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.
*
* @return the 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 extends Thread{
static{
System.out.println("静态代码块的打印：" + Thread.currentThread().getName());
}
public Thread01(){
System.out.println("构造函数的打印：" +Thread.currentThread().getName());
}
@Override
public void run() {
System.out.println("run方法的打印：" + Thread.currentThread().getName());
}
}
public class Test {
public static void main(String[] args){
Thread01 thread01 = new Thread01();
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 class Test {
public static void main(String[] args){
Thread01 thread01 = new Thread01();
// 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 extends Thread{
public Thread01(){
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());
}
@Override
public void run() {
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 class Test {
public static void main(String[] args){
Thread01 thread01 = new Thread01();
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-0
run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 class Test {
public static void main(String[] args){
Thread01 thread01 = new Thread01();
// 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.
*
* @param millis
* the length of time to sleep in milliseconds
*
* @throws IllegalArgumentException
* if the value of {@code millis} is negative
*
* @throws InterruptedException
* if any thread has interrupted the current thread. The
* <i>interrupted status</i> 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 extends Thread{
@Override
public void run() {
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 class Test {
public static void main(String[] args){
System.out.println("main 开始===" + System.currentTimeMillis());
Thread01 thread01 = new Thread01();
thread01.start();
System.out.println("main 结束=====" + System.currentTimeMillis());
}
}
结果：
main 开始===1552401515206
main 结束=====1552401515208
run 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.
*
* <p> 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.
*
* <p> 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
* {@link java.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 extends Thread{
@Override
public void run() {
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 extends Thread{
@Override
public void run() {
for(int i = 0; i < 500000; i++) {
List<Integer> list = new ArrayList<>();
list.add(i);
}
}
}
public class Test {
public static void main(String[] args) {
Thread01 thread01 = new Thread01();
thread01.start();
//根据Thread02多开几个线程
Thread02 thread02 = new Thread02();
thread02.start();
Thread02 thread021 = new Thread02();
thread021.start();
Thread02 thread022 = new Thread02();
thread022.start();
Thread02 thread023 = new Thread02();
thread023.start();
Thread02 thread024 = new Thread02();
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
* <i>interrupted status</i> 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).
*
* <p>A thread interruption ignored because a thread was not alive
* at the time of the interrupt will be reflected by this method
* returning false.
*
* @return <code>true</code> if the current thread has been interrupted;
* <code>false</code> otherwise.
* @see #isInterrupted()
* @revised 6.0
*/
public static boolean interrupted() {
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 class Test {
public static void main(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？true
main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：