线程池的实现核心之一是FutureTask。在提交任务时，用户实现的Callable实例task会被包装为FutureTask实例ftask；提交后任务异步执行，无需用户关心；当用户需要时，再调用FutureTask#get()获取结果——或异常。

随之而来的问题是，如何优雅的获取ftask的结果并处理异常？本文讨论使用FutureTask的正确姿势。

今天换个风格。

源码分析

从提交一个Callable实例task开始。

submit()

ThreadPoolExecutor直接继承AbstractExecutorService的实现。

public abstract class AbstractExecutorService implements ExecutorService {
...
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }
...
}

后续流程可参考源码|从串行线程封闭到对象池、线程池。最终会在ThreadPoolExecutor#runWorker()中执行task.run()。

task即5行创建的ftask，看newTaskFor()。

newTaskFor()

AbstractExecutorService#newTaskFor()创建一个RunnableFuture类型的FutureTask。

public abstract class AbstractExecutorService implements ExecutorService {
...
    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }
...
}

看FutureTask的实现。

FutureTask

public class FutureTask<V> implements RunnableFuture<V> {
...
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;
...
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }
...
}

构造方法的重点是初始化ftask状态为NEW。

状态机

状态转换比较少，直接给状态序列：

* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED

状态在后面有用.

run()

简化如下：

public class FutureTask<V> implements RunnableFuture<V> {
...
    public void run() {
        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);
        }
    }
...
}

如果执行时未抛出异常

如果未抛出异常，则ran==true，FutureTask#set()设置结果。

public class FutureTask<V> implements RunnableFuture<V> {
...
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
...
}

• outcome中保存结果result
• 连续两步设置状态到NORMAL
• finishCompletion()执行一些清理

记住outcome。

相当于4行获取独占锁，5-6行执行锁中的操作（注意，7行是不加锁的）。

如果执行时抛出了异常

如果运行时抛出了异常，则被12行catch捕获，FutureTask#setException()设置结果；同时，ran==false，因此不执行FutureTask#set()。

public class FutureTask<V> implements RunnableFuture<V> {
...
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }
...
}