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，而线程又有自己私有的内存地址，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

原文地址https://www.cnblogs.com/dslx/p/12664004.html