Java多线程基础学习（一）

1. 创建线程

1.1 通过构造函数：public Thread(Runnable target, String name){} 或：public Thread(Runnable target){}示例:

Thread thread1 = new Thread(new MyThread(), "mythread");
class MyThread extends Thread(){
        public void run(){
             System.out.println("My First Thread');
        }
}

1.2 直接实现Runnable接口：示例：

Thread thread2 = new Thread(new Runnable{}{
         public void run(){
                  System.out.println("This is my thread.");
          }
});

2. 运行线程 thead1.start() 3. sleep

try{
    #休眠1000ms
    Thread.sleep(1000);
}catch(InterruptedException e){
    e.printStackTrace();
}

4. getName() 获取线程名字， getId()获取线程idSystem.out.println(Thread.currentThread().getName() + “:”+ Thread.currentThread().getId); 5. 停止线程，千万不用stop()，stop会立即终止线程。通过interrupt()中断线程，但是中断并没有停止线程，配合异常来实现：

public class Main {
   public static void main(String[] args) throws InterruptedException {
      try{
          Thread thread1=new Thread(new TheThread(),"thread1");
          thread1.start();
          Thread.sleep(2000);
          thread1.interrupt();
      }catch (InterruptedException e){
                  e.printStackTrace();
       }
   }
}
   class TheThread extends Thread{
     public void run() {
        super.run();
        for (int i = 0; i < 10; i++) {
           if(this.interrupted()){
              break;
        }
        System.out.println(Thread.currentThread().getName() + ":" + i);
     }
   }
}

注意，如果在TheThread类里加入catch InterruptException的话，可能会导致interrupt被捕获，而绕过if（this.interrupted()）的判断而无法终止线程。

6. 等待和通知 线程等待：当前线程就处于等待状态，直到其他线程调用了notify()方法，线程才会继续执行

public final void wait() throws InterruptedException

线程通知：

public final native void notify()

注意：在notify()方法后，当前线程不会马上释放该对象锁，要等到执行notify()方法的线程将程序执行完，也就是退出同步代码块中。

 package wait.notify; public class ThreadWaitNotifyTest {
     final static Object object=new Object();
     public static class T1 extends Thread{
         public void run(){
             System.out.println(System.currentTimeMillis()+": T1 start");
             synchronized (object){
                 try {
                     System.out.println(System.currentTimeMillis()+": T1 wait");
                     object.wait();
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             }
             System.out.println(System.currentTimeMillis()+": T1 end");
         }
     }
     public static class T2 extends Thread{
         public void run(){
             System.out.println(System.currentTimeMillis()+": T2 start");
             synchronized (object){
                 System.out.println("T2 synchonized code start.");
                 object.notify();
                 try {
                     Thread.sleep(2000);
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }finally{
                     System.out.println("T2 synchonized code end.");
                 }             }
             try {
                 Thread.sleep(2000);
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println(System.currentTimeMillis()+": T2 end");
         }
     }
     public static void main(String[] args){
         Thread thread1=new T1();
         Thread thread2=new T2();
         thread1.start();
         thread2.start();
     }
 }

输出结果：

7. 线程优先级高优先级的线程将会获得更多的CPU资源。一共分为10个优先级。public final void setPriority(int newPriority) 源码分析：

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);
        }
}

public final static int MIN_PRIORITY = 1;
public final static int NORM_PRIORITY = 5;
public final static int MAX_PRIORITY = 10;

可见线程最高优先级为10, 最低为1，默认为5.当设定的newPriority高于该线程组ThreadGroup的最高Priority时，只能分配该线程组的最高Priority 8. 守护线程类似守护进程，Java存在两种线程：用户线程和守护线程。它是一种特殊线程，执行的是一种后台服务，当一个系统中不存在非守护线程的时候，守护线程会自己销毁。典型的守护线程：JVM的垃圾回收线程。public final void setDaemon(boolean on) 示例：

public class Main {
    public static void main(String[] args) throws InterruptedException {
       TheThread theThread=new TheThread();
        theThread.setDaemon(true);//设置守护线程
        theThread.start();
        Thread.sleep(5000);
        System.out.println("全都退出啦");
    }
    public static class TheThread extends Thread{
        public void run(){
            int i = 0;
            while (true){
                i++;
                System.out.println(Thread.currentThread().getId()+":"+i);
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

源码分析：设置线程为用户线程（user thread）或守护线程（daemon thread），当剩余运行的线程均为守护线程时，JVM会退出。

 public final void setDaemon(boolean on) {
        checkAccess();
        if (isAlive()) {
            throw new IllegalThreadStateException();
        }
        daemon = on;
    }

其中checkAccesss()方法如下：

 public final void checkAccess() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkAccess(this);
        }
    }

该方法用于判断当前运行的线程是否有修改此线程的权限。而public final native boolean isAlive();用于判断该线程是否处于alive状态，即该线程是否已经start，且没有die。当isAlive的话就会抛出IllegalThreadStateException异常。所以，设置守护线程的方法，逻辑就是先判断当前线程是否有修改的权限，再判断是否处于alive状态，如果不处于alive状态，则根据boolean变量on的值更改它的状态，即true：设为daemon线程，false：设为user线程。

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