【143期】你知道 Java 是如何实现线程间通信的吗?
共 1126字,需浏览 3分钟
·
2021-02-26 07:18
阅读本文大概需要 8.5 分钟。
作者:wingjay
来源:http://wingjay.com
thread.join(),
object.wait(),
object.notify(),
CountdownLatch,
CyclicBarrier,
FutureTask,
Callable 。
本文涉及代码:
https://github.com/wingjay/HelloJava/blob/master/multi-thread/src/ForArticle.java
如何让两个线程依次执行?
那如何让 两个线程按照指定方式有序交叉运行呢?
四个线程 A B C D,其中 D 要等到 A B C 全执行完毕后才执行,而且 A B C 是同步运行的
三个运动员各自准备,等到三个人都准备好后,再一起跑
子线程完成某件任务后,把得到的结果回传给主线程
如何让两个线程依次执行?
private static void demo1() {
Thread A = new Thread(new Runnable() {
@Override
public void run() {
printNumber("A");
}
});
Thread B = new Thread(new Runnable() {
@Override
public void run() {
printNumber("B");
}
});
A.start();
B.start();
}
private static void printNumber(String threadName) {
int i=0;
while (i++ < 3) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(threadName + " print: " + i);
}
}
B print: 1
A print: 1
B print: 2
A print: 2
B print: 3
A print: 3
private static void demo2() {
Thread A = new Thread(new Runnable() {
@Override
public void run() {
printNumber("A");
}
});
Thread B = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("B 开始等待 A");
try {
A.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
printNumber("B");
}
});
B.start();
A.start();
}
B 开始等待 A
A print: 1
A print: 2
A print: 3B print: 1
B print: 2
B print: 3
那如何让 两个线程按照指定方式有序交叉运行呢?
/**
* A 1, B 1, B 2, B 3, A 2, A 3
*/
private static void demo3() {
Object lock = new Object();
Thread A = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
System.out.println("A 1");
try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("A 2");
System.out.println("A 3");
}
}
});
Thread B = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
System.out.println("B 1");
System.out.println("B 2");
System.out.println("B 3");
lock.notify();
}
}
});
A.start();
B.start();
}
A 1
A waiting…B 1
B 2
B 3
A 2
A 3
首先创建一个 A 和 B 共享的对象锁 lock = new Object();
当 A 得到锁后,先打印 1,然后调用 lock.wait() 方法,交出锁的控制权,进入 wait 状态;
对 B 而言,由于 A 最开始得到了锁,导致 B 无法执行;直到 A 调用 lock.wait() 释放控制权后, B 才得到了锁;
B 在得到锁后打印 1, 2, 3;然后调用 lock.notify() 方法,唤醒正在 wait 的 A;
A 被唤醒后,继续打印剩下的 2,3。
private static void demo3() {
Object lock = new Object();
Thread A = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("INFO: A 等待锁 ");
synchronized (lock) {
System.out.println("INFO: A 得到了锁 lock");
System.out.println("A 1");
try {
System.out.println("INFO: A 准备进入等待状态,放弃锁 lock 的控制权 ");
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("INFO: 有人唤醒了 A, A 重新获得锁 lock");
System.out.println("A 2");
System.out.println("A 3");
}
}
});
Thread B = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("INFO: B 等待锁 ");
synchronized (lock) {
System.out.println("INFO: B 得到了锁 lock");
System.out.println("B 1");
System.out.println("B 2");
System.out.println("B 3");
System.out.println("INFO: B 打印完毕,调用 notify 方法 ");
lock.notify();
}
}
});
A.start();
B.start();
}
INFO: A 等待锁
INFO: A 得到了锁 lock
A 1
INFO: A 准备进入等待状态,调用 lock.wait() 放弃锁 lock 的控制权
INFO: B 等待锁
INFO: B 得到了锁 lock
B 1
B 2
B 3
INFO: B 打印完毕,调用 lock.notify() 方法
INFO: 有人唤醒了 A, A 重新获得锁 lock
A 2
A 3
四个线程 A B C D,其中 D 要等到 A B C 全执行完毕后才执行,而且 A B C 是同步运行的
创建一个计数器,设置初始值,CountdownLatch countDownLatch = new CountDownLatch(2);
在 等待线程 里调用 countDownLatch.await() 方法,进入等待状态,直到计数值变成 0;
在 其他线程 里,调用 countDownLatch.countDown() 方法,该方法会将计数值减小 1;
当 其他线程 的 countDown() 方法把计数值变成 0 时,等待线程 里的 countDownLatch.await() 立即退出,继续执行下面的代码。
private static void runDAfterABC() {
int worker = 3;
CountDownLatch countDownLatch = new CountDownLatch(worker);
new Thread(new Runnable() {
@Override
public void run() {
System.out.println("D is waiting for other three threads");
try {
countDownLatch.await();
System.out.println("All done, D starts working");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
for (char threadName='A'; threadName <= 'C'; threadName++) {
final String tN = String.valueOf(threadName);
new Thread(new Runnable() {
@Override
public void run() {
System.out.println(tN + " is working");
try {
Thread.sleep(100);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(tN + " finished");
countDownLatch.countDown();
}
}).start();
}
}
D is waiting for other three threads
A is working
B is working
C is workingA finished
C finished
B finished
All done, D starts working
三个运动员各自准备,等到三个人都准备好后,再一起跑
先创建一个公共 CyclicBarrier 对象,设置 同时等待 的线程数,CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
这些线程同时开始自己做准备,自身准备完毕后,需要等待别人准备完毕,这时调用 cyclicBarrier.await(); 即可开始等待别人;
当指定的 同时等待 的线程数都调用了 cyclicBarrier.await();时,意味着这些线程都准备完毕好,然后这些线程才 同时继续执行。
private static void runABCWhenAllReady() {
int runner = 3;
CyclicBarrier cyclicBarrier = new CyclicBarrier(runner);
final Random random = new Random();
for (char runnerName='A'; runnerName <= 'C'; runnerName++) {
final String rN = String.valueOf(runnerName);
new Thread(new Runnable() {
@Override
public void run() {
long prepareTime = random.nextInt(10000) + 100;
System.out.println(rN + " is preparing for time: " + prepareTime);
try {
Thread.sleep(prepareTime);
} catch (Exception e) {
e.printStackTrace();
}
try {
System.out.println(rN + " is prepared, waiting for others");
cyclicBarrier.await(); // 当前运动员准备完毕,等待别人准备好
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(rN + " starts running"); // 所有运动员都准备好了,一起开始跑
}
}).start();
}
}
A is preparing for time: 4131
B is preparing for time: 6349
C is preparing for time: 8206
A is prepared, waiting for others
B is prepared, waiting for others
C is prepared, waiting for others
C starts running
A starts running
B starts running
public interface Runnable {
public abstract void run();
}
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
private static void doTaskWithResultInWorker() {
Callablecallable = new Callable () {
@Override
public Integer call() throws Exception {
System.out.println("Task starts");
Thread.sleep(1000);
int result = 0;
for (int i=0; i<=100; i++) {
result += i;
}
System.out.println("Task finished and return result");
return result;
}
};
FutureTaskfutureTask = new FutureTask<>(callable);
new Thread(futureTask).start();
try {
System.out.println("Before futureTask.get()");
System.out.println("Result: " + futureTask.get());
System.out.println("After futureTask.get()");
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
Before futureTask.get()
Task starts
Task finished and return result
Result: 5050
After futureTask.get()
小结
推荐阅读:
【142期】阿里面试:分析为什么B+树更适合作为索引的结构以及索引原理
【138期】面试官:谈谈常用的Iterator中hasNext()、next()、remove()方法吧
【137期】面试官:问点儿基础的,你能说说Java深拷贝和浅拷贝区别吗
微信扫描二维码,关注我的公众号
朕已阅