Java Synchronization with multiple objects/locks
concurrency, java, locks, multithreading, synchronization
Solution
Step 1
Provide a natural ordering for your resources. For example, if your resources were letters, `A` would come before `B`, `B` would come before `C`, and so on.
Step 2
Only allow your threads to grab resources in order.
Now your threads cannot possibly reach deadlock in any situation.
Example
- Thread 1 needs locks on resources `A`, `B`, `D`, and `E`
- Thread 2 needs locks on resources `B` and `E`
Our threads must fight for locks on resources `B` and `D`. Because we have enforced a natural order, the thread that obtains the lock on `B` first is guaranteed to get the lock on `D` and proceed smoothly. The losing thread will be waiting for `B` to be released.
Problem
I'm wondering if there's a package or model that will help me solve this scenario. Let's say I have 3 threads and a bunch of objects A,B,C,D,E,F - T1 needs locks A,B - T2 needs locks B,C,D - T3 needs locks E,F In this scenario, it'd be ok if T1 & T3 ran simultaneously. Also, T2 & T3 can run simultaneously. But T1 & T2 should never run simultaneously. Also, note that - Threads can obtain an arbitrary number of locks, not just 2. (I saw an elegant solution to this problem with a fixed number of locks but not sure I can apply it here.) - Obviously, I'd like each thread to acquire all the needed locks at the same time to prevent deadlock. If someone can point me to a package that supports this use case, or some snippet of code that resolves this, please let me know. Thanks so much.