同步模式之顺序控制

固定运行顺序

比如，两个线程，一个打印1，一个打印2。要求必须先打印2再打印1

wait/notify实现(lock/unlock相似)

import lombok.extern.slf4j.Slf4j;

@Slf4j(topic = "fixedOrder")
public class FixOrderTest {
    static Object obj = new Object();
    static boolean flag = false;

    public static void main(String[] args) {
        new Thread(()->{
            synchronized (obj){
                while (!flag){
                    try {
                        obj.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.info("1");
            }

        },"t1").start();

        new Thread(()->{
            synchronized (obj){
                log.info("2");
                flag = true;
                obj.notify();
            }
        },"t2").start();
    }
}

park/unpark实现

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.LockSupport;

@Slf4j(topic = "fixedOrder")
public class FixOrderTest {

    public static void main(String[] args) {
        Thread t1 = new Thread(()->{
            LockSupport.park();
            log.info("1");
        },"t1");
        t1.start();

        new Thread(()->{
            log.info("2");
            LockSupport.unpark(t1);
        },"t2").start();
    }
}

交替输出

比如，线程1输出a 5次，线程2输出b 5次，线程3输出c 5次.
现在要求输出abcabcabcabcabc，怎么实现

wait/notify

分析：

• 三个线程，不能用Boolean值作为标记
• 输出有先后顺序要求，a输出完b，b输出完c，c输出完a，需要控制输出次序

import lombok.extern.slf4j.Slf4j;


@Slf4j(topic = "fixedOrder")
public class FixOrderTest {
    public static void main(String[] args) {
        WaitNotify obj = new WaitNotify(1, 5);

        new Thread(()->{obj.print("a",1,2);},"t1").start();
        new Thread(()->{obj.print("b",2,3);},"t2").start();
        new Thread(()->{obj.print("c",3,1);},"t3").start();
    }
}

@Slf4j(topic = "waitNotify")
class WaitNotify{
    private int flag;
    private int loopNum;

    public WaitNotify(int flag, int loopNum) {
        this.flag = flag;
        this.loopNum = loopNum;
    }

    public void print(String str,int flag,int nextFlag){
        for (int i = 0; i < loopNum; i++) {
            synchronized (this) {
                while (flag != this.flag){
                    try {
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.info(str);
                this.flag = nextFlag;
                this.notifyAll();
            }
        }
    }
}

ReentrantLock的Await/Signal

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


@Slf4j(topic = "fixedOrder")
public class FixOrderTest {
    public static void main(String[] args) {
        AwaitSignal awaitSignal = new AwaitSignal(5);
        Condition a = awaitSignal.newCondition();
        Condition b = awaitSignal.newCondition();
        Condition c = awaitSignal.newCondition();

        new Thread(()->{awaitSignal.print("a",a,b);},"t1").start();
        new Thread(()->{awaitSignal.print("b",b,c);},"t2").start();
        new Thread(()->{awaitSignal.print("c",c,a);},"t3").start();

        //启动
        awaitSignal.lock();
        try {
            a.signal();
        } finally {
            awaitSignal.unlock();
        }
    }
}

@Slf4j(topic = "awaitSignal")
class AwaitSignal extends ReentrantLock {
    private int loopNum;

    public AwaitSignal(int loopNum) {
        this.loopNum = loopNum;
    }

    public void print(String str, Condition cur ,Condition next){
        for (int i = 0; i < loopNum; i++) {
            lock();
            try {
                cur.await();
                log.info(str);
                next.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                unlock();
            }
        }
    }
}

park/unpark

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;


@Slf4j(topic = "fixedOrder")
public class FixOrderTest {
    static Thread t1,t2,t3;
    public static void main(String[] args) {
        ParkUnpark parkUnpark = new ParkUnpark(5);
        t1 = new Thread(()->{parkUnpark.print("a",t2);},"t1");
        t2 = new Thread(()->{parkUnpark.print("b",t3);},"t2");
        t3 = new Thread(()->{parkUnpark.print("c",t1);},"t3");

        t1.start();
        t2.start();
        t3.start();

        LockSupport.unpark(t1);
    }
}


@Slf4j(topic = "parkUnpark")
class ParkUnpark{
    private int loopNum;

    public ParkUnpark(int loopNum) {
        this.loopNum = loopNum;
    }

    public void print(String str,Thread next){
        for (int i = 0; i < loopNum; i++) {
            LockSupport.park();
            log.info(str);
            LockSupport.unpark(next);
        }
    }
}