并发之固定运行和交替运行方案

例如,按规定先输出2再输出1

1 固定运行顺序

1 wait/notify方法

// 用来同步的对象
static Object obj = new Object();
// t2 运行标记， 代表 t2 是否执行过
static boolean t2runed = false;
public static void main(String[] args) {
     Thread t1 = new Thread(() -> {
         synchronized (obj) {
             // 如果 t2 没有执行过
             while (!t2runed) { 
                 try {
                 // t1 先等一会
                 obj.wait(); 
                 } catch (InterruptedException e) {
                 e.printStackTrace()
                 }
             }
         }
     System.out.println(1);
     });
     Thread t2 = new Thread(() -> {
     System.out.println(2);
         synchronized (obj) {
         // 修改运行标记
         t2runed = true;
         // 通知 obj 上等待的线程（可能有多个，因此需要用 notifyAll）
         obj.notifyAll();
         }
     });
    
     t1.start();
     t2.start();
    
}
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按照wait/notify方法:

1 需要保证先wait再notify,否者wait线程永远都不能唤醒. 所以上面添加了标识来判断是否wait.

2 如存在干扰线程错误notify了wait线程,条件标识不满足需要重新等待,添加了while循环控制.

3 唤醒对象上的wait线程需要使用notifyAll,同步线程可能存在多个.

2 park/unpark方法

Thread t1 = new Thread(() -> {
 try { 
     Thread.sleep(1000); 
     } catch (InterruptedException e) {
 }
 // 当没有『许可』时，当前线程暂停运行；有『许可』时，用掉这个『许可』，当前线程恢复运行
 LockSupport.park();
 System.out.println("1");
});
Thread t2 = new Thread(() -> {
 System.out.println("2");
 // 给线程 t1 发放『许可』（多次连续调用 unpark 只会发放一个『许可』）
 LockSupport.unpark(t1);
});
t1.start();
t2.start();

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与上面wait/notify方法相比,park 和 unpark 方法比较灵活:

1 他俩谁先调用，谁后调用无所谓。

2 是以线程为单位进行暂停和恢复

3 不需要额外同步对象和运行标记

2 交替输出

列如,线程 1 输出 a 5 次，线程 2 输出 b 5 次，线程 3 输出 c 5 次。要求输出 abcabcabcabcabc

1 wait/notify方法

class SyncWaitNotify {
     private int flag;
     private int loopNumber;
     public SyncWaitNotify(int flag, int loopNumber) {
     this.flag = flag;
     this.loopNumber = loopNumber;
 }
 public void print(int waitFlag, int nextFlag, String str) {
     for (int i = 0; i < loopNumber; i++) {
         synchronized (this) {
             while (this.flag != waitFlag) {
                 try {
                 this.wait();
                 } catch (InterruptedException e) {
                 e.printStackTrace();
                 }
             }
         System.out.print(str);
         flag = nextFlag;
         this.notifyAll();
         }
     }
  }
}
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测试类

SyncWaitNotify syncWaitNotify = new SyncWaitNotify(1, 5);
new Thread(() -> {
 syncWaitNotify.print(1, 2, "a");
}).start();
new Thread(() -> {
 syncWaitNotify.print(2, 3, "b");
}).start();
new Thread(() -> {
 syncWaitNotify.print(3, 1, "c");
}).start();

/*
运行结果:
abcabcabcabcabc 
*/
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2 lock/unlock方法

class AwaitSignal extends ReentrantLock {
     public void start(Condition first) {
     this.lock();
     try {
          log.debug("start");
         first.signal();
     } finally {
         this.unlock();
         }
 }
 public void print(String str, Condition current, Condition next) {
     for (int i = 0; i < loopNumber; i++) {
         this.lock();
         try {
         current.await();
         log.debug(str);
         next.signal();
         } catch (InterruptedException e) {
         e.printStackTrace();
         } finally {
         this.unlock();
         }
     }
 }
     // 循环次数
     private int loopNumber;
     public AwaitSignal(int loopNumber) {
     this.loopNumber = loopNumber;
     }
}
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测试类

    AwaitSignal as = new AwaitSignal(5);
    Condition aWaitSet = as.newCondition();
    Condition bWaitSet = as.newCondition();
    Condition cWaitSet = as.newCondition();

    new Thread(() -> {
     as.print("a", aWaitSet, bWaitSet);
    }).start();
    new Thread(() -> {
     as.print("b", bWaitSet, cWaitSet);
    }).start();
    new Thread(() -> {
     as.print("c", cWaitSet, aWaitSet);
    }).start();

    as.start(aWaitSet);
/*
运行结果:
abcabcabcabcabc 
*/
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3 prak/unprak方法

class SyncPark {
     private int loopNumber;
     private Thread[] threads;
    
     public SyncPark(int loopNumber) {
     	this.loopNumber = loopNumber;
     }
     public void setThreads(Thread... threads) {
  	   this.threads = threads;
     }
    
     public void print(String str) {
         for (int i = 0; i < loopNumber; i++) {
         LockSupport.park();
         System.out.print(str);
         LockSupport.unpark(nextThread());
         }
     }
    
     private Thread nextThread() {
         Thread current = Thread.currentThread();
         int index = 0;

         for (int i = 0; i < threads.length; i++) {
             if(threads[i] == current) {
             index = i;
             break;
             }
         }
         if(index < threads.length - 1) {
         return threads[index+1];
         } else {
         return threads[0];
         }
     }
    
     public void start() {
         for (Thread thread : threads) {
         thread.start();
         }
        LockSupport.unpark(threads[0]);
     }
}
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测试类

	SyncPark syncPark = new SyncPark(5);

    Thread t1 = new Thread(() -> {
     syncPark.print("a");
    });
    Thread t2 = new Thread(() -> {
     syncPark.print("b");
    });
    Thread t3 = new Thread(() -> {
     syncPark.print("c\n");
    });

    syncPark.setThreads(t1, t2, t3);
    syncPark.start();
/*
运行结果:
abcabcabcabcabc 
*/
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