Java的多线程1：线程的使用

概述

进程是线程的容器，线程共享进程的内存空间，所以线程之间彼此通信是比较容易的，而线程又有自己本地内存地址（jmm抽象概念，并不真实存在），其他线程无法访问。了解进程和线程关系，可以看我另一篇博客《进程与线程》

Java创建线程的两种方式

继承Thread类

public class ThreadDemo1 extends Thread {

    @Override
    public void run(){
        for (int i = 0; i < 10; i++) {
            System.out.println("当前执行的线程是" + Thread.currentThread().getName());
        }
    }

    public static void main(String[] args) {
        ThreadDemo1 threadDemo1 = new ThreadDemo1();
        ThreadDemo1 threadDemo2 = new ThreadDemo1();
        threadDemo1.start();
        threadDemo2.start();
    }
}

执行结果是不确定的

实现Runnable

public class ThreadDemo2 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            for (int j = 0;j < 1000; ++j){
                System.out.println(i + "当前执行的线程是" + Thread.currentThread().getName());
            }
        }
    }

    public static void main(String[] args) {
        ThreadDemo2 threadDemo1 = new ThreadDemo2();
        ThreadDemo2 threadDemo2 = new ThreadDemo2();
        Thread thread1 = new Thread(threadDemo1);
        Thread thread2 = new Thread(threadDemo2);
        thread1.start();
        thread2.start();
        System.out.println("当前线程是===>" + Thread.currentThread().getName());
    }
}

主线程的名字为main，非主线程的名字是由虚拟机来指定的，同时，我们也可以为线程指定具体的名称。

 我们保证每个线程都能正常启动，并不意味着它会按顺序的执行，因为调度程序是无法保证它的执行次序的，同时，run（）函数结束时，意味着该线程的任务完成了。

注意：调用线程要调用start，如果调用run，那仅仅是简单的对象调用。

线程生命周期

    public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

新建状态

线程对象创建后，就进入新建状态  Thread thread = new Thread

就绪状态

调用start()方法，线程进入就绪状态，但并不意味着线程就立即执行，只是说明此线程已经做好准备，随时等待CPU调度执行。

阻塞状态

多个线程同时竞争一个独占锁，其他未抢到锁的线程，就进入阻塞状态被放置到锁池中，直到获取到锁，进入就绪状态

等待状态

该线程需要等待其他线程做出一些特定动作，通知或者是中断，等待其被其他线程唤醒，像CountDownLatch就可以等待一个或者几个线程结束。

超时等待状态

和等待状态不同的是，它可以在指定的时间自行的返回，sheep（long）函数就会让线程进入超时等待状态，时间到了才会转入就绪状态。

运行状态（Running）

CPU调度处于就绪状态的线程时，这个线程才真正执行，进入运行状态。

终止状态

线程正常执行完毕后或提前强制性终止或出现异常，线程就要销毁，释放资源。

线程的方法调用

获取线程基本信息

public class ThreadDemo6 {
    public static void main(String[] args) {
        Thread thread = new Thread(){
            @Override
            public void run(){
                /*获取线程唯一id标识*/
                long id = this.getId();
                System.out.println("thread的ID==>" + id);

                /*获取线程名字*/
                String name = this.getName();
                System.out.println("thread的名字==>" + name);

                /*获取线程的优先级 默认5 1-10*/
                int priority = this.getPriority();
                System.out.println("thread的优先等级==>" + priority);

                /*查看当前线程是否为守护线程*/
                boolean isDaemon = this.isDaemon();
                System.out.println("thread是否为守护线程==>" + isDaemon);

                /*查看线程是否被中断*/
                boolean isInterrupted = this.isInterrupted();
                System.out.println("thread是否被中断==>" + isInterrupted);
            }
        };
        thread.start();
    }
}

执行结果

thread的ID==>11
thread的名字==>Thread-0
thread的优先等级==>5
thread是否为守护线程==>false
thread是否被中断==>false

Thread.yield()

public class ThreadDemo1 implements Runnable {
    protected int countDown = 10;
    private static int taskCount = 0;
    private final int id = taskCount++;
    public ThreadDemo1(){}
    public ThreadDemo1(int countDown){
        this.countDown = countDown;
    }
    public String status(){
        return "#" + id + "(" + (countDown > 0 ? countDown : "stop!") + ")";
    }


    @Override
    public void run() {
        while (countDown-- > 0){
            System.out.println(status() + "  ");
            Thread.yield();
        }
    }

    public static void main(String[] args) {
        for (int i = 0; i < 3; i++){
            new Thread(new ThreadDemo1()).start();
        }
    }
}

#0(9)#0(8)#0(7)#0(6)#0(5)#0(4)#0(3)#0(2)#0(1)#0(stop!)
#1(9)#1(8)#1(7)#1(6)#1(5)#1(4)#1(3)#1(2)#1(1)#1(stop!)
#2(9)#2(8)#2(7)#2(6)#2(5)#2(4)#2(3)#2(2)#2(1)#2(stop!)

这个是一个倒计时的任务，对Thread.yield()调用是对线程调度器的一种建议，它在声明“我已经执行完生命周期中最重要的部分了，此刻正是切换给其他任务执行一段时间的大好时机”，说白就是自己按暂停键，让出自己CPU的使用权限，转为就绪状态，至于下一次什么时候能获得CPU调度就不一定了，有时很快，有时得等上一会。

Thread.sleep

public class ThreadDemo1 implements Runnable {
    protected int countDown = 10;
    private static int taskCount = 0;
    private final int id = taskCount++;
    public ThreadDemo1(){}
    public ThreadDemo1(int countDown){
        this.countDown = countDown;
    }
    public String status(){
        return "#" + id + "(" + (countDown > 0 ? countDown : "stop!") + ")";
    }


    @Override
    public void run() {
        try {
            while (countDown-- > 0){
                System.out.println(status());
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

    public static void main(String[] args) {
        for (int i = 0; i < 3; i++){
            new Thread(new ThreadDemo1()).start();
        }
    }
}

Thread.sleep(long)将使“正在执行的任务“中止执行给定的时间（暂停执行）并且让出CPU使用权，这个语句相当于说在接下来的1秒内，你都不要叫我，我想睡一会，1秒睡眠时间过后，它自动转为就绪状态，但CPU不一定马上执行这个睡醒的线程，这要取决于是否抢到CPU时间片段。值得注意的是如果sleep和yield上下文被加锁了，它们依然使用锁，不会去释放。而sleep与yield最大的不同是，yield不会让线程进入等待状态，只是把线程转为就绪状态，并把CPU执行机会让步给优先级相同或者更高的线程，而sleep能控制具体交出CPU的使用时间。

Thread.currentThread()

public class ThreadDemo2 extends Thread {
    static {
        System.out.println("静态块执行的线程===>" + Thread.currentThread().getName());
    }
    {
        System.out.println("非静态块执行的线程是====>" + Thread.currentThread().getName());
        System.out.println("this.getName()1=====>" + this.getName());
    }

    public ThreadDemo2(){
        System.out.println("构造方法内执行的线程====>" + Thread.currentThread().getName());
        System.out.println("this.getName()2=====>" + this.getName());
    }

    @Override
    public void run() {
        System.out.println("当前执行的线程为====>" + Thread.currentThread().getName());
        System.out.println("this.getName()3=====>" + this.getName());
    }

    public static void main(String[] args) {
        ThreadDemo2 threadDemo2 = new ThreadDemo2();
        threadDemo2.start();
    }
}

执行结果

静态块执行的线程===>main
非静态块执行的线程是====>main
this.getName()1=====>Thread-0
构造方法内执行的线程====>main
this.getName()2=====>Thread-0
当前执行的线程为====>Thread-0
this.getName()3=====>Thread-0

currentThread返回的是当前正在执行线程对象的引用，它与this.getName()有明显的不同，执行静态块，非静态块，构造方法的线程是main，而非ThreadDemo2，在执行run（）方法的才是实例化的线程threadDemo2。所以在当前执行的Thread未必就是Thread本身。

isAlive()

public class ThreadDemo3 extends Thread {
    @Override
    public void run(){
        System.out.println("执行执行====" + this.isAlive());
    }

    public static void main(String[] args) {
        ThreadDemo3 threadDemo3 = new ThreadDemo3();
        System.out.println("begin===>" + threadDemo3.isAlive());
        threadDemo3.start();
        System.out.println("end==>" + threadDemo3.isAlive());
    }
}

begin===>false
end==>true
执行执行====true

isAlive（）检测线程是否处于活动状态，活动状态返回true

setPriority（）

优先级设定，优先级高的线程越容易获取CPU使用权，

public class ThreadDemo4 {
    public static void main(String[] args) {
        for (int i = 0; i < 5; ++i){
            Thread1 thread1 = new Thread1();
            thread1.setPriority(6);
            Thread2 thread2 = new Thread2();
            thread2.setPriority(4);
            thread2.start();
            thread1.start();
        }
    }
}
class Thread1 extends  Thread{
    @Override
    public void run(){
        for (int i = 0; i < 100000; ++i){
            System.out.println("+++++++++++++");
        }
    }
}
class Thread2 extends Thread{

    @Override
    public void run(){
        for (int i = 0; i < 100000; ++i){
            System.out.println("--------------");
        }
    }
}

执行结果

+++++++++++++
+++++++++++++
+++++++++++++
+++++++++++++
+++++++++++++
+++++++++++++
+++++++++++++
+++++++++++++
...

CPU会将资源尽量让给优先级高的线程

setDaemon()

守护线程，也有人叫后天线程，我们创建出来的线程默认都是前台线程，在使用上来说，守护线程和前台线程是没啥区别，区别在于进程结束，当一个进程中的所有前台线程都结束时，无论这个进程中的守护线程是否还在运行都要强制将他们结束。也就是说前台线程都结束了，守护线程也会自动销毁，它是为其他线程提供便利而存在的。

/*rose与jack*/
public class ThreadDemo5 {
    public static void main(String[] args) {
        Rose rose = new Rose();
        Jack jack = new Jack();
        /*设置为守护线程必须在线程未启动之前*/
        jack.setDaemon(true);
        rose.start();
        jack.start();
    }
}
class Rose extends Thread{
    @Override
    public void run(){
        for (int i = 0; i < 5; ++i){
            System.out.println("rose: let me go!");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("成功跳水");
    }
}
class Jack extends Thread{
    @Override
    public void run(){
        while (true){
            System.out.println("jack:you jump! i jump!");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

执行结果

rose: let me go!
jack:you jump! i jump!
rose: let me go!
jack:you jump! i jump!
rose: let me go!
jack:you jump! i jump!
rose: let me go!
jack:you jump! i jump!
rose: let me go!
jack:you jump! i jump!
成功跳水

jack守护着rose，jack是守护线程，当rose跳水后，jack认为自己也没有活着的必要了，也自己销毁了，但注意一点是这当中还有一个第三者main，需要main也运行完jack线程才会销毁。

join（）

这个方法可以协调多个线程同步运行，多线程运行本身是设计异步运行的，但在程序运行业务中，有可能线程A的计算需要线程B的返回结果，这就需要他们执行各自任务时要有先后，join就需要协调这些线程同步运行。

public class ThreadDemo6 {
    private static boolean isFinish = false;

    public static void main(String[] args) {
        Thread download = new Thread(){
            @Override
            public void run(){
                System.out.println("下载图片中.....");
                for (int i = 1; i <= 100; ++i){
                    System.out.println("下载进度" + i + "%");
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("图片下载完毕");
                isFinish = true;
            }
        };
        Thread show = new Thread(){
            @Override
            public void run(){
                System.out.println("开始显示图片...");
                try {
                    download.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                if (!isFinish){
                    throw new RuntimeException("图片下载出错");
                }
                System.out.println("图片正常展示。。。");
            }
        };
        download.start();
        show.start();
    }
}

执行结果

下载图片中.....
开始显示图片...
下载进度1%
下载进度2%
...
下载进度100%
图片下载完毕
图片正常展示。。。

show调用join会使show无限阻塞，直到down线程销毁为止，它和sleep最大的区别是join会释放锁，而sleep不会。

涉及到jmm内存模型，线程安全等，后面在介绍