关于多线程的思考

多线程的意义

1. 充分的利用多核机器性能，如果在单核系统上运行，多线程只是利用时间片的概念实现的假多线程。
2. 防止阻塞

多线程的创建

1. 继承Thread类
public class myThread_1 extends Thread{
    @Override
    public void run(){
        System.out.println("My thread is running");
    }

    public void createThread_1(){
        Thread thread = new Thread();
        thread.start();
    }

    public void createThread_1_1(){
        Thread thread = new Thread(){
            @Override
            public void run(){
                System.out.println("Thread Running");
            }
        };
        thread.start();
    }
}

2.实现Runnable方法
public class myThread_2 implements Runnable {

    @Override
    public void run() {
        System.out.println("MyRunnable running");
    }

    public void createMyThread_2(){
        Thread thread = new Thread(new myThread_2());
        thread.start();
    }

    public void createMyThread_2_1(){
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                System.out.println("MyRunnable running");
            }
        };
        Thread thread = new Thread(runnable);
        thread.start();
    }
}

3. 实现Callable方法
public class myThread_3{
    public void createCallableThread_3 (){
        Callable callable = new Callable() {
            @Override
            public Integer call() throws Exception {
                return new Random().nextInt(100);
            }
        };

        FutureTask future = new FutureTask(callable);
        new Thread(future).start();
        try {
            Thread.sleep(5000);
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

Runnable()与Callable()方式

1. Runnable实现的run()方法没有返回值，单纯执行run中的代码。
   2.Callable可以获取线程执行的状态和执行进度。通过与Future的联合使用，可以获得多线程的运行结果，并且进行一些操作。

如何实现线程间的通信
在多线程中，不可避免的需要线程之间进行通信 来完成。这里涉及到的概念有：thread.join(), object.wait(), object.notify(), CountdownLatch, CyclicBarrier, FutureTask, Callable
比如说：
如何让两个线程依次执行？我们可以使用thread.join().

    public void demo_join(){
         final Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("A is running");
            }
        });

        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("B is waiting A");
                try {
                    A.join();
                }catch(InterruptedException e){
                    e.printStackTrace();
                }
                System.out.println("B is running");
            }
        });

        B.start();
        A.start();
    }

那如何让 两个线程按照指定方式有序交叉运行呢？

public void demo_notify(){
        final Object lock = new Object();
        Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized(lock){
                    System.out.println("A-1");
                    try{
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("A-2");
                }
            }
        });

        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized(lock){
                    System.out.println("B-1");
                    lock.notify();
                    System.out.println("B-2");
                }
            }
        });

        A.start();
        B.start();
    }

四个线程 A B C D，其中 D 要等到 A B C 全执行完毕后才执行，而且 A B C 是同步运行的。

public void demo_countDownlatch(){
        int worker = 3;
        final CountDownLatch countDownLatch = new CountDownLatch(worker);
        new Thread(new Runnable() {
            @Override
            public void run() {
            System.out.println("D is waiting for other three threads");
                try {
                    countDownLatch.await();
                    System.out.println("D is running");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();

        for (char threadName='A'; threadName <= 'C'; threadName++) {
            final String tN = String.valueOf(threadName);
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(tN + "is working");
                    try {
                        Thread.sleep(100);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    System.out.println(tN + "finished");
                    countDownLatch.countDown();
                }
            }).start();
        }
    }

三个运动员各自准备，等到三个人都准备好后，再一起跑

    public void demo_CyclicBarrier(){
        int runner = 3;
        final CyclicBarrier cyclicBarrier = new CyclicBarrier(runner);

        for(char name='A';name<='C';name++){
            final String name_temp = String.valueOf(name);
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        cyclicBarrier.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } catch (BrokenBarrierException e) {
                        e.printStackTrace();
                    }
                }
            });
        }
    }