Choosing the object-to-lock in explicit locks (ReentrantLock example)

https://coderanch.com/t/665262/java/Choosing-object-lock-explicit-locks

So, I just finished studying synchronized keyword (Blocks and methods). And here is what i basically know.

A synchronized keyword guarantees atomicity and visibility and it has to operate on two things.
1- A thread. (The calling thread)
2- An object. (The monitor object)

When using

synchronized (object) {
    //do stuff
}

The object "object" will be locked on the calling thread only. until the lock is released.

So, When i started attempting to study explicit locks, I came across this example
[Source] Java 8 Concurrency Tutorial: Synchronization and Locks - Part 2: Synchronization and Locks
https://winterbe.com/posts/2015/04/30/java8-concurrency-tutorial-synchronized-locks-examples/

ReentrantLock

The class ReentrantLock is a mutual exclusion lock with the same basic behavior as the implicit monitors accessed via the synchronized keyword but with extended capabilities. As the name suggests this lock implements reentrant characteristics just as implicit monitors.
Let's see how the above sample looks like using ReentrantLock

ReentrantLock lock = new ReentrantLock();
int count = 0;
 
void increment() {
    lock.lock();
    try {
        count++;
    } finally {
        lock.unlock();
    }
}

Which brings me to my questions:

1- Is what's previously mentioned correct and generally accurate?
2- When using explicit locks (in the above ReentrantLock example), How do i get to choose the object that i want locked? Assuming that the calling thread is the thread that will have the lock.

You decide the policy for how and which objects to lock.

In general, you create a lock for each set of operations that should not be allowed to run concurrently.

Let's say you have an object that has two completely unrelated fields:

final class MyObject {
 
  private final Thing a = Thing.something();
  private final Thing b = Thing.something();
 
  private final Lock aLock = new ReentrantLock();
  private final Lock bLock = new ReentrantLock();
 
  int readFromA() {
    aLock.lock();
    try { return a.read(); }
    finally { aLock.unlock(); }
  }
 
  void writeToA(int val) {
    aLock.lock();
    try { a.write(val); }
    finally { aLock.unlock(); }
  }
 
  int readFromB() {
    bLock.lock();
    try { return b.read(); }
    finally { bLock.unlock(); }
  }
 
  void writeB(int val) {
    bLock.lock();
    try { b.write(val); }
    finally { bLock.unlock(); }
  }
}

Since the two fields are unrelated and the validity of the object doesn't depend on certain combinations of a and b, one thread should be allowed to interact with a while another interacts with b. To facilitate this, you need two different locks.

A big advantage of using explicit locks over implicit ones, is that a malicious class can't lock an object indefinitely while it goes off and does other things. For instance, if readFromA() used synchronized(this), any class that has access to an instance of MyObject can use a synchronized block to lock the instance, and then keep it locked forever. With explicit locks this doesn't happen, as long as you keep locks private.

Question. What "atomicity and visibility" guarantees are you referring to? Method calls? Access to instance variables?

Simply, there are no method or variable guarantees, with owning a synchronization lock on an object. The only guarantee is that only one thread can own the lock (on the object) at a time. Synchronization locks are cooperative. It is the developer's responsibility to take the "only one thread can own the lock" guarantee, and make that into visibility and atomicity guarantees.

Henry

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