[英]Java Multi Threading synchronized block Looping forever
我实现了示例程序以确保一次执行特定进程的线程不超过 5 个。 但结果并不如预期。
public class MiscUtils{
private volatile static int count = 0;
public synchronized static int getCount() {
return count;
}
public synchronized static boolean incrementCounter() throws InterruptedException {
System.out.println("count - " + count);
while(MiscUtils.getCount() > 4) {
System.out.println(Thread.currentThread().getName() + " Sleeping..");
Thread.sleep(1000 * 5);
}
System.out.println("Thread acquired " + Thread.currentThread().getName());
if(count > 4) {
return false;
}
count++;
return true;
}
public synchronized static boolean decrementCounter() {
System.out.println("Decrement Called " + Thread.currentThread().getName());
count--;
return true;
}
public static void main(String[] args) throws InterruptedException {
Runnable r = new Runnable() {
@Override
public void run() {
try {
//System.out.println(new Date() + " :: Thread incremented");
MiscUtils.incrementCounter();
Thread.sleep(10 * 1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
MiscUtils.decrementCounter();
System.out.println(Thread.currentThread().getName());
}
};
int count = 10;
for(int i = 0; i < count; i++) {
Thread s = new Thread(r);
System.out.println(new Date() + "Thread " + i + " starting..");
s.start();
System.out.println(new Date() + "Thread " + i + " started..");
Thread.sleep(1 * 1000);
}
}
}
实际输出:
Mon Dec 30 19:29:20 IST 2019Thread 0 starting..
Mon Dec 30 19:29:20 IST 2019Thread 0 started..
count - 0
Thread acquired Thread-0
Mon Dec 30 19:29:21 IST 2019Thread 1 starting..
Mon Dec 30 19:29:21 IST 2019Thread 1 started..
count - 1
Thread acquired Thread-1
Mon Dec 30 19:29:22 IST 2019Thread 2 starting..
Mon Dec 30 19:29:22 IST 2019Thread 2 started..
count - 2
Thread acquired Thread-2
Mon Dec 30 19:29:23 IST 2019Thread 3 starting..
Mon Dec 30 19:29:23 IST 2019Thread 3 started..
count - 3
Thread acquired Thread-3
Mon Dec 30 19:29:24 IST 2019Thread 4 starting..
Mon Dec 30 19:29:24 IST 2019Thread 4 started..
count - 4
Thread acquired Thread-4
Mon Dec 30 19:29:25 IST 2019Thread 5 starting..
Mon Dec 30 19:29:25 IST 2019Thread 5 started..
count - 5
Thread-5 Sleeping..
Mon Dec 30 19:29:26 IST 2019Thread 6 starting..
Mon Dec 30 19:29:26 IST 2019Thread 6 started..
Mon Dec 30 19:29:27 IST 2019Thread 7 starting..
Mon Dec 30 19:29:27 IST 2019Thread 7 started..
Mon Dec 30 19:29:28 IST 2019Thread 8 starting..
Mon Dec 30 19:29:28 IST 2019Thread 8 started..
Mon Dec 30 19:29:29 IST 2019Thread 9 starting..
Mon Dec 30 19:29:29 IST 2019Thread 9 started..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread-5 Sleeping..
预期输出 -
Mon Dec 30 19:29:20 IST 2019Thread 0 starting..
Mon Dec 30 19:29:20 IST 2019Thread 0 started..
count - 0
Thread acquired Thread-0
Mon Dec 30 19:29:21 IST 2019Thread 1 starting..
Mon Dec 30 19:29:21 IST 2019Thread 1 started..
count - 1
Thread acquired Thread-1
Mon Dec 30 19:29:22 IST 2019Thread 2 starting..
Mon Dec 30 19:29:22 IST 2019Thread 2 started..
count - 2
Thread acquired Thread-2
Mon Dec 30 19:29:23 IST 2019Thread 3 starting..
Mon Dec 30 19:29:23 IST 2019Thread 3 started..
count - 3
Thread acquired Thread-3
Mon Dec 30 19:29:24 IST 2019Thread 4 starting..
Mon Dec 30 19:29:24 IST 2019Thread 4 started..
count - 4
Thread acquired Thread-4
Mon Dec 30 19:29:25 IST 2019Thread 5 starting..
Mon Dec 30 19:29:25 IST 2019Thread 5 started..
count - 5
Thread-5 Sleeping..
Mon Dec 30 19:29:26 IST 2019Thread 6 starting..
Mon Dec 30 19:29:26 IST 2019Thread 6 started..
Mon Dec 30 19:29:27 IST 2019Thread 7 starting..
Mon Dec 30 19:29:27 IST 2019Thread 7 started..
Mon Dec 30 19:29:28 IST 2019Thread 8 starting..
Mon Dec 30 19:29:28 IST 2019Thread 8 started..
Mon Dec 30 19:29:29 IST 2019Thread 9 starting..
Mon Dec 30 19:29:29 IST 2019Thread 9 started..
Thread-5 Sleeping..
Thread-5 Sleeping..
Thread acquired Thread-5
Thread-6 Sleeping..
Thread acquired Thread-6
and so on...
DecrementCounter() 方法永远不会被调用。 此外,我希望同步块应该更新 getCount() 方法中的计数变量以获取最新值。 但它总是返回值 5。请让我知道我哪里出错了。
当您在类中同步静态方法时,这意味着两者都锁定在同一个监视器上,即类本身。
因此,当Thread-5
进入 while 循环时,它将无限期地持有锁,而线程 1-4 仍在等待获取锁以进入decrementCounter
。
您的代码的主要问题如下:
在 Java 中,同一类中的静态同步方法内部只能执行一个线程。 [ 参考]
MiscUtils.incrementCounter
的执行。MiscUtils.decrementCounter
的执行。count
达到 5 后, thread-5
将卡在循环中while(MiscUtils.getCount() > 4)
thread-5
之后的所有线程都将启动,但将进入睡眠状态,因为同步静态方法MiscUtils.incrementCounter
仍然停留在thread-5
。这肯定会导致无限循环。
我希望这是一个明确的答案。
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