使用多線程的方式3---callable（帶傳回值）及FutureTask原理

目錄

實作callable接口（無傳回值）

建立FutureTask執行個體（無傳回值）

運作測試（無傳回值）

實作callable接口（有傳回值）

建立FutureTask執行個體（有傳回值）

運作測試（有傳回值）

探究一下futureTask

多線程使用方式---實作callable接口

實作callable接口（無傳回值）

package my.notes;

import java.util.concurrent.Callable;

public class MyCallable implements Callable {
    @Override
    public Object call() throws Exception {
        System.out.println("實作callable");
        return null;
    }
}
           

建立FutureTask執行個體（無傳回值）

package my.notes;

import java.util.concurrent.FutureTask;

public class MyTest {
    public static void main(String[] args) {

        System.out.println("主線程開始執行");
        MyCallable myCallable = new MyCallable();
        FutureTask futureTask = new FutureTask(myCallable);
        futureTask.run();
        System.out.println("主線程執行完畢");

    }
}
           

運作測試（無傳回值）

實作callable接口（有傳回值）

package my.notes;

import java.util.concurrent.Callable;

public class MyCallable implements Callable {
    @Override
    public Object call() throws Exception {
        System.out.println("實作callable");
        return "這是一個傳回值曉得吧？";
    }
}
           

建立FutureTask執行個體（有傳回值）

package my.notes;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

public class MyTest {
    public static void main(String[] args) throws ExecutionException, InterruptedException {

        System.out.println("主線程開始執行");
        MyCallable myCallable = new MyCallable();
        FutureTask futureTask = new FutureTask(myCallable);
        futureTask.run();
        System.out.println("callable傳回值：" + futureTask.get());
        System.out.println("主線程執行完畢");
    }
}
           

運作測試（有傳回值）

探究一下futureTask

public class FutureTask<V> implements RunnableFuture<V> 
           

構造方法：

public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }
           

來看一眼我們用到的.get方法，畢竟有一個猜測，如果那邊線程沒執行完，我們去get，會怎麼辦

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }
           

嗯哼？發現了一個什麼，等待，哦~ get不到它會等待，阻塞嘛這不是

都到這兒了，我們繼續往下看這個等待是幹嘛的

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);
        }
    }
           

Lock，這下就明白了

但是也不態光一直等待啊，有等待肯定就必須有喚醒，繼續往下

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 must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }
           

可以看到有一個boolean ran，是一個狀态，當我們的callable執行完傳回結果，會執行一個set，還把傳回值傳過去了，我們看看這個set

protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
           

又調用了一個finishCompletion，那我們繼續

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
    }
           

注意到了沒，釋放了線程，還記得我們從哪兒開始看的嗎？從get方法開始看的，那我們再回去

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }
           

這個時候是不是注意到了最後一個return了？于是我們就拿到了傳回值