Don't load classes on secondary threads. ADDRESSES TICKET-27.

We'll move the "load class" call to the main thread, hopefully fixing
ticket 27. The secondary thread will still call the "define class"
protected method, which may not be thread safe.

ProtocolLib/src/main/java/com/comphenix/protocol/reflect/compiler/BackgroundCompiler.java

@@ -150,41 +150,59 @@ public class BackgroundCompiler {
 			if (executor == null || executor.isShutdown())
 				return;
 			
-			try {
-				executor.submit(new Callable<Object>() {
-					@Override
-					public Object call() throws Exception {
-						StructureModifier<TKey> modifier = uncompiled;
+			// Create the worker that will compile our modifier
+			Callable<?> worker = new Callable<Object>() {
+				@Override
+				public Object call() throws Exception {
+					StructureModifier<TKey> modifier = uncompiled;
 					
-						// Do our compilation
-						try {
-							modifier = compiler.compile(modifier);
-							listener.onCompiled(modifier);
+					// Do our compilation
+					try {
+						modifier = compiler.compile(modifier);
+						listener.onCompiled(modifier);
 
-						} catch (Throwable e) {
-							// Disable future compilations!
-							setEnabled(false);
+					} catch (Throwable e) {
+						// Disable future compilations!
+						setEnabled(false);
 						
-							// Inform about this error as best as we can
-							if (reporter != null) {
-								reporter.reportDetailed(BackgroundCompiler.this, 
-										"Cannot compile structure. Disabing compiler.", e, uncompiled);
-							} else {
-								System.err.println("Exception occured in structure compiler: ");
-								e.printStackTrace();
-							}
+						// Inform about this error as best as we can
+						if (reporter != null) {
+							reporter.reportDetailed(BackgroundCompiler.this, 
+									"Cannot compile structure. Disabing compiler.", e, uncompiled);
+						} else {
+							System.err.println("Exception occured in structure compiler: ");
+							e.printStackTrace();
 						}
-						
-						// We'll also return the new structure modifier
-						return modifier;
-						
 					}
-				});
+					
+					// We'll also return the new structure modifier
+					return modifier;
+					
+				}
+			};
+			
+			try {
+				// Lookup the previous class name on the main thread.
+				// This is necessary as the Bukkit class loaders are not thread safe
+				if (compiler.lookupClassLoader(uncompiled)) {
+					try {
+						worker.call();
+					} catch (Exception e) {
+						// Impossible!
+						e.printStackTrace();
+					}
+					
+				} else {
+					
+					// Perform the compilation on a seperate thread
+					executor.submit(worker);
+				}
+				
 			} catch (RejectedExecutionException e) {
 				// Occures when the underlying queue is overflowing. Since the compilation  
 				// is only an optmization and not really essential we'll just log this failure 
 				// and move on.
-				Logger.getLogger("Minecraft").log(Level.WARNING, "Unable to schedule compilation task.", e);
+				reporter.reportWarning(this, "Unable to schedule compilation task.", e);
 			}
 		}
 	}
@@ -209,7 +227,7 @@ public class BackgroundCompiler {
 		try {
 			executor.awaitTermination(timeout, unit);
 		} catch (InterruptedException e) {
-			// Unlikely to ever occur.
+			// Unlikely to ever occur - it's the main thread
 			e.printStackTrace();
 		}
 	}

ProtocolLib/src/main/java/com/comphenix/protocol/reflect/compiler/StructureCompiler.java

@@ -21,9 +21,9 @@ import java.lang.reflect.Field;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import com.comphenix.protocol.reflect.StructureModifier;
 import com.google.common.base.Objects;
@@ -98,6 +98,10 @@ public final class StructureCompiler {
 		private Class targetType;
 		private Class fieldType;
 		
+		public StructureKey(StructureModifier<?> source) {
+			this(source.getTargetType(), source.getFieldType());
+		}
+		
 		public StructureKey(Class targetType, Class fieldType) {
 			this.targetType = targetType;
 			this.fieldType = fieldType;
@@ -123,7 +127,7 @@ public final class StructureCompiler {
 	private volatile static Method defineMethod;
 	
 	@SuppressWarnings("rawtypes")
-	private Map<StructureKey, Class> compiledCache = new HashMap<StructureKey, Class>();
+	private Map<StructureKey, Class> compiledCache = new ConcurrentHashMap<StructureKey, Class>();
 	
 	// The class loader we'll store our classes
 	private ClassLoader loader;
@@ -142,6 +146,37 @@ public final class StructureCompiler {
 		this.loader = loader;
 	}
 	
+	/**
+	 * Lookup the current class loader for any previously generated classes before we attempt to generate something.
+	 * @param source - the structure modifier to look up.
+	 * @return TRUE if we successfully found a previously generated class, FALSE otherwise.
+	 */
+	public <TField> boolean lookupClassLoader(StructureModifier<TField> source) {
+		StructureKey key = new StructureKey(source);
+		
+		// See if there's a need to lookup the class name
+		if (compiledCache.containsKey(key)) {
+			return true;
+		}
+		
+		try {
+			String className = getCompiledName(source);
+			
+			// This class might have been generated before. Try to load it.
+			Class<?> before = loader.loadClass(PACKAGE_NAME.replace('/', '.') + "." + className);
+			
+			if (before != null) {
+				compiledCache.put(key, before);
+				return true;
+			}
+		} catch (ClassNotFoundException e) {
+			// That's ok. 
+		}
+		
+		// We need to compile the class
+		return false;
+	}
+	
 	/**
 	 * Compiles the given structure modifier.
 	 * <p>
@@ -158,7 +193,7 @@ public final class StructureCompiler {
 			return source;
 		}
 		
-		StructureKey key = new StructureKey(source.getTargetType(), source.getFieldType());
+		StructureKey key = new StructureKey(source);
 		Class<?> compiledClass = compiledCache.get(key);
 		
 		if (!compiledCache.containsKey(key)) {
@@ -195,29 +230,35 @@ public final class StructureCompiler {
 		return type.getCanonicalName().replace("[]", "Array").replace(".", "_");
 	}
 	
+	/**
+	 * Retrieve the compiled name of a given structure modifier.
+	 * @param source - the structure modifier.
+	 * @return The unique, compiled name of a compiled structure modifier.
+	 */
+	private String getCompiledName(StructureModifier<?> source) {
+		Class<?> targetType = source.getTargetType();
+	
+		// Concat class and field type
+		return "CompiledStructure$" + 
+				getSafeTypeName(targetType) + "$" + 
+				getSafeTypeName(source.getFieldType());
+	}
+	
+	/**
+	 * Compile a structure modifier.
+	 * @param source - structure modifier.
+	 * @return The compiled structure modifier.
+	 */
 	private <TField> Class<?> generateClass(StructureModifier<TField> source) {
 		
 		ClassWriter cw = new ClassWriter(0);
+		Class<?> targetType = source.getTargetType();
 		
-		@SuppressWarnings("rawtypes")
-		Class targetType = source.getTargetType();
-		
-		String className = "CompiledStructure$" + 
-				getSafeTypeName(targetType) + "$" + 
-				getSafeTypeName(source.getFieldType());
+		String className = getCompiledName(source);
 		String targetSignature = Type.getDescriptor(targetType);
 		String targetName = targetType.getName().replace('.', '/');
 		
-		try {
-			// This class might have been generated before. Try to load it.
-			Class<?> before = loader.loadClass(PACKAGE_NAME.replace('/', '.') + "." + className);
-			
-			if (before != null)
-				return before;
-		} catch (ClassNotFoundException e) {
-			// That's ok. 
-		}
-		
+		// Define class
 		cw.visit(Opcodes.V1_6, Opcodes.ACC_PUBLIC + Opcodes.ACC_SUPER, PACKAGE_NAME + "/" + className, 
 				 null, COMPILED_CLASS, null);