Wu Yudong's Blog

本文主要总结在java中线程的优先级

在java中设置线程优先级使用setPriority，在jdk中的源代码如下：

    public final void setPriority(int newPriority) {
        ThreadGroup g;
        checkAccess();
        if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
            throw new IllegalArgumentException();
        }
        if((g = getThreadGroup()) != null) {
            if (newPriority > g.getMaxPriority()) {
                newPriority = g.getMaxPriority();
            }
            setPriority0(priority = newPriority);
        }
    }

在java中，线程的优先级分为1～10这10个等级，小于1或大于10，则会抛出IllegalArgumentException异常

在JDK中使用3个常量来预定义优先级：

    /**
     * The minimum priority that a thread can have.
     */
    public final static int MIN_PRIORITY = 1;

   /**
     * The default priority that is assigned to a thread.
     */
    public final static int NORM_PRIORITY = 5;

    /**
     * The maximum priority that a thread can have.
     */
    public final static int MAX_PRIORITY = 10;

线程优先级的继承性

在java中，线程的优先级具有继承性，例如A线程启动B线程，则A和B的优先级是一样的。举个例子：

public class MyThread1 extends Thread {
	@Override
	public void run() {
		System.out.println("MyThread1 run priority=" + this.getPriority());
		MyThread2 thread2 = new MyThread2();
		thread2.start();
	}
}
public class MyThread2 extends Thread {
	@Override
	public void run() {
		System.out.println("MyThread2 run priority=" + this.getPriority());
	}
}
public class Run {
	public static void main(String[] args) throws InterruptedException {
		System.out.println("main thread begin priority="
				+ Thread.currentThread().getPriority());
		System.out.println("main thread end priority="
				+ Thread.currentThread().getPriority());
		MyThread1 thread1 = new MyThread1();
		thread1.start();
	}
}

运行结果如下：

main thread begin priority=5
main thread end priority=5
MyThread1 run priority=5
MyThread2 run priority=5

优先级具有规则性

虽然setPriority()方法可以设置线程的优先级，但是没有看到设置优先级所带来的效果

举例如下：

public class MyThread1 extends Thread {
	@Override
	public void run() {
		long beginTime = System.currentTimeMillis();
		long addResult = 0;
		for (int i = 0; i < 10; i++) {
			for (int j = 0; j < 50000; j++) {
				Random random = new Random();
				random.nextInt();
				addResult += j;
			}
		}
		long endTime = System.currentTimeMillis();
		System.out.println("* * * * * thread 1 use time=" + (endTime - beginTime));
	}
}
public class MyThread2 extends Thread {
	@Override
	public void run() {
		long beginTime = System.currentTimeMillis();
		long addResult = 0;
		for (int i = 0; i < 10; i++) {
			for (int j = 0; j < 50000; j++) {
				Random random = new Random();
				random.nextInt();
				addResult += j;
			}
		}
		long endTime = System.currentTimeMillis();
		System.out.println("* * * * * thread 2 use time=" + (endTime - beginTime));
	}
}
public class Run {
	public static void main(String[] args) throws InterruptedException {
		
		for(int i=0;i<5;i++){
			MyThread1 thread1=new MyThread1();
			thread1.setPriority(10);
			thread1.start();
			MyThread2 thread2 =new MyThread2();
			thread2.setPriority(1);
			thread2.start();
		}
	}
}

运行结果如下：

* * * * * thread 1 use time=174
* * * * * thread 1 use time=221
* * * * * thread 1 use time=224
* * * * * thread 2 use time=360
* * * * * thread 1 use time=202
* * * * * thread 2 use time=185
* * * * * thread 1 use time=169
* * * * * thread 2 use time=466
* * * * * thread 2 use time=425
* * * * * thread 2 use time=98

从上面的结果可以看出，高优先级的线程总是大部分先执行完，但是不是所有的先执行完。先执行完也不是因为先调用，如果更改优先级，先执行完和和代码的调用顺序无关。

优先级具有一定的规则性，CPU总是尽量将执行资源让给优先级比较高的线程

优先级具有随机性

优先级较高的线程不一定每一次都先执行完，举个例子：

public class MyThread1 extends Thread {
	@Override
	public void run() {
		long beginTime = System.currentTimeMillis();
		for (int i = 0; i < 1000; i++) {
			Random random = new Random();
			random.nextInt();
		}
		long endTime = System.currentTimeMillis();
		System.out.println("* * * * * thread 1 use time="
				+ (endTime - beginTime));
	}
}
public class MyThread2 extends Thread {
	@Override
	public void run() {
		long beginTime = System.currentTimeMillis();
		for (int i = 0; i < 1000; i++) {
			Random random = new Random();
			random.nextInt();
		}
		long endTime = System.currentTimeMillis();
		System.out.println("* * * * * thread 2 use time="
				+ (endTime - beginTime));
	}
}
public class Run {
	public static void main(String[] args) throws InterruptedException {
		for (int i = 0; i < 5; i++) {
			MyThread1 thread1 = new MyThread1();
			thread1.setPriority(5);
			thread1.start();
			MyThread2 thread2 = new MyThread2();
			thread2.setPriority(6);
			thread2.start();
		}
	}
}

运行结果如下：

* * * * * thread 1 use time=5
* * * * * thread 2 use time=4
* * * * * thread 1 use time=5
* * * * * thread 1 use time=4
* * * * * thread 1 use time=4
* * * * * thread 2 use time=6
* * * * * thread 2 use time=3
* * * * * thread 2 use time=5
* * * * * thread 1 use time=2
* * * * * thread 2 use time=6

线程的优先级与打印顺序无关，它们的关系具有不确定性和随机性