Tomcat example source code file (SingleRemoveSynchronizedAddLock.java)

This example Tomcat source code file (SingleRemoveSynchronizedAddLock.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 - Tomcat tags/keywords

interruptedexception, interruptedexception, singleremovesynchronizedaddlock, singleremovesynchronizedaddlock, thread, thread

The Tomcat SingleRemoveSynchronizedAddLock.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.catalina.tribes.transport.bio.util;

/**
 * The class <b>SingleRemoveSynchronizedAddLock implement locking for accessing the queue
 * by a single remove thread and multiple add threads.
 *
 * A thread is only allowed to be either the remove or
 * an add thread.
 *
 * The lock can either be owned by the remove thread
 * or by a single add thread.
 *
 * If the remove thread tries to get the lock,
 * but the queue is empty, it will block (poll)
 * until an add threads adds an entry to the queue and
 * releases the lock.
 * 
 * If the remove thread and add threads compete for
 * the lock and an add thread releases the lock, then
 * the remove thread will get the lock first.
 *
 * The remove thread removes all entries in the queue
 * at once and proceeses them without further
 * polling the queue.
 *
 * The lock is not reentrant, in the sense, that all
 * threads must release an owned lock before competing
 * for the lock again!
 *
 * @author Rainer Jung
 * @author Peter Rossbach
 * @version 1.1
 */
 
public class SingleRemoveSynchronizedAddLock {
    
    public SingleRemoveSynchronizedAddLock() {
    }
    
    public SingleRemoveSynchronizedAddLock(boolean dataAvailable) {
        this.dataAvailable=dataAvailable;
    }
    
    /**
     * Time in milliseconds after which threads
     * waiting for an add lock are woken up.
     * This is used as a safety measure in case
     * thread notification via the unlock methods
     * has a bug.
     */
    private long addWaitTimeout = 10000L;

    /**
     * Time in milliseconds after which threads
     * waiting for a remove lock are woken up.
     * This is used as a safety measure in case
     * thread notification via the unlock methods
     * has a bug.
     */
    private long removeWaitTimeout = 30000L;

    /**
     * The current remove thread.
     * It is set to the remove thread polling for entries.
     * It is reset to null when the remove thread
     * releases the lock and proceeds processing
     * the removed entries.
     */
    private Thread remover = null;

    /**
     * A flag indicating, if an add thread owns the lock.
     */
    private boolean addLocked = false;

    /**
     * A flag indicating, if the remove thread owns the lock.
     */
    private boolean removeLocked = false;

    /**
     * A flag indicating, if the remove thread is allowed
     * to wait for the lock. The flag is set to false, when aborting.
     */
    private boolean removeEnabled = true;

    /**
     * A flag indicating, if the remover needs polling.
     * It indicates, if the locked object has data available
     * to be removed.
     */
    private boolean dataAvailable = false;

    /**
     * @return Value of addWaitTimeout
     */
    public synchronized long getAddWaitTimeout() {
        return addWaitTimeout;
    }

    /**
     * Set value of addWaitTimeout
     */
    public synchronized void setAddWaitTimeout(long timeout) {
        addWaitTimeout = timeout;
    }

    /**
     * @return Value of removeWaitTimeout
     */
    public synchronized long getRemoveWaitTimeout() {
        return removeWaitTimeout;
    }

    /**
     * Set value of removeWaitTimeout
     */
    public synchronized void setRemoveWaitTimeout(long timeout) {
        removeWaitTimeout = timeout;
    }

    /**
     * Check if the locked object has data available
     * i.e. the remover can stop poling and get the lock.
     * @return True iff the lock Object has data available.
     */
    public synchronized boolean isDataAvailable() {
        return dataAvailable;
    }

    /**
     * Check if an add thread owns the lock.
     * @return True iff an add thread owns the lock.
     */
    public synchronized boolean isAddLocked() {
        return addLocked;
    }

    /**
     * Check if the remove thread owns the lock.
     * @return True iff the remove thread owns the lock.
     */
    public synchronized boolean isRemoveLocked() {
        return removeLocked;
    }

    /**
     * Check if the remove thread is polling.
     * @return True iff the remove thread is polling.
     */
    public synchronized boolean isRemovePolling() {
        if ( remover != null ) {
            return true;
        }
        return false;
    }

    /**
     * Acquires the lock by an add thread and sets the add flag.
     * If any add thread or the remove thread already acquired the lock
     * this add thread will block until the lock is released.
     */
    public synchronized void lockAdd() {
        if ( addLocked || removeLocked ) {
            do {
                try {
                    wait(addWaitTimeout);
                } catch ( InterruptedException e ) {
                    Thread.currentThread().interrupted();
                }
            } while ( addLocked || removeLocked );
        }
        addLocked=true;
    }

    /**
     * Acquires the lock by the remove thread and sets the remove flag.
     * If any add thread already acquired the lock or the queue is
     * empty, the remove thread will block until the lock is released
     * and the queue is not empty.
     */
    public synchronized boolean lockRemove() {
        removeLocked=false;
        removeEnabled=true;
        if ( ( addLocked || ! dataAvailable ) && removeEnabled ) {
            remover=Thread.currentThread();
            do {
                try {
                    wait(removeWaitTimeout);
                } catch ( InterruptedException e ) {
                    Thread.currentThread().interrupted();
                }
            } while ( ( addLocked || ! dataAvailable ) && removeEnabled );
            remover=null;
        }
        if ( removeEnabled ) {
            removeLocked=true;
        } 
        return removeLocked;
    }

    /**
     * Releases the lock by an add thread and reset the remove flag.
     * If the reader thread is polling, notify it.
     */
    public synchronized void unlockAdd(boolean dataAvailable) {
        addLocked=false;
        this.dataAvailable=dataAvailable;
        if ( ( remover != null ) && ( dataAvailable || ! removeEnabled ) ) {
            remover.interrupt();
        } else {
            notifyAll();
        }
    }

    /**
     * Releases the lock by the remove thread and reset the add flag.
     * Notify all waiting add threads,
     * that the lock has been released by the remove thread.
     */
    public synchronized void unlockRemove() {
        removeLocked=false;
        dataAvailable=false;
        notifyAll();
    }

    /**
     * Abort any polling remover thread
     */
    public synchronized void abortRemove() {
        removeEnabled=false;
        if ( remover != null ) {
            remover.interrupt();
        }
    }

}