我是多线程新手,希望避免以下代码中出现的竞争条件。在release()方法中有一行available.add(resource),在remove()方法中有一行available.remove(resource)。所以我的问题是如何同步资源'变量以避免这种竞争条件?
package threadpool;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
public class ResourcePoolImpl<R> implements ResourcePool<R> {
private static final String CLOSED_POOL_EXCEPTION = "Pool is closed,cannot aquire resource.";
private static final String RELEASE_EXCEPTION = "Unaquired resource, cannot release it.";
private volatile boolean open = false;
private final BlockingQueue<R> available = new LinkedBlockingQueue<R>();
private final ConcurrentMap<R, CountDownLatch> aquired = new ConcurrentHashMap<R, CountDownLatch>();
public R acquire() throws InterruptedException {
if ( !open ) { throw new IllegalStateException( CLOSED_POOL_EXCEPTION ); }
final R resource = available.take();
if ( resource != null ) {
aquired.put( resource, new CountDownLatch( 1 ) );
}
return resource;
}
public R acquire( final long timeout, final TimeUnit timeUnit ) throws InterruptedException {
if ( !open ) { throw new IllegalStateException( CLOSED_POOL_EXCEPTION ); }
final R resource = available.poll( timeout, timeUnit );
if ( resource != null ) {
aquired.put( resource, new CountDownLatch( 1 ) );
}
return resource;
}
public boolean add( final R resource )
{
return available.add( resource );
}
public void close() throws InterruptedException {
open = false;
for ( final CountDownLatch latch : aquired.values() ) {
latch.await();
}
}
public void closeNow() {
open = false;
}
public boolean isOpen() {
return open;
}
public void open() {
open = true;
}
public void release( final R resource )
{
final CountDownLatch latch = aquired.get( resource );
if ( latch == null ) { throw new IllegalArgumentException( RELEASE_EXCEPTION ); }
available.add( resource );
latch.countDown();
}
public boolean remove( final R resource ) throws InterruptedException
{
final CountDownLatch latch = aquired.get( resource );
if ( latch != null ) {
latch.await();
}
return available.remove( resource );
}
public boolean removeNow( final R resource ) {
return available.remove( resource );
}
}
答案 0 :(得分:2)
宣布
final Object mutex = new Object();
让所有对共享集合执行读/写操作的方法在执行操作之前获取互斥锁,或者根据共享数据做出决策,在同步块中执行:
synchronized (mutex) {
// .. guaranteed single-threaded access here
// (for instance, contents of aquire() or release();
// also add() or any other collection access)
}
然后,您可以使用更简单的非并发集合类,因为在互斥保护区域内,不能进行任何多线程访问。
并发集合只是将他们的访问权限包含在他们自己的内部互斥锁中 - 但正如您在评论中解释的那样,问题是aquired
和available
可以彼此独立地更新,你绝对不想要。
因此:通过为所有关键区域访问声明和使用单个互斥锁来简化代码。