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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.
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/*
 * Copyright 2005 Joe Walker
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.directwebremoting.extend;

import java.io.IOException;

import javax.servlet.ServletOutputStream;

import org.directwebremoting.ScriptBuffer;

/**
 * While a Marshaller is processing a request it can register a ScriptConduit
 * with the ScriptSession to say - pass scripts straight to me and bypass the
 * temporary storage area.
 * This interface allows this to happen.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class ScriptConduit implements Comparable
{
    /**
     * All ScriptConduit need a rank
     * @param rank How does this ScriptConduit sort
     */
    public ScriptConduit(int rank)
    {
        this.rank = rank;
    }

    /**
     * ScriptConduits have a rank that indicates the priority order in which we
     * should send scripts to them.
     * The rank is a number from 1 to 10, and should only be one of the defined
     * values: ScriptConduit.RANK_*.
     * @see ScriptConduit#RANK_PROCEDURAL
     * @see ScriptConduit#RANK_FAST
     * @see ScriptConduit#RANK_SLOW
     * @return The rank of this ScriptConduit
     */
    public int getRank()
    {
        return rank;
    }

    /**
     * Indicates that this ScriptConduit is used for control-flow and will
     * probably not actually convey the script, but does need to tell someone
     * else about it
     */
    public static final int RANK_PROCEDURAL = 10;

    /**
     * Indicates that this ScriptConduit is a very good way of getting scripts
     * to the client and should be used as a preferred method
     */
    public static final int RANK_FAST = 5;

    /**
     * Indicates that this ScriptConduit is a poor way of getting scripts to the
     * client and should only be used as a last resort.
     */
    public static final int RANK_SLOW = 1;

    /**
     * Add a script to the list bound for remote execution.
     * It is not an error to refuse to handle the script and return false, it
     * just indicates that this ScriptConduit did not accept the script.
     * If the ScriptConduit can no longer function then it should throw an
     * exception and it will be assumed to be no longer useful.
     * If you want to implement this method then you will probably be doing
     * something like calling {@link ServletOutputStream#print(String)} and
     * passing in the results of calling ScriptBufferUtil.createOutput().
     * @param script The script to execute
     * @return true if this ScriptConduit handled the script.
     * @throws IOException If this conduit is broken and should not be used
     * @throws MarshallException If objects in the script can not be marshalled
     */
    public abstract boolean addScript(ScriptBuffer script) throws IOException, MarshallException;

    /* (non-Javadoc)
     * @see java.lang.Comparable#compareTo(java.lang.Object)
     */
    @SuppressWarnings({"SubtractionInCompareTo", "NumericCastThatLosesPrecision"})
    public int compareTo(ScriptConduit that)
    {
        int rankdiff = that.getRank() - this.getRank();
        if (rankdiff != 0)
        {
            return rankdiff;
        }

        return (int) (this.id - that.id);
    }

    /* (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object obj)
    {
        if (obj == null)
        {
            return false;
        }

        if (obj == this)
        {
            return true;
        }

        if (!this.getClass().equals(obj.getClass()))
        {
            return false;
        }

        ScriptConduit that = (ScriptConduit) obj;
        return this.id == that.id;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @SuppressWarnings({"NumericCastThatLosesPrecision"})
    @Override
    public int hashCode()
    {
        return 17 + (int) id;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString()
    {
        return getClass().getSimpleName() + "[id=" + id + "]";
    }

    /**
     * The rank of this ScriptConduit
     */
    private int rank;

    /**
     * Our ID, to get around serialization issues
     */
    private final long id = getNextId();

    /**
     * Get the next unique ID in a thread safe way
     * @return a unique id
     */
    private static synchronized long getNextId()
    {
        nextId++;
        return nextId;
    }

    /**
     * The next ID, to get around serialization issues
     */
    private static long nextId = 0L;
}