Deep Dive into ThreadPoolExecutor: Uncover How Java Manages Thread Pools

ThreadPoolExecutor Source Code Analysis

Table of Contents

TreadPoolexecutor source code analysis

Class diagram

Executor interface

ExecutorService interface

AbstractExecutorService

Member variables

Constructor

Worker class analysis

Worker overview

Member variables

Constructor

execute() analysis

addWorker() analysis

runWorker() analysis

getTask() analysis

processWorkerExit() analysis

tryTerminate() analysis

interruptIdleWorker() analysis

Thread pool monitoring

Class Diagram

Executor Interface

Executor provides the execute() method to start a task.

public interface Executor {
    // start a task
    void execute(Runnable command);
}

ExecutorService Interface

ExecutorService extends Executor and adds methods for managing thread pools and task execution, such as shutdown, submit, invokeAll, and invokeAny.

public interface ExecutorService extends Executor {
    // shutdown the pool, allowing queued tasks to finish
    void shutdown();
    // shutdown now, interrupting running tasks
    List<Runnable> shutdownNow();
    boolean isShutdown();
    boolean isTerminated();
    boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException;
    <T> Future<T> submit(Callable<T> task);
    <T> Future<T> submit(Runnable task, T result);
    Future<?> submit(Runnable task);
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException;
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException;
    <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException;
    <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
}

AbstractExecutorService

AbstractExecutorService overrides several methods and defines newTaskFor() to construct a RunnableFuture.

Member Variables

/**
  * The thread pool uses a single int to store both state and worker count.
  * High 3 bits store state, low 29 bits store worker count.
  */
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3; // 29
private static final int CAPACITY = (1 << COUNT_BITS) - 1; // max workers = 536870911

private static final int RUNNING    = -1 << COUNT_BITS;
private static final int SHUTDOWN   =  0 << COUNT_BITS;
private static final int STOP       =  1 << COUNT_BITS;
private static final int TIDYING    =  2 << COUNT_BITS;
private static final int TERMINATED =  3 << COUNT_BITS;

private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }

private final BlockingQueue<Runnable> workQueue;
private final ReentrantLock mainLock = new ReentrantLock();
private final HashSet<Worker> workers = new HashSet<>();
private final Condition termination = mainLock.newCondition();
private int largestPoolSize;
private long completedTaskCount;
private volatile ThreadFactory threadFactory;
private volatile RejectedExecutionHandler handler;
private volatile long keepAliveTime;
private volatile boolean allowCoreThreadTimeOut;
private volatile int corePoolSize;
private volatile int maximumPoolSize;
private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();
private static final RuntimePermission shutdownPerm = new RuntimePermission("modifyThread");

Constructor

The main constructor validates parameters and initializes core fields.

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

Worker Class Analysis

Worker Overview

The Worker class is an inner class of ThreadPoolExecutor that extends AbstractQueuedSynchronizer, implements Runnable, and runs tasks via runWorker().

Member Variables

// Thread created by the thread factory
final Thread thread;
// Task supplied at construction time
Runnable firstTask;
// Number of completed tasks by this worker
volatile long completedTasks;

Constructor

Worker(Runnable firstTask) {
    // state -1 prevents interruption before task starts
    setState(-1);
    this.firstTask = firstTask;
    // thread is created by the factory; it is not the user task thread
    this.thread = getThreadFactory().newThread(this);
}

execute() Analysis

public void execute(Runnable command) {
    if (command == null) throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        if (!isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    } else if (!addWorker(command, false))
        reject(command);
}

addWorker() Analysis

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        if (rs >= SHUTDOWN && !(rs == SHUTDOWN && firstTask == null && !workQueue.isEmpty()))
            return false;
        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();
            if (runStateOf(c) != rs)
                continue retry;
        }
    }
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask);
        Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                int rs = runStateOf(ctl.get());
                if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive())
                        throw new IllegalThreadStateException();
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (!workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

runWorker() Analysis

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            w.lock();
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) { thrown = x; throw x; }
                catch (Error x) { thrown = x; throw x; }
                catch (Throwable x) { thrown = x; throw new Error(x); }
                finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

getTask() Analysis

private Runnable getTask() {
    boolean timedOut = false;
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }
        int wc = workerCountOf(c);
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
        if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }
        try {
            Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

processWorkerExit() Analysis

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly)
        decrementWorkerCount();
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks;
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
    int c = ctl.get();
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && !workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                return;
        }
        addWorker(null, false);
    }
}

tryTerminate() Analysis

final void tryTerminate() {
    for (;;) {
        int c = ctl.get();
        if (isRunning(c) ||
            runStateAtLeast(c, TIDYING) ||
            (runStateOf(c) == SHUTDOWN && !workQueue.isEmpty()))
            return;
        if (workerCountOf(c) != 0) {
            interruptIdleWorkers(ONLY_ONE);
            return;
        }
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                try {
                    terminated();
                } finally {
                    ctl.set(ctlOf(TERMINATED, 0));
                    termination.signalAll();
                }
                return;
            }
        } finally {
            mainLock.unlock();
        }
    }
}

interruptIdleWorker() Analysis

private void interruptIdleWorkers(boolean onlyOne) {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        for (Worker w : workers) {
            Thread t = w.thread;
            if (!t.isInterrupted() && w.tryLock()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {}
                finally {
                    w.unlock();
                }
            }
            if (onlyOne)
                break;
        }
    } finally {
        mainLock.unlock();
    }
}

Thread Pool Monitoring

Use the thread pool's exposed metrics for monitoring:

getTaskCount : total number of tasks submitted.

getCompletedTaskCount : number of tasks that have completed.

getPoolSize : current number of threads in the pool.

getLargestPoolSize : largest number of threads that have ever simultaneously been in the pool.

getActiveCount : number of threads actively executing tasks.