Java FutureTask source code analysis and usage details

Understand what is FutureTask?

FutureTask is a cancelable asynchronous calculation.

FutureTask Provides a basic implementation of Future. You can call methods to start and cancel a calculation, query whether the calculation is completed, and obtain the calculation results.

FutureTask can only obtain the calculation result after the calculation is completed. Once the calculation is completed, it cannot be restarted or canceled unless the runAndReset method is called.

In addition to implementing the Future interface, FutureTask also implements the Runnable interface. Therefore, FutureTask can be executed by the Executor of the thread pool, or it can be executed directly using an asynchronous thread call. (futureTask.run()).

How is FutureTask implemented?

First, let’s take a look at the inheritance structure of the FutureTask class, as shown below. It implements the RunnableFuture interface, and RunnableFuture inherits from Future and functional interface Runnable, so FutureTask is essentially a runnable Future.

The Future interface stipulates some functions that asynchronous calculation classes must implement. The source code is as follows:

package java.util.concurrent;
public interface Future<V> {
    /**
     * 尝试取消任务的执行，并返回取消结果。
     * 参数mayInterruptIfRunning：是否中断线程。
     */
    boolean cancel(boolean mayInterruptIfRunning);
    /**
     * 判断任务是否被取消（正常结束之前被被取消返回true）
     */
    boolean isCancelled();
    /**
     * 判断当前任务是否执行完毕，包括正常执行完毕、执行异常或者任务取消。
     */
    boolean isDone();
    /**
     * 获取任务执行结果，任务结束之前会阻塞。
     */
    V get() throws InterruptedException, ExecutionException;
    /**
     * 在指定时间内尝试获取执行结果。若超时则抛出超时异常TimeoutException
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

Runnable We are all familiar with the interface. It is a functional interface, which we often use to create a thread.

package java.lang;
?
@FunctionalInterface
public interface Runnable {
 ? ?
 ? ?public abstract void run();
}

FutureTask is a task that will be executed. It contains the specific implementation of the above interface. FutureTask internally defines the state of the task and some state constants. Its internal core is a Callable callable. We pass The constructor can pass in callable or runnable, and it will eventually be converted to callable internally, because we need to obtain the execution result of the asynchronous task, and only threads created through Callable will return the result.

We can judge the return results of isCancelled() and isDone() in Future through the status at this time.

The following is the FutureTask source code, including core source code analysis comments

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
public class FutureTask<V> implements RunnableFuture<V> {
    /**
     * 任务的运行状态
     */
    private volatile int state;
    private static final int NEW          = 0; // 新建
    private static final int COMPLETING   = 1; // 完成
    private static final int NORMAL       = 2; // 正常
    private static final int EXCEPTIONAL  = 3; // 异常
    private static final int CANCELLED    = 4; // 取消
    private static final int INTERRUPTING = 5; // 中断中
    private static final int INTERRUPTED  = 6; // 中断的
    private Callable<V> callable;
    /**
 	 * 返回结果
 	 */
    private Object outcome; 
    private volatile Thread runner;
    private volatile WaitNode waiters;
    ...
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       
    }
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       
    }
    public boolean isCancelled() {
        return state >= CANCELLED;
    }
    public boolean isDone() {
        return state != NEW;
    }
    /*
     * 取消任务实现
     *   如果任务还没有启动就调用了cancel(true)，任务将永远不会被执行。
     *   如果任务已经启动，参数mayInterruptIfRunning将决定任务是否应该中断执行该任务的线程，以尝试中断该任务。
     *   如果任务任务已经取消、已经完成或者其他原因不能取消，尝试将失败。
     */
    public boolean cancel(boolean mayInterruptIfRunning) {
        if (!(state == NEW &&
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }
        /*
     * 等待获取结果
     *   获取当前状态，判断是否执行完成。并且判断时间是否超时
     *   如果任务没有执行完成，就阻塞等待完成，若超时抛出超时等待异常。
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }
  /*
     * 等待获取结果
     *   获取当前状态，判断是否执行完成。
     *   如果任务没有执行完成，就阻塞等待完成。
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }
    /**
     * 根据状态判断返回结果还是异常
     */
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }
    protected void done() { }
    /**
     * 设置结果借助CAS确认状态是否完成状态
     */
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
    /**
     * 设置异常，当运行完成出现异常，设置异常状态
     */
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }
    /*
     * 执行callable获取结果，或者异常
     *   判断状态是不是启动过的，如果是新建才可以执行run方法
     */
    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }
    /**
     * 重新执行
     */
    protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don&#39;t set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            runner = null;
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }
    /*
     * 处理可能取消的中断
     */
    private void handlePossibleCancellationInterrupt(int s) {
        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); 
    }
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }
    /**
     * 移除并唤醒所有等待线程,执行done,置空callable
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        done();
        callable = null;        // to reduce footprint
    }
    /**
     * 等待完成
     * 首先判断是否超时
     * 处理中断的，然后处理异常状态的，处理完成的...
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }
    /**
     * 去除等待
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
                    else if (pred != null) {
                        pred.next = s;
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }
    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

FutureTask running process

Generally speaking, we can think of FutureTask Has the following three states:

Not started: The newly created FutureTask is not started before run() is executed.

private static final int NEW          = 0; // 新建

Started: FutureTaskWhen the run method of the object is started and executed, the FutureTask is in the started state.

Completed: FutureTask execution ends normally, or FutureTask execution is canceled (FutureTask object cancel method), or FutureTask object run method throws an exception and causes interruption. At the end, the FutureTask is in the completed state.

private static final int COMPLETING   = 1; // 完成
private static final int NORMAL       = 2; // 完成后正常设置结果
private static final int EXCEPTIONAL  = 3; // 完成后异常设置异常
private static final int CANCELLED    = 4; // 执行取消
private static final int INTERRUPTING = 5; // 中断中
private static final int INTERRUPTED  = 6; // 中断的

Usage of FutureTask

Use one (directly create a new thread call):

FutureTask<Integer> task = new FutureTask<>(new Callable() {
	@Override
    public Integer call() throws Exception {
    	return sum();
    }
});
new Thread(task).stat();
Integer result = task.get();

Use two (used in conjunction with the thread pool)

FutureTask<Integer> task = new FutureTask<>(new Callable() {
	@Override
    public Integer call() throws Exception {
    	return sum();
    }
});
Executors.newCachedThreadPool().submit(task);
Integer result = task.get();

The above is the detailed content of Java FutureTask source code analysis and usage details. For more information, please follow other related articles on the PHP Chinese website!