EventHandler.java

/* java.beans.EventHandler
   Copyright (C) 2004, 2005 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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General Public License for more details.

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02110-1301 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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modules, and to copy and distribute the resulting executable under
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independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.beans;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

/**
 * <p>EventHandler forms a bridge between dynamically created listeners and
 * arbitrary properties and methods.</p>
 *
 * <p>You can use this class to easily create listener implementations for
 * some basic interactions between an event source and its target. Using
 * the three static methods named <code>create</code> you can create
 * these listener implementations.</p>
 *
 * <p>See the documentation of each method for usage examples.</p>
 *
 * @author Jerry Quinn (jlquinn@optonline.net)
 * @author Robert Schuster (thebohemian@gmx.net)
 * @since 1.4
 */
public class EventHandler implements InvocationHandler
{
  // The name of the method that will be implemented.  If null, any method.
  private String listenerMethod;

  // The object to call action on.
  private Object target;

  // The name of the method or property setter in target.
  private String action;

  // The property to extract from an event passed to listenerMethod.
  private String property;

  // The target objects Class.
  private Class targetClass;

  // String class doesn't already have a capitalize routine.
  private String capitalize(String s)
  {
    return s.substring(0, 1).toUpperCase() + s.substring(1);
  }

  /**
   * Creates a new <code>EventHandler</code> instance.
   *
   * <p>Typical creation is done with the create method, not by knewing an
   * EventHandler.</p>
   *
   * <p>This constructs an EventHandler that will connect the method
   * listenerMethodName to target.action, extracting eventPropertyName from
   * the first argument of listenerMethodName. and sending it to action.</p>
   *
   * <p>Throws a <code>NullPointerException</code> if the <code>target</code>
   * argument is <code>null</code>.
   *
   * @param target Object that will perform the action.
   * @param action A property or method of the target.
   * @param eventPropertyName A readable property of the inbound event.
   * @param listenerMethodName The listener method name triggering the action.
   */
  public EventHandler(Object target, String action, String eventPropertyName,
                      String listenerMethodName)
  {
    this.target = target;

    // Retrieving the class is done for two reasons:
    // 1) The class object is needed very frequently in the invoke() method.
    // 2) The constructor should throw a NullPointerException if target is null.
    targetClass = target.getClass();

    this.action = action;       // Turn this into a method or do we wait till
                // runtime
    property = eventPropertyName;
    listenerMethod = listenerMethodName;
  }

  /**
   * Returns the event property name.
   */
  public String getEventPropertyName()
  {
    return property;
  }

  /**
   * Returns the listener's method name.
   */
  public String getListenerMethodName()
  {
    return listenerMethod;
  }

  /**
   * Returns the target object.
   */
  public Object getTarget()
  {
    return target;
  }

  /**
   * Returns the action method name.
   */
  public String getAction()
  {
    return action;
  }

  // Fetch a qualified property like a.b.c from object o.  The properties can
  // be boolean isProp or object getProp properties.
  //
  // Returns a length 2 array with the first entry containing the value
  // extracted from the property, and the second entry contains the class of
  // the method return type.
  //
  // We play this game because if the method returns a native type, the return
  // value will be a wrapper.  If we then take the type of the wrapper and use
  // it to locate the action method that takes the native type, it won't match.
  private Object[] getProperty(Object o, String prop)
  {
    // Isolate the first property name from a.b.c.
    int pos;
    String rest = null;
    if ((pos = prop.indexOf('.')) != -1)
      {
        rest = prop.substring(pos + 1);
        prop = prop.substring(0, pos);
      }

    // Find a method named getProp.  It could be isProp instead.
    Method getter;
    try
      {
        // Look for boolean property getter isProperty
        getter = o.getClass().getMethod("is" + capitalize(prop));
      }
    catch (NoSuchMethodException nsme1)
      {
        try {
          // Look for regular property getter getProperty
          getter = o.getClass().getMethod("get" + capitalize(prop));
        } catch(NoSuchMethodException nsme2) {
            try {
            // Finally look for a method of the name prop
            getter = o.getClass().getMethod(prop);
            } catch(NoSuchMethodException nsme3) {
                // Ok, give up with an intelligent hint for the user.
                throw new RuntimeException("Method not called: Could not find a property or method '" + prop
                        + "' in " + o.getClass() + " while following the property argument '" + property + "'.");
            }
        }
      }
    try {
      Object val = getter.invoke(o);

      if (rest != null)
        return getProperty(val, rest);

      return new Object[] {val, getter.getReturnType()};
    } catch(InvocationTargetException ite) {
        throw new RuntimeException("Method not called: Property or method '" + prop + "' has thrown an exception.", ite);
    } catch(IllegalAccessException iae) {
        // This cannot happen because we looked up method with Class.getMethod()
        // which returns public methods only.
        throw (InternalError) new InternalError("Non-public method was invoked.").initCause(iae);
    }
  }

  /**
   * Invokes the <code>EventHandler</code>.
   *
   * <p>This method is normally called by the listener's proxy implementation.</p>
   *
   * @param proxy The listener interface that is implemented using
   * the proxy mechanism.
   * @param method The method that was called on the proxy instance.
   * @param arguments The arguments which where given to the method.
   * @throws Throwable <code>NoSuchMethodException</code> is thrown when the EventHandler's
   * action method or property cannot be found.
   */
  public Object invoke(Object proxy, Method method, Object[] arguments)
  {
      try {
      // The method instance of the target object. We have to find out which
      // one we have to invoke.
      Method actionMethod = null;

    // Listener methods that weren't specified are ignored.  If listenerMethod
    // is null, then all listener methods are processed.
    if (listenerMethod != null && !method.getName().equals(listenerMethod))
      return null;

    // If a property is defined we definitely need a valid object at
    // arguments[0] that can be used to retrieve a value to which the
    // property of the target gets set.
    if(property != null) {
      // Extracts the argument. We will let it fail with a NullPointerException
      // the caller used a listener method that has no arguments.
      Object event = arguments[0];

      // Obtains the property XXX propertyType keeps showing up null - why?
      // because the object inside getProperty changes, but the ref variable
      // can't change this way, dolt!  need a better way to get both values out
      // - need method and object to do the invoke and get return type
      Object v[] = getProperty(event, property);
      Object[] args = new Object[] { v[0] };

      // Changes the class array that controls which method signature we are going
      // to look up in the target object.
      Class[] argTypes = new Class[] { initClass((Class) v[1]) };

      // Tries to  find a setter method to which we can apply the
      while(argTypes[0] != null) {
      try
      {
        // Look for a property setter for action.
        actionMethod = targetClass.getMethod("set" + capitalize(action), argTypes);

        return actionMethod.invoke(target, args);
      }
    catch (NoSuchMethodException e)
      {
        // If action as property didn't work, try as method later.
      }

      argTypes[0] = nextClass(argTypes[0]);
      }

      // We could not find a suitable setter method. Now we try again interpreting
      // action as the method name itself.
      // Since we probably have changed the block local argTypes array
      // we need to rebuild it.
      argTypes = new Class[] { initClass((Class) v[1]) };

      // Tries to  find a setter method to which we can apply the
      while(argTypes[0] != null) {
        try
        {
          actionMethod = targetClass.getMethod(action, argTypes);

          return actionMethod.invoke(target, args);
        }
        catch (NoSuchMethodException e)
        {
        }

        argTypes[0] = nextClass(argTypes[0]);
      }

        throw new RuntimeException("Method not called: Could not find a public method named '"
                + action + "' in target " + targetClass + " which takes a '"
                + v[1] + "' argument or a property of this type.");
      }

    // If property was null we will search for a no-argument method here.
    // Note: The ordering of method lookups is important because we want to prefer no-argument
    // calls like the JDK does. This means if we have actionMethod() and actionMethod(Event) we will
    // call the first *EVEN* if we have a valid argument for the second method. This is behavior compliant
    // to the JDK.
    // If actionMethod() is not available but there is a actionMethod(Event) we take this. That makes us
    // more specification compliant than the JDK itself because this one will fail in such a case.
    try
      {
      actionMethod = targetClass.getMethod(action);
      }
    catch(NoSuchMethodException nsme)
      {
        // Note: If we want to be really strict the specification says that a no-argument method should
        // accept an EventObject (or subclass I guess). However since the official implementation is broken
        // anyways, it's more flexible without the EventObject restriction and we are compatible on everything
        // else this can stay this way.
        if(arguments != null && arguments.length >= 1/* && arguments[0] instanceof EventObject*/) {
            Class[] targetArgTypes = new Class[] { initClass(arguments[0].getClass()) };

            while(targetArgTypes[0] != null) {
                try
                {
                  // If no property exists we expect the first element of the arguments to be
                  // an EventObject which is then applied to the target method.

                  actionMethod = targetClass.getMethod(action, targetArgTypes);

                  return actionMethod.invoke(target, new Object[] { arguments[0] });
                }
                catch(NoSuchMethodException nsme2)
                {

                }

                targetArgTypes[0] = nextClass(targetArgTypes[0]);
            }

        }
      }

    // If we do not have a Method instance at this point this means that all our tries
    // failed. The JDK throws an ArrayIndexOutOfBoundsException in this case.
    if(actionMethod == null)
      throw new ArrayIndexOutOfBoundsException(0);

    // Invoke target.action(property)
    return actionMethod.invoke(target);
      } catch(InvocationTargetException ite) {
         throw new RuntimeException(ite.getCause());
      } catch(IllegalAccessException iae) {
          // Cannot happen because we always use getMethod() which returns public
          // methods only. Otherwise there is something seriously broken in
          // GNU Classpath.
          throw (InternalError) new InternalError("Non-public method was invoked.").initCause(iae);
      }
  }

  /**
   * <p>Returns the primitive type for every wrapper class or the
   * class itself if it is no wrapper class.</p>
   *
   * <p>This is needed because to be able to find both kinds of methods:
   * One that takes a wrapper class as the first argument and one that
   * accepts a primitive instead.</p>
   */
  private Class initClass(Class klass) {
   if(klass == Boolean.class) {
    return Boolean.TYPE;
   } else if(klass == Byte.class) {
    return Byte.TYPE;
   } else if(klass == Short.class) {
    return Short.TYPE;
   } else if(klass == Integer.class) {
    return Integer.TYPE;
   } else if(klass == Long.class) {
    return Long.TYPE;
   } else if(klass == Float.class) {
    return Float.TYPE;
   } else if(klass == Double.class) {
    return Double.TYPE;
   } else {
    return klass;
   }
  }

  /**
   *
   *
   * @param klass
   * @return
   */
  private Class nextClass(Class klass) {
    if(klass == Boolean.TYPE) {
    return Boolean.class;
   } else if(klass == Byte.TYPE) {
    return Byte.class;
   } else if(klass == Short.TYPE) {
    return Short.class;
   } else if(klass == Integer.TYPE) {
    return Integer.class;
   } else if(klass == Long.TYPE) {
    return Long.class;
   } else if(klass == Float.TYPE) {
    return Float.class;
   } else if(klass == Double.TYPE) {
    return Double.class;
   } else {
    return klass.getSuperclass();
   }
   }

  /**
   * <p>Constructs an implementation of <code>listenerInterface</code>
   * to dispatch events.</p>
   *
   * <p>You can use such an implementation to simply call a public
   * no-argument method of an arbitrary target object or to forward
   * the first argument of the listener method to the target method.</p>
   *
   * <p>Call this method like:</p>
   * <code>
   * button.addActionListener((ActionListener)
   *    EventHandler.create(ActionListener.class, target, "dispose"));
   * </code>
   *
   * <p>to achieve the following behavior:</p>
   * <code>
   * button.addActionListener(new ActionListener() {
   *    public void actionPerformed(ActionEvent ae) {
   *        target.dispose();
   *    }
   * });
   * </code>
   *
   * <p>That means if you need a listener implementation that simply calls a
   * a no-argument method on a given instance for <strong>each</strong>
   * method of the listener interface.</p>
   *
   * <p>Note: The <code>action</code> is interpreted as a method name. If your target object
   * has no no-argument method of the given name the EventHandler tries to find
   * a method with the same name but which can accept the first argument of the
   * listener method. Usually this will be an event object but any other object
   * will be forwarded, too. Keep in mind that using a property name instead of a
   * real method here is wrong and will throw an <code>ArrayIndexOutOfBoundsException</code>
   * whenever one of the listener methods is called.<p/>
   *
   * <p>The <code>EventHandler</code> will automatically convert primitives
   * to their wrapper class and vice versa. Furthermore it will call
   * a target method if it accepts a superclass of the type of the
   * first argument of the listener method.</p>
   *
   * <p>In case that the method of the target object throws an exception
   * it will be wrapped in a <code>RuntimeException</code> and thrown out
   * of the listener method.</p>
   *
   * <p>In case that the method of the target object cannot be found an
   * <code>ArrayIndexOutOfBoundsException</code> will be thrown when the
   * listener method is invoked.</p>
   *
   * <p>A call to this method is equivalent to:
   * <code>create(listenerInterface, target, action, null, null)</code></p>
   *
   * @param listenerInterface Listener interface to implement.
   * @param target Object to invoke action on.
   * @param action Target property or method to invoke.
   * @return A constructed proxy object.
   */
  public static <T> T create(Class<T> listenerInterface, Object target,
                             String action)
  {
    return create(listenerInterface, target, action, null, null);
  }

  /**
   * <p>Constructs an implementation of <code>listenerInterface</code>
   * to dispatch events.</p>
   *
   * <p>Use this method if you want to create an implementation that retrieves
   * a property value from the <b>first</b> argument of the listener method
   * and applies it to the target's property or method. This first argument
   * of the listener is usually an event object but any other object is
   * valid, too.</p>
   *
   * <p>You can set the value of <code>eventPropertyName</code> to "prop"
   * to denote the retrieval of a property named "prop" from the event
   * object. In case that no such property exists the <code>EventHandler</code>
   * will try to find a method with that name.</p>
   *
   * <p>If you set <code>eventPropertyName</code> to a value like this "a.b.c"
   * <code>EventHandler</code> will recursively evaluate the properties "a", "b"
   * and "c". Again if no property can be found the <code>EventHandler</code>
   * tries a method name instead. This allows mixing the names, too: "a.toString"
   * will retrieve the property "a" from the event object and will then call
   * the method "toString" on it.</p>
   *
   * <p>An exception thrown in any of these methods will provoke a
   * <code>RuntimeException</code> to be thrown which contains an
   * <code>InvocationTargetException</code> containing the triggering exception.</p>
   *
   * <p>If you set <code>eventPropertyName</code> to a non-null value the
   * <code>action</code> parameter will be interpreted as a property name
   * or a method name of the target object.</p>
   *
   * <p>Any object retrieved from the event object and applied to the
   * target will converted from primitives to their wrapper class or
   * vice versa or applied to a method that accepts a superclass
   * of the object.</p>
   *
   * <p>Examples:</p>
   * <p>The following code:</p><code>
   * button.addActionListener(
   *    new ActionListener() {
   *        public void actionPerformed(ActionEvent ae) {
   *            Object o = ae.getSource().getClass().getName();
   *            textField.setText((String) o);
   *        }
   *    });
   * </code>
   *
   * <p>Can be expressed using the <code>EventHandler</code> like this:</p>
   * <p>
   * <code>button.addActionListener((ActionListener)
   *    EventHandler.create(ActionListener.class, textField, "text", "source.class.name");
   * <code>
   * </p>
   *
   * <p>As said above you can specify the target as a method, too:</p>
   * <p>
   * <code>button.addActionListener((ActionListener)
   *    EventHandler.create(ActionListener.class, textField, "setText", "source.class.name");
   * <code>
   * </p>
   *
   * <p>Furthermore you can use method names in the property:</p>
   * <p>
   * <code>button.addActionListener((ActionListener)
   *    EventHandler.create(ActionListener.class, textField, "setText", "getSource.getClass.getName");
   * <code>
   * </p>
   *
   * <p>Finally you can mix names:</p>
   * <p>
   * <code>button.addActionListener((ActionListener)
   *    EventHandler.create(ActionListener.class, textField, "setText", "source.getClass.name");
   * <code>
   * </p>
   *
   * <p>A call to this method is equivalent to:
   * <code>create(listenerInterface, target, action, null, null)</code>
   * </p>
   *
   * @param listenerInterface Listener interface to implement.
   * @param target Object to invoke action on.
   * @param action Target property or method to invoke.
   * @param eventPropertyName Name of property to extract from event.
   * @return A constructed proxy object.
   */
  public static <T> T create(Class<T> listenerInterface, Object target,
                             String action, String eventPropertyName)
  {
    return create(listenerInterface, target, action, eventPropertyName, null);
  }

  /**
   * <p>Constructs an implementation of <code>listenerInterface</code>
   * to dispatch events.</p>
   *
   * <p>Besides the functionality described for {@link create(Class, Object, String)}
   * and {@link create(Class, Object, String, String)} this method allows you
   * to filter the listener method that should have an effect. Look at these
   * method's documentation for more information about the <code>EventHandler</code>'s
   * usage.</p>
   *
   * <p>If you want to call <code>dispose</code> on a <code>JFrame</code> instance
   * when the <code>WindowListener.windowClosing()</code> method was invoked use
   * the following code:</p>
   * <p>
   * <code>
   * EventHandler.create(WindowListener.class, jframeInstance, "dispose", null, "windowClosing");
   * </code>
   * </p>
   *
   * <p>A <code>NullPointerException</code> is thrown if the <code>listenerInterface</code>
   * or <code>target</code> argument are <code>null</code>.
   *
   * @param listenerInterface Listener interface to implement.
   * @param target Object to invoke action on.
   * @param action Target method name to invoke.
   * @param eventPropertyName Name of property to extract from event.
   * @param listenerMethodName Listener method to implement.
   * @return A constructed proxy object.
   */
  public static <T> T create(Class<T> listenerInterface, Object target,
                             String action, String eventPropertyName,
                             String listenerMethodName)
  {
    // Create EventHandler instance
    EventHandler eh = new EventHandler(target, action, eventPropertyName,
                                       listenerMethodName);

    // Create proxy object passing in the event handler
    Object proxy = Proxy.newProxyInstance(listenerInterface.getClassLoader(),
                                          new Class<?>[] {listenerInterface},
                                          eh);

    return (T) proxy;
  }
}