java多线程（一）

start和run的区别

run执行的是函数体内容，并没有真正创建线程，star()才创建了线程，并回调了run()方法

public class test extends Thread{
    @Override
    public void run(){
        System.out.println(Thread.currentThread().getName()+"开始学习");
    }
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            test test = new test();
            test.start();
            System.out.println("main线程");
        }
    }
}

执行结果：

main线程
main线程
Thread-0开始学习
main线程
main线程
Thread-1开始学习
main线程
Thread-2开始学习
Thread-3开始学习
main线程
Thread-4开始学习

public class test extends Thread{
    @Override
    public void run(){
        System.out.println(Thread.currentThread().getName()+"开始学习");
    }
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            test test = new test();
            test.run();
            System.out.println("main线程");
        }
    }
}

执行结果：

main开始学习
main线程
main开始学习
main线程
main开始学习
main线程
main开始学习
main线程
main开始学习
main线程

可以看到并没有真正创建线程，并且是以同步的方式运行（即严格按照代码顺序执行）。

start在Thread类中的源码：

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()//start0()真正启动了一个线程
        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 */
        }
    }
}