If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.
In Java, when a thread attempts to acquire a lock on an object using the synchronized keyword or by using the Lock interface, and it cannot acquire the lock because another thread already holds it, the thread will be blocked. The thread will enter a state where it is waiting for the lock to be released by the thread currently holding it.
There are two scenarios when a thread cannot acquire a lock:
- Lock is held by another thread: If another thread already holds the lock, the requesting thread will wait until the lock is released. Once the lock is released, the waiting thread will be given an opportunity to acquire it.
- Timeout while attempting to acquire the lock: If a thread specifies a timeout when attempting to acquire a lock, and the lock is not available within the specified time, the thread may choose to do something else instead of waiting indefinitely. In this case, the attempt to acquire the lock will return false or throw an exception, depending on the specific mechanism used.
It’s important to handle synchronization carefully to avoid potential deadlocks or other issues related to thread contention. Deadlocks can occur when two or more threads are blocked forever, each waiting for the other to release a lock. To prevent deadlocks, it’s essential to acquire locks in a consistent order and release them in the reverse order. Additionally, using tools like java.util.concurrent package can help manage concurrency more effectively.