Further improvements.

The main component here is the total revamp of the plugin channel identifier system - instead of Legacy/Modern channel IDs, you can have a modern channel or a modern channel paired with a legacy channel, which is much less confusing to work with.

api/src/main/java/com/velocitypowered/api/event/connection/PluginMessageEvent.java

@@ -11,9 +11,9 @@ import com.google.common.io.ByteArrayDataInput;
 import com.velocitypowered.api.event.ResultedEvent;
 import com.velocitypowered.api.proxy.connection.Player;
 import com.velocitypowered.api.proxy.connection.ServerConnection;
-import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.api.proxy.messages.ChannelMessageSink;
 import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
+import com.velocitypowered.api.proxy.messages.PluginChannelId;
 import java.io.ByteArrayInputStream;
 
 /**
@@ -26,7 +26,7 @@ public interface PluginMessageEvent extends ResultedEvent<PluginMessageEvent.For
 
   ChannelMessageSink sink();
 
-  ChannelIdentifier channel();
+  PluginChannelId channel();
 
   byte[] rawMessage();
 

api/src/main/java/com/velocitypowered/api/event/connection/PluginMessageEventImpl.java

@@ -12,9 +12,9 @@ import com.google.common.io.ByteArrayDataInput;
 import com.google.common.io.ByteStreams;
 import com.velocitypowered.api.proxy.connection.Player;
 import com.velocitypowered.api.proxy.connection.ServerConnection;
-import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.api.proxy.messages.ChannelMessageSink;
 import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
+import com.velocitypowered.api.proxy.messages.PluginChannelId;
 import java.io.ByteArrayInputStream;
 import java.util.Arrays;
 
@@ -26,7 +26,7 @@ public final class PluginMessageEventImpl implements PluginMessageEvent {
 
   private final ChannelMessageSource source;
   private final ChannelMessageSink target;
-  private final ChannelIdentifier identifier;
+  private final PluginChannelId identifier;
   private final byte[] data;
   private ForwardResult result;
 
@@ -39,7 +39,7 @@ public final class PluginMessageEventImpl implements PluginMessageEvent {
    * @param data the payload of the plugin message
    */
   public PluginMessageEventImpl(ChannelMessageSource source, ChannelMessageSink target,
-      ChannelIdentifier identifier, byte[] data) {
+      PluginChannelId identifier, byte[] data) {
     this.source = Preconditions.checkNotNull(source, "source");
     this.target = Preconditions.checkNotNull(target, "target");
     this.identifier = Preconditions.checkNotNull(identifier, "identifier");
@@ -68,7 +68,7 @@ public final class PluginMessageEventImpl implements PluginMessageEvent {
   }
 
   @Override
-  public ChannelIdentifier channel() {
+  public PluginChannelId channel() {
     return identifier;
   }
 

api/src/main/java/com/velocitypowered/api/event/player/PlayerChannelRegisterEvent.java

@@ -8,7 +8,7 @@
 package com.velocitypowered.api.event.player;
 
 import com.velocitypowered.api.proxy.connection.Player;
-import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
+import com.velocitypowered.api.proxy.messages.PluginChannelId;
 import java.util.List;
 
 /**
@@ -19,5 +19,5 @@ public interface PlayerChannelRegisterEvent {
 
   Player player();
 
-  List<ChannelIdentifier> channels();
+  List<PluginChannelId> channels();
 }

api/src/main/java/com/velocitypowered/api/event/player/PlayerChannelRegisterEventImpl.java

@@ -9,7 +9,7 @@ package com.velocitypowered.api.event.player;
 
 import com.google.common.base.Preconditions;
 import com.velocitypowered.api.proxy.connection.Player;
-import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
+import com.velocitypowered.api.proxy.messages.PluginChannelId;
 import java.util.List;
 
 /**
@@ -19,9 +19,9 @@ import java.util.List;
 public final class PlayerChannelRegisterEventImpl implements PlayerChannelRegisterEvent {
 
   private final Player player;
-  private final List<ChannelIdentifier> channels;
+  private final List<PluginChannelId> channels;
 
-  public PlayerChannelRegisterEventImpl(Player player, List<ChannelIdentifier> channels) {
+  public PlayerChannelRegisterEventImpl(Player player, List<PluginChannelId> channels) {
     this.player = Preconditions.checkNotNull(player, "player");
     this.channels = Preconditions.checkNotNull(channels, "channels");
   }
@@ -32,7 +32,7 @@ public final class PlayerChannelRegisterEventImpl implements PlayerChannelRegist
   }
 
   @Override
-  public List<ChannelIdentifier> channels() {
+  public List<PluginChannelId> channels() {
     return channels;
   }
 

api/src/main/java/com/velocitypowered/api/plugin/PluginManager.java

@@ -39,7 +39,7 @@ public interface PluginManager {
    *
    * @return the plugins
    */
-  Collection<PluginContainer> getPlugins();
+  Collection<PluginContainer> plugins();
 
   /**
    * Checks if a plugin is loaded based on its ID.

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelIdentifier.java

@@ -1,21 +0,0 @@
-/*
- * Copyright (C) 2018 Velocity Contributors
- *
- * The Velocity API is licensed under the terms of the MIT License. For more details,
- * reference the LICENSE file in the api top-level directory.
- */
-
-package com.velocitypowered.api.proxy.messages;
-
-/**
- * Represents a channel identifier for use with plugin messaging.
- */
-public interface ChannelIdentifier {
-
-  /**
-   * Returns the textual representation of this identifier.
-   *
-   * @return the textual representation of the identifier
-   */
-  String id();
-}

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSink.java

@@ -19,5 +19,5 @@ public interface ChannelMessageSink {
    * @param data the data to send
    * @return whether or not the message could be sent
    */
-  boolean sendPluginMessage(ChannelIdentifier identifier, byte[] data);
+  boolean sendPluginMessage(PluginChannelId identifier, byte[] data);
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java

@@ -10,7 +10,7 @@ package com.velocitypowered.api.proxy.messages;
 import com.velocitypowered.api.event.connection.PluginMessageEventImpl;
 
 /**
- * Represents an interface to register and unregister {@link ChannelIdentifier}s for the proxy to
+ * Represents an interface to register and unregister {@link PluginChannelId}s for the proxy to
  * listen on.
  */
 public interface ChannelRegistrar {
@@ -21,12 +21,12 @@ public interface ChannelRegistrar {
    *
    * @param identifiers the channel identifiers to register
    */
-  void register(ChannelIdentifier... identifiers);
+  void register(PluginChannelId... identifiers);
 
   /**
    * Removes the intent to listen for the specified channel.
    *
    * @param identifiers the identifiers to unregister
    */
-  void unregister(ChannelIdentifier... identifiers);
+  void unregister(PluginChannelId... identifiers);
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

@@ -1,64 +0,0 @@
-/*
- * Copyright (C) 2018 Velocity Contributors
- *
- * The Velocity API is licensed under the terms of the MIT License. For more details,
- * reference the LICENSE file in the api top-level directory.
- */
-
-package com.velocitypowered.api.proxy.messages;
-
-import com.google.common.base.Preconditions;
-import com.google.common.base.Strings;
-import java.util.Objects;
-import org.checkerframework.checker.nullness.qual.Nullable;
-
-/**
- * Reperesents a legacy channel identifier (for Minecraft 1.12 and below). For modern 1.13 plugin
- * messages, please see {@link MinecraftChannelIdentifier}. This class is immutable and safe for
- * multi-threaded use.
- */
-public final class LegacyChannelIdentifier implements ChannelIdentifier {
-
-  private final String name;
-
-  /**
-   * Creates a new legacy channel identifier.
-   *
-   * @param name the name for the channel
-   */
-  public LegacyChannelIdentifier(String name) {
-    Preconditions.checkArgument(!Strings.isNullOrEmpty(name), "provided name is empty");
-    this.name = name;
-  }
-
-  public String getName() {
-    return name;
-  }
-
-  @Override
-  public String toString() {
-    return name + " (legacy)";
-  }
-
-  @Override
-  public boolean equals(@Nullable Object o) {
-    if (this == o) {
-      return true;
-    }
-    if (o == null || getClass() != o.getClass()) {
-      return false;
-    }
-    LegacyChannelIdentifier that = (LegacyChannelIdentifier) o;
-    return Objects.equals(name, that.name);
-  }
-
-  @Override
-  public int hashCode() {
-    return Objects.hash(name);
-  }
-
-  @Override
-  public String id() {
-    return this.getName();
-  }
-}

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

@@ -1,114 +0,0 @@
-/*
- * Copyright (C) 2018 Velocity Contributors
- *
- * The Velocity API is licensed under the terms of the MIT License. For more details,
- * reference the LICENSE file in the api top-level directory.
- */
-
-package com.velocitypowered.api.proxy.messages;
-
-import com.google.common.base.Preconditions;
-import com.google.common.base.Strings;
-import java.util.Objects;
-import java.util.regex.Pattern;
-import org.checkerframework.checker.nullness.qual.Nullable;
-
-/**
- * Represents a Minecraft 1.13+ channel identifier. This class is immutable and safe for
- * multi-threaded use.
- */
-public final class MinecraftChannelIdentifier implements ChannelIdentifier {
-
-  private static final Pattern VALID_IDENTIFIER_REGEX = Pattern.compile("[a-z0-9/\\-_]*");
-
-  private final String namespace;
-  private final String name;
-
-  private MinecraftChannelIdentifier(String namespace, String name) {
-    this.namespace = namespace;
-    this.name = name;
-  }
-
-  /**
-   * Creates an identifier in the default namespace ({@code minecraft}). Plugins are strongly
-   * encouraged to provide their own namespace.
-   *
-   * @param name the name in the default namespace to use
-   * @return a new channel identifier
-   */
-  public static MinecraftChannelIdentifier forDefaultNamespace(String name) {
-    return new MinecraftChannelIdentifier("minecraft", name);
-  }
-
-  /**
-   * Creates an identifier in the specified namespace.
-   *
-   * @param namespace the namespace to use
-   * @param name the channel name inside the specified namespace
-   * @return a new channel identifier
-   */
-  public static MinecraftChannelIdentifier create(String namespace, String name) {
-    Preconditions.checkArgument(!Strings.isNullOrEmpty(namespace), "namespace is null or empty");
-    Preconditions.checkArgument(name != null, "namespace is null or empty");
-    Preconditions.checkArgument(VALID_IDENTIFIER_REGEX.matcher(namespace).matches(),
-        "namespace is not valid");
-    Preconditions
-        .checkArgument(VALID_IDENTIFIER_REGEX.matcher(name).matches(), "name is not valid");
-    return new MinecraftChannelIdentifier(namespace, name);
-  }
-
-  /**
-   * Creates an channel identifier from the specified Minecraft identifier.
-   *
-   * @param identifier the Minecraft identifier
-   * @return a new channel identifier
-   */
-  public static MinecraftChannelIdentifier from(String identifier) {
-    int colonPos = identifier.indexOf(':');
-    if (colonPos == -1) {
-      throw new IllegalArgumentException("Identifier does not contain a colon.");
-    }
-    if (colonPos + 1 == identifier.length()) {
-      throw new IllegalArgumentException("Identifier is empty.");
-    }
-    String namespace = identifier.substring(0, colonPos);
-    String name = identifier.substring(colonPos + 1);
-    return create(namespace, name);
-  }
-
-  public String getNamespace() {
-    return namespace;
-  }
-
-  public String getName() {
-    return name;
-  }
-
-  @Override
-  public String toString() {
-    return namespace + ":" + name + " (modern)";
-  }
-
-  @Override
-  public boolean equals(@Nullable Object o) {
-    if (this == o) {
-      return true;
-    }
-    if (o == null || getClass() != o.getClass()) {
-      return false;
-    }
-    MinecraftChannelIdentifier that = (MinecraftChannelIdentifier) o;
-    return Objects.equals(namespace, that.namespace)
-        && Objects.equals(name, that.name);
-  }
-
-  @Override
-  public int hashCode() {
-    return Objects.hash(namespace, name);
-  }
-
-  @Override
-  public String id() {
-    return namespace + ":" + name;
-  }
-}

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftPluginChannelId.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2018 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.proxy.messages;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Strings;
+import java.util.Objects;
+import java.util.regex.Pattern;
+import net.kyori.adventure.key.Key;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Represents a Minecraft 1.13+ channel identifier.
+ */
+public final class MinecraftPluginChannelId implements PluginChannelId {
+
+  private final Key key;
+
+  MinecraftPluginChannelId(Key key) {
+    this.key = Preconditions.checkNotNull(key, "key");
+  }
+
+  public Key key() {
+    return key;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+
+    MinecraftPluginChannelId that = (MinecraftPluginChannelId) o;
+
+    return key.equals(that.key);
+  }
+
+  @Override
+  public int hashCode() {
+    return key.hashCode();
+  }
+
+  @Override
+  public String toString() {
+    return key.asString();
+  }
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/PairedPluginChannelId.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright (C) 2018 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.proxy.messages;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Strings;
+import net.kyori.adventure.key.Key;
+
+/**
+ * Reperesents a legacy channel identifier (for Minecraft 1.12 and below). For modern 1.13 plugin
+ * messages, please see {@link MinecraftPluginChannelId}. This class is immutable and safe for
+ * multi-threaded use.
+ */
+public final class PairedPluginChannelId implements PluginChannelId {
+
+  private final String legacyChannel;
+  private final Key modernChannelKey;
+
+  /**
+   * Creates a new legacy channel identifier.
+   *
+   * @param legacyChannel the name for the legacy channel name
+   * @param modernChannelKey the modern channel key to use
+   */
+  PairedPluginChannelId(String legacyChannel, Key modernChannelKey) {
+    Preconditions.checkArgument(!Strings.isNullOrEmpty(legacyChannel), "provided name is empty");
+    this.legacyChannel = legacyChannel;
+    this.modernChannelKey = Preconditions.checkNotNull(modernChannelKey, "modernChannelKey");
+  }
+
+  public String legacyChannel() {
+    return legacyChannel;
+  }
+
+  public Key modernChannelKey() {
+    return modernChannelKey;
+  }
+
+  @Override
+  public String toString() {
+    return legacyChannel + "/" + modernChannelKey.asString();
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+
+    PairedPluginChannelId that = (PairedPluginChannelId) o;
+
+    if (!legacyChannel.equals(that.legacyChannel)) {
+      return false;
+    }
+    return modernChannelKey.equals(that.modernChannelKey);
+  }
+
+  @Override
+  public int hashCode() {
+    int result = legacyChannel.hashCode();
+    result = 31 * result + modernChannelKey.hashCode();
+    return result;
+  }
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/PluginChannelId.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2018 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.proxy.messages;
+
+import net.kyori.adventure.key.Key;
+
+/**
+ * Represents a channel identifier for use with plugin messaging.
+ */
+public interface PluginChannelId {
+
+  /**
+   * Wraps the specified Minecraft key so it can be used as a {@link PluginChannelId}.
+   * If the client is connected using Minecraft 1.12.2 or earlier, use the key as the
+   * channel name.
+   *
+   * @param key the key instance to wrap
+   * @return a wrapped plugin channel ID
+   */
+  static MinecraftPluginChannelId wrap(Key key) {
+    return new MinecraftPluginChannelId(key);
+  }
+
+  /**
+   * Wraps the specified Minecraft key so it can be used as a {@link PluginChannelId},
+   * with the specified {@code legacyChannel} for clients onnected using Minecraft 1.12.2
+   * or earlier.
+   *
+   * @param legacyChannel the legacy channel name
+   * @param modernChannelKey the key instance to wrap
+   * @return a wrapped plugin channel ID
+   */
+  static PairedPluginChannelId withLegacy(String legacyChannel, Key modernChannelKey) {
+    return new PairedPluginChannelId(legacyChannel, modernChannelKey);
+  }
+}

api/src/test/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifierTest.java

@@ -1,63 +0,0 @@
-/*
- * Copyright (C) 2018 Velocity Contributors
- *
- * The Velocity API is licensed under the terms of the MIT License. For more details,
- * reference the LICENSE file in the api top-level directory.
- */
-
-package com.velocitypowered.api.proxy.messages;
-
-import static com.velocitypowered.api.proxy.messages.MinecraftChannelIdentifier.create;
-import static com.velocitypowered.api.proxy.messages.MinecraftChannelIdentifier.from;
-import static org.junit.jupiter.api.Assertions.assertAll;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertThrows;
-
-import org.junit.jupiter.api.Test;
-
-class MinecraftChannelIdentifierTest {
-
-  @Test
-  void createAllowsValidNamespaces() {
-    create("minecraft", "brand");
-  }
-
-  @Test
-  void createAllowsEmptyName() {
-    create("minecraft", "");
-  }
-
-  @Test
-  void createDisallowsNull() {
-    assertAll(
-        () -> assertThrows(IllegalArgumentException.class, () -> create(null, "")),
-        () -> assertThrows(IllegalArgumentException.class, () -> create("", "")),
-        () -> assertThrows(IllegalArgumentException.class, () -> create("minecraft", null))
-    );
-  }
-
-  @Test
-  void fromIdentifierIsCorrect() {
-    MinecraftChannelIdentifier expected = MinecraftChannelIdentifier.create("velocity", "test");
-    assertEquals(expected, MinecraftChannelIdentifier.from("velocity:test"));
-  }
-
-  @Test
-  void createAllowsSlashes() {
-    create("velocity", "test/test2");
-  }
-
-  @Test
-  void fromIdentifierThrowsOnBadValues() {
-    assertAll(
-        () -> assertThrows(IllegalArgumentException.class, () -> from("")),
-        () -> assertThrows(IllegalArgumentException.class, () -> from(":")),
-        () -> assertThrows(IllegalArgumentException.class, () -> from(":a")),
-        () -> assertThrows(IllegalArgumentException.class, () -> from("a:")),
-        () -> assertThrows(IllegalArgumentException.class, () -> from("hello:$$$$$$")),
-        () -> assertThrows(IllegalArgumentException.class, () -> from("hello::"))
-    );
-  }
-
-
-}