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Posted 19 Jan 2011

A remote dynamic Command pattern with GWT and LinkSet

, 19 Jan 2011 LGPL3
Command pattern is a very useful solution, but it causes some problems when used in a distributed environment.
// -----------------------------------------------------------------------------
//    LinkSet
//    Copyright: 2010 Lukasz Bownik (lukasz.bownik@gmail.com)
//
//    This file is part of LinkSet.
//
//    LinkSet is free software; you can redistribute it and/or modify
//    it under the terms of the Lesser GNU General Public License as published by
//    the Free Software Foundation; either version 2 of the License, or
//    (at your option) any later version.
//
//    LinkSet is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    Lesser GNU General Public License for more details.
//
//    You should have received a copy of the Lesser GNU General Public License
//    along with LinkSet; if not, write to the Free Software
//    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
// -----------------------------------------------------------------------------
package org.linkset;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Arrays;

/*******************************************************************************
 * The pointer to a listener
 * 
 * @author Lukasz Bownik (lukasz.bownik@gmail.com)
 ******************************************************************************/
final public class MethodPointer {

    /***************************************************************************
     * Contructor
     *
     * @param methodId
     *            an id of a handler method
     * @throws NullPointerException
     *             id methodId == null or target == null
     * @throws IllegalArgumentException
     *             if methodId is empty
     **************************************************************************/
    public MethodPointer(final Object target, final String methodId) {

        if (target == null) {
            throw new NullPointerException("Null target");
        }

        this.target = target;

        if (methodId == null) {
            throw new NullPointerException("Null methodId");
        }
        this.methodId = methodId.trim();

        if (this.methodId.isEmpty()) {
            throw new IllegalArgumentException("Empty mehotdId.");
        }

        if (this.target instanceof Class<?>) {
            this.method = getMethod((Class<?>) target, methodId);
            if (!Modifier.isStatic(this.method.getModifiers())) {
                throw new IllegalArgumentException("Non-static method not allowed");
            }
        } else {
            this.method = getMethod(target.getClass(), methodId);
            if (Modifier.isStatic(this.method.getModifiers())) {
                throw new IllegalArgumentException("Static method not allowed");
            }
        }
    }
    /***************************************************************************
     * @see Object#toString()
     **************************************************************************/
    @Override
    public String toString() {

        return super.toString() + " ; " + this.methodId;
    }
    /***************************************************************************
     * @see Object#hashCode()
     **************************************************************************/
    @Override
    public int hashCode() {

        return 17 * this.target.hashCode() + this.method.hashCode();
    }
    /***************************************************************************
     * @see Object#equals(Object)
     **************************************************************************/
    @Override
    public boolean equals(final Object obj) {

        if (obj.getClass().equals(this.getClass())) {
            final MethodPointer other = (MethodPointer) obj;
            // use identity not equality
            return this.target == other.target && this.method.equals(other.method);
        }
        return false;
    }
    /***************************************************************************
     * Exerutes a method pointed by this pointer
     *
     * @param args
     *            method arguments
     * @return value returned by a called method or null
     * @throws ExceptionInInitializerError  if the initialization provoked by this method fails.
     * @throws InvocationTargetException if the underlying method throws an exception.
     **************************************************************************/
    public Object invoke(final Object[] args) throws InvocationTargetException,
            ExceptionInInitializerError {

        try {
            return this.method.invoke(this.target, args);
        } catch (final IllegalAccessException e) {
            // should never happen
            throw new RuntimeException(e);
        } catch (final IllegalArgumentException e) {
            // should never happen
            throw new RuntimeException(e);
        }
    }
    /***************************************************************************
     * This method is equivalent to invoke((Object[])null);
     * @see MethodPointer#invoke(java.lang.Object[]) 
     **************************************************************************/
    public Object invoke() throws InvocationTargetException,
            ExceptionInInitializerError {

        return invoke((Object[]) null);
    }
    /***************************************************************************
     * Returns a method
     *
     * @param cls
     **************************************************************************/
    private static Method getMethod(final Class<?> cls, final String methodId) {

        for (final Method method : cls.getDeclaredMethods()) {

            final HandlerMethod handler = method.getAnnotation(HandlerMethod.class);
            if (handler != null && handler.id().equals(methodId)) {
                method.setAccessible(true);
                return method;
            }
        }
        // handler not found
        throw new IllegalArgumentException(methodId + " not found in " + cls);
    }
    /***************************************************************************
     * ensures that method parameters are conformant to the specified ones.
     * Throws exception otherwise.
     * @param parameterTypes the array of parameter types of event handler methods
     **************************************************************************/
    void ensureParameterTypes(final Class<?>[] parameterTypes) {

        if (!Arrays.equals(parameterTypes,
                this.method.getParameterTypes())) {
            throw new IllegalArgumentException("Nonconformant parameter types.");
        }
    }
    /***************************************************************************
     *
     **************************************************************************/
    private final Method method;
    final Object target;
    private final String methodId;
}

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License

This article, along with any associated source code and files, is licensed under The GNU Lesser General Public License (LGPLv3)

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