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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 org.directwebremoting.ConversionException;

/**
 * An interface for converting types from a string to some other type.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public interface Converter
{
    /**
     * If we are a compound converter that farms out part of the conversion
     * to other converters then you farm the conversion out via a configuration.
     * @param converterManager The configuration object
     */
    void setConverterManager(ConverterManager converterManager);

    /**
     * Attempt to coerce the data from a string to an Object.
     * If anything goes wrong with inbound conversion then we generally throw
     * an exception because we are converting data from the untrusted Internet
     * so we take the assumption that anything wrong is someone hacking.
     * @param paramType The type to convert to
     * @param data The data to convert
     * @return The converted data, or null if the conversion was not possible
     * @throws ConversionException If the conversion failed for some reason
     */
    Object convertInbound(Class paramType, InboundVariable data) throws ConversionException;

    /**
     * Return a javascript string that defines the variable named varName to
     * have the contents of the converted object data.
     * In contrast to convertInbound() failures in converting
     * data on the way out should not stop processing, and we should carry on
     * if we can. Failures are probably down to some misconfiguration so as much
     * information about the error as can be safely generated to console logs is
     * good. In other words if you need to loop in outbound conversion then it
     * might be a good idea to catch issues inside the loop, log, and carry on.
     * @param data The data to convert
     * @param outctx A collection of objects already converted and the results
     * @return The OutboundVariable that represents the data to convert
     * @throws ConversionException If the conversion failed for some reason
     */
    OutboundVariable convertOutbound(Object data, OutboundContext outctx) throws ConversionException;
}