ReadWrite locks in Java

Question

While a write lock is being held, Read lock can be acquired. But not vice versa. What is the rationale for this design choice.

  public static void main(String[] args)
    {
        System.out.println("read to write test");
        ReadWriteLock lock = new ReentrantReadWriteLock();
        
        lock.writeLock().lock();
        lock.readLock().lock();
        
        System.out.println("Read locks can be acquired after  Write locks are acquired as well.");
    }

Output of above code:

read to write test
Read locks can be acquired after  Write locks are acquired as well.

Vice versa doesn't work.

public static void main(String[] args) throws InterruptedException {


        ReadWriteLock lock = new ReentrantReadWriteLock();


        lock.readLock().lock();
        
        System.out.println("Read lock is  acquired");
        System.out.print("Trying to get write lock. ");

        boolean  writeLockAcquired =  lock.writeLock().tryLock(120, TimeUnit.SECONDS);
        if (! writeLockAcquired){
            System.out.println("Even after 120 seconds, we  couldn't get write lock");
        }
        else{
            System.out.println("We could get write lock.");
        }
        
    }

Here is the output:

Read lock is  acquired
Trying to get write lock. Even after 120 seconds, we  couldn't get write lock

Can some one point to snippets in Java open source projects to look at practical usage of Read Write locks. I understand that there are a lot of Java open source projects like ElasticSearch / Hadoop / Spark etc.

score 2 · Accepted Answer · answered Dec 30 '22 at 22:19

While a write lock is being held, Read lock can be acquired.

Locking is to protect against access by concurrent threads.

A read-write lock allows multiple concurrent readers OR a single writer thread.

If a thread owns the write lock, it already has exclusive access, so there's no harm in granting it read access. It will not conflict with itself.

If a thread owns the read lock, there is no guarantee it has exclusive access, so granting the write lock is disallowed. There is no benefit in special-casing the 'single thread wanting both locks' situation.

In other systems, the lock modes are referred to as 'exclusive' (= write) and 'shared' (= read) modes. Considering those terms might help clarify the situation.

There is further rationale in the documentation for the reentrant read-write lock:

Additionally, a writer can acquire the read lock, but not vice-versa. Among other applications, reentrancy can be useful when write locks are held during calls or callbacks to methods that perform reads under read locks. If a reader tries to acquire the write lock it will never succeed.

In other words, the behavior I see is because I am using the same main thread ? If we use different threads, when one of them is writing, other threads can't access the read lock ? — Vineel, Dec 30 '22 at 22:45
That's correct. And the rationale for that is - the data protected by the lock are being written, so may be inconsistent. — undefined symbol, Dec 30 '22 at 22:46

score 1 · Answer 2 · edited Dec 31 '22 at 06:56

Allowing a read lock to be upgraded to a write lock would be a source of deadlocks:

Thread A takes a read lock (now no one can take a write lock)
Thread B takes a read lock
Thread A decides to upgrade to a write lock -- blocks
Thread B decides to upgrade to a write lock -- deadlock

The correct way to do this (when you are doing an operation which may need a write lock, but usually doesn't) is to retry it:

Thread A takes a read lock
Thread B takes a read lock
Thread A decides it needs a write lock
Thread A drops the read lock
Thread A tries to take the write lock -- blocks because B currently holds the read lock
Thread B decides it needs a write lock
Thread B drops the read lock
Thread A gets the write lock
Thread B tries to take the write lock and blocks
Thread A finishes its work and releases the lock
Thread B takes the write lock

ReadWrite locks in Java

2 Answers2