/*
 *  This file is part of the Jikes RVM project (http://jikesrvm.org).
 *
 *  This file is licensed to You under the Eclipse Public License (EPL);
 *  You may not use this file except in compliance with the License. You
 *  may obtain a copy of the License at
 *
 *      http://www.opensource.org/licenses/eclipse-1.0.php
 *
 *  See the COPYRIGHT.txt file distributed with this work for information
 *  regarding copyright ownership.
 */
package org.jikesrvm.runtime;

import java.util.Enumeration;

import org.jikesrvm.VM;
import org.jikesrvm.classloader.Atom;
import org.jikesrvm.classloader.RVMClass;
import org.jikesrvm.classloader.RVMMethod;
import org.jikesrvm.classloader.RVMType;
import org.jikesrvm.scheduler.RVMThread;

/**
 * A class for managing various callbacks from the VM.
 *
 * <p>Consumers should register an implementation of the needed interface with
 * a given callback method, and will get notified when the event happens.
 *
 * <p>Note: callback consumers should not rely on any particular order of
 * callback invocation.
 *
 * <p>TODO: allow limited control over callback order.
 *
 * <p>
 * The following events are currently implemented.  See code for exact
 * invocation syntax.
 * <ul>
 * <li> ClassLoaded       - called after a RVMClass is loaded
 * <li> ClassResolved     - called after a RVMClass is resolved
 * <li> ClassInstantiated - called after a RVMClass is instantiated
 * <li> ClassInitialized  - called after a RVMClass is initialized
 * <li> MethodOverride    - called when a method in a newly loaded class
 *                          overrides a method in an existing class
 * <li> MethodCompile     - called before a method is compiled
 * <li> ForName           - called when java.lang.Class.forName() is invoked
 * <li> BootImageWriting  - called when boot image writing is started
 * <li> Startup           - called when the VM has completed booting
 * <li> Exit              - called when the VM is about to exit  (note: this is very fragile; TODO: remove???)
 * <li> AppStart          - called before the application starts executing
 *                          all runs -- needs application support)
 * <li> AppComplete       - called after the application completes executing
 *                          all runs --- needs application support)
 * <li> AppRunStart       - called before the application starts a run
 *                          (many applications have several runs -- needs
 *                          application support)
 * <li> AppRunComplete    - called after the application completes a run
 *                          (many applications have several runs --- needs
 *                          application support)
 * <li> RecompileAllDynamicallyLoadedMethods - called when the application
 *                          wants to recompile all methods that were previously
 *                          dynamically compiled.  Could be useful for
 *                          studying the the impact of how much of
 *                          class hierarchy being loaded effects compilation
 *                          performance
 *                          (application must call this explicitly for anything
 *                           to happen)
 * </ul>
 */
public final class Callbacks {
  ///////////////
  // INTERFACE //
  ///////////////

  /**
   * Interface for monitoring class loading.
   */
  public interface ClassLoadedMonitor {
    /**
     * Notify the monitor that a class has been loaded.
     * @param klass the class that was loaded
     */
    void notifyClassLoaded(RVMClass klass);
  }

  /**
   * Class loading callback list.
   */
  private static CallbackList classLoadedCallbacks = null;
  private static final Object classLoadedLock = new Object();
  private static boolean classLoadedEnabled = true;

  /**
   * Register a callback for class loading.
   * @param cb the object to notify when event happens
   */
  public static void addClassLoadedMonitor(ClassLoadedMonitor cb) {
    synchronized (classLoadedLock) {
      if (TRACE_ADDMONITOR || TRACE_CLASSLOADED) {
        VM.sysWrite("adding class loaded monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      classLoadedCallbacks = new CallbackList(cb, classLoadedCallbacks);
    }
  }

  /**
   * Notify the callback manager that a class has been loaded.
   * @param klass the class that was loaded
   */
  public static void notifyClassLoaded(RVMClass klass) {
    // NOTE: will need synchronization if allowing unregistering
    if (!classLoadedEnabled) return;
    classLoadedEnabled = false;
    if (TRACE_CLASSLOADED) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking class loaded monitors: ");
      VM.sysWrite(klass.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = classLoadedCallbacks; l != null; l = l.next) {
      if (TRACE_CLASSLOADED) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((ClassLoadedMonitor) l.callback).notifyClassLoaded(klass);
    }
    classLoadedEnabled = true;
  }

  /**
   * Interface for monitoring class resolution.
   */
  public interface ClassResolvedMonitor {
    /**
     * Notify the monitor that a class has been resolved.
     * @param klass the class that was resolved
     */
    void notifyClassResolved(RVMClass klass);
  }

  /**
   * Class resolution callback list.
   */
  private static CallbackList classResolvedCallbacks = null;
  private static final Object classResolvedLock = new Object();
  private static boolean classResolvedEnabled = true;

  /**
   * Register a callback for class resolution.
   * @param cb the object to notify when event happens
   */
  public static void addClassResolvedMonitor(ClassResolvedMonitor cb) {
    synchronized (classResolvedLock) {
      if (TRACE_ADDMONITOR || TRACE_CLASSRESOLVED) {
        VM.sysWrite("adding class resolved monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      classResolvedCallbacks = new CallbackList(cb, classResolvedCallbacks);
    }
  }

  /**
   * Notify the callback manager that a class has been resolved.
   * @param klass the class that was resolved
   */
  public static void notifyClassResolved(RVMClass klass) {
    // NOTE: will need synchronization if allowing unregistering
    if (!classResolvedEnabled) return;
    classResolvedEnabled = false;
    if (TRACE_CLASSRESOLVED) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking class resolved monitors: ");
      VM.sysWrite(klass.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = classResolvedCallbacks; l != null; l = l.next) {
      if (TRACE_CLASSRESOLVED) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((ClassResolvedMonitor) l.callback).notifyClassResolved(klass);
    }
    classResolvedEnabled = true;
  }

  /**
   * Interface for monitoring class instantiation.
   */
  public interface ClassInstantiatedMonitor {
    /**
     * Notify the monitor that a class has been instantiated.
     * @param klass the class that was instantiated
     */
    void notifyClassInstantiated(RVMClass klass);
  }

  /**
   * Class instantiation callback list.
   */
  private static CallbackList classInstantiatedCallbacks = null;
  private static final Object classInstantiatedLock = new Object();
  private static boolean classInstantiatedEnabled = true;

  /**
   * Register a callback for class instantiation.
   * @param cb the object to notify when event happens
   */
  public static void addClassInstantiatedMonitor(ClassInstantiatedMonitor cb) {
    synchronized (classInstantiatedLock) {
      if (TRACE_ADDMONITOR || TRACE_CLASSINSTANTIATED) {
        VM.sysWrite("adding class instantiated monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      classInstantiatedCallbacks = new CallbackList(cb, classInstantiatedCallbacks);
    }
  }

  /**
   * Notify the callback manager that a class has been instantiated.
   * @param klass the class that was instantiated
   */
  public static void notifyClassInstantiated(RVMClass klass) {
    // NOTE: will need synchronization if allowing unregistering
    if (!classInstantiatedEnabled) return;
    classInstantiatedEnabled = false;
    if (TRACE_CLASSINSTANTIATED) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking class instantiated monitors: ");
      VM.sysWrite(klass.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = classInstantiatedCallbacks; l != null; l = l.next) {
      if (TRACE_CLASSINSTANTIATED) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((ClassInstantiatedMonitor) l.callback).notifyClassInstantiated(klass);
    }
    classInstantiatedEnabled = true;
  }

  /**
   * Interface for monitoring class initialization.
   */
  public interface ClassInitializedMonitor {
    /**
     * Notify the monitor that a class has been initialized.
     * @param klass the class that was initialized
     */
    void notifyClassInitialized(RVMClass klass);
  }

  /**
   * Class initialization callback list.
   */
  private static CallbackList classInitializedCallbacks = null;
  private static final Object classInitializedLock = new Object();
  private static boolean classInitializedEnabled = true;

  /**
   * Register a callback for class initialization.
   * @param cb the object to notify when event happens
   */
  public static void addClassInitializedMonitor(ClassInitializedMonitor cb) {
    synchronized (classInitializedLock) {
      if (TRACE_ADDMONITOR || TRACE_CLASSINITIALIZED) {
        VM.sysWrite("adding class initialized monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      classInitializedCallbacks = new CallbackList(cb, classInitializedCallbacks);
    }
  }

  /**
   * Notify the callback manager that a class has been initialized.
   * @param klass the class that was initialized
   */
  public static void notifyClassInitialized(RVMClass klass) {
    // NOTE: will need synchronization if allowing unregistering
    if (!classInitializedEnabled) return;
    classInitializedEnabled = false;
    if (TRACE_CLASSINITIALIZED) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking class initialized monitors: ");
      VM.sysWrite(klass.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = classInitializedCallbacks; l != null; l = l.next) {
      if (TRACE_CLASSINITIALIZED) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((ClassInitializedMonitor) l.callback).notifyClassInitialized(klass);
    }
    classInitializedEnabled = true;
  }

  /**
   * Interface for monitoring method override.
   */
  public interface MethodOverrideMonitor {
    /**
     * Notify the monitor that a method has been overridden.
     * @param method the method that was loaded
     * @param parent the method that it overrides (null if none)
     */
    void notifyMethodOverride(RVMMethod method, RVMMethod parent);
  }

  /**
   * Method override callback list.
   */
  private static CallbackList methodOverrideCallbacks = null;
  private static final Object methodOverrideLock = new Object();
  private static boolean methodOverrideEnabled = true;

  /**
   * Register a callback for method override.
   * @param cb the object to notify when event happens
   */
  public static void addMethodOverrideMonitor(MethodOverrideMonitor cb) {
    synchronized (methodOverrideLock) {
      if (TRACE_ADDMONITOR || TRACE_METHODOVERRIDE) {
        VM.sysWrite("adding method override monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      methodOverrideCallbacks = new CallbackList(cb, methodOverrideCallbacks);
    }
  }

  /**
   * Notify the callback manager that a method has been overridden.
   * @param method the method that was loaded
   * @param parent the method that it overrides (null if none)
   */
  public static void notifyMethodOverride(RVMMethod method, RVMMethod parent) {
    // NOTE: will need synchronization if allowing unregistering
    if (!methodOverrideEnabled) return;
    methodOverrideEnabled = false;
    if (TRACE_METHODOVERRIDE) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking method override monitors: ");
      VM.sysWrite(method);
      VM.sysWrite(":");
      if (parent != null) {
        VM.sysWrite(parent);
      } else {
        VM.sysWrite("null");
      }
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = methodOverrideCallbacks; l != null; l = l.next) {
      if (TRACE_METHODOVERRIDE) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((MethodOverrideMonitor) l.callback).notifyMethodOverride(method, parent);
    }
    methodOverrideEnabled = true;
  }

  /**
   * Interface for monitoring method compile.
   */
  public interface MethodCompileMonitor {
    /**
     * Notify the monitor that a method is about to be compiled.
     * NOTE: use VM.runningVM and VM.writingBootImage to determine
     *       whether the VM is running
     * @param method the method that will be compiled
     * @param compiler the compiler that will be invoked.
     *        Values are constants in CompiledMethod
     */
    void notifyMethodCompile(RVMMethod method, int compiler);
  }

  /**
   * Method compile callback list.
   */
  private static CallbackList methodCompileCallbacks = null;
  private static final Object methodCompileLock = new Object();
  private static boolean methodCompileEnabled = true;

  /**
   * Register a callback for method compile.
   * @param cb the object to notify when event happens
   */
  public static void addMethodCompileMonitor(MethodCompileMonitor cb) {
    synchronized (methodCompileLock) {
      if (TRACE_ADDMONITOR || TRACE_METHODCOMPILE) {
        VM.sysWrite("adding method compile monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      methodCompileCallbacks = new CallbackList(cb, methodCompileCallbacks);
    }
  }

  /**
   * Notify the callback manager that a method is about to be compiled.
   * NOTE: use VM.runningVM and VM.writingBootImage to determine
   *       whether the VM is running
   * @param method the method that will be compiled
   * @param compiler the compiler that will be invoked
   *        Values are constants in CompiledMethod
   */
  public static void notifyMethodCompile(RVMMethod method, int compiler) {
    // NOTE: will need synchronization if allowing unregistering
    if (!methodCompileEnabled) return;
    methodCompileEnabled = false;
    if (TRACE_METHODCOMPILE) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking method compile monitors: ");
      VM.sysWrite(method);
      VM.sysWrite(":");
      VM.sysWrite(compiler);
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = methodCompileCallbacks; l != null; l = l.next) {
      if (TRACE_METHODCOMPILE) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((MethodCompileMonitor) l.callback).notifyMethodCompile(method, compiler);
    }
    methodCompileEnabled = true;
  }

  /**
   * Interface for monitoring forName calls.
   */
  public interface ForNameMonitor {
    /**
     * Notify the monitor that java.lang.Class.forName was called.
     * @param type the type that will be returned
     */
    void notifyForName(RVMType type);
  }

  /**
   * forName call callback list.
   */
  private static CallbackList forNameCallbacks = null;
  private static final Object forNameLock = new Object();
  private static boolean forNameEnabled = true;

  /**
   * Register a callback for forName call.
   * @param cb the object to notify when event happens
   */
  public static void addForNameMonitor(ForNameMonitor cb) {
    synchronized (forNameLock) {
      if (TRACE_ADDMONITOR || TRACE_FORNAME) {
        VM.sysWrite("adding forName monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      forNameCallbacks = new CallbackList(cb, forNameCallbacks);
    }
  }

  /**
   * Notify the monitor that java.lang.Class.forName was called.
   * @param type the type that will be returned
   */
  public static void notifyForName(RVMType type) {
    // NOTE: will need synchronization if allowing unregistering
    if (!forNameEnabled) return;
    forNameEnabled = false;
    if (TRACE_FORNAME) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking forName monitors: ");
      VM.sysWrite(type.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = forNameCallbacks; l != null; l = l.next) {
      if (TRACE_FORNAME) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((ForNameMonitor) l.callback).notifyForName(type);
    }
    forNameEnabled = true;
  }

  /**
   * Interface for monitoring defineClass calls.
   */
  public interface DefineClassMonitor {
    /**
     * Notify the monitor that java.lang.Class.defineclass was called.
     * @param type the type that will be returned
     */
    void notifyDefineClass(RVMType type);
  }

  /**
   * defineclass call callback list.
   */
  private static CallbackList defineClassCallbacks = null;
  private static final Object defineClassLock = new Object();
  private static boolean defineClassEnabled = true;

  /**
   * Register a callback for defineClass call.
   * @param cb the object to notify when event happens
   */
  public static void addDefineClassMonitor(DefineClassMonitor cb) {
    synchronized (defineClassLock) {
      if (TRACE_ADDMONITOR || TRACE_DEFINECLASS) {
        VM.sysWrite("adding defineclass monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      defineClassCallbacks = new CallbackList(cb, defineClassCallbacks);
    }
  }

  /**
   * Notify the monitor that java.lang.Class.defineclass was called.
   * @param type the type that will be returned
   */
  public static void notifyDefineClass(RVMType type) {
    // NOTE: will need synchronization if allowing unregistering
    if (!defineClassEnabled) return;
    defineClassEnabled = false;
    if (TRACE_DEFINECLASS) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking defineclass monitors: ");
      VM.sysWrite(type.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = defineClassCallbacks; l != null; l = l.next) {
      if (TRACE_DEFINECLASS) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((DefineClassMonitor) l.callback).notifyDefineClass(type);
    }
    defineClassEnabled = true;
  }

  /**
   * Interface for monitoring loadClass calls.
   */
  public interface LoadClassMonitor {
    /**
     * Notify the monitor that java.lang.Class.loadclass was called.
     * @param type the type that will be returned
     */
    void notifyLoadClass(RVMType type);
  }

  /**
   * loadclass call callback list.
   */
  private static CallbackList loadClassCallbacks = null;
  private static final Object loadClassLock = new Object();
  private static boolean loadClassEnabled = true;

  /**
   * Register a callback for loadClass call.
   * @param cb the object to notify when event happens
   */
  public static void addLoadClassMonitor(LoadClassMonitor cb) {
    synchronized (loadClassLock) {
      if (TRACE_ADDMONITOR || TRACE_LOADCLASS) {
        VM.sysWrite("adding loadclass monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      loadClassCallbacks = new CallbackList(cb, loadClassCallbacks);
    }
  }

  /**
   * Notify the monitor that java.lang.Class.loadclass was called.
   * @param type the type that will be returned
   */
  public static void notifyLoadClass(RVMType type) {
    // NOTE: will need synchronization if allowing unregistering
    if (!loadClassEnabled) return;
    loadClassEnabled = false;
    if (TRACE_LOADCLASS) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking loadclass monitors: ");
      VM.sysWrite(type.getDescriptor());
      VM.sysWrite("\n");
      //printStack("From: ");
    }
    for (CallbackList l = loadClassCallbacks; l != null; l = l.next) {
      if (TRACE_LOADCLASS) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((LoadClassMonitor) l.callback).notifyLoadClass(type);
    }
    loadClassEnabled = true;
  }

  /**
   * Interface for monitoring boot image writing.
   */
  public interface BootImageMonitor {
    /**
     * Notify the monitor that boot image writing is in progress.
     * @param types the types that are included in the boot image
     */
    void notifyBootImage(Enumeration<String> types);
  }

  /**
   * Boot image writing callback list.
   */
  private static CallbackList bootImageCallbacks = null;
  private static final Object bootImageLock = new Object();
  private static boolean bootImageEnabled = true;

  /**
   * Register a callback for boot image writing.
   * @param cb the object to notify when event happens
   */
  public static void addBootImageMonitor(BootImageMonitor cb) {
    synchronized (bootImageLock) {
      if (TRACE_ADDMONITOR || TRACE_BOOTIMAGE) {
        VM.sysWrite("adding boot image writing monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      bootImageCallbacks = new CallbackList(cb, bootImageCallbacks);
    }
  }

  /**
   * Notify the monitor that boot image writing is in progress.
   * @param types the types that are included in the boot image
   */
  public static void notifyBootImage(Enumeration<String> types) {
    // NOTE: will need synchronization if allowing unregistering
    if (!bootImageEnabled) return;
    bootImageEnabled = false;
    if (TRACE_BOOTIMAGE) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking boot image writing monitors\n");
    }
    for (CallbackList l = bootImageCallbacks; l != null; l = l.next) {
      if (TRACE_BOOTIMAGE) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((BootImageMonitor) l.callback).notifyBootImage(types);
    }
    bootImageEnabled = true;
  }

  /**
   * Interface for monitoring VM startup.
   */
  public interface StartupMonitor {
    /**
     * Notify the monitor that the VM has started up.
     */
    void notifyStartup();
  }

  /**
   * VM startup callback list.
   */
  private static CallbackList startupCallbacks = null;
  private static final Object startupLock = new Object();
  private static boolean startupEnabled = true;

  /**
   * Register a callback for VM startup.
   * @param cb the object to notify when event happens
   */
  public static void addStartupMonitor(StartupMonitor cb) {
    synchronized (startupLock) {
      if (TRACE_ADDMONITOR || TRACE_STARTUP) {
        VM.sysWrite("adding startup monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      startupCallbacks = new CallbackList(cb, startupCallbacks);
    }
  }

  /**
   * Notify the callback manager that the VM has started up.
   * NOTE: Runs in the main thread!
   */
  public static void notifyStartup() {
    // NOTE: will need synchronization if allowing unregistering
    if (!startupEnabled) return;
    startupEnabled = false;
    if (TRACE_STARTUP) {
      //VM.sysWrite(getThread(), false);
      //VM.sysWrite(": ");
      VM.sysWrite("invoking startup monitors\n");
    }
    for (CallbackList l = startupCallbacks; l != null; l = l.next) {
      if (TRACE_STARTUP) {
        VM.sysWrite("    ");
        VM.sysWrite(getClass(l.callback));
        VM.sysWrite("\n");
      }
      ((StartupMonitor) l.callback).notifyStartup();
    }
    startupEnabled = true;
  }

  /**
   * Interface for monitoring VM exit.
   */
  public interface ExitMonitor {
    /**
     * Notify the monitor that the VM is about to exit.
     * @param value the exit value
     */
    void notifyExit(int value);
  }

  /**
   * VM exit callback list.
   */
  private static CallbackList exitCallbacks = null;
  private static final Object exitLock = new Object();
  private static boolean exitCallbacksStarted = false;

  /**
   * Register a callback for VM exit.
   * @param cb the object to notify when event happens
   */
  public static void addExitMonitor(ExitMonitor cb) {
    synchronized (exitLock) {
      if (TRACE_ADDMONITOR || TRACE_EXIT) {
        VM.sysWrite("adding exit monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      exitCallbacks = new CallbackList(cb, exitCallbacks);
    }
  }

  /**
   * Notify the callback manager that the VM is about to exit.
   * Will return once all the callbacks are invoked.
   * @param value the exit value
   */
  public static void notifyExit(final int value) {
    synchronized (exitLock) {
      if (exitCallbacksStarted) return;
      if (exitCallbacks == null) return;
      exitCallbacksStarted = true;
      if (TRACE_EXIT) {
        //VM.sysWrite(Callbacks.getThread(), false);
        //VM.sysWrite(": ");
        VM.sysWrite("invoking exit monitors: ");
        VM.sysWriteln(value);
        //printStack("From: ");
      }
      for (CallbackList l = exitCallbacks; l != null; l = l.next) {
        if (TRACE_EXIT) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((ExitMonitor) l.callback).notifyExit(value);
      }
    }
  }

  /**
   * Interface for monitoring when an application starts executing
   */
  public interface AppStartMonitor {
    /**
     * Notify the monitor that the application has started executing
     * @param app application name
     */
    void notifyAppStart(String app);
  }

  /**
   * Application Start executing callback list.
   */
  private static CallbackList appStartCallbacks = null;
  private static final Object appStartLock = new Object();

  /**
   * Register a callback for when the application starts executing
   * @param cb the object to notify when event happens
   */
  public static void addAppStartMonitor(AppStartMonitor cb) {
    synchronized (appStartLock) {
      if (TRACE_ADDMONITOR || TRACE_APP_START) {
        VM.sysWrite("adding application start monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      appStartCallbacks = new CallbackList(cb, appStartCallbacks);
    }
  }

  /**
   * Notify the callback manager that the application started executing
   * Will return once all the callbacks are invoked.
   * @param app name of application
   */
  public static void notifyAppStart(String app) {
    synchronized (appStartLock) {
      if (appStartCallbacks == null) return;
      if (TRACE_APP_START) {
        VM.sysWrite("invoking application start monitors\n");
      }
      for (CallbackList l = appStartCallbacks; l != null; l = l.next) {
        if (TRACE_APP_START) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((AppStartMonitor) l.callback).notifyAppStart(app);
      }
    }
  }

  /**
   * Interface for monitoring when an application completes executing
   */
  public interface AppCompleteMonitor {
    /**
     * Notify the monitor that the application has completed executing
     * @param app  name of application
     */
    void notifyAppComplete(String app);
  }

  /**
   * Application Execution Complete callback list.
   */
  private static CallbackList appCompleteCallbacks = null;
  private static final Object appCompleteLock = new Object();

  /**
   * Register a callback for when the application completes executing
   * @param cb the object to notify when event happens
   */
  public static void addAppCompleteMonitor(AppCompleteMonitor cb) {
    synchronized (appCompleteLock) {
      if (TRACE_ADDMONITOR || TRACE_APP_COMPLETE) {
        VM.sysWrite("adding application complete monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      appCompleteCallbacks = new CallbackList(cb, appCompleteCallbacks);
    }
  }

  /**
   * Notify the callback manager that the application completed executing
   * Will return once all the callbacks are invoked.
   * @param app name of application
   */
  public static void notifyAppComplete(String app) {
    synchronized (appCompleteLock) {
      if (appCompleteCallbacks == null) return;
      if (TRACE_APP_COMPLETE) {
        VM.sysWrite("invoking application complete monitors for application ");
        VM.sysWrite(app);
        VM.sysWrite("\n");
      }
      for (CallbackList l = appCompleteCallbacks; l != null; l = l.next) {
        if (TRACE_APP_COMPLETE) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((AppCompleteMonitor) l.callback).notifyAppComplete(app);
      }
    }
  }

  /**
   * Interface for monitoring when an application starts a run
   */
  public interface AppRunStartMonitor {
    /**
     * Notify the monitor that the application has started a run
     * @param app application name
     * @param run run number
     */
    void notifyAppRunStart(String app, int run);
  }

  /**
   * Application Run Start callback list.
   */
  private static CallbackList appRunStartCallbacks = null;
  private static final Object appRunStartLock = new Object();

  /**
   * Register a callback for when the application starts a run
   * @param cb the object to notify when event happens
   */
  public static void addAppRunStartMonitor(AppRunStartMonitor cb) {
    synchronized (appRunStartLock) {
      if (TRACE_ADDMONITOR || TRACE_APP_RUN_START) {
        VM.sysWrite("adding application run start monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      appRunStartCallbacks = new CallbackList(cb, appRunStartCallbacks);
    }
  }

  /**
   * Notify the callback manager that the application started a run
   * Will return once all the callbacks are invoked.
   * @param app application name
   * @param run run number
   */
  public static void notifyAppRunStart(String app, int run) {
    synchronized (appRunStartLock) {
      if (appRunStartCallbacks == null) return;
      if (TRACE_APP_RUN_START) {
        //VM.sysWrite(getThread(), false);
        //VM.sysWrite(": ");
        VM.sysWrite("invoking the start monitor for application ");
        VM.sysWrite(app);
        VM.sysWrite(" at run ");
        VM.sysWrite(run);
        VM.sysWrite("\n");
      }
      for (CallbackList l = appRunStartCallbacks; l != null; l = l.next) {
        if (TRACE_APP_RUN_START) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((AppRunStartMonitor) l.callback).notifyAppRunStart(app, run);
      }
    }
  }

  /**
   * Interface for monitoring when an application completes a run
   */
  public interface AppRunCompleteMonitor {
    /**
     * Notify the monitor that the application has completed a run
     * @param app name of application
     * @param run run number
     */
    void notifyAppRunComplete(String app, int run);
  }

  /**
   * Application Run Complete callback list.
   */
  private static CallbackList appRunCompleteCallbacks = null;
  private static final Object appRunCompleteLock = new Object();

  /**
   * Register a callback for when the application completes a run
   * @param cb the object to notify when event happens
   */
  public static void addAppRunCompleteMonitor(AppRunCompleteMonitor cb) {
    synchronized (appRunCompleteLock) {
      if (TRACE_ADDMONITOR || TRACE_APP_RUN_COMPLETE) {
        VM.sysWrite("adding application run complete monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      appRunCompleteCallbacks = new CallbackList(cb, appRunCompleteCallbacks);
    }
  }

  /**
   * Notify the callback manager that the application completed a run
   * Will return once all the callbacks are invoked.
   * @param app name of application
   * @param run run number
   */
  public static void notifyAppRunComplete(String app, int run) {
    synchronized (appRunCompleteLock) {
      if (appRunCompleteCallbacks == null) return;
      if (TRACE_APP_RUN_COMPLETE) {
        //VM.sysWrite(getThread(), false);
        //VM.sysWrite(": ");
        VM.sysWrite("invoking the complete monitor for application ", app);
        VM.sysWriteln(" at run ", run);
      }
      for (CallbackList l = appRunCompleteCallbacks; l != null; l = l.next) {
        if (TRACE_APP_RUN_COMPLETE) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((AppRunCompleteMonitor) l.callback).notifyAppRunComplete(app, run);
      }
    }
  }

  /**
   * Interface for requesting VM to recompile all previously dynamically compiled methods
   */
  public interface RecompileAllDynamicallyLoadedMethodsMonitor {
    /**
     * Notify the monitor that the application has requested the recompile
     */
    void notifyRecompileAll();
  }

  /**
   * Recompile all callback list.
   */
  private static CallbackList recompileAllCallbacks = null;
  private static final Object recompileAllLock = new Object();

  /**
   * Register a callback for when the application requests to recompile all
   *  dynamically loaded classes
   * @param cb the object to notify when event happens
   */
  public static void addRecompileAllDynamicallyLoadedMethodsMonitor(RecompileAllDynamicallyLoadedMethodsMonitor cb) {
    synchronized (recompileAllLock) {
      if (TRACE_ADDMONITOR || TRACE_RECOMPILE_ALL) {
        VM.sysWrite("adding recompile all monitor: ");
        VM.sysWrite(getClass(cb));
        VM.sysWrite("\n");
      }
      recompileAllCallbacks = new CallbackList(cb, recompileAllCallbacks);
    }
  }

  /**
   * Notify the callback manager that the application requested a recompile all
   * Will return once all the callbacks are invoked.
   */
  public static void recompileAllDynamicallyLoadedMethods() {
    synchronized (recompileAllLock) {
      if (recompileAllCallbacks == null) return;
      if (TRACE_RECOMPILE_ALL) {
        VM.sysWriteln("invoking the recompile all monitor");
      }
      for (CallbackList l = recompileAllCallbacks; l != null; l = l.next) {
        if (TRACE_RECOMPILE_ALL) {
          VM.sysWrite("    ");
          VM.sysWrite(Callbacks.getClass(l.callback));
          VM.sysWrite("\n");
        }
        ((RecompileAllDynamicallyLoadedMethodsMonitor) l.callback).notifyRecompileAll();
      }
    }
  }

  ////////////////////
  // IMPLEMENTATION //
  ////////////////////

  /**
   * Initialize callbacks.
   */
  public static void init() { }

  /**
   * Perform boot-time actions.
   */
  public static void boot() { }

  /**
   * Linked list of callbacks.
   */
  private static class CallbackList {
    CallbackList(Object cb, CallbackList n) {
      callback = cb;
      next = n;
    }

    public final Object callback;
    public final CallbackList next;
  }

  private static final boolean TRACE_ADDMONITOR = false;
  private static final boolean TRACE_CLASSLOADED = false;
  private static final boolean TRACE_CLASSRESOLVED = false;
  private static final boolean TRACE_CLASSINITIALIZED = false;
  private static final boolean TRACE_CLASSINSTANTIATED = false;
  private static final boolean TRACE_METHODOVERRIDE = false;
  private static final boolean TRACE_METHODCOMPILE = false;
  private static final boolean TRACE_FORNAME = false;
  private static final boolean TRACE_DEFINECLASS = false;
  private static final boolean TRACE_LOADCLASS = false;
  private static final boolean TRACE_BOOTIMAGE = false;
  private static final boolean TRACE_STARTUP = false;
  private static final boolean TRACE_EXIT = false;
  private static final boolean TRACE_APP_RUN_START = false;
  private static final boolean TRACE_APP_RUN_COMPLETE = false;
  private static final boolean TRACE_APP_START = false;
  private static final boolean TRACE_APP_COMPLETE = false;
  private static final boolean TRACE_RECOMPILE_ALL = false;

  /**
   * Return class name of the object.
   * @param o the object
   * @return class name of the object
   */
  private static Atom getClass(Object o) {
    if (VM.runningVM) {
      return java.lang.JikesRVMSupport.getTypeForClass(o.getClass()).getDescriptor();
    } else {
      return Atom.findOrCreateAsciiAtom(o.getClass().getName());
    }
  }

  /**
   * Return current thread id.
   * @return current thread id
   */
  @SuppressWarnings("unused")
  private static int getThread() {
    if (VM.runningVM) {
      return RVMThread.getCurrentThread().getThreadSlot();
    } else {
      return System.identityHashCode(Thread.currentThread());
    }
  }

  /**
   * Print current stack trace.
   * @param message error message
   */
  @SuppressWarnings("unused")
  private static void printStack(String message) {
    if (VM.runningVM) {
      RVMThread.traceback(message);
    } else {
      new Throwable(message).printStackTrace();
    }
  }
}