如何控制线程执行的顺序？

 方法一：通过共享对象锁加上可见变量来实现。

public class MyService {
 
    private volatile int orderNum = 1;
 
    public synchronized void methodA() {
        try {
            while (orderNum != 1) {
                wait();
            }
            for (int i = 0; i < 2; i++) {
                System.out.println("AAAAA");
            }
            orderNum = 2;
            notifyAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
 
    public synchronized void methodB() {
        try {
            while (orderNum != 2) {
                wait();
            }
            for (int i = 0; i < 2; i++) {
                System.out.println("BBBBB");
            }
            orderNum = 3;
            notifyAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
 
    public synchronized void methodC() {
        try {
            while (orderNum != 3) {
                wait();
            }
            for (int i = 0; i < 2; i++) {
                System.out.println("CCCCC");
            }
            orderNum = 1;
            notifyAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

 
import service.MyService;
public class ThreadAA extends Thread {
 
	private MyService dbtools;
 
	public ThreadAA(MyService dbtools) {
		super();
		this.dbtools = dbtools;
	}
 
	@Override
	public void run() {
		dbtools.methodA();
	}
 
}

import service.MyService;
public class ThreadBB extends Thread {
 
	private MyService dbtools;
 
	public ThreadBB(MyService dbtools) {
		super();
		this.dbtools = dbtools;
	}
 
	@Override
	public void run() {
		dbtools.methodB();
	}
 
}

import service.MyService;
public class ThreadCC extends Thread {
 
	private MyService dbtools;
 
	public ThreadCC(MyService dbtools) {
		this.dbtools = dbtools;
	}
 
	@Override
	public void run() {
		dbtools.methodC();
	}
 
}

import extthread.ThreadCC;
import service.MyService;
import extthread.ThreadAA;
import extthread.ThreadBB;
 
public class Run {
 
	public static void main(String[] args) {
		MyService myService = new MyService();
		for (int i = 0; i < 2; i++) {
			ThreadBB output = new ThreadBB(myService);
			output.start();

			ThreadAA input = new ThreadAA(myService);
			input.start();

             ThreadCC threadCC = new ThreadCC(myService);
             threadCC.start();
		}
	}
 
}

执行结果：

可以看到线程的启动按顺序执行了。共享对象锁，可以保证每个方法只能同时有一个线程进入，配合wait和notifyall方法，可以启动或者唤醒线程。

 

方法二：通过主线程Join()

thread.Join把指定的线程加入到当前线程，可以将两个交替执行的线程合并为顺序执行的线程。

比如在线程B中调用了线程A的Join()方法，直到线程A执行完毕后，才会继续执行线程B。

class T11 extends Thread {
    public void run() {
        System.out.println("in T1");
    }
}
 
class T22 extends Thread {
    public void run() {
        System.out.println("in T2");
    }
}
 
class T33 extends Thread {
    public void run() {
        System.out.println("in T3");
    }
}
 
public class Test2 {
    public static void main(String[] args) throws InterruptedException {
        T11 t1 = new T11();
        T22 t2 = new T22();
        T33 t3 = new T33();
        t1.start();
/**join的意思是使得放弃当前线程的执行，并返回对应的线程，例如下面代码的意思就是：
         程序在main线程中调用t1线程的join方法，则main线程放弃cpu控制权，并返回t1线程继续执行直到线程t1执行完毕
         所以结果是t1线程执行完后，才到主线程执行，相当于在main线程中同步t1线程，t1执行完了，main线程才有执行的机会
         */
        t1.join();
        t2.start();
        t2.join();
        t3.start();
    }
}

 方法三：通过线程执行时Join():

class T1 extends Thread {
    public void run(){
        Random random = new Random();
        try {
            Thread.sleep(random.nextInt(1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("in T1");
    }
}
 
class T2 extends Thread{
    private Thread thread;
    public T2(Thread thread) {
        this.thread = thread;
    }
 
    public void run(){
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("in T2");
    }
}
 
class T3 extends Thread{
    private Thread thread;
    public T3(Thread thread) {
        this.thread = thread;
    }
 
    public void run(){
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("in T3");
    }
}
 
public class Test {
    public static void main(String[] args) throws InterruptedException {
        T1 t1 = new T1();
        T2 t2 = new T2(t1);
        T3 t3 = new T3(t2);
        t2.start();
        t1.start();
        t3.start();
    }
}