Java多线程

Thread的start()方法

public class ThreadTest extends Thread{

    public void run(){
        try {
            for(int i =0 ;i<5;i++) {
                Thread.sleep(1000);
                //Thread.currentThread().xxx() 表示执行当前代码的线程方法
                //this.xxx 当前线程的方法
                System.out.println("Thread = " + Thread.currentThread().getName());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args){

        ThreadTest th = new ThreadTest();
        th.start();
        //main也相当于一个线程
        for(int i=0;i<5;i++){
            try {
                Thread.sleep(1000);
                System.out.println("Thread = " + Thread.currentThread().getName());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }

    }
}

执行结果

意味着cpu执行线程的顺序是无序的，随机的

那么start()方法对于线程的执行顺序是否有影响呢？

public class ThreadTest extends Thread{

    public void run(){
        //Thread.currentThread().xxx() 表示执行当前代码的线程方法
        System.out.println("Thread = " + Thread.currentThread().getName());
    }
    public static void main(String[] args){

        ThreadTest th = new ThreadTest();
        ThreadTest th0 = new ThreadTest();
        ThreadTest th1 = new ThreadTest();
        ThreadTest th2 = new ThreadTest();
        ThreadTest th3 = new ThreadTest();
        th.start();
        th0.start();
        th1.start();
        th2.start();
        th3.start();
    }
}

结果

欧豁，start()方法的启动顺序和最终线程的执行顺序是不同的

看下源码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();
            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 */
            }
        }
    }

有个start0()的本地方法，同时还有一个registerNatives()

registerNatives()方法就是注册本地方法让Thread使用

有个Thread.c文件

 

vmSymbols.hpp中发现

所以执行顺序是 start()-->start0()--->JVM_StratThread---->run()_method_name

run()

public class ThreadTest extends Thread{

    public void run(){
        for(int i=1;i<5;i++) {
            try {
                Thread.sleep(1000);
                System.out.println("Thread = " + Thread.currentThread().getName());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args){

        ThreadTest th = new ThreadTest();
        th.run();

        for(int i=1; i<5;i++){
            try {
                Thread.sleep(1000);
                System.out.println("Thread = " + Thread.currentThread().getName());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

结果总共执行了8次

所以用run()启动没有多线程的效果

isAlive() 只有线程在启动的过程中，返回true

getId() 获得线程的id

getName() 获得线程名，上边的代码结果中的Thread-0就是默认的，也可以在new的时候指定线程的名字

getPriority()和setPriority(int newPriority) 获取/设置优先级，优先级越高越容易执行，默认优先级是5，优先级可以被继承

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