一、Thread类中的静态方法

Thread类中的静态方法是通过Thread.方法名来调用的，那么问题来了，这个Thread指的是哪个Thread，是所在位置对应的那个Thread嘛？通过下面的例子可以知道，Thread类中的静态方法所操作的线程是“正在执行该静态方法的线程”，不一定是其所在位置的线程。为什么Thread类中要有静态方法，这样就能对CPU当前正在运行的线程进行操作。下面来看一下Thread类中的静态方法：

1、currentThread()

/*** Returns a reference to the currently executing thread object.

*

*@returnthe currently executing thread.*/

public static native Thread currentThread();

currentThread()方法返回的是对当前正在执行的线程对象的引用。

举例：

public class Thread01 extendsThread{static{

System.out.println("静态代码块的打印：" +Thread.currentThread().getName());

}publicThread01(){

System.out.println("构造函数的打印：" +Thread.currentThread().getName());

}

@Overridepublic voidrun() {

System.out.println("run方法的打印：" +Thread.currentThread().getName());

}

}

public classTest {public static voidmain(String[] args){

Thread01 thread01= newThread01();

thread01.start();

}

}

结果：

静态代码块的打印：main

构造函数的打印：main

run方法的打印：Thread-0

可以看到，Thread01类中的三个相同的静态方法Thread.currentThread()所操作的不是同一个线程，虽然写在了Thread01内，但是静态代码块和构造函数中的静态方法是随着main线程而被调用的，run方法中的静态方法则是thread01线程调用的。把thread01.start()注释掉

public classTest {public static voidmain(String[] args){

Thread01 thread01= newThread01();//thread01.start();

}

}

结果：

静态代码块的打印：main

构造函数的打印：main

因为Thread01中的静态代码块和构造方法都是在main线程中被调用的，而run方法是thread01这个线程调用的，所以不一样。

举例说明上篇说的"this.XXX()"和"Thread.currentThread().XXX()"的区别，this表示的线程是线程实例本身，后一种表示的线程是正在执行"Thread.currentThread.XXX()这块代码的线程"

public class Thread01 extendsThread{publicThread01(){

System.out.println("构造函数中通过this调用：" + this.getName());

System.out.println("构造函数中通过静态方法调用：" +Thread.currentThread().getName());

}

@Overridepublic voidrun() {

System.out.println("run方法中通过this调用：" + this.getName());

System.out.println("run方法中通过静态方法调用：" +Thread.currentThread().getName());

}

}

public classTest {public static voidmain(String[] args){

Thread01 thread01= newThread01();

thread01.start();

}

}

结果：

构造函数中通过this调用：Thread-0构造函数中通过静态方法调用：main

run方法中通过this调用：Thread-0run方法中通过静态方法调用：Thread-0

同样的，把thread01.start()这一行注释掉以后

public classTest {public static voidmain(String[] args){

Thread01 thread01= newThread01();//thread01.start();

}

}

结果：

构造函数中通过this调用：Thread-0构造函数中通过静态方法调用：main

所以，在Thread01里面通过Thread.currentThread得到的线程对象的引用不一定就是Thread01，要看该方法所在的代码会被哪个线程调用。

2、sleep(long millis)

/*** Causes the currently executing thread to sleep (temporarily cease

* execution) for the specified number of milliseconds, subject to

* the precision and accuracy of system timers and schedulers. The thread

* does not lose ownership of any monitors.

*

*@parammillis

* the length of time to sleep in milliseconds

*

*@throwsIllegalArgumentException

* if the value of {@codemillis} is negative

*

*@throwsInterruptedException

* if any thread has interrupted the current thread. The

* interrupted status of the current thread is

* cleared when this exception is thrown.*/

public static native void sleep(long millis) throws InterruptedException;

sleep(long millis)方法的作用是在指定的毫秒内让当前"正在执行的线程"休眠(暂停执行)。这个"正在执行的线程"是关键，指的是Thread.currentThread()返回的线程。根据JDK API的说法，"该线程不丢失任何监视器的所属权"，简单说就是sleep代码上下文如果被加锁了，锁依然在，但是CPU资源会让出给其他线程。

举例：

public class Thread01 extendsThread{

@Overridepublic voidrun() {try{

System.out.println("run threadName：" + this.getName());

System.out.println("调用Thread.sleep方法休眠3秒");

Thread.sleep(3000);

System.out.println("run threadName：" +Thread.currentThread().getName());

}catch(InterruptedException e) {

e.printStackTrace();

}

}

}

public classTest {public static voidmain(String[] args){

System.out.println("main 开始===" +System.currentTimeMillis());

Thread01 thread01= newThread01();

thread01.start();

System.out.println("main 结束=====" +System.currentTimeMillis());

}

}

结果：

main 开始===1552401515206main 结束=====1552401515208run threadName：Thread-0调用Thread.sleep方法休眠3秒

run threadName：Thread-0

3、yield()

/*** A hint to the scheduler that the current thread is willing to yield

* its current use of a processor. The scheduler is free to ignore this

* hint.

*

*

Yield is a heuristic attempt to improve relative progression

* between threads that would otherwise over-utilise a CPU. Its use

* should be combined with detailed profiling and benchmarking to

* ensure that it actually has the desired effect.

*

*

It is rarely appropriate to use this method. It may be useful

* for debugging or testing purposes, where it may help to reproduce

* bugs due to race conditions. It may also be useful when designing

* concurrency control constructs such as the ones in the

* {@linkjava.util.concurrent.locks} package.*/

public static native void yield();

暂停当前执行的线程，并执行其他的线程。这个暂停是会放弃CPU资源的，并且放弃CPU的时间不确定，有可能刚放弃，就获得CPU资源了，也有可能放弃好一会儿，才会被CPU执行。

举例说明yield()放弃CPU的时间是不一定的，用户无法指定

public class Thread01 extendsThread{

@Overridepublic voidrun() {for(int i = 1; i <= 500; i++) {long beginTime =System.currentTimeMillis();

Thread.yield();long endTime =System.currentTimeMillis();

System.out.println(" 第" + i + "次yield 的时长为：" + (endTime - beginTime) + "ms");

System.out.println("i = " +i);

}

}

}

public class Thread02 extendsThread{

@Overridepublic voidrun() {for(int i = 0; i < 500000; i++) {

List list = new ArrayList<>();

list.add(i);

}

}

}

public classTest {public static voidmain(String[] args) {

Thread01 thread01= newThread01();

thread01.start();//根据Thread02多开几个线程

Thread02 thread02 = newThread02();

thread02.start();

Thread02 thread021= newThread02();

thread021.start();

Thread02 thread022= newThread02();

thread022.start();

Thread02 thread023= newThread02();

thread023.start();

Thread02 thread024= newThread02();

thread024.start();

}

}

结果：

.......................................

.......................................

i= 48第48次yield 的时长为：0ms

i= 49第49次yield 的时长为：0ms

i= 50第50次yield 的时长为：1ms

i= 51第51次yield 的时长为：0ms

i= 52第52次yield 的时长为：4ms

i= 53第53次yield 的时长为：0ms

i= 54第54次yield 的时长为：0ms

.......................................

.......................................

i= 442第442次yield 的时长为：0ms

i= 443第443次yield 的时长为：0ms

i= 444第444次yield 的时长为：1ms

i= 445第445次yield 的时长为：0ms

i= 446第446次yield 的时长为：0ms

.......................................

.......................................

可以看到，yield()方法放弃CPU的时间是不确定的，可能立马就被CPU执行，也可能要等待一会再被CPU执行。

4、interrupted()

/*** Tests whether the current thread has been interrupted. The

* interrupted status of the thread is cleared by this method. In

* other words, if this method were to be called twice in succession, the

* second call would return false (unless the current thread were

* interrupted again, after the first call had cleared its interrupted

* status and before the second call had examined it).

*

*

A thread interruption ignored because a thread was not alive

* at the time of the interrupt will be reflected by this method

* returning false.

*

*@returntrue if the current thread has been interrupted;

* false otherwise.

*@see#isInterrupted()

* @revised 6.0*/

public static booleaninterrupted() {return currentThread().isInterrupted(true);

}

测试当前线程是否处于中断状态，调用该方法，线程中断状态的标识被清除(置为false)，也就是说，如果这个方法被连续调用两次，第二次一定会返回false

public classTest {public static voidmain(String[] args) {

Thread.currentThread().interrupt();

System.out.println(Thread.currentThread().getName()+ "线程是否被中断？" +Thread.interrupted());

System.out.println(Thread.currentThread().getName()+ "线程是否被中断？" +Thread.interrupted());

}

}

结果：

main线程是否被中断？truemain线程是否被中断？false

当然，这也涉及Java的中断机制，留在后面的一篇文章专门讲解。

参考资料：