Chapter 21 并发

7

将时间改为0
原输出

DaemonSpawn 0 started, d.isDaemon() = true, DaemonSpawn 1 started, DaemonSpawn 2 started, DaemonSpawn 3 started, DaemonSpawn 4 started, DaemonSpawn 5 started, DaemonSpawn 6 started, DaemonSpawn 7 started, DaemonSpawn 8 started, DaemonSpawn 9 started, t[0].isDaemon() = true, t[1].isDaemon() = true, t[2].isDaemon() = true, t[3].isDaemon() = true, t[4].isDaemon() = true, t[5].isDaemon() = true, t[6].isDaemon() = true, t[7].isDaemon() = true, t[8].isDaemon() = true, t[9].isDaemon() = true, 

修改后的输出

d.isDaemon() = true, DaemonSpawn 0 started, DaemonSpawn 1 started, DaemonSpawn 2 started, DaemonSpawn 3 started, DaemonSpawn 4 started, DaemonSpawn 5 started, DaemonSpawn 6 started, DaemonSpawn 7 started, DaemonSpawn 8 started, DaemonSpawn 9 started, t[0].isDaemon() = true, t[1].isDaemon() = true, t[2].isDaemon() = true, 

说明后台线程再执行过程中就被终止

8

//: concurrency/SimpleThread.java
package concurrency; /* Added by Eclipse.py */

// Inheriting directly from the Thread class.

public class SimpleThread extends Thread {
    private int countDown = 5;
    private static int threadCount = 0;

    public SimpleThread() {
        // Store the thread name:
        super(Integer.toString(++threadCount));
        this.setDaemon(true);
        start();
    }

    public String toString() {
        return "#" + getName() + "(" + countDown + "), ";
    }

    public void run() {
        while (true) {
            System.out.print(this);
            if (--countDown == 0)
                return;
        }
    }

    public static void main(String[] args) {

        for (int i = 0; i < 5; i++)
            new SimpleThread();
    }
} /*
     * Output: #1(5), #1(4), #1(3), #1(2), #1(1), #2(5), #2(4), #2(3), #2(2),
     * #2(1), #3(5), #3(4), #3(3), #3(2), #3(1), #4(5), #4(4), #4(3), #4(2),
     * #4(1), #5(5), #5(4), #5(3), #5(2), #5(1),
     */// :~

//: concurrency/SimplePriorities.java
package concurrency; /* Added by Eclipse.py */

// Shows the use of thread priorities.

import java.util.concurrent.*;

import net.mindview.util.DaemonThreadFactory;

public class SimplePriorities implements Runnable {
    private int countDown = 5;
    private volatile double d; // No optimization
    private int priority;

    public SimplePriorities(int priority) {
        this.priority = priority;
    }

    public String toString() {
        return Thread.currentThread() + ": " + countDown;
    }

    public void run() {
        System.out.println("Thread Start");
        Thread.currentThread().setPriority(priority);
        while (true) {
            // An expensive, interruptable operation:
            for (int i = 1; i < 100000; i++) {
                d += (Math.PI + Math.E) / (double) i;
                if (i % 1000 == 0)
                    Thread.yield();
            }
            System.out.println(this);
            if (--countDown == 0) {
                System.out.println("Thread return");        //Thread 结束
                return;
            }
        }

    }

    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool(new DaemonThreadFactory());
        for (int i = 0; i < 5; i++)
            exec.execute(new SimplePriorities(Thread.MIN_PRIORITY));
        exec.execute(new SimplePriorities(Thread.MAX_PRIORITY));
        exec.shutdown();
        System.out.println("main return");
    }
}

9

11

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Mod7Generator {
    private int a = 0;
    private int b = 0;
    private volatile boolean cancled = false; 
    
//  错误的代码
    public int next() {
        a += 4;
        b += 3;
        return a+b;
    }

//  正确的代码
//  public synchronized int next() {
//      a += 4;
//      b += 3;
//      return a+b;
//  }
    
    public void cancle() {
        cancled = true;
    }
    
    public boolean isCancled() {
        return cancled;
    }
}

class EvenChecker implements Runnable {
    private Mod7Generator m7g;
    private int id;
    
    public EvenChecker(Mod7Generator m7g, int id) {
        this.id = id;
        this.m7g = m7g;
    }
    
    @Override
    public void run() {
        int a;
        while(!m7g.isCancled()) {
            a = m7g.next();
            if(a%7 != 0) {
                System.out.println(a + " not even!");
                m7g.cancle();
                return;
            }
        }
    }
}

public class Main {
    
    public static void test(Mod7Generator m7g, int ct) {
        ExecutorService exec = Executors.newCachedThreadPool();
        for(int i=0; i<ct; i++) {
            exec.execute(new EvenChecker(m7g, i));
        }
        exec.shutdown();
    }
    public static void main(String[] args) {
        Mod7Generator m7g = new Mod7Generator();
        test(m7g, 10);
    }
}

输出:

284 not even!
277 not even!
291 not even!

14

import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.TimeUnit;

public class Main {
    
    static final int N = 5000; 
    static int id = 0;
    public static void main(String[] args) {
        Timer timer = new Timer();
        for(int i=0; i<N; i++) {
            timer.schedule(new TimerTask() {
                
                @Override
                public void run() {
                    if(id == 0) System.gc();  //Garbage Collection
                    System.out.println("id= " + id++);
                }
            }, 5000);
        }
//      try {
//          TimeUnit.MILLISECONDS.sleep(1000);
//      } catch (InterruptedException e) {
//          e.printStackTrace();
//      }
//      System.gc();
    }
}

发现如果不进行垃圾回收System.gc(),进程就不会停止,查阅了一下相关博客发现应该在线程里加上垃圾回收,但是每个都加上后发现运行速度明显降低,再次查阅博客发现只在其中一个GC即可参考链接