Ant example source code file (Parallel.java)

This example Ant source code file (Parallel.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 - Ant tags/keywords

buildexception, buildexception, interruptedexception, object, parallel, reflection, task, taskrunnable, taskrunnable, thread, thread, threadgroup, throwable, throwable, util, vector

The Parallel.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.tools.ant.taskdefs;

import java.lang.reflect.Method;
import java.util.Enumeration;
import java.util.Vector;
import java.util.List;
import java.util.ArrayList;
import org.apache.tools.ant.BuildException;
import org.apache.tools.ant.Location;
import org.apache.tools.ant.Task;
import org.apache.tools.ant.TaskContainer;
import org.apache.tools.ant.util.StringUtils;

/**
 * Executes the contained tasks in separate threads, continuing
 * once all are completed.
 * <p>
 * New behavior allows for the ant script to specify a maximum number of
 * threads that will be executed in parallel.  One should be very careful about
 * using the <code>waitFor task when specifying threadCount
 * as it can cause deadlocks if the number of threads is too small or if one of
 * the nested tasks fails to execute completely.  The task selection algorithm
 * will insure that the tasks listed before a task have started before that
 * task is started, but it will not insure a successful completion of those
 * tasks or that those tasks will finish first (i.e. it's a classic race
 * condition).
 * </p>
 * @since Ant 1.4
 *
 * @ant.task category="control"
 */
public class Parallel extends Task
                      implements TaskContainer {

    /** Class which holds a list of tasks to execute */
    public static class TaskList implements TaskContainer {
        /** Collection holding the nested tasks */
        private List tasks = new ArrayList();

        /**
         * Add a nested task to execute parallel (asynchron).
         * <p>
         * @param nestedTask  Nested task to be executed in parallel.
         *                    must not be null.
         */
        public void addTask(Task nestedTask) {
            tasks.add(nestedTask);
        }
    }

    /** Collection holding the nested tasks */
    private Vector nestedTasks = new Vector();

    /** Semaphore to notify of completed threads */
    private final Object semaphore = new Object();

    /** Total number of threads to run */
    private int numThreads = 0;

    /** Total number of threads per processor to run.  */
    private int numThreadsPerProcessor = 0;

    /** The timeout period in milliseconds */
    private long timeout;

    /** Indicates threads are still running and new threads can be issued */
    private volatile boolean stillRunning;

    /** Indicates that the execution timedout */
    private boolean timedOut;

    /**
     * Indicates whether failure of any of the nested tasks should end
     * execution
     */
    private boolean failOnAny;

    /** The dameon task list if any */
    private TaskList daemonTasks;

    /** Accumulation of exceptions messages from all nested tasks */
    private StringBuffer exceptionMessage;

    /** Number of exceptions from nested tasks */
    private int numExceptions = 0;

    /** The first exception encountered */
    private Throwable firstException;

    /** The location of the first exception */
    private Location firstLocation;

    /**
     * Add a group of daemon threads
     * @param daemonTasks The tasks to be executed as daemon.
     */
    public void addDaemons(TaskList daemonTasks) {
        if (this.daemonTasks != null) {
            throw new BuildException("Only one daemon group is supported");
        }
        this.daemonTasks = daemonTasks;
    }

    /**
     * Interval to poll for completed threads when threadCount or
     * threadsPerProcessor is specified.  Integer in milliseconds.; optional
     *
     * @param pollInterval New value of property pollInterval.
     */
    public void setPollInterval(int pollInterval) {
    }

    /**
     * Control whether a failure in a nested task halts execution. Note that
     * the task will complete but existing threads will continue to run - they
     * are not stopped
     *
     * @param failOnAny if true any nested task failure causes parallel to
     *        complete.
     */
    public void setFailOnAny(boolean failOnAny) {
        this.failOnAny = failOnAny;
    }

    /**
     * Add a nested task to execute in parallel.
     * @param nestedTask  Nested task to be executed in parallel
     */
    public void addTask(Task nestedTask) {
        nestedTasks.addElement(nestedTask);
    }

    /**
     * Dynamically generates the number of threads to execute based on the
     * number of available processors (via
     * <code>java.lang.Runtime.availableProcessors()). Requires a J2SE
     * 1.4 VM, and it will overwrite the value set in threadCount.
     * If used in a 1.1, 1.2, or 1.3 VM then the task will defer to
     * <code>threadCount.; optional
     * @param numThreadsPerProcessor Number of threads to create per available
     *        processor.
     *
     */
    public void setThreadsPerProcessor(int numThreadsPerProcessor) {
        this.numThreadsPerProcessor = numThreadsPerProcessor;
    }

    /**
     * Statically determine the maximum number of tasks to execute
     * simultaneously.  If there are less tasks than threads then all will be
     * executed at once, if there are more then only <code>threadCount
     * tasks will be executed at one time.  If <code>threadsPerProcessor
     * is set and the JVM is at least a 1.4 VM then this value is
     * ignored.; optional
     *
     * @param numThreads total number of threads.
     *
     */
    public void setThreadCount(int numThreads) {
        this.numThreads = numThreads;
    }

    /**
     * Sets the timeout on this set of tasks. If the timeout is reached
     * before the other threads complete, the execution of this
     * task completes with an exception.
     *
     * Note that existing threads continue to run.
     *
     * @param timeout timeout in milliseconds.
     */
    public void setTimeout(long timeout) {
        this.timeout = timeout;
    }



    /**
     * Execute the parallel tasks
     *
     * @exception BuildException if any of the threads failed.
     */
    public void execute() throws BuildException {
        updateThreadCounts();
        if (numThreads == 0) {
            numThreads = nestedTasks.size();
        }
        spinThreads();
    }

    /**
     * Determine the number of threads based on the number of processors
     */
    private void updateThreadCounts() {
        if (numThreadsPerProcessor != 0) {
            int numProcessors = getNumProcessors();
            if (numProcessors != 0) {
                numThreads = numProcessors * numThreadsPerProcessor;
            }
        }
    }

    private void processExceptions(TaskRunnable[] runnables) {
        if (runnables == null) {
            return;
        }
        for (int i = 0; i < runnables.length; ++i) {
            Throwable t = runnables[i].getException();
            if (t != null) {
                numExceptions++;
                if (firstException == null) {
                    firstException = t;
                }
                if (t instanceof BuildException
                    && firstLocation == Location.UNKNOWN_LOCATION) {
                    firstLocation = ((BuildException) t).getLocation();
                }
                exceptionMessage.append(StringUtils.LINE_SEP);
                exceptionMessage.append(t.getMessage());
            }
        }
    }

    /**
     * Spin up required threads with a maximum number active at any given time.
     *
     * @exception BuildException if any of the threads failed.
     */
    private void spinThreads() throws BuildException {
        final int numTasks = nestedTasks.size();
        TaskRunnable[] runnables = new TaskRunnable[numTasks];
        stillRunning = true;
        timedOut = false;

        int threadNumber = 0;
        for (Enumeration e = nestedTasks.elements(); e.hasMoreElements();
             threadNumber++) {
            Task nestedTask = (Task) e.nextElement();
            runnables[threadNumber]
                = new TaskRunnable(nestedTask);
        }

        final int maxRunning = numTasks < numThreads ? numTasks : numThreads;
        TaskRunnable[] running = new TaskRunnable[maxRunning];

        threadNumber = 0;
        ThreadGroup group = new ThreadGroup("parallel");

        TaskRunnable[] daemons = null;
        if (daemonTasks != null && daemonTasks.tasks.size() != 0) {
            daemons = new TaskRunnable[daemonTasks.tasks.size()];
        }

        synchronized (semaphore) {
            // When we leave this block we can be sure all data is really
            // stored in main memory before the new threads start, the new
            // threads will for sure load the data from main memory.
            //
            // This probably is slightly paranoid.
        }

        synchronized (semaphore) {
            // start any daemon threads
            if (daemons != null) {
                for (int i = 0; i < daemons.length; ++i) {
                    daemons[i] = new TaskRunnable((Task) daemonTasks.tasks.get(i));
                    Thread daemonThread =  new Thread(group, daemons[i]);
                    daemonThread.setDaemon(true);
                    daemonThread.start();
                }
            }

            // now run main threads in limited numbers...
            // start initial batch of threads
            for (int i = 0; i < maxRunning; ++i) {
                running[i] = runnables[threadNumber++];
                Thread thread =  new Thread(group, running[i]);
                thread.start();
            }

            if (timeout != 0) {
                // start the timeout thread
                Thread timeoutThread = new Thread() {
                    public synchronized void run() {
                        try {
                            wait(timeout);
                            synchronized (semaphore) {
                                stillRunning = false;
                                timedOut = true;
                                semaphore.notifyAll();
                            }
                        } catch (InterruptedException e) {
                            // ignore
                        }
                    }
                };
                timeoutThread.start();
            }

            // now find available running slots for the remaining threads
            outer:
            while (threadNumber < numTasks && stillRunning) {
                for (int i = 0; i < maxRunning; i++) {
                    if (running[i] == null || running[i].isFinished()) {
                        running[i] = runnables[threadNumber++];
                        Thread thread =  new Thread(group, running[i]);
                        thread.start();
                        // continue on outer while loop to get another
                        // available slot
                        continue outer;
                    }
                }

                // if we got here all slots in use, so sleep until
                // something happens
                try {
                    semaphore.wait();
                } catch (InterruptedException ie) {
                    // doesn't java know interruptions are rude?
                    // just pretend it didn't happen and go about out business.
                    // sheesh!
                }
            }

            // are all threads finished
            outer2:
            while (stillRunning) {
                for (int i = 0; i < maxRunning; ++i) {
                    if (running[i] != null && !running[i].isFinished()) {
                        //System.out.println("Thread " + i + " is still alive ");
                        // still running - wait for it
                        try {
                            semaphore.wait();
                        } catch (InterruptedException ie) {
                            // who would interrupt me at a time like this?
                        }
                        continue outer2;
                    }
                }
                stillRunning = false;
            }
        }

        if (timedOut) {
            throw new BuildException("Parallel execution timed out");
        }

        // now did any of the threads throw an exception
        exceptionMessage = new StringBuffer();
        numExceptions = 0;
        firstException = null;
        firstLocation = Location.UNKNOWN_LOCATION;
        processExceptions(daemons);
        processExceptions(runnables);

        if (numExceptions == 1) {
            if (firstException instanceof BuildException) {
                throw (BuildException) firstException;
            } else {
                throw new BuildException(firstException);
            }
        } else if (numExceptions > 1) {
            throw new BuildException(exceptionMessage.toString(),
                                     firstLocation);
        }
    }

    /**
     * Determine the number of processors. Only effective on later VMs
     *
     * @return the number of processors available or 0 if not determinable.
     */
    private int getNumProcessors() {
        try {
            Class[] paramTypes = {};
            Method availableProcessors =
                Runtime.class.getMethod("availableProcessors", paramTypes);

            Object[] args = {};
            Integer ret = (Integer) availableProcessors.invoke(Runtime.getRuntime(), args);
            return ret.intValue();
        } catch (Exception e) {
            // return a bogus number
            return 0;
        }
    }

    /**
     * thread that execs a task
     */
    private class TaskRunnable implements Runnable {
        private Throwable exception;
        private Task task;
        private boolean finished;

        /**
         * Construct a new TaskRunnable.<p>
         *
         * @param task the Task to be executed in a separate thread
         */
        TaskRunnable(Task task) {
            this.task = task;
        }

        /**
         * Executes the task within a thread and takes care about
         * Exceptions raised within the task.
         */
        public void run() {
            try {
                task.perform();
            } catch (Throwable t) {
                exception = t;
                if (failOnAny) {
                    stillRunning = false;
                }
            } finally {
                synchronized (semaphore) {
                    finished = true;
                    semaphore.notifyAll();
                }
            }
        }

        /**
         * get any exception that got thrown during execution;
         * @return an exception or null for no exception/not yet finished
         */
        public Throwable getException() {
            return exception;
        }

        /**
         * Provides the indicator that the task has been finished.
         * @return Returns true when the task is finished.
         */
        boolean isFinished() {
            return finished;
        }
    }

}