Why Does Java Need Lock When Synchronized Exists? Exploring Deadlock Solutions

Characteristics of synchronized's lock acquisition

When a thread attempts to acquire a synchronized lock and fails, it enters a blocked state, cannot perform any work, cannot release resources it already holds, and cannot be interrupted; it must wait until another thread releases the lock.

Problems caused by synchronized blocking

The blocking behavior can lead to deadlocks, where two or more threads wait indefinitely for each other’s resources.

A deadlock occurs when two or more threads are each waiting for a lock held by another, creating a circular wait.

Example: Thread 1 locks resource A, Thread 2 locks resource B. Later Thread 1 tries to lock B and blocks, while Thread 2 tries to lock A and also blocks, resulting in a deadlock that cannot be broken because synchronized cannot be interrupted.

How Lock solves the problem

Lock provides APIs that allow a thread to give up acquiring a lock without blocking, or to be interruptible, thus avoiding deadlock.

For instance, if a thread gives up after a timeout, the other thread can acquire the needed lock and progress.

Lock API methods

void lockInterruptibly() throws InterruptedException;

This method acquires the lock interruptibly; if the thread is interrupted while waiting, it aborts and throws InterruptedException, allowing the program to continue. boolean tryLock(); Attempts to acquire the lock; if it fails, it returns false immediately without blocking.

boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

Tries to acquire the lock, waiting up to the specified time; if the lock is still unavailable, it returns false and can be interrupted.

Lock implementations are built on Java’s AbstractQueuedSynchronizer (AQS), which maintains a state field and a double‑linked queue of waiting threads. ReentrantLock is a common Lock implementation based on AQS.

Summary

Because synchronized blocks and cannot be interrupted, it can cause deadlocks. The Lock interface offers interruptible and non‑blocking acquisition methods, providing flexible ways to avoid such deadlocks.