我正在编写一个用于两阶段锁定的java实现。所以，我正在使用Reentrant lock（ReadWrite lock）。问题是，当一个线程执行lock.readLock.lock（）或lock.writeLock（）。lock（）并且锁已经被锁定时，即使使用lock.readLock（）锁解锁，它也会永远陷入困境。 （）或lock.writeLock（）。unlock（）。所以，看起来解锁不会唤醒服务员！ 这是导致问题的代码：
class LockTable
{
// /*******************************************************************************
// * This class is used to represent an individual lock.
// * @param tid the id of the transaction holding the lock
// * @param shared whether the lock is shared (true) or exclusive (false)
// */
// void Lock (int tid, boolean shared)
// {
// Semaphore sem = new Semaphore (0);
// } // Lock class
/** Associative map of locks held by transactions of the form (key = oid, value = lock)
*/
private HashMap<Integer,MyLock> locks;
public LockTable(){
locks= new HashMap<Integer,MyLock>();
}
/*******************************************************************************
* Acquire a shared/read lock on data object oid.
* @param tid the transaction id
* @param oid the data object id
*/
void rl (int tid, int oid) throws InterruptedException
{
MyLock lock=null;
boolean wait = false;
synchronized(this) {
try {
lock = locks.get(oid); // find the lock
if((lock != null) && (lock.lock.isWriteLocked())){
wait = true;
// System.out.println(locks.get(oid).shared);
}
if(lock == null){
lock = new MyLock(tid, true);
lock.lock.readLock().lock();
lock.readers.add(tid);
locks.put(oid, lock);
}
} catch(Exception e) {
System.out.println(e.getStackTrace()); // lock not found, so oid is not locked;
} // try
}//synch
if (wait){
System.out.println("Transaction " + tid + " is waiting..");
Main.g.addEdge(tid, lock.tid);
if(Main.g.hasCycle())
restart(tid);
//to exclude the restarted thread
if(!Main.trans[tid].terminate){
lock.lock.readLock().lock();
Main.g.removeEdge(tid, lock.tid);
synchronized(this){
lock.readers.add(tid);
}//synchronized
}//if isInturrupted
else
return;
}
else
synchronized(this) {
lock.lock.readLock().lock();
lock.readers.add(tid);
} // synchronized
} // rl
/*******************************************************************************
* Acquire an exclusive/write lock on data object oid.
* @param tid the transaction id
* @param oid the data object id
*/
void wl (int tid, int oid) throws InterruptedException
{
//type to determine the last lock type in order
//to be able to remove the edges from waitfor graph
int type = 0;
MyLock lock = null;
boolean wait = false;
synchronized(this) {
try {
lock = locks.get(oid); // find the lock
if(lock != null && (lock.lock.isWriteLocked() || lock.readers.size() > 0))
{
wait = true;
}
if(lock == null){
lock = new MyLock(tid);
lock.lock.writeLock().lock();
locks.put(oid,lock);
}
} catch(Exception e) {
System.out.println(e.getStackTrace()); // lock not found, so oid is not locked;
} // try
}
if (wait){
System.out.println("Transaction " + tid + " is waiting..");
if(lock.lock.isWriteLocked())
Main.g.addEdge(tid, lock.tid);
else{
type = 1;
for(int reader : lock.readers)
Main.g.addEdge(tid, reader);
}//else
if(Main.g.hasCycle())
{
restart(tid);
}//if
if(!Main.trans[tid].terminate){
System.out.println("I'm waiting here in wl");
lock.lock.writeLock().lock();
System.out.println("Wakeup..");
if(type == 0)
Main.g.removeEdge(tid, lock.tid);
else
for(int reader : lock.readers)
Main.g.removeEdge(tid, reader);
lock.tid = tid;
}
else
return;
}// if(wait) ==> for the lock to be released
else
lock.lock.writeLock().lock();
} // wl
void restart(int tid){
synchronized(this) {
MyLock lock;
List<Integer> toRemove = new ArrayList();
for(int i : locks.keySet()){
lock = locks.get(i);
//lock.sem.release();
if(lock.lock.isWriteLockedByCurrentThread()){
System.out.println("Transaction"+tid+" unlock object "+ i +" in order to restart");
lock.lock.writeLock().unlock();
System.out.println("number of write holders: " + lock.lock.writeLock().getHoldCount());
System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
System.out.println("number of waiters: " + lock.lock.getQueueLength());
toRemove.add(i);
}
if(!lock.lock.isWriteLocked())
if(lock.readers.contains(tid)){
// lock.numberOfReaders --;
System.out.println("Transaction"+tid+" unlock object "+ i +" in order to restart");
lock.readers.remove(lock.readers.indexOf(tid));
lock.lock.readLock().unlock();
System.out.println("number of write holders: " + lock.lock.getWriteHoldCount());
System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
System.out.println("number of waiters: " + lock.lock.getQueueLength());
toRemove.add(i);
}//if
}//for
for(int i = 0; i < toRemove.size() ; i ++)
locks.remove(toRemove.get(i));
Main.g.removeEdges(tid);
// Thread.currentThread().interrupt();
Main.trans[tid].terminate = true;
System.out.println("Transaction" + tid + " restarted");
}//sync
}
/*******************************************************************************
* Unlock/release the lock on data object oid.
* @param tid the transaction id
* @param oid the data object id
*/
void ul (int tid, int oid)
{
MyLock lock = null;
boolean error = false;
synchronized(this) {
try {
lock = locks.get(oid); // find the lock
if( lock == null)
System.out.println("println: lock not found");
} catch(Exception e) {
System.out.println("lock not found"); // lock not found
} // try
}//sync
if((lock != null) && (lock.lock.isWriteLockedByCurrentThread())){
System.out.println("tid: " + tid + " unlock object: " + oid);
lock.lock.writeLock().unlock();
System.out.println("done with unlock");
System.out.println("number of write holders: " + lock.lock.writeLock().getHoldCount());
System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
System.out.println("number of waiters: " + lock.lock.getQueueLength());
}// if lock != null
else
if((lock != null) && (lock.readers.size()>0)){
if(lock.readers.contains(tid)){
lock.readers.remove(lock.readers.indexOf(tid));
lock.lock.readLock().unlock();
System.out.println("Transaction"+tid+" unlocked shared lock on object "+oid);
//System.out.println("number of write holders: " + lock.lock.readLock().);
System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
System.out.println("number of waiters: " + lock.lock.getQueueLength());
}//if lock.readers
}//if
if (error)
System.out.println ("Error: ul: no lock for oid = " + oid + " found/owned");
} // ul
答案 0 :(得分：4)
Method A:
...snip...
synchronized(this) {
...snip...
lock.lock.readLock().lock();
...snip...
}
Method B:
...snip...
synchronized(this) {
...snip...
lock.lock.readLock().unlock();
...snip...
}
你遇到的问题是基本上阻塞一个同步器而另一个同步器出现死锁。当您在Lock.lock()代码块内拨打synchronized(this)时，Thread将进入BLOCKING状态，同时继续保持this上的内部锁定。由于实际持有Lock以获取this上的内部锁定并且内部锁定的线程永远不会被释放，因此您有一个死锁。
例如：
Thread1进入方法A，获得this上的锁定
Thread2进入方法A，无法获得this所以阻止
Thread1获得Readlock
Thread1发布this
Thread2获得this
Thread2阻塞等待Readlock
Thread1进入方法B，无法获得this所以阻止
死锁
答案 1 :(得分：0)
另外，不要忘记不快乐的道路。确保将你的unlock（）置于finally中，否则如果抛出异常，则永远不会调用unlock（）并最终导致死锁。