Thread系列：run()方法和start()方法有什么区别

系统通过调用线程类的start()方法来启动一个新线程，此时线程处于可运行状态，而非运行状态。这个线程可以被JVM来调度执行。在调度过程中，JVM通过调用线程类的run()方法来完成操作,run()执行结束，该线程也会终止。

直接调用run()方法，只是把run()方法当做一个普通的函数了调用，程序中仍然只有一个主线程。

也就是说，start()能够异步调用run()方法，直接调用run()方法是同步的，要实现多线程的目的只用调用start()方法。

实例1: 演示一下，调用start()方法会创建一个新的线程，当run()方法执行结束，新线程终止。

public class UP1013 {

    public static void main(String[] args) {
	System.out.println("*********启动 检测有几个线程*******");
	Thread mainThread = Thread.currentThread();
	ThreadGroup mainThreadThreadGroup = mainThread.getThreadGroup();
	// 获取线程组中的线程。
	int count = mainThreadThreadGroup.activeCount(); //
	Thread[] threads = new Thread[count];
	mainThreadThreadGroup.enumerate(threads, true);  //enumerate方法用来将ThreadGroup线程组中的active线程全部复制到Thread类型的数组中，并且返回数组中元素个数，即线程组中active线程数量
	System.out.println("线程数量 = " + count);
	Stream.of(threads).forEach(thread -> System.out.println("Name = " +thread.getName()));
	// 调用线程的start()方法，创建一个新的线程
	Thread thread = new Thread(new myThread(), "myThread");
	thread.start();
	try {
	    thread.join();
	} catch (InterruptedException e) {
	    // TODO Auto-generated catch block
	    e.printStackTrace();
	}
	System.out.println("*********run() end 重新检测有几个线程*******");
	count = mainThreadThreadGroup.activeCount();
	Thread[] threads3 = new Thread[count];
	mainThreadThreadGroup.enumerate(threads3, true);
	System.out.println("线程数量 = " + count);
	Stream.of(threads3).forEach(thread3 -> System.out.println("Name = " + thread3.getName()));
    }

}

class myThread implements Runnable {

    @Override
    public void run() {
	System.out.println("*********run() ing 重新检测有几个线程*******");
	Thread mainThread = Thread.currentThread();
	ThreadGroup mainThreadThreadGroup = mainThread.getThreadGroup();
	// 获取线程组中的线程。
	int count = mainThreadThreadGroup.activeCount(); 
	Thread[] threads = new Thread[count];
	mainThreadThreadGroup.enumerate(threads, true);  
	System.out.println("线程数量 = " + count);
	Stream.of(threads).forEach(thread -> System.out.println("Name = " +thread.getName()));
	try {
	    Thread.sleep(5 * 1000); // 等待5s
	} catch (InterruptedException e) {
	    e.printStackTrace();
	}
	System.out.println("myThread run end");
    }

}

运行结果

实例2: 演示一下，直接调用run()方法，只有主线程。将thread.start();调整为thread.run();

public class UP1013 {

    public static void main(String[] args) {
	System.out.println("*********启动 检测有几个线程*******");
	Thread mainThread = Thread.currentThread();
	ThreadGroup mainThreadThreadGroup = mainThread.getThreadGroup();
	// 获取线程组中的线程。
	int count = mainThreadThreadGroup.activeCount(); //
	Thread[] threads = new Thread[count];
	mainThreadThreadGroup.enumerate(threads, true);  //enumerate方法用来将ThreadGroup线程组中的active线程全部复制到Thread类型的数组中，并且返回数组中元素个数，即线程组中active线程数量
	System.out.println("线程数量 = " + count);
	Stream.of(threads).forEach(thread -> System.out.println("Name = " +thread.getName()));
	// 调用线程的start()方法，创建一个新的线程
	Thread thread = new Thread(new myThread(), "myThread");
	thread.run();
	System.out.println("*********run() end 重新检测有几个线程*******");
	count = mainThreadThreadGroup.activeCount();
	Thread[] threads3 = new Thread[count];
	mainThreadThreadGroup.enumerate(threads3, true);
	System.out.println("线程数量 = " + count);
	Stream.of(threads3).forEach(thread3 -> System.out.println("Name = " + thread3.getName()));
    }

}

class myThread implements Runnable {

    @Override
    public void run() {
	System.out.println("*********run() ing 重新检测有几个线程*******");
	Thread mainThread = Thread.currentThread();
	ThreadGroup mainThreadThreadGroup = mainThread.getThreadGroup();
	// 获取线程组中的线程。
	int count = mainThreadThreadGroup.activeCount(); 
	Thread[] threads = new Thread[count];
	mainThreadThreadGroup.enumerate(threads, true);  
	System.out.println("线程数量 = " + count);
	Stream.of(threads).forEach(thread -> System.out.println("Name = " +thread.getName()));
	try {
	    Thread.sleep(5 * 1000); // 等待5s
	} catch (InterruptedException e) {
	    e.printStackTrace();
	}
	System.out.println("myThread run end");
    }

}

运行结果：

实例3: 演示一下，调用start(）方法异步执行run()方法

public class UP1013 {

    public static void main(String[] args) {
	Thread thread = new Thread(new myThread());
	thread.start();
	System.out.println("finish");
    }

}

class myThread implements Runnable {
    
    @Override
    public void run() {
	System.out.println("myThread run start");
	try {
	    Thread.sleep(5 * 1000); // 等待5s
	} catch (InterruptedException e) {
	    e.printStackTrace();
	}
	System.out.println("myThread run end");
    }

}

运行结果：

finish
myThread run start
myThread run end

main()方法中的System.out.println(“finish”);语句，不需要等待thread.start()运行结果就可以执行，顾是异步。

实例4: 演示一下，直接调用run()方法的执行是同步的。

public class UP1013 {

    public static void main(String[] args) {
	Thread thread = new Thread(new myThread());
	thread.run();
	System.out.println("finish");
    }

}

class myThread implements Runnable {
    
    @Override
    public void run() {
	System.out.println("myThread run start");
	try {
	    Thread.sleep(5 * 1000); // 等待5s
	} catch (InterruptedException e) {
	    e.printStackTrace();
	}
	System.out.println("myThread run end");
    }

}

运行结果：

myThread run start
myThread run end
finish

main()方法中的System.out.println(“finish”);语句，只有等待thread.run()调用结束才可以执行，顾是同步。