hand over hand locking

[Mike Hays]

Hello,

I've read that masterLock.lock() in the following code should come before the try block because it might throw an exception. If it is placed inside the try block and an exception is thrown, the finally block will attempt to unlock a lock that isn't locked.

Can masterLock.unlock() also throw an exception? If it does then recordLock might not get unlocked. Is the following code not a correct way to implement hand over hand locking?

    masterLock.lock();

    try {
        recordLock = getRecordLock...

        recordLock.lock();
    } finally {
        masterLock.unlock();
    }

    try {
        ...
    } finally {
        recordLock.unlock();
    }

[Mike Hays]

Hi all,

I'm also trying to understand how to acquire 2 locks in a row. For example,

 
masterLock.lock();

        try {
            ...
            recordLock.lock();
            ...
        } finally {
            masterLock.unlock();
            recordLock.unlock();  // throws exception if recordLock.lock() fails.
        }
 

 
masterLock.lock();
recordLock.lock();  // if this fails masterLock won't get unlocked.

        try {
            ...
        } finally {
            masterLock.unlock();
            recordLock.unlock();
        }
 

Are either of the above approaches correct? As noted in the comments they both seem to have problems.

How about

 
masterLock.lock();

        try {
            recordLock.lock();

            try {
                ...
            } finally {
                recordLock.unlock();
            }
        } finally {
            masterLock.unlock();
        }
 

But I've read that nested try blocks are undesirable. Anyone know how to do this correctly?