交叉输出字符串A或B，各输出若干次

public class DBTools {

    private volatile boolean flag = false;

    public synchronized void backupA(){
        try {
            while(flag){    //当为true阻塞
                this.wait();
            }

            for (int i = 0; i < 5; i ++){
                System.out.println("AAAAAAAA");
            }

            flag = true;
            this.notifyAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void backupB(){
        try {
            while(!flag){    //当为false阻塞
                this.wait();
            }

            for (int i = 0; i < 5; i ++){
                System.out.println("BBBBBBBB");
            }

            flag = false;
            this.notifyAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class ThreadA extends Thread{

    private DBTools dbTools;

    public ThreadA(DBTools dbTools){
        this.dbTools = dbTools;
    }

    @Override
    public void run() {
        dbTools.backupA();
    }
}

public class ThreadB extends Thread{

    private DBTools dbTools;

    public ThreadB(DBTools dbTools){
        this.dbTools = dbTools;
    }

    @Override
    public void run() {
        dbTools.backupB();
    }
}

public class Run {

    public static void main(String[] args) {
        DBTools tools = new DBTools();
        for (int i = 0; i < 10; i ++){
            ThreadA a = new ThreadA(tools);
            a.start();

            ThreadB b = new ThreadB(tools);
            b.start();
        }
    }
}

备注：来源于书籍（高洪岩）Java多线程编程核心技术-177页