Integer 对象的同步块
我刚刚在 Java 中遇到了同步块，并编写了一个小程序来测试它是如何工作的.
I just came across the synchronized block in Java and wrote a small programm to test how it works.
我创建了 10 个线程，并让每个线程将一个 Integer 对象递增 1000 次.
I create 10 threads and let each thread increment an Integer object 1000 times.
所以在同步的情况下，我会假设所有线程完成工作后的结果为 10000，而没有同步的结果小于 10000.
So with synchronization I would assume a result of 10000 after all threads have finished their work and a result of less than 10000 without synchronization .
但是，同步并没有像我预期的那样工作.
However the synchronization is not wokring as I expected.
我猜这与对象的不变性有关.
I guess it has something to do with immutability of the object or so.
我的程序:
public class SyncTest extends Thread{
private static Integer syncObj = new Integer(0);
private static SyncTest[] threads = new SyncTest[10];
private boolean done = false;
public void run(){
for(int i = 0; i < 1000; i++){
synchronized(syncObj){
syncObj ++;
}
}
done = true;
}
public static void main(String[] args) {
for(int i=0; i < threads.length; i++){
threads[i] = new SyncTest();
threads[i].start();
}
while(!allDone()); //wait until all threads finished
System.out.println(syncObj);
}
private static boolean allDone(){
boolean done = true;
for(int i = 0; i < threads.length; i++){
done &= threads[i].done;
}
return done;
}
}
有人可以澄清一下吗?
推荐答案
syncObject 每次 ++ 时都会发生变化(++ 将其转换为原始 int，递增，然后自动装箱回 Integer 对象. 整数对象是不可变的……一旦被创建，就无法改变.
syncObject is changing each time you ++ it (the ++ is converting it to a primitive int, incrementing it, and then autoboxing it back to the Integer object. Integer objects are immutable ... once they are created, they cannot change.
底线是你没有在所有线程中使用相同的syncPObj，不同的线程在不同的时间使用不同的syncObjects来同步.
Bottom ine is that you are not using the same syncPObj in all the threads, different threads use different syncObjects at different times to sync on.
使用一个对象作为同步对象(称为syncObj)，并将其声明为最终对象:
use one object as the synchronization (call it syncObj), and declare it as a final Object:
private static final Object syncObject = new Object();
那么你的计数器应该是性能的原始(int)，称之为计数器"或其他东西.
Then your counter should be a primitive (int) for perofrmance, call it 'counter' or something.
在syncObject上同步，并增加计数器.
Synchronize on syncObject, and increment counter.
根据@jsn，done 标志也被破坏，因为您的代码在 isAllDone() 方法上有一个紧密循环"，这是不好的做法.您应该使用 thread[i].join() 等待(阻塞)每个线程的完成，然后从中检查状态.使用 ExecutorService 是正确的方法".
as per @jsn, the done flag is also broken in that your code has a 'tight loop' on the isAllDone() method, and that is bad practice. You should use thread[i].join() to wait (blocking) on each thread's completion, and then check the status from that. Using an ExecutorService is the 'right way'.
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