从头认识多线程-1.15 优先级特性-继承性

这一章节我们来讨论一下优先级的特性-继承性。

1代码清单

package com.ray.deepintothread.ch01.topic_15;

public class PrioritySample {
	public static void main(String[] args) throws InterruptedException {
		ThreadOne threadOne = new ThreadOne();
		Thread thread = new Thread(threadOne);
		System.out.println("before set , priority : " + Thread.currentThread().getPriority());
		Thread.currentThread().setPriority(6);
		System.out.println("after set , priority : " + Thread.currentThread().getPriority());
		thread.start();
	}
}

class ThreadOne implements Runnable {

	@Override
	public void run() {
		System.out.println("ThreadOne priority : " + Thread.currentThread().getPriority());
		ThreadTwo threadTwo = new ThreadTwo();
		Thread thread = new Thread(threadTwo);
		thread.start();
	}
}

class ThreadTwo implements Runnable {

	@Override
	public void run() {
		System.out.println("ThreadTwo priority : " + Thread.currentThread().getPriority());
	}
}

输出：

before set , priority : 5
after set , priority : 6
ThreadOne priority : 5
ThreadTwo priority : 5

package com.ray.deepintothread.ch01.topic_15;

public class PrioritySample2 {
	public static void main(String[] args) throws InterruptedException {
		System.out.println("before set , priority : " + Thread.currentThread().getPriority());
		Thread.currentThread().setPriority(6);
		System.out.println("after set , priority : " + Thread.currentThread().getPriority());
		ThreadThree threadThree = new ThreadThree();
		Thread thread = new Thread(threadThree);
		thread.start();
	}
}

class ThreadThree implements Runnable {

	@Override
	public void run() {
		System.out.println("ThreadThree priority : " + Thread.currentThread().getPriority());
		ThreadFour threadFour = new ThreadFour();
		Thread thread = new Thread(threadFour);
		thread.start();
	}
}

class ThreadFour implements Runnable {

	@Override
	public void run() {
		System.out.println("ThreadFour priority : " + Thread.currentThread().getPriority());
	}
}

输出：

before set , priority : 5
after set , priority : 6
ThreadThree priority : 6
ThreadFour priority : 6

2.对比

对比两组代码：不同之处就是把创建线程的代码顺序调换了一下

对比两组输出：截然不同的结果

为什么？

因为线程的优先级具有继承特性

3.创建线程的源码

通过一步步debug创建线程的源码，我们可以找到下面的一个方法：

private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name.toCharArray();

        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();

        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();//创建线程的时候，是获取父线程的优先级，然后设置到线程里面
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);//设置线程优先级
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

我们可以看看我的注释，其实在创建线程的过程中，他是先获取了父线程的优先级，然后再设置到新建的线程里面去。

总结：这一章节我们讨论了线程优先级的继承特性。

我的github：https://github.com/raylee2015/DeepIntoThread