我正在为我的应用程序实现一个网络线程管理器。我创建了一个JUnit测试,通过调用以下两个方法快速请求和释放网络线程索引:
protected static final List <Integer> currThreads = new ArrayList <Integer>();
protected static int maxThreads = 5;
protected static int lastGrantedId = 0;
public static synchronized int reqNewThread(){
if (currThreads.size() >= maxThreads) return -1;
++lastGrantedId;
currThreads.add(lastGrantedId);
return lastGrantedId;
}
public static void threadFinished(final int threadId) throws InternalError{
if (threadId == -1) return;
synchronized (currThreads) {
boolean works = currThreads.remove(Integer.valueOf(threadId));
assert works : ("threadId: " + threadId);
}
}
线程完成工作后,currThreads
不为空,但 reqNewThread
和 threadFinished
具有相同的调用计数,并且 remove( )
总是产生 true
。如果我同步整个 threadFinished
方法,它就可以正常工作。问题是——为什么?唯一使用的全局变量已经同步了,不是吗?
JUnit4测试代码:
final int iters = 15;
final Runnable getAndFree = new GetAndFree(iters);
final int sz = 15;
final Thread[] t = new Thread[sz];
for (int i = 0; i < sz; i++)
t[i] = new Thread(getAndFree);
for (int i = 0; i < sz; i++)
t[i].start();
for (int i = 0; i < sz; i++)
t[i].join();
assertEquals(0, currThreads.size());
测试线程来源:
private class GetAndFree implements Runnable {
int iters;
public GetAndFree(int iters){
this.iters = iters;
}
@Override
public void run(){
try {
int id = -1;
for (int i = 0; i < iters; i++) {
while ((id = reqNewThread()) == -1) {
Thread.sleep(25);
};
System.out.println("Strarted: " + id);
Thread.sleep((long)(Math.random() * 10));
threadFinished(id);
System.out.println("Finished: " + id);
} // for
} catch(final Exception ex) {
ex.printStackTrace();
}
}
}
最佳答案
The only used global variable is already synchronized, isn't it?
没有。
第一个方法是在该方法的封闭类的 Class
对象上进行同步。
第二种方法是在 currThreads
对象上进行同步。
将第一种方法更改为以下方法,应该可以解决这两种方法的同步问题。
public static int reqNewThread(){
synchronize(currThread) {
if (currThreads.size() >= maxThreads) return -1;
++lastGrantedId;
currThreads.add(lastGrantedId);
return lastGrantedId;
}
}
关于Java同步帮助请求,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/5593786/