Java并发编程：线程的创建

1. Thread
2. Runnable
3. start() 和 run()

在Java中线程的创建主要有两种，一种是通过继承抽象类Thread，一种是通过实现Runnable接口。当然，还有Concurent包里面的Callable和Future也可以算是一种。

1 Thread

我们先来看一下，使用Thread如何创建线程：

public class ThreadDemo extends Thread {
    @Override
    public void run() {
        System.out.println("Current thread name is: " + Thread.currentThread().getName());
    }
 
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Thread thread = new ThreadDemo();
            thread.start();
        }
    }
}

Current thread name is: Thread-0
Current thread name is: Thread-2
Current thread name is: Thread-1
Current thread name is: Thread-3
Current thread name is: Thread-5
Current thread name is: Thread-4
Current thread name is: Thread-6
Current thread name is: Thread-7
Current thread name is: Thread-8
Current thread name is: Thread-9

可以看到，我们创建来10个线程，但是执行的顺序是不确定的。可以设置优先级，默认是5，最大是10，最小是1.但是即使设置来优先级，顺序也是不能保证。

public static void main(String[] args) {
    for (int i = 0; i < 10; i++) {
        Thread thread = new ThreadDemo();
        thread.setPriority(i + 1);
        thread.start();
    }
}

优先级逐渐上升，但执行但结果还是一样。

2 Runnable

我们再来使用Runnable创建线程：

public class RunnableDemo implements Runnable {
    @Override
    public void run() {
        System.out.println("Current runnable thread name is: " + Thread.currentThread().getName());
    }
 
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new RunnableDemo());
            thread.start();
        }
    }
}

Current runnable thread name is: Thread-0
Current runnable thread name is: Thread-3
Current runnable thread name is: Thread-2
Current runnable thread name is: Thread-5
Current runnable thread name is: Thread-1
Current runnable thread name is: Thread-6
Current runnable thread name is: Thread-7
Current runnable thread name is: Thread-4
Current runnable thread name is: Thread-8
Current runnable thread name is: Thread-9

可以看出，实现Runnable接口接口之后但RunnableDemo类但实例还是无法直接运行的，它必须将实例对象传入Thread类，然后，才能调用Thread对象中的start()进行启动。

3 start() 和 run()

我们接下来来看一下start() 和 run()的区别：
当我们中定义新的线程类的时候，唯一覆写的方法就是run()。那么，我们能不能直接调用run()方法呢？
答案是肯定的。
我们将start()替换为run()再试一下：

public static void main(String[] args) {
    for (int i = 0; i < 10; i++) {
        Thread thread = new Thread(new RunnableDemo());
        thread.run();
    }
}

Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main
Current runnable thread name is: main

结果全是主线程在运行，也就是说根据就没有新的线程启动运行。所以，使用run()方法调用时，就和一般的函数调用一样，是由当前线程进行调用的，并不会启动新的线程，然后在新的线程中运行这个run()方法。只有使用start()方法去调用，才会启动新的线程，然后，在新的线程中运行run()方法。

Date: 2017-07-04 21:37

Author: WEN YANG

Created: 2017-07-04 Tue 22:44

Emacs 25.2.1 (Org mode 8.2.10)

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