Ease ClassLoader Deadlocks Where Possible

When on Java 7 we can register the classloader as parallel capable to prevent deadlocks caused by certain scenarios. Due to the nature of PluginClassLoader this isn't completely safe, but we can make it safer by switching to concurrency focused collections. Either way this is far better than crashing the server.

src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java

@@ -48,7 +48,7 @@
 public final class JavaPluginLoader implements PluginLoader {
     final Server server;
     private final Pattern[] fileFilters = new Pattern[] { Pattern.compile("\\.jar$"), };
-    private final Map<String, Class<?>> classes = new HashMap<String, Class<?>>();
+    private final Map<String, Class<?>> classes = new java.util.concurrent.ConcurrentHashMap<String, Class<?>>(); // Spigot
     private final Map<String, PluginClassLoader> loaders = new LinkedHashMap<String, PluginClassLoader>();
     public static final CustomTimingsHandler pluginParentTimer = new CustomTimingsHandler("** Plugins"); // Spigot
 

src/main/java/org/bukkit/plugin/java/PluginClassLoader.java

@@ -17,14 +17,38 @@
  */
 final class PluginClassLoader extends URLClassLoader {
     private final JavaPluginLoader loader;
-    private final Map<String, Class<?>> classes = new HashMap<String, Class<?>>();
+    private final Map<String, Class<?>> classes = new java.util.concurrent.ConcurrentHashMap<String, Class<?>>(); // Spigot
     private final PluginDescriptionFile description;
     private final File dataFolder;
     private final File file;
     final JavaPlugin plugin;
     private JavaPlugin pluginInit;
     private IllegalStateException pluginState;
 
+    // Spigot Start
+    static
+    {
+        try
+        {
+            java.lang.reflect.Method method = ClassLoader.class.getDeclaredMethod( "registerAsParallelCapable" );
+            if ( method != null )
+            {
+                boolean oldAccessible = method.isAccessible();
+                method.setAccessible( true );
+                method.invoke( null );
+                method.setAccessible( oldAccessible );
+                org.bukkit.Bukkit.getLogger().log( java.util.logging.Level.INFO, "Set PluginClassLoader as parallel capable" );
+            }
+        } catch ( NoSuchMethodException ex )
+        {
+            // Ignore
+        } catch ( Exception ex )
+        {
+            org.bukkit.Bukkit.getLogger().log( java.util.logging.Level.WARNING, "Error setting PluginClassLoader as parallel capable", ex );
+        }
+    }
+    // Spigot End
+
     PluginClassLoader(final JavaPluginLoader loader, final ClassLoader parent, final PluginDescriptionFile description, final File dataFolder, final File file) throws InvalidPluginException, MalformedURLException {
         super(new URL[] {file.toURI().toURL()}, parent);
         Validate.notNull(loader, "Loader cannot be null");