java 多线程 练习 答案 java多线程例题

1、建立三个线程，A线程打印10次A，B线程打印10次B,C线程打印10次C，要求线程同时运行，交替打印10次ABC

　  首先使用Java多线程，使用Object.wait()和Object.notify()来对对象释放和唤醒操作。先创建三个对象锁a、b、c，每个打印线程需要获取前一个对象和自身对象才可以执行打印操作，否则等待。打印完后，立即释放自身对象及前一个对象，唤醒等待自身对象的线程。为了避免JVM调用线程的时间片轮转时间小于一个打印线程所需的时间，也就是说为了避免打印线程在还没来得及释放对象时，CPU写换到其他线程引起其他结果，在创建线程时，需让主线程sleep一下。

public class ABCThreadTest {

    /**
     * @param args
     * @throws InterruptedException 
     */
    public static void main(String[] args) throws InterruptedException {
        Object a = new Object();     
        Object b = new Object();     
        Object c = new Object();     
        MyThreadPrinter pa = new MyThreadPrinter("A", c, a);     
        MyThreadPrinter pb = new MyThreadPrinter("B", a, b);     
        MyThreadPrinter pc = new MyThreadPrinter("C", b, c);     
             
        new Thread(pa).start();  
        Thread.sleep(120);
        new Thread(pb).start();  
        Thread.sleep(120);
        new Thread(pc).start();
        Thread.sleep(120);
    }

}

class MyThreadPrinter  implements Runnable{
    private String name;     
    private Object prev;     
    private Object self;     
    
    MyThreadPrinter (String name, Object prev, Object self) {     
        this.name = name;     
        this.prev = prev;     
        this.self = self;     
    }     
    
    @Override    
    public void run() {     
        int count = 10;     
        while (count > 0) {     
            synchronized (prev) {     
                synchronized (self) {     
                    System.out.print(name);     
                    count--;    
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    self.notify();     
                }     
                try {     
                    if(count>0) prev.wait();     
                } catch (InterruptedException e) {     
                    e.printStackTrace();     
                }     
            }     
        }  
    } 
}

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2、建立一个线程，子线程循环5次，主线程循环10次，一共执行10次

　 在主线程main函数内新建一个子线程，执行Business中的sub方法，然后主线程执行Business中的main方法，sub方法与main方法通过变量bool实行同步。 

//子线程循环5次，主线程循环10次，如此循环5次，好像是空中网的笔试题
public class ThreadCircle {
    public static void main(String[] args) {  
        final Business business = new Business();  
        new Thread(new Runnable() {  
            @Override  
            public void run() {  
                threadExecute(business, "sub");  
            }  
        }).start();  
        threadExecute(business, "main");  
    }     
    public static void threadExecute(Business business, String threadType) {  
        for(int i = 0; i < 5; i++) {  
            try {  
                if("main".equals(threadType)) {  
                    business.main(i);  
                } else {  
                    business.sub(i);  
                }  
            } catch (InterruptedException e) {  
                e.printStackTrace();  
            }  
        }  
    }  
}

class Business {  
    private boolean bool = true;  
    public synchronized void main(int loop) throws InterruptedException {  
        while(bool) {  
            this.wait();  
        }  
        for(int i = 0; i < 10; i++) {  
            System.out.println("main thread seq of " + i + ", loop of " + loop);  
        }  
        bool = true;  
        this.notify();  
    }     
    public synchronized void sub(int loop) throws InterruptedException {  
        while(!bool) {  
            this.wait();  
        }  
        for(int i = 0; i < 5; i++) {  
            System.out.println("sub thread seq of " + i + ", loop of " + loop);  
        }  
        bool = false;  
        this.notify();  
    }  
}  
//此处wait时用while而不是用if，官方也推荐用while，因为在线程wait后，线程有可能被假唤醒，用while的话，假唤醒还会继续检测bool是否符合要求

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