Java高并发（七）——Future模式

       大家想下，多线程处理提高性能的根本本质在哪？其实就是将串行的处理步骤进行并行的处理，其实总时间是没有缩短的。也就是以前一个人干活需要10个小时，而十个人干同样的活需要1小时，从而缩短处理时间。但是如果干活有先后限制怎么办？例如工作中：测试前必须编码，编码前必须设计，设计前必须需求分析，分析前……如何提高这种情况的性能呢？或者说是如何让这中情况下的线程更加充分利用呢？Future模式——异步调用。好，在总结Future模式前，我们来先看一篇文章https://www.cnblogs.com/cz123/p/7693064.html。

       一，流程对比：对上边厨房做饭的例子，进行用Future模式的前后执行流程对比，从图中可以看出，其实就将能够并行的处理步骤通过异步调用进行了并行，并行不了的只能串行。

       二，Future模式：

      1，Future的核心结构图：

     2，看下实现简易代码：

/**
 * 数据接口
 */
public interface Data {
    /**
     * 获取数据
     * @return
     */
    String getResult();
}


/**
 * RealData类
 */
public class RealData implements Data {
    protected final String result;

    public RealData(String para) {
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 10; i++) {
            sb.append(para);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        result = sb.toString();
    }


    @Override
    public String getResult() {
        return result;
    }
}


/**
 * FutureData类
 */
public class FutureData implements Data {
    //FutureData是realData的包装
    protected RealData realData = null;
    protected boolean isReady = false;

    public synchronized void setRealData(RealData realData) {
        if (isReady) {
            return;
        }
        this.realData = realData;
        isReady = true;
        notifyAll();
    }


    @Override
    public synchronized String getResult() {
        while (!isReady) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return realData.getResult();
    }
}



public class Client {

    public Data request(final String queryStr) {
        final FutureData futureData = new FutureData();
        //单起个线程进行数据处理
        new Thread() {
            @Override
            public void run() {
                RealData realData = new RealData(queryStr);
                futureData.setRealData(realData);
            }
        }.start();
        //立即返回
        return futureData;
    }


    public static void main(String[] args) {
        Client client = new Client();

        Data data = client.request("ljhname");
        System.out.println("请求完毕");

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("数据" + data.getResult());
    }
}

       三，JDK中的Future模式：JDK已经帮我们准备了一套完整的实现，我们可以利用其进行非常方便的实现功能。

      1，先把上边的例子改为使用jdk的：

public class RealData implements Callable {
    private String para;

    public RealData (String para){
        this.para = para;
    }

    @Override
    public String call() throws Exception {
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 10; i++) {
            sb.append(para);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        return sb.toString();
    }
}



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

        FutureTask futureTask = new FutureTask(new RealData("ljh"));
        ExecutorService executorService = Executors.newFixedThreadPool(1);

        executorService.submit(futureTask);

        System.out.println("请求完毕");

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("数据=" + futureTask.get());

    }
}

       2，看下FutureTask的类关系结构图：

       3，FutureTask源码分析：

   /**
     * 一，属性：几个处理状态流程 callable,outcome runner,waiter...
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    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;

    /** The underlying callable; nulled out after running */
    private Callable callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;


    //二，两个构造方法
    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     * @param runnable the runnable task
     * @param result the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future f = new FutureTask(runnable, null)}
     * @throws NullPointerException if the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }


    //三，取消任务
    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;
    }

   //四，是否已经取消
    public boolean isCancelled() {
        return state >= CANCELLED;
    }


    //五，取的返回对象，get()直到去的为止，get(long timeout, TimeUnit unit)有取的等待时间，超过则报超时异常
    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    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);
    }

    //六，是否完成
    public boolean isDone() {
        return state != NEW;
    }

    //七，执行
    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable 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);
        }
    }

       好，Future模式先这样吧，主要理解其通过线程异步处理的过程，当然JDK8中又增加了CompletableFuture，功能更加强大，后边我会总结上。继续中……