Understanding the Meaning of "Locked" in Java Stack Traces

What does the term "locked" mean in a Java stack trace?

In Java, the term "locked" in a stack trace typically indicates that a thread is blocked because it is waiting to acquire a lock on an object that is currently held by another thread. This situation commonly arises in multi-threading scenarios where two or more threads interact with shared resources.

// Example of synchronized method in Java
class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

Causes

• A thread attempts to access a synchronized block or method, but another thread is holding the lock on the same object.
• Deadlocks occur when two or more threads are each waiting for the other to release locks, resulting in a standstill.
• Long-running operations within synchronized blocks can prevent other threads from acquiring the lock, leading to increased wait times.

Solutions

• Optimize the design of your application to minimize the need for synchronized methods, possibly using alternative concurrency utilities such as `java.util.concurrent` package.
• Utilize timeouts when attempting to acquire locks, allowing threads to handle lock acquisition failures gracefully.
• Analyze the code for potential deadlocks by reviewing lock acquisition order and ensuring that they are consistent across threads.

Mistake: Not keeping lock acquisition order consistent across threads, leading to deadlocks.

Solution: Establish a strict ordering for lock acquisitions and make sure all threads follow it.

Mistake: Using synchronized blocks excessively, which can lead to performance bottlenecks.

Solution: Refactor the code to minimize synchronized usage or use concurrent data structures.