Do not allow plugins to make multiple instances of their main class.

Prevents foot shooting, eg #2772

api/src/main/java/net/md_5/bungee/api/plugin/Plugin.java

@@ -1,5 +1,6 @@
 package net.md_5.bungee.api.plugin;
 
+import com.google.common.base.Preconditions;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import java.io.File;
 import java.io.InputStream;
@@ -27,6 +28,22 @@ public class Plugin
     @Getter
     private Logger logger;
 
+    public Plugin()
+    {
+        ClassLoader classLoader = getClass().getClassLoader();
+        Preconditions.checkState( classLoader instanceof PluginClassloader, "Plugin requires " + PluginClassloader.class.getName() );
+
+        ( (PluginClassloader) classLoader ).init( this );
+    }
+
+    protected Plugin(ProxyServer proxy, PluginDescription description)
+    {
+        ClassLoader classLoader = getClass().getClassLoader();
+        Preconditions.checkState( !( classLoader instanceof PluginClassloader ), "Cannot use initialization constructor at runtime" );
+
+        // init( proxy, description );
+    }
+
     /**
      * Called when the plugin has just been loaded. Most of the proxy will not
      * be initialized, so only use it for registering

api/src/main/java/net/md_5/bungee/api/plugin/PluginClassloader.java

@@ -1,37 +1,47 @@
 package net.md_5.bungee.api.plugin;
 
+import com.google.common.base.Preconditions;
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;
+import net.md_5.bungee.api.ProxyServer;
 
-public class PluginClassloader extends URLClassLoader
+final class PluginClassloader extends URLClassLoader
 {
 
     private static final Set<PluginClassloader> allLoaders = new CopyOnWriteArraySet<>();
+    //
+    private final ProxyServer proxy;
+    private final PluginDescription desc;
+    //
+    private Plugin plugin;
 
     static
     {
         ClassLoader.registerAsParallelCapable();
     }
 
-    public PluginClassloader(URL[] urls)
+    public PluginClassloader(ProxyServer proxy, PluginDescription desc, URL[] urls)
     {
         super( urls );
+        this.proxy = proxy;
+        this.desc = desc;
+
         allLoaders.add( this );
     }
 
     @Override
-    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
+    protected Class<?> findClass(String name) throws ClassNotFoundException
     {
-        return loadClass0( name, resolve, true );
+        return findClass0( name, true );
     }
 
-    private Class<?> loadClass0(String name, boolean resolve, boolean checkOther) throws ClassNotFoundException
+    private Class<?> findClass0(String name, boolean checkOther) throws ClassNotFoundException
     {
         try
         {
-            return super.loadClass( name, resolve );
+            return super.findClass( name );
         } catch ( ClassNotFoundException ex )
         {
         }
@@ -43,7 +53,7 @@ public class PluginClassloader extends URLClassLoader
                 {
                     try
                     {
-                        return loader.loadClass0( name, resolve, false );
+                        return loader.findClass0( name, false );
                     } catch ( ClassNotFoundException ex )
                     {
                     }
@@ -52,4 +62,17 @@ public class PluginClassloader extends URLClassLoader
         }
         throw new ClassNotFoundException( name );
     }
+
+    void init(Plugin plugin)
+    {
+        Preconditions.checkArgument( plugin != null, "plugin" );
+        Preconditions.checkArgument( plugin.getClass().getClassLoader() == this, "Plugin has incorrect ClassLoader" );
+        if ( this.plugin != null )
+        {
+            throw new IllegalArgumentException( "Plugin already initialized!" );
+        }
+
+        this.plugin = plugin;
+        plugin.init( proxy, desc );
+    }
 }

api/src/main/java/net/md_5/bungee/api/plugin/PluginManager.java

@@ -320,14 +320,13 @@ public class PluginManager
         {
             try
             {
-                URLClassLoader loader = new PluginClassloader( new URL[]
+                URLClassLoader loader = new PluginClassloader( proxy, plugin, new URL[]
                 {
                     plugin.getFile().toURI().toURL()
                 } );
                 Class<?> main = loader.loadClass( plugin.getMain() );
                 Plugin clazz = (Plugin) main.getDeclaredConstructor().newInstance();
 
-                clazz.init( proxy, plugin );
                 plugins.put( plugin.getName(), clazz );
                 clazz.onLoad();
                 ProxyServer.getInstance().getLogger().log( Level.INFO, "Loaded plugin {0} version {1} by {2}", new Object[]

proxy/src/test/java/net/md_5/bungee/api/plugin/DummyPlugin.java

@@ -1,11 +1,12 @@
 package net.md_5.bungee.api.plugin;
 
-import lombok.AccessLevel;
+public final class DummyPlugin extends Plugin
-import lombok.NoArgsConstructor;
-
-@NoArgsConstructor(access = AccessLevel.PRIVATE)
-public class DummyPlugin extends Plugin
 {
 
     public static final DummyPlugin INSTANCE = new DummyPlugin();
+
+    private DummyPlugin()
+    {
+        super( null, null );
+    }
 }