Add preview of automatic library support

Example plugin.yml usage:
```
    libraries:
      - com.squareup.okhttp3:okhttp:4.9.0
```

Libraries will only be accessible to plugins and their transitive depends, allowing for multiple versions of the same library to be used by different plugins.

This is a preview feature. Feedback is welcome so that it may be refined before being made widely available.

api/pom.xml

@@ -49,6 +49,27 @@
             <version>${netty.version}</version>
             <scope>compile</scope>
         </dependency>
+        <dependency>
+            <groupId>org.apache.maven</groupId>
+            <artifactId>maven-resolver-provider</artifactId>
+            <version>3.8.1</version>
+            <!-- not part of the API proper -->
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.resolver</groupId>
+            <artifactId>maven-resolver-connector-basic</artifactId>
+            <version>1.6.2</version>
+            <!-- not part of the API proper -->
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.resolver</groupId>
+            <artifactId>maven-resolver-transport-http</artifactId>
+            <version>1.6.2</version>
+            <!-- not part of the API proper -->
+            <scope>provided</scope>
+        </dependency>
         <dependency>
             <groupId>org.yaml</groupId>
             <artifactId>snakeyaml</artifactId>

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

@@ -0,0 +1,123 @@
+package net.md_5.bungee.api.plugin;
+
+import java.io.File;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import org.apache.maven.repository.internal.MavenRepositorySystemUtils;
+import org.eclipse.aether.DefaultRepositorySystemSession;
+import org.eclipse.aether.RepositorySystem;
+import org.eclipse.aether.artifact.Artifact;
+import org.eclipse.aether.artifact.DefaultArtifact;
+import org.eclipse.aether.collection.CollectRequest;
+import org.eclipse.aether.connector.basic.BasicRepositoryConnectorFactory;
+import org.eclipse.aether.graph.Dependency;
+import org.eclipse.aether.impl.DefaultServiceLocator;
+import org.eclipse.aether.repository.LocalRepository;
+import org.eclipse.aether.repository.RemoteRepository;
+import org.eclipse.aether.repository.RepositoryPolicy;
+import org.eclipse.aether.resolution.ArtifactResult;
+import org.eclipse.aether.resolution.DependencyRequest;
+import org.eclipse.aether.resolution.DependencyResolutionException;
+import org.eclipse.aether.resolution.DependencyResult;
+import org.eclipse.aether.spi.connector.RepositoryConnectorFactory;
+import org.eclipse.aether.spi.connector.transport.TransporterFactory;
+import org.eclipse.aether.transfer.AbstractTransferListener;
+import org.eclipse.aether.transfer.TransferCancelledException;
+import org.eclipse.aether.transfer.TransferEvent;
+import org.eclipse.aether.transport.http.HttpTransporterFactory;
+
+class LibraryLoader
+{
+
+    private final Logger logger;
+    private final RepositorySystem repository;
+    private final DefaultRepositorySystemSession session;
+    private final List<RemoteRepository> repositories;
+
+    public LibraryLoader(Logger logger)
+    {
+        this.logger = logger;
+
+        DefaultServiceLocator locator = MavenRepositorySystemUtils.newServiceLocator();
+        locator.addService( RepositoryConnectorFactory.class, BasicRepositoryConnectorFactory.class );
+        locator.addService( TransporterFactory.class, HttpTransporterFactory.class );
+
+        this.repository = locator.getService( RepositorySystem.class );
+        this.session = MavenRepositorySystemUtils.newSession();
+
+        session.setChecksumPolicy( RepositoryPolicy.CHECKSUM_POLICY_FAIL );
+        session.setLocalRepositoryManager( repository.newLocalRepositoryManager( session, new LocalRepository( "libraries" ) ) );
+        session.setTransferListener( new AbstractTransferListener()
+        {
+            @Override
+            public void transferStarted(TransferEvent event) throws TransferCancelledException
+            {
+                logger.log( Level.INFO, "Downloading {0}", event.getResource().getRepositoryUrl() + event.getResource().getResourceName() );
+            }
+        } );
+        session.setReadOnly();
+
+        this.repositories = repository.newResolutionRepositories( session, Arrays.asList( new RemoteRepository.Builder( "central", "default", "https://repo.maven.apache.org/maven2" ).build() ) );
+    }
+
+    public ClassLoader createLoader(PluginDescription desc)
+    {
+        if ( desc.getLibraries().isEmpty() )
+        {
+            return null;
+        }
+        logger.log( Level.INFO, "[{0}] Loading {1} libraries... please wait", new Object[]
+        {
+            desc.getName(), desc.getLibraries().size()
+        } );
+
+        List<Dependency> dependencies = new ArrayList<>();
+        for ( String library : desc.getLibraries() )
+        {
+            Artifact artifact = new DefaultArtifact( library );
+            Dependency dependency = new Dependency( artifact, null );
+
+            dependencies.add( dependency );
+        }
+
+        DependencyResult result;
+        try
+        {
+            result = repository.resolveDependencies( session, new DependencyRequest( new CollectRequest( (Dependency) null, dependencies, repositories ), null ) );
+        } catch ( DependencyResolutionException ex )
+        {
+            throw new RuntimeException( "Error resolving libraries", ex );
+        }
+
+        List<URL> jarFiles = new ArrayList<>();
+        for ( ArtifactResult artifact : result.getArtifactResults() )
+        {
+            File file = artifact.getArtifact().getFile();
+
+            URL url;
+            try
+            {
+                url = file.toURI().toURL();
+            } catch ( MalformedURLException ex )
+            {
+                throw new AssertionError( ex );
+            }
+
+            jarFiles.add( url );
+            logger.log( Level.INFO, "[{0}] Loaded library {1}", new Object[]
+            {
+                desc.getName(), file
+            } );
+        }
+
+        URLClassLoader loader = new URLClassLoader( jarFiles.toArray( new URL[ jarFiles.size() ] ) );
+
+        return loader;
+    }
+}

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

@@ -1,12 +1,23 @@
 package net.md_5.bungee.api.plugin;
 
 import com.google.common.base.Preconditions;
+import com.google.common.io.ByteStreams;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
 import java.net.URL;
 import java.net.URLClassLoader;
+import java.security.CodeSigner;
+import java.security.CodeSource;
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.jar.JarEntry;
+import java.util.jar.JarFile;
+import java.util.jar.Manifest;
+import lombok.ToString;
 import net.md_5.bungee.api.ProxyServer;
 
+@ToString(of = "desc")
 final class PluginClassloader extends URLClassLoader
 {
 
@@ -14,6 +25,10 @@ final class PluginClassloader extends URLClassLoader
     //
     private final ProxyServer proxy;
     private final PluginDescription desc;
+    private final JarFile jar;
+    private final Manifest manifest;
+    private final URL url;
+    private final ClassLoader libraryLoader;
     //
     private Plugin plugin;
 
@@ -22,11 +37,18 @@ final class PluginClassloader extends URLClassLoader
         ClassLoader.registerAsParallelCapable();
     }
 
-    public PluginClassloader(ProxyServer proxy, PluginDescription desc, URL[] urls)
+    public PluginClassloader(ProxyServer proxy, PluginDescription desc, File file, ClassLoader libraryLoader) throws IOException
     {
-        super( urls );
+        super( new URL[]
+        {
+            file.toURI().toURL()
+        } );
         this.proxy = proxy;
         this.desc = desc;
+        this.jar = new JarFile( file );
+        this.manifest = jar.getManifest();
+        this.url = file.toURI().toURL();
+        this.libraryLoader = libraryLoader;
 
         allLoaders.add( this );
     }
@@ -34,10 +56,10 @@ final class PluginClassloader extends URLClassLoader
     @Override
     protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
     {
-        return loadClass0( name, resolve, true );
+        return loadClass0( name, resolve, true, true );
     }
 
-    private Class<?> loadClass0(String name, boolean resolve, boolean checkOther) throws ClassNotFoundException
+    private Class<?> loadClass0(String name, boolean resolve, boolean checkOther, boolean checkLibraries) throws ClassNotFoundException
     {
         try
         {
@@ -45,6 +67,17 @@ final class PluginClassloader extends URLClassLoader
         } catch ( ClassNotFoundException ex )
         {
         }
+
+        if ( checkLibraries && libraryLoader != null )
+        {
+            try
+            {
+                return libraryLoader.loadClass( name );
+            } catch ( ClassNotFoundException ex )
+            {
+            }
+        }
+
         if ( checkOther )
         {
             for ( PluginClassloader loader : allLoaders )
@@ -53,13 +86,66 @@ final class PluginClassloader extends URLClassLoader
                 {
                     try
                     {
-                        return loader.loadClass0( name, resolve, false );
+                        return loader.loadClass0( name, resolve, false, proxy.getPluginManager().isTransitiveDepend( desc, loader.desc ) );
                     } catch ( ClassNotFoundException ex )
                     {
                     }
                 }
             }
         }
+
+        throw new ClassNotFoundException( name );
+    }
+
+    @Override
+    protected Class<?> findClass(String name) throws ClassNotFoundException
+    {
+        String path = name.replace( '.', '/' ).concat( ".class" );
+        JarEntry entry = jar.getJarEntry( path );
+
+        if ( entry != null )
+        {
+            byte[] classBytes;
+
+            try ( InputStream is = jar.getInputStream( entry ) )
+            {
+                classBytes = ByteStreams.toByteArray( is );
+            } catch ( IOException ex )
+            {
+                throw new ClassNotFoundException( name, ex );
+            }
+
+            int dot = name.lastIndexOf( '.' );
+            if ( dot != -1 )
+            {
+                String pkgName = name.substring( 0, dot );
+                if ( getPackage( pkgName ) == null )
+                {
+                    try
+                    {
+                        if ( manifest != null )
+                        {
+                            definePackage( pkgName, manifest, url );
+                        } else
+                        {
+                            definePackage( pkgName, null, null, null, null, null, null, null );
+                        }
+                    } catch ( IllegalArgumentException ex )
+                    {
+                        if ( getPackage( pkgName ) == null )
+                        {
+                            throw new IllegalStateException( "Cannot find package " + pkgName );
+                        }
+                    }
+                }
+            }
+
+            CodeSigner[] signers = entry.getCodeSigners();
+            CodeSource source = new CodeSource( url, signers );
+
+            return defineClass( name, classBytes, 0, classBytes.length, source );
+        }
+
         throw new ClassNotFoundException( name );
     }
 

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

@@ -2,6 +2,8 @@ package net.md_5.bungee.api.plugin;
 
 import java.io.File;
 import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
 import java.util.Set;
 import lombok.AllArgsConstructor;
 import lombok.Data;
@@ -48,4 +50,8 @@ public class PluginDescription
      * Optional description.
      */
     private String description = null;
+    /**
+     * Optional libraries.
+     */
+    private List<String> libraries = new LinkedList<>();
 }

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

@@ -4,10 +4,12 @@ import com.google.common.base.Preconditions;
 import com.google.common.collect.ArrayListMultimap;
 import com.google.common.collect.Multimap;
 import com.google.common.eventbus.Subscribe;
+import com.google.common.graph.GraphBuilder;
+import com.google.common.graph.Graphs;
+import com.google.common.graph.MutableGraph;
 import java.io.File;
 import java.io.InputStream;
 import java.lang.reflect.Method;
-import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.Arrays;
 import java.util.Collection;
@@ -49,6 +51,8 @@ public final class PluginManager
     private final Yaml yaml;
     private final EventBus eventBus;
     private final Map<String, Plugin> plugins = new LinkedHashMap<>();
+    private final MutableGraph<String> dependencyGraph = GraphBuilder.directed().build();
+    private final LibraryLoader libraryLoader;
     private final Map<String, Command> commandMap = new HashMap<>();
     private Map<String, PluginDescription> toLoad = new HashMap<>();
     private final Multimap<Plugin, Command> commandsByPlugin = ArrayListMultimap.create();
@@ -67,6 +71,17 @@ public final class PluginManager
         yaml = new Yaml( yamlConstructor );
 
         eventBus = new EventBus( proxy.getLogger() );
+
+        LibraryLoader libraryLoader = null;
+        try
+        {
+            libraryLoader = new LibraryLoader( proxy.getLogger() );
+        } catch ( NoClassDefFoundError ex )
+        {
+            // Provided depends were not added back
+            proxy.getLogger().warning( "Could not initialize LibraryLoader (missing dependencies?)" );
+        }
+        this.libraryLoader = libraryLoader;
     }
 
     /**
@@ -309,6 +324,7 @@ public final class PluginManager
                 status = false;
             }
 
+            dependencyGraph.putEdge( plugin.getName(), dependName );
             if ( !status )
             {
                 break;
@@ -320,10 +336,7 @@ public final class PluginManager
         {
             try
             {
-                URLClassLoader loader = new PluginClassloader( proxy, plugin, new URL[]
+                URLClassLoader loader = new PluginClassloader( proxy, plugin, plugin.getFile(), ( libraryLoader != null ) ? libraryLoader.createLoader( plugin ) : null );
-                {
-                    plugin.getFile().toURI().toURL()
-                } );
                 Class<?> main = loader.loadClass( plugin.getMain() );
                 Plugin clazz = (Plugin) main.getDeclaredConstructor().newInstance();
 
@@ -335,7 +348,7 @@ public final class PluginManager
                 } );
             } catch ( Throwable t )
             {
-                proxy.getLogger().log( Level.WARNING, "Error enabling plugin " + plugin.getName(), t );
+                proxy.getLogger().log( Level.WARNING, "Error loading plugin " + plugin.getName(), t );
             }
         }
 
@@ -463,4 +476,19 @@ public final class PluginManager
     {
         return Collections.unmodifiableCollection( commandMap.entrySet() );
     }
+
+    boolean isTransitiveDepend(PluginDescription plugin, PluginDescription depend)
+    {
+        Preconditions.checkArgument( plugin != null, "plugin" );
+        Preconditions.checkArgument( depend != null, "depend" );
+
+        if ( dependencyGraph.nodes().contains( plugin.getName() ) )
+        {
+            if ( Graphs.reachableNodes( dependencyGraph, plugin.getName() ).contains( depend.getName() ) )
+            {
+                return true;
+            }
+        }
+        return false;
+    }
 }

proxy/pom.xml

@@ -91,6 +91,31 @@
             <version>5.1.49</version>
             <scope>runtime</scope>
         </dependency>
+        <!-- add these back in as they are not exposed by the API -->
+        <dependency>
+            <groupId>org.apache.maven</groupId>
+            <artifactId>maven-resolver-provider</artifactId>
+            <version>3.8.1</version>
+            <scope>compile</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.resolver</groupId>
+            <artifactId>maven-resolver-connector-basic</artifactId>
+            <version>1.6.2</version>
+            <scope>compile</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.resolver</groupId>
+            <artifactId>maven-resolver-transport-http</artifactId>
+            <version>1.6.2</version>
+            <scope>compile</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-jdk14</artifactId>
+            <version>1.7.30</version>
+            <scope>compile</scope>
+        </dependency>
     </dependencies>
 
     <build>