What is Deadlock in Java?

A deadlock in Java is a situation where two or more threads are permanently blocked, waiting for each other to release resources. Because each thread holds a lock that another thread needs, none of them can continue execution, and the program gets stuck.

Deadlocks are one of the most common problems in multithreaded applications.

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🔹 How Deadlock Occurs?

Deadlock happens when:

1. Thread-1 locks Resource-A and waits for Resource-B.

2. Thread-2 locks Resource-B and waits for Resource-A.

3. Both threads keep waiting forever.

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🔹 Deadlock Example

class Test {
    public static void main(String[] args) {

        final String resource1 = "Java";
        final String resource2 = "Thread";

        Thread t1 = new Thread(() -> {
            synchronized(resource1) {
                System.out.println("Thread 1 locked resource1");

                synchronized(resource2) {
                    System.out.println("Thread 1 locked resource2");
                }
            }
        });

        Thread t2 = new Thread(() -> {
            synchronized(resource2) {
                System.out.println("Thread 2 locked resource2");

                synchronized(resource1) {
                    System.out.println("Thread 2 locked resource1");
                }
            }
        });

        t1.start();
        t2.start();
    }
}

In this example, both threads wait for each other’s lock, causing a deadlock.

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🔹 Conditions Required for Deadlock

Deadlock occurs when these four conditions exist:

1️⃣ Mutual Exclusion – Resource is used by one thread at a time
2️⃣ Hold and Wait – Thread holds one resource while waiting for another
3️⃣ No Preemption – Resource cannot be forcefully taken away
4️⃣ Circular Wait – Threads wait in a circular chain

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🔹 How to Prevent Deadlock?

✅ Acquire locks in the same order
✅ Avoid nested synchronization when possible
✅ Use timeout locks (tryLock() in advanced concurrency)
✅ Keep synchronized blocks small

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✅ Conclusion

Deadlock is a critical issue in Java multithreading where threads stop progressing due to circular waiting. Understanding synchronization and proper lock management helps developers build stable and high-performance applications.

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