.event.contact;

common/src/main/java/envoy/event/package-info.java

@@ -0,0 +1,10 @@
+/**
+ * This package contains all events that can be sent or received by Envoy Client
+ * or Envoy Server Standalone.
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian Käfer
+ * @author Kai S.K. Engelbart
+ * @since Envoy common v0.1-beta
+ */
+package envoy.event;

common/src/main/java/envoy/exception/EnvoyException.java

@@ -0,0 +1,35 @@
+package envoy.exception;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>EnvoyException.java</strong><br>
+ * Created: <strong>27 Oct 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy v0.1-alpha
+ */
+public class EnvoyException extends Exception {
+
+	private static final long serialVersionUID = 2096147309395387479L;
+
+	/**
+	 * @param message the message to display once this Exception is thrown
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public EnvoyException(String message) { super(message); }
+
+	/**
+	 * @param message the message to display once this Exception is thrown
+	 * @param cause   the {@link Throwable} which resulted in the throw of an
+	 *                EnvoyException
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public EnvoyException(String message, Throwable cause) { super(message, cause); }
+
+	/**
+	 * @param cause the {@link Throwable} which resulted in the throw of an
+	 *              EnvoyException
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public EnvoyException(Throwable cause) { super(cause); }
+}

common/src/main/java/envoy/exception/package-info.java

@@ -0,0 +1,9 @@
+/**
+ * This package contains all Envoy-specific exceptions.
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian Käfer
+ * @author Kai S.K. Engelbart
+ * @since Envoy common v0.1-beta
+ */
+package envoy.exception;

common/src/main/java/envoy/util/Bounds.java

@@ -0,0 +1,40 @@
+package envoy.util;
+
+import java.util.regex.Pattern;
+
+/**
+ * Implements contact name validation.
+ * <p>
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>Bounds.java</strong><br>
+ * Created: <strong>25.06.2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.1-beta
+ */
+public class Bounds {
+
+	private Bounds() {}
+
+	/**
+	 * The regular expression against which contact names should be validated.
+	 *
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static final Pattern CONTACT_NAME_PATTERN = Pattern.compile("^\\w[a-zA-Z0-9-]{2,15}$");
+
+	/**
+	 * @param contactName the contact name to validate
+	 * @return {@code true} if the given contact name is valid
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static boolean isValidContactName(String contactName) { return CONTACT_NAME_PATTERN.matcher(contactName).matches(); }
+
+	/**
+	 * @return the maximum size allowed for a user/ group name.
+	 * @apiNote has to be updated manually if {@link Bounds#CONTACT_NAME_PATTERN}
+	 *          gets updated.
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static int maximumUsernameSize() { return 16; }
+}

common/src/main/java/envoy/util/EnvoyLog.java

@@ -0,0 +1,126 @@
+package envoy.util;
+
+import java.io.File;
+import java.io.IOException;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.logging.*;
+
+import envoy.data.Config;
+
+/**
+ * Configures the {@link java.util.logging} API to output the log into the
+ * console and a log file.
+ * <p>
+ * Call the {@link EnvoyLog#attach(String)} method to configure a part of the
+ * logger hierarchy.
+ * <p>
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>EnvoyLogger.java</strong><br>
+ * Created: <strong>14 Dec 2019</strong><br>
+ *
+ * @author Leon Hofmeister
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.1-beta
+ */
+public class EnvoyLog {
+
+	private static FileHandler		fileHandler;
+	private static StreamHandler	consoleHandler;
+	private static boolean			initialized;
+
+	private EnvoyLog() {}
+
+	/**
+	 * Initializes logging. Call this method before calling the
+	 * {@link EnvoyLog#attach(String)} method.
+	 *
+	 * @param config the config providing the console and log file barriers
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static void initialize(Config config) {
+		if (initialized) throw new IllegalStateException("EnvoyLog is already initialized");
+
+		// Remove default console handler
+		LogManager.getLogManager().reset();
+
+		// Configure log file
+		final File logFile = new File((File) config.get("homeDirectory").get(),
+				"log/envoy_user_" + DateTimeFormatter.ofPattern("yyyy-MM-dd--hh-mm-mm").format(LocalDateTime.now()) + ".log");
+		logFile.getParentFile().mkdirs();
+
+		// Configure formatting
+		// Sample log entry: [2020-06-13 16:50:26] [INFORMATION] [envoy.client.ui.Startup] Closing connection...
+		System.setProperty("java.util.logging.SimpleFormatter.format", "[%1$tF %1$tT] [%4$-7s] [%3$s] %5$s %6$s%n");
+		final SimpleFormatter formatter = new SimpleFormatter();
+
+		try {
+			fileHandler = new FileHandler(logFile.getAbsolutePath());
+			fileHandler.setLevel((Level) config.get("fileLevelBarrier").get());
+			fileHandler.setFormatter(formatter);
+		} catch (SecurityException | IOException e) {
+			e.printStackTrace();
+		}
+		consoleHandler = new StreamHandler(System.out, formatter) {
+
+			@Override
+			public synchronized void publish(LogRecord record) {
+				super.publish(record);
+				flush();
+			}
+		};
+		consoleHandler.setLevel((Level) config.get("consoleLevelBarrier").get());
+		consoleHandler.setFormatter(formatter);
+
+		initialized = true;
+	}
+
+	/**
+	 * Configures all loggers that are contained within the hierarchy of a specific
+	 * path to use the console and file handlers.
+	 *
+	 * @param path the path to the loggers to configure
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static void attach(String path) {
+		if (!initialized) throw new IllegalStateException("EnvoyLog is not initialized");
+
+		// Get root logger
+		final Logger logger = Logger.getLogger(path);
+
+		// Add handlers
+		if (fileHandler != null) logger.addHandler(fileHandler);
+		logger.addHandler(consoleHandler);
+
+		// Delegate logger level filtering to the handlers
+		logger.setLevel(Level.ALL);
+	}
+
+	/**
+	 * Creates a logger for a specified class, which output will be displayed inside
+	 * the console and written to the log file.
+	 *
+	 * @param logClass the class in which the logger is used
+	 * @return the created logger
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static Logger getLogger(Class<?> logClass) { return Logger.getLogger(logClass.getCanonicalName()); }
+
+	/**
+	 * Defines the logger level required for a record to be written to the log file.
+	 *
+	 * @param fileLevelBarrier the log file level
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static void setFileLevelBarrier(Level fileLevelBarrier) { if (fileHandler != null) fileHandler.setLevel(fileLevelBarrier); }
+
+	/**
+	 * Defines the logger level required for a record to be written to the console.
+	 *
+	 * @param consoleLevelBarrier the console logger level
+	 * @since Envoy Common v0.1-beta
+	 */
+	public static void setConsoleLevelBarrier(Level consoleLevelBarrier) {
+		if (consoleHandler != null) consoleHandler.setLevel(consoleLevelBarrier);
+	}
+}

common/src/main/java/envoy/util/SerializationUtils.java

@@ -0,0 +1,153 @@
+package envoy.util;
+
+import java.io.*;
+
+/**
+ * Defines utility methods related to serialization.
+ * <p>
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>SerializationUtils.java</strong><br>
+ * Created: <strong>23.12.2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Common v0.2-alpha
+ */
+public class SerializationUtils {
+
+	private SerializationUtils() {}
+
+	/**
+	 * Converts an integer into a byte array.
+	 *
+	 * @param n the integer to convert
+	 * @return a byte array of length 4
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static byte[] intToBytes(int n) { return new byte[] { (byte) (n >>> 24), (byte) (n >>> 16), (byte) (n >>> 8), (byte) n }; }
+
+	/**
+	 * Converts four bytes in byte array to an integer
+	 *
+	 * @param bytes  the bytes to convert from
+	 * @param offset the offset at which four bytes are read
+	 * @return the converted integer
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static int bytesToInt(byte[] bytes, int offset) {
+		return ((bytes[offset] & 0xFF) << 24) | ((bytes[offset + 1] & 0xFF) << 16) | ((bytes[offset + 2] & 0xFF) << 8)
+				| ((bytes[offset + 3] & 0xFF) << 0);
+	}
+
+	/**
+	 * Deserializes an arbitrary {@link Serializable} object from a file.
+	 *
+	 * @param <T>             the type of the serialized object
+	 * @param file            the file to deserialize from
+	 * @param serializedClass the class of the object to deserialize
+	 * @return the deserialized object
+	 * @throws IOException            if something failed while deserializing the
+	 *                                object
+	 * @throws ClassNotFoundException if the deserialized object can not be linked
+	 *                                to a class
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static <T extends Serializable> T read(File file, Class<T> serializedClass) throws IOException, ClassNotFoundException {
+		if (file == null) throw new NullPointerException("File is null");
+		return read(new FileInputStream(file), serializedClass);
+	}
+
+	/**
+	 * Deserializes an arbitrary {@link Serializable} object from a byte array.
+	 *
+	 * @param <T>             the type of the serialized object
+	 * @param bytes           the array in which the serialized object is stored
+	 * @param serializedClass the class of the serialized object
+	 * @return the deserialized object
+	 * @throws IOException            if something failed while deserializing the
+	 *                                object
+	 * @throws ClassNotFoundException if the deserialized object can not be linked
+	 *                                to a class
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static <T extends Serializable> T read(byte[] bytes, Class<T> serializedClass) throws IOException, ClassNotFoundException {
+		return read(new ByteArrayInputStream(bytes), serializedClass);
+	}
+
+	/**
+	 * Deserializes an arbitrary {@link Serializable} object from a stream.
+	 *
+	 * @param <T>             the type of the serialized object
+	 * @param in              the {@link InputStream} of a serialized Object
+	 * @param serializedClass the object type to convert the deserialized object
+	 *                        into
+	 * @return the deserialized object
+	 * @throws IOException            if something failed while deserializing the
+	 *                                object
+	 * @throws ClassNotFoundException if the deserialized object can not be linked
+	 *                                to a class
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static <T extends Serializable> T read(InputStream in, Class<T> serializedClass) throws IOException, ClassNotFoundException {
+		try (ObjectInputStream oin = new ObjectInputStream(in)) {
+			return serializedClass.cast(oin.readObject());
+		}
+	}
+
+	/**
+	 * Serializes arbitrary objects to a file.
+	 *
+	 * @param file the file to serialize to
+	 * @param objs the objects to serialize
+	 * @throws IOException if an error occurred during serialization
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static void write(File file, Object... objs) throws IOException {
+		if (file == null) throw new NullPointerException("File is null");
+		if (objs == null) throw new NullPointerException("Null array passed to serialize");
+		if (!file.exists()) {
+			file.getParentFile().mkdirs();
+			file.createNewFile();
+		}
+		try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(file))) {
+			for (var obj : objs)
+				out.writeObject(obj);
+		}
+	}
+
+	/**
+	 * Serializes an arbitrary object to a byte array.
+	 *
+	 * @param obj the object to serialize
+	 * @return a byte array containing the serialized object
+	 * @throws IOException if the serialization failed
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static byte[] writeToByteArray(Object obj) throws IOException {
+		ByteArrayOutputStream baos = new ByteArrayOutputStream();
+		try (ObjectOutputStream oout = new ObjectOutputStream(baos)) {
+			oout.writeObject(obj);
+		}
+		return baos.toByteArray();
+	}
+
+	/**
+	 * Serializes an object and writes it into an output stream preceded by 4 bytes
+	 * containing the number of serialized bytes.
+	 *
+	 * @param obj the object to serialize
+	 * @param out the output stream to serialize to
+	 * @throws IOException if an error occurred during serialization
+	 * @since Envoy Common v0.2-alpha
+	 */
+	public static void writeBytesWithLength(Object obj, OutputStream out) throws IOException {
+		// Serialize object to byte array
+		byte[] objBytes = writeToByteArray(obj);
+
+		// Get length of byte array in bytes
+		byte[] objLen = intToBytes(objBytes.length);
+
+		// Write length and byte array
+		out.write(objLen);
+		out.write(objBytes);
+	}
+}

common/src/main/java/envoy/util/package-info.java

@@ -0,0 +1,11 @@
+/**
+ * This package contains general useful classes that can be used by both Envoy
+ * Client and Envoy Server Standalone and that could not be assigned to any
+ * other package.
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian Käfer
+ * @author Kai S.K. Engelbart
+ * @since Envoy common v0.1-beta
+ */
+package envoy.util;

common/src/main/java/module-info.java

@@ -0,0 +1,19 @@
+/**
+ * This module contains all packages that are used by Envoy Client and Envoy
+ * Server Standalone at the same time.
+ *
+ * @author Leon Hofmeister
+ * @author Maximilian Käfer
+ * @author Kai S.K. Engelbart
+ * @since Envoy Common v0.1-beta
+ */
+module envoy.common {
+
+	exports envoy.data;
+	exports envoy.util;
+	exports envoy.exception;
+	exports envoy.event;
+	exports envoy.event.contact;
+
+	requires transitive java.logging;
+}