java多线程面试题

1、有三个线程ID分别是A、B、C,请有多线编程实现，在屏幕上循环打印10次ABCABC…

由于线程执行的不确定性，要保证这样有序的输出，必须控制好多线程的同步。

线程同步有两种基本方法：

（1）    synchronized

（2）    wait,notify,notifyAll

现在分别采用这两种方法来解答这道题目。

 注：此面试题主要考查，面试者线程并发的问题。可以参考：生产者-消费者模型。

方法1：使用 synhronized同步代码块

class Method1 implements Runnable {
    private String name = "";
    private StartLock lock = null;
    private int count = 10;

    public Method1(String name, StartLock lock) {
        this.name = name;
        this.lock = lock;

    }

    public void run() {
        while (count > 0) {
            synchronized (lock) {
                if (lock.getName().equalsIgnoreCase(this.name)) {
                    System.out.print("当前线程名："
                            + Thread.currentThread().getName() + " " + name
                            + "    ");

                    if (name == "C") {
                        System.out.println();
                    }

                    count--;

                    if (this.name.equals("A")) {
                        lock.setName("B");
                    } else if (this.name.equals("B")) {
                        lock.setName("C");
                    } else if (this.name.equals("C")) {
                        lock.setName("A");
                    }
                }
            }
        }
    }
}

方法2：使用 wait,notify,notifyAll

class Method2 implements Runnable {
    private String name = "";
    private StartLock lock = null;
    private int count = 10;
    
    public Method2(String name, StartLock lock) {
        this.name = name;
        this.lock = lock;
    }

    public void run() {
        while (count > 0) {
            synchronized (lock) {
                while (!lock.getName().equalsIgnoreCase(this.name)) {
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

                System.out.print(name);

                count--;

                if (this.name.equals("A")) {
                    lock.setName("B");
                } else if (this.name.equals("B")) {
                    lock.setName("C");
                } else if (this.name.equals("C")) {
                    lock.setName("A");
                }
                lock.notifyAll();
            }
        }
    }
}

测试：

class StartLock {//锁对象
    public String name = "A";
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

public class Test  {
    public static void main(String[] args) {
        StartLock lock = new StartLock();
        
        //new Thread(new Method1("A", lock)).start();
        //new Thread(new Method1("B", lock)).start();
        //new Thread(new Method1("C", lock)).start();
        
        new Thread(new Method2("A", lock)).start();
        new Thread(new Method2("B", lock)).start();
        new Thread(new Method2("C", lock)).start();
    }
}