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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.util.IdentityHashMap;
import java.util.Map;

/**
 * We need to keep track of stuff while we are converting on the way out to
 * prevent recursion.
 * This class helps track the conversion process.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public final class OutboundContext
{
    /**
     * Contexts need to know if they are producing JSON output
     * @param jsonMode Are we producing JSON output?
     */
    public OutboundContext(boolean jsonMode)
    {
        this.jsonMode = jsonMode;
    }

    /**
     * Have we already converted an object?
     * @param object The object to check
     * @return How it was converted last time or null if we've not seen it before
     */
    public OutboundVariable get(Object object)
    {
        return map.get(object);
    }

    /**
     * @param object We have converted a new object, remember it
     * @param ss How the object was converted
     */
    public void put(Object object, OutboundVariable ss)
    {
        map.put(object, ss);
    }

    /**
     * Create a new variable name to keep everything we declare separate
     * @return A new unique variable name
     */
    public String getNextVariableName()
    {
        String varName = OUTBOUND_VARIABLE_PREFIX + nextVarIndex;
        nextVarIndex++;

        return varName;
    }

    /**
     * @return Are we in JSON mode where everything is inline?
     */
    public boolean isJsonMode()
    {
        return jsonMode;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString()
    {
        return map.toString();
    }

    /**
     * The prefix for outbound variable names the we generate
     */
    private static final String OUTBOUND_VARIABLE_PREFIX = "s";

    /**
     * The map of objects to how we converted them last time
     */
    private final Map map = new IdentityHashMap();

    /**
     * What index do we tack on the next variable name that we generate
     */
    private int nextVarIndex = 0;

    /**
     * Are we in JSON mode where everything is inline?
     */
    private boolean jsonMode = true;
}