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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.impl;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * @author David Marginian [david at butterdev dot com].
 *
 * Represent the accessLogLevel's that can be set as init-params
 * on the DWR Servlet (see DefaultRemoter). Each type contains a name
 * and a hierarchy.  The hierarchy is number value (byte) that is used
 * to determine what/when to log based on the accessLogLevel.
 *
 */
public enum AccessLogLevel
{
    CALL ("call", (byte) 0),
    EXCEPTION   ("exception", (byte) 1),
    RUNTIMEEXCEPTION   ("runtimeexception", (byte) 2),
    ERROR   ("error", (byte) 3),
    OFF   ("off", (byte) 4);

    // We only want one accessLogLevelInstance, and we ensure this
    // in the getValue method.
    private static AccessLogLevel accessLogLevelInstance;
    private final String description;
    // Indicates the position in the hierarchy.  All types
    // with a higher hierarchy will be displayed in the log.
    private final byte hierarchy;

    private AccessLogLevel(String description, byte hierarchy) {
        this.description = description;
        this.hierarchy = hierarchy;
    }

    /**
     * This is a safe valueOf method.  Since we cannot override valueOf this method
     * should be used to retrieve an AccessLogLevel based on a String value.  This method
     * will always return an AccessLogLevel.  If accessLogLevelString is not valid a default
     * of OFF will be returned unless we are in debug mode when EXCEPTION will be returned.
     *
     * @param accessLogLevelString
     * @param debug - are we in debug mode
     * @return AccessLogLevel
     */
    public static AccessLogLevel getValue(String accessLogLevelString, boolean debug)
    {
        // May be overkill but use double-checked locking
        // to reduce the effect of the synchronization.
        if (null == accessLogLevelInstance)
        {
            synchronized (AccessLogLevel.class)
            {
                if (null == accessLogLevelInstance)
                {
                    try
                    {
                        accessLogLevelString = accessLogLevelString.toUpperCase();
                        accessLogLevelInstance = AccessLogLevel.valueOf(accessLogLevelString);
                    }
                    catch (Exception e)
                    {
                        // If debug is disabled and the user has attempted to set an accessLogLevel that does not exist
                        // we will default to OFF.  Let the user know why they aren't seeing anything in their logs.
                        if (!debug)
                        {
                            // only log if an accessLogLevel exists.  This means the user attempted to set the accessLogLevel
                            // but it was invalid.  In this case let the user know.
                            if (null != accessLogLevelString)
                            {
                                log.info("DWR is disabling logging.  " + accessLogLevelString + " is not a valid accessLogLevel.  Valid accessLogLevel's are: CALL, EXCEPTION, RUNTIMEEXCEPTION, and ERROR");
                            }
                            accessLogLevelInstance = AccessLogLevel.OFF;
                        }
                        else
                        {
                            if (null != accessLogLevelString)
                            {
                                log.info(accessLogLevelString + " is not a valid accessLogLevel.  Valid accessLogLevel's are: CALL, EXCEPTION, RUNTIMEEXCEPTION, and ERROR.  Since the debug init-param has been set to true DWR will log all Exceptions." );
                            }
                            accessLogLevelInstance = AccessLogLevel.EXCEPTION;
                        }
                    }
                }
            }
        }
        return accessLogLevelInstance;
    }

    public String description()   { return description; }
    public byte hierarchy() { return hierarchy; }

    /**
     * The log stream
     */
    private static final Log log = LogFactory.getLog(AccessLogLevel.class);
}