24/365 java 观测线程状态 线程优先级

1.观测线程

JDK中定义的线程的六个状态

可以用getState()来观测线程

 public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(
                ()->{
                    for (int i = 0; i < 10; i++) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException e) {
                            throw new RuntimeException(e);
                        }
                        System.out.println("Thread is running " + i);
                    }
                }

        );

        Thread.State state = thread.getState();
        System.out.println("Thread state is " + state);

        thread.start();
        while(state!=Thread.State.TERMINATED){
            Thread.sleep(100);
            state = thread.getState();
            System.out.println("Thread state is " + state);
        }

    }

观测结果

线程一旦死亡terminated状态就不能再start()了

2.线程优先级

从低到高1-10一般默认为5

线程的优先级不能保证线程的执行顺序因为具体调度策略是由OS决定的。

每当线程调度器有机会选择新线程时它首先选择具有较高优先级的线程。但是线程的优先级高度依赖于OS。

“当虚拟机依赖于宿主机平台的线程实现时Java线程的优先级会映射到宿主平台的优先级平台的线程优先级别可能比Java的10个级别多也可能更少。
例如Windows有7个优先级别。Java的一些优先级会映射到同一个操作系统优先级。在Oracle为Linux提供的Java虚拟机中会完全忽略线程优先级————所有线程都具有相同的优先级。”

“线程优先级不能作为程序正确性的依赖因为操作系统可以完全不用理会JAVA线程对于优先级的设定”

默认情况下一个线程会继承构造它的那个线程的优先级。

在线程启动之前设置优先级。

package oop;

public class D18 {
    public static void main(String[] args) {
        System.out.println("Thread " + Thread.currentThread().getName() + " Priority is " + Thread.currentThread().getPriority());

        TestPriority testPriority = new TestPriority();

        Thread thread = new Thread(testPriority);
        Thread thread2 = new Thread(testPriority);
        Thread thread3 = new Thread(testPriority);
        Thread thread4 = new Thread(testPriority);
        Thread thread5 = new Thread(testPriority);

        thread.start();

        thread2.setPriority(3);
        thread2.start();

        thread3.setPriority(10);
        thread3.start();

        thread4.setPriority(8);
        thread4.start();

        thread5.setPriority(9);
        thread5.start();

    }
}

class TestPriority implements Runnable{
    @Override
    public void run() {
        System.out.println("Thread " + Thread.currentThread().getName() + " Priority is " + Thread.currentThread().getPriority());
    }
}

可以看到结果并不是完全按照线程优先级来执行的