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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.dwrp;

import java.util.Collection;

import org.directwebremoting.extend.JsonModeMarshallException;
import org.directwebremoting.extend.NonNestedOutboundVariable;
import org.directwebremoting.extend.OutboundContext;
import org.directwebremoting.extend.OutboundVariable;

/**
 * A helper class for people that want to implement {@link OutboundVariable}.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class NonJsonNestedOutboundVariable implements OutboundVariable
{
    /**
     * @param outboundContext the OutboundContext to set
     */
    protected NonJsonNestedOutboundVariable(OutboundContext outboundContext)
    {
        this.outboundContext = outboundContext;
    }

    /**
     * We might want to force us into predefined mode.
     */
    protected void forceOutline()
    {
        // if we are in JSON mode then we are not allowed to be non-inline.
        if (outboundContext.isJsonMode())
        {
            throw new JsonModeMarshallException(NonJsonNestedOutboundVariable.class, "Outline in JSON mode is illegal");
        }

        this.outline = true;
    }

    /**
     * @return the outline
     */
    public boolean isOutline()
    {
        return outline;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.OutboundVariable#getDeclareCode()
     */
    public String getDeclareCode()
    {
        if (declareCode == null || children == null)
        {
            throw new NullPointerException();
        }

        return declareCode + getChildDeclareCodes();
    }

    /**
     * @param declareCode the declareCode to set
     */
    public void setBaseDeclareCode(String declareCode)
    {
        this.declareCode = declareCode;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.OutboundVariable#getBuildCode()
     */
    public String getBuildCode()
    {
        if (declareCode == null || children == null)
        {
            throw new NullPointerException();
        }

        return buildCode + getChildBuildCodes();
    }

    /**
     * @param buildCode the buildCode to set
     */
    public void setBaseBuildCode(String buildCode)
    {
        this.buildCode = buildCode;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.OutboundVariable#getAssignCode()
     */
    public String getAssignCode()
    {
        return assignCode;
    }

    /**
     * @param assignCode the assignCode to set
     */
    public void setAssignCode(String assignCode)
    {
        this.assignCode = assignCode;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.OutboundVariable#getReference()
     */
    public OutboundVariable getReferenceVariable()
    {
        if (reference == null)
        {
            reference = new NonNestedOutboundVariable(getVariableName());
            forceOutline();
        }

        return reference;
    }

    /**
     * In JSON mode we need to recursively prepare assign codes because the
     * order in which they are created matters.
     */
    protected void prepareChildAssignCodes()
    {
        for (OutboundVariable child : children)
        {
            child.prepareAssignCode();
        }
    }

    /**
     * Grab all the build codes together
     * @return A build string
     */
    private String getChildBuildCodes()
    {
        if (children == null)
        {
            return "";
        }

        StringBuffer buffer = new StringBuffer();

        // Make sure the nested things are declared
        for (OutboundVariable nested : children)
        {
            buffer.append(nested.getBuildCode());
        }

        return buffer.toString();
    }

    /**
     * Grab all the declare codes together
     * @return A declare string
     */
    private String getChildDeclareCodes()
    {
        if (children == null)
        {
            return "";
        }

        StringBuffer buffer = new StringBuffer();

        // Make sure the nested things are declared
        for (OutboundVariable nested : children)
        {
            buffer.append(nested.getDeclareCode());
        }

        return buffer.toString();
    }

    /**
     * @return the varName
     */
    protected String getVariableName()
    {
        if (varName == null)
        {
            varName = outboundContext.getNextVariableName();
        }

        return varName;
    }

    /**
     * When we create buildCode and declareCode, what do we need to add?
     * @param children The list of dependent {@link OutboundVariable}s
     */
    public void setChildren(Collection children)
    {
        this.children = children;
    }

    /**
     * Does anything refer to us?
     */
    private OutboundVariable reference;

    /**
     * Are we known to be recursive
     */
    private boolean outline = false;

    /**
     * The code to be executed to do basic initialization
     */
    private String declareCode;

    /**
     * The code to be executed to setup the data structure
     */
    private String buildCode;

    /**
     * The code to use to refer to this data structure
     */
    private String assignCode;

    /**
     * If we get recursive, this is the variable name we declare
     */
    private String varName;

    /**
     * When we create buildCode and declareCode, what do we need to add?
     */
    protected Collection children;

    /**
     * The conversion context
     */
    private OutboundContext outboundContext;
}