java Callable接口和Future接口创建线程示例详解

public interface Future<V> {//Future源码
    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when {@code cancel} is called,
     * this task should never run.  If the task has already started,
     * then the {@code mayInterruptIfRunning} parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return {@code true}.  Subsequent calls to {@link #isCancelled}
     * will always return {@code true} if this method returned {@code true}.
     *
     * @param mayInterruptIfRunning {@code true} if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed normally;
     * {@code true} otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);
    //用于停止任务，如果未启动，将停止任务，已启动，仅在mayInterrupt为true时才会中断任务
    /**
     * Returns {@code true} if this task was cancelled before it completed
     * normally.
     *
     * @return {@code true} if this task was cancelled before it completed
     */
    boolean isCancelled();
    /**
     * Returns {@code true} if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * {@code true}.
     *
     * @return {@code true} if this task completed
     */
    boolean isDone();//判断任务是否完成，完成true，未完成false
    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;//任务完成返回结果，未完成，等待完成
    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

public class FutureTask<V> implements RunnableFuture<V> {//实现了RunnableFuture
	//该类的构造函数有两个
    public FutureTask(Callable<V> callable) {//
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }
    //用此方法创建的线程，最后start()调用的其实是Executors.RunnableAdapter类的call(); 
    public FutureTask(Runnable runnable, V result) {//返回的结果，就是构造函数传入的值
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }
    //原因如下
    //使用FutureTask对象的方式，创建的Thread，在开启线程.start后，底层执行的是
   //Thread的方法 start()方法会调用start0()，但是调用这个start0()(操作系统创建线程)的时机，是操作系统决定的，但是这个start0()最后会调用run()方法，此线程的执行内容就在传入的对象的run()方法中
    public void run() {
        if (target != null) {
            target.run();
            //执行到这 target就是在创建Thread时，传递的Runnable接口的子类对象，FUtureTask方式就是传入的FutureTask对象
            //会执行FutureTask中的run()方法
        }
    }
    //FutureTask的run()
    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;//类的属性
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    //会执行callable的call()，callable是创建TutureTask时，根据传入的Runnable的对象封装后的一个对象
                    //this.callable = Executors.callable(runnable, result);
                    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);
        }
    }
    //Executors类的callable
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }
    //最后执行到这里 Executors类的静态内部类RunnableAdapter
    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }
}
public interface RunnableFuture<V> extends Runnable, Future<V> {//继承了Runnable, Future<V>接口
	void run();//
}

/**
 * @author 长名06
 * @version 1.0
 * 演示Callable创建线程 需要使用到Runnable的实现类FutureTask类
 */
public class CallableDemo {
    public static void main(String[] args) {
        FutureTask<Integer> futureTask = new FutureTask<>(() -> {
            System.out.println(Thread.currentThread().getName() +  "使用Callable接口创建的线程");
            return 100;
        });
        new Thread(futureTask,"线程1").start();
    }
}