我们如何使用最后尝试Lock.lock和Lock.unlock [英] How would we use try finally for Lock.lock and Lock.unlock

问题描述

请考虑以下代码。为了在调用 listIterator 时阻止 IndexOutOfBoundsException ，我们使用读取器锁来检索基于索引的iteartor和writer锁否则在 stockCodes 上写操作时。

Consider the following code. In order to prevent IndexOutOfBoundsException while calling listIterator, we use a reader lock to retrieve the index based iteartor, and writer lock else where when write operation on stockCodes.

请注意，我们没有使用任何锁定机制进行迭代使用 listIterator ，因为它来自 CopyOnWriteArrayList 。不需要锁定，因为不会抛出 ConcurrentModificationException 。

Take note that, we didn't use any locking mechanism to iterate using listIterator, as it is from CopyOnWriteArrayList. Locking is not required as ConcurrentModificationException shall not be thrown.

// stockCodesReaderLock is reader lock from java.util.concurrent.locks.ReadWriteLock
// stockCodes is CopyOnWriteArrayList
// Acquire iterator in a safe way.
stockCodesReaderLock.lock();
final int stockCodesSize = stockCodes.size();
if (currIndex < stockCodesSize) {
    listIterator = stockCodes.listIterator(currIndex);
}
stockCodesReaderLock.unlock();

我想知道，我是否应该尝试/终于阻止，因为我看不出有任何异常出现的机会？如果使用 try / finally 是必须的，我应该使用（A）还是（B）？

I was wondering, whether I should have try/finally block, as I cannot see any chance for exception to arise? If using try/finally is a must, should I use (A) or (B)?

我有什么需要吗？

（A）

try {
    stockCodesReaderLock.lock();
    final int stockCodesSize = stockCodes.size();
    if (currIndex < stockCodesSize) {
        listIterator = stockCodes.listIterator(currIndex);
    }
} finally {
    stockCodesReaderLock.unlock();
}

---

（B ）

stockCodesReaderLock.lock();
try  {
    final int stockCodesSize = stockCodes.size();
    if (currIndex < stockCodesSize) {
        listIterator = stockCodes.listIterator(currIndex);
    }
} finally {
    stockCodesReaderLock.unlock();
}

推荐答案

其他受访者是对的：你应该总是使用try / finally。

The other respondents are right: you should always use try/finally.

关于（A）或（B）是否正确，Sun似乎建议（B）in ReDrantReadWriteLock的JavaDoc （搜索终于看到它）。我想这是因为 lock（）方法如果失败就会抛出异常：例如，JavaDoc说它会抛出错误在晦涩的情况下，同一个线程试图以递归方式获取锁定超过65535次。

Regarding whether (A) or (B) is correct, Sun seems to recommend (B) in the JavaDoc of ReentrantReadWriteLock (search for "finally" to see it). I suppose this is because the lock() method could throw an exception if it fails: for example, the JavaDoc says it will throw an Error in the obscure case where the same thread attempts to acquire the lock recursively more than 65535 times.

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