Thread.currentThread 和this 区别

笔者最近在看《java多线程编程核心技术》一书时，看到一个例子说明如何区分Thread.currentThread 和this ，特此分享。

例子如下：

package test;

public class CountOperate extends Thread {

	public CountOperate(){
	      
		System.out.println("CountOperate-----begin");
		System.out.println("Thread.currentThread().getName() = " + Thread.currentThread().getName());
		System.out.println("Thread.currentThread().isAlive() = " + Thread.currentThread().isAlive());
		System.out.println("this.getName() = " + this.getName());
		System.out.println("this.isAlive() = " + this.isAlive());
		System.out.println("CountOperate-----end");
	}
	
	@Override
	public void run(){
		
		System.out.println("run-----begin");
		System.out.println("Thread.currentThread().getName() = " + Thread.currentThread().getName());
		System.out.println("Thread.currentThread().isAlive() = " + Thread.currentThread().isAlive());
		System.out.println("this.getName() = " + this.getName());
		System.out.println("this.isAlive() = " + this.isAlive());
		System.out.println("run-----end");
	}
	
}



package test;

public class Run {

	public static void main(String[] args) {

		CountOperate c = new CountOperate();
		Thread t1 = new Thread(c);
		System.out.println("main begin t1 isAlive = " + t1.isAlive());
		t1.setName("A");
		t1.start();
		System.out.println("main end t1 isAlive = " + t1.isAlive());
	}

}

打印结果如下：

CountOperate-----begin
Thread.currentThread().getName() = main
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
CountOperate-----end
main begin t1 isAlive = false
main end t1 isAlive = true
run-----begin
Thread.currentThread().getName() = A
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
run-----end

结果分析：

首先

isAlive（）判断线程是否处于活动状态，活动状态就是指线程已经启动，尚未终止。

其次，Thread t1 = new Thread(c);调用的是如下构造函数（jdk源码如下）

  /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, target, gname)}, where {@code gname} is a newly generated
     * name. Automatically generated names are of the form
     * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this classes {@code run} method does
     *         nothing.
     */
    public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

本例中一共有三个线程

main  主线程

thread-0 : 也就是CoutOperate产生的线程

thread-1: 也就是“A”

提前说明一下：currentThread（）方法返回的是代码段正在被哪个线程调用的信息

调用CountOperate产生结果：

CountOperate-----begin
Thread.currentThread().getName() = main
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
CountOperate-----end

很好理解，main函数调用的CountOperate，所以Thread.currentThread()是main， 此时main肯定是活跃的(否则怎么调用别的线程呢)，this肯定指的是当前类了，这个线程thread-0只是生成并没有被start或run所以isAlive是false；

main begin t1 isAlive = false
main end t1 isAlive = true

这个结果很好立即，请看Run（）函数，这两个打印之间存在一个t1.start();代码，所以一个start之前是false，之后是true(也不一定，如果中间存在时间较长的话，t1就结束了，也可能是false)。

至于结果：

run-----begin
Thread.currentThread().getName() = A
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
run-----end

是在t1进行start产生的，具体细节过程如下：

Thread对象进行start启动线程，我们直接启动的是Thread（Runnable target），所以c作为构造参数传给Thread中的属性target,之后，在Thread的run方法中，调用target.run()，此时Thread.currentThread()指的就是Thread的引用,也就是为什么这里Thread.currentThread()是A（thread-1）的原因了，而this依旧是myThread的引用，也就是说，this的这个线程还是thread-0，从这里可以看出二者区别了，值得注意的是笔者认为thread-0这个线程从头至尾一致没有执行过，就是说，这个的iaAlive一致是false，如果不对之处，请指正，感谢！