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ActiveMQ example source code file (MultiExpressionEvaluator.java)

This example ActiveMQ source code file (MultiExpressionEvaluator.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - ActiveMQ tags/keywords

binaryexpression, binaryexpression, cacheexpression, cacheexpression, expressionlistener, expressionlistenerset, expressionlistenerset, hashmap, iterator, map, object, string, unaryexpression, unaryexpression, util

The ActiveMQ MultiExpressionEvaluator.java source code

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.activemq.filter;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import javax.jms.JMSException;

/**
 * A MultiExpressionEvaluator is used to evaluate multiple expressions in single
 * method call. <p/> Multiple Expression/ExpressionListener pairs can be added
 * to a MultiExpressionEvaluator object. When the MultiExpressionEvaluator
 * object is evaluated, all the registed Expressions are evaluated and then the
 * associated ExpressionListener is invoked to inform it of the evaluation
 * result. <p/> By evaluating multiple expressions at one time, some
 * optimizations can be made to reduce the number of computations normally
 * required to evaluate all the expressions. <p/> When this class adds an
 * Expression it wrapps each node in the Expression's AST with a CacheExpression
 * object. Then each CacheExpression object (one for each node) is placed in the
 * cachedExpressions map. The cachedExpressions map allows us to find the sub
 * expressions that are common across two different expressions. When adding an
 * Expression in, if a sub Expression of the Expression is allready in the
 * cachedExpressions map, then instead of wrapping the sub expression in a new
 * CacheExpression object, we reuse the CacheExpression allready int the map.
 * <p/> To help illustrate what going on, lets try to give an exmample: If we
 * denote the AST of a Expression as follows:
 * [AST-Node-Type,Left-Node,Right-Node], then A expression like: "3*5+6" would
 * result in "[*,3,[+,5,6]]" <p/> If the [*,3,[+,5,6]] expression is added to
 * the MultiExpressionEvaluator, it would really be converted to:
 * [c0,[*,3,[c1,[+,5,6]]]] where c0 and c1 represent the CacheExpression
 * expression objects that cache the results of the * and the + operation.
 * Constants and Property nodes are not cached. <p/> If later on we add the
 * following expression [=,11,[+,5,6]] ("11=5+6") to the
 * MultiExpressionEvaluator it would be converted to: [c2,[=,11,[c1,[+,5,6]]]],
 * where c2 is a new CacheExpression object but c1 is the same CacheExpression
 * used in the previous expression. <p/> When the expressions are evaluated, the
 * c1 CacheExpression object will only evaluate the [+,5,6] expression once and
 * cache the resulting value. Hence evauating the second expression costs less
 * because that [+,5,6] is not done 2 times. <p/> Problems: - cacheing the
 * values introduces overhead. It may be possible to be smarter about WHICH
 * nodes in the AST are cached and which are not. - Current implementation is
 * not thread safe. This is because you need a way to invalidate all the cached
 * values so that the next evaluation re-evaluates the nodes. By going single
 * threaded, chache invalidation is done quickly by incrementing a 'view'
 * counter. When a CacheExpressionnotices it's last cached value was generated
 * in an old 'view', it invalidates its cached value.
 * 
 *  $Date: 2005/08/27 03:52:36 $
 */
public class MultiExpressionEvaluator {

    Map<String, ExpressionListenerSet> rootExpressions = new HashMap();
    Map<Expression, CacheExpression> cachedExpressions = new HashMap();

    int view;

    /**
     * A UnaryExpression that caches the result of the nested expression. The
     * cached value is valid if the
     * CacheExpression.cview==MultiExpressionEvaluator.view
     */
    public class CacheExpression extends UnaryExpression {
        short refCount;
        int cview = view - 1;
        Object cachedValue;
        int cachedHashCode;

        public CacheExpression(Expression realExpression) {
            super(realExpression);
            cachedHashCode = realExpression.hashCode();
        }

        /**
         * @see org.apache.activemq.filter.Expression#evaluate(MessageEvaluationContext)
         */
        public Object evaluate(MessageEvaluationContext message) throws JMSException {
            if (view == cview) {
                return cachedValue;
            }
            cachedValue = right.evaluate(message);
            cview = view;
            return cachedValue;
        }

        public int hashCode() {
            return cachedHashCode;
        }

        public boolean equals(Object o) {
            if (o == null) {
                return false;
            }
            return ((CacheExpression)o).right.equals(right);
        }

        public String getExpressionSymbol() {
            return null;
        }

        public String toString() {
            return right.toString();
        }

    }

    /**
     * Multiple listeners my be interested in the results of a single
     * expression.
     */
    static class ExpressionListenerSet {
        Expression expression;
        List<ExpressionListener> listeners = new ArrayList();
    }

    /**
     * Objects that are interested in the results of an expression should
     * implement this interface.
     */
    static interface ExpressionListener {
        void evaluateResultEvent(Expression selector, MessageEvaluationContext message, Object result);
    }

    /**
     * Adds an ExpressionListener to a given expression. When evaluate is
     * called, the ExpressionListener will be provided the results of the
     * Expression applied to the evaluated message.
     */
    public void addExpressionListner(Expression selector, ExpressionListener c) {
        ExpressionListenerSet data = rootExpressions.get(selector.toString());
        if (data == null) {
            data = new ExpressionListenerSet();
            data.expression = addToCache(selector);
            rootExpressions.put(selector.toString(), data);
        }
        data.listeners.add(c);
    }

    /**
     * Removes an ExpressionListener from receiving the results of a given
     * evaluation.
     */
    public boolean removeEventListner(String selector, ExpressionListener c) {
        String expKey = selector;
        ExpressionListenerSet d = rootExpressions.get(expKey);
        // that selector had not been added.
        if (d == null) {
            return false;
        }
        // that selector did not have that listeners..
        if (!d.listeners.remove(c)) {
            return false;
        }

        // If there are no more listeners for this expression....
        if (d.listeners.size() == 0) {
            // Un-cache it...
            removeFromCache((CacheExpression)d.expression);
            rootExpressions.remove(expKey);
        }
        return true;
    }

    /**
     * Finds the CacheExpression that has been associated with an expression. If
     * it is the first time the Expression is being added to the Cache, a new
     * CacheExpression is created and associated with the expression. <p/> This
     * method updates the reference counters on the CacheExpression to know when
     * it is no longer needed.
     */
    private CacheExpression addToCache(Expression expr) {

        CacheExpression n = cachedExpressions.get(expr);
        if (n == null) {
            n = new CacheExpression(expr);
            cachedExpressions.put(expr, n);
            if (expr instanceof UnaryExpression) {

                // Cache the sub expressions too
                UnaryExpression un = (UnaryExpression)expr;
                un.setRight(addToCache(un.getRight()));

            } else if (expr instanceof BinaryExpression) {

                // Cache the sub expressions too.
                BinaryExpression bn = (BinaryExpression)expr;
                bn.setRight(addToCache(bn.getRight()));
                bn.setLeft(addToCache(bn.getLeft()));

            }
        }
        n.refCount++;
        return n;
    }

    /**
     * Removes an expression from the cache. Updates the reference counters on
     * the CacheExpression object. When the refernce counter goes to zero, the
     * entry int the Expression to CacheExpression map is removed.
     * 
     * @param cn
     */
    private void removeFromCache(CacheExpression cn) {
        cn.refCount--;
        Expression realExpr = cn.getRight();
        if (cn.refCount == 0) {
            cachedExpressions.remove(realExpr);
        }
        if (realExpr instanceof UnaryExpression) {
            UnaryExpression un = (UnaryExpression)realExpr;
            removeFromCache((CacheExpression)un.getRight());
        }
        if (realExpr instanceof BinaryExpression) {
            BinaryExpression bn = (BinaryExpression)realExpr;
            removeFromCache((CacheExpression)bn.getRight());
        }
    }

    /**
     * Evaluates the message against all the Expressions added to this object.
     * The added ExpressionListeners are notified of the result of the
     * evaluation.
     * 
     * @param message
     */
    public void evaluate(MessageEvaluationContext message) {
        Collection<ExpressionListenerSet> expressionListeners = rootExpressions.values();
        for (Iterator<ExpressionListenerSet> iter = expressionListeners.iterator(); iter.hasNext();) {
            ExpressionListenerSet els = iter.next();
            try {
                Object result = els.expression.evaluate(message);
                for (Iterator<ExpressionListener> iterator = els.listeners.iterator(); iterator.hasNext();) {
                    ExpressionListener l = iterator.next();
                    l.evaluateResultEvent(els.expression, message, result);
                }
            } catch (Throwable e) {
                e.printStackTrace();
            }
        }
    }
}

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