并发编程线程基础(一)---线程的创建

2021-04-01 15:26

阅读：484

标签：ssi thread sof zed dstar group until run obj

创建线程的几种方式：

1. 通过继承 Thread 类

 public static class MyThreadOne extends Thread{
        @Override
        public void run() {
            System.out.println("MyThreadOne running ... ");
        }
    }

2. 通过实现Runnable接口

 public static class MyThreadTwo implements Runnable{
        @Override
        public void run() {
            System.out.println("myThreadTwo running ... ");
        }
    }

3. 通过Callable接口和FutureTask实现有返回值的线程

 public static class MyThreadThree implements Callable{

        @Override
        public String call() throws Exception {
            System.out.println("MyThreadThree running .... ");
            return "success";
        }
}

具体的线程执行

 public static void main(String[] args) {
        MyThreadOne myThreadOne  = new MyThreadOne();
        myThreadOne.start();

        Thread thread = new Thread(new MyThreadTwo());
        thread.start();


        // futureTask 类实现了  RunnableFuture 接口 该接口继承了 Runable 接口  继承runnable 接口 就能通过  new Thread(new Runable())方式
        // 通过线程执行  futureTask 中的 run 方法 ； 继承 future 接口 能通过 get 方法 阻塞线程 获取放回直
        FutureTask stringFutureTask = new FutureTask(new MyThreadThree());


        new Thread(stringFutureTask).start();

        try {
            // get 时通过  LockSupport.park()  阻塞线程
            String s = stringFutureTask.get();
            System.out.println(s);
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }

}

原理探究：

1. 关于第三种方式是如何实现线程执行实现 Callable 接口中的 call 方法的？

通过debug 一步一步查看执行过程可以发现

thread.start方法创建线程，当线程被CPU分配时间片时， Runable实现类中的 run 方法，因为 FutureTask 类实现了 RunnableFuture 接口该接口继承了 Runnable 和 Future接口

实际上执行的是FutureTask 中的 run 方法，run 方法中执行 Callable接口中run 方法也就是自己实现类中的run 方法；

执行流程：

FutureTask stringFutureTask = new FutureTask(new MyThreadThree());
new Thread(stringFutureTask).start();

线程启动：

public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */
        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        /* Notify the group that this thread is about to be started
         * so that it can be added to the group‘s list of threads
         * and the group‘s unstarted count can be decremented. */
        group.add(this);

        boolean started = false;
        try {
            start0();
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

当线程分配CPU 时间后执行 run 方法：

/* What will be run. */

private Runnable target;


public void run() {
        if (target != null) {
            target.run();
        }
    }

RunableFuture 继承 Runnable 接口和 Future 接口，而 FutureTask s实现 Runnable接口，执行 FutureTask 中的 run 方法

public class FutureTask implements RunnableFuture {

    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 {
　　　　　　　　　　　　// 执行 Callale 实现类中的 call方法
                    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);
        }
    }

}

　该方法会调用 Callable 的接口的实现类，也就是我们自己的Callable 接口实现类中的call方法

2. get方法是如何获取到线程的返回值的？

可以发现在执行FutureTask 中的 run 方法中，在执行Callable 的call 放回会获取到返回值放到 FutureTask 的局部变量 outcoome 中，然后在通过 get方法当线程执行完成时，获取该临时变量的值返回；

执行 run 方法，执行完成后执行set 方法

    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);
        }
    }

set方法将方法的返回值放到全局变量outcome 中

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

当执行get方法时

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s

　当线程执行完成会执行report 方法从里面拿到返回值

    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);
    }

3. get方法为啥会阻塞线程？

执行get方法时候，会判断线程的状态，若call方法还未执行完成，会执行 awaiter方法，该方法会通过循环判断线程状态，若线程状态被中断，则通过异常放回

若线程执行完成，则结束循环返回，若为其他状态，通过 LockSupport.park()加锁

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s

执行awaitNode

 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

万事预则立，不预则废~~~加油

并发编程线程基础(一)---线程的创建

标签：ssi thread sof zed dstar group until run obj

原文地址：https://www.cnblogs.com/sunyangCoder/p/13532556.html

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