Java Interview Question: What is Deadlock and How to Create It in Java (With Example & Prevention)

 

🔹 Introduction

Deadlock is one of the most critical concepts in Java multithreading and is frequently asked in interviews.

👉 Interviewers often ask:
“What is deadlock? Can you create one in Java?”

In this article, we will:

• Understand deadlock
• Create a deadlock
• Learn how to prevent it

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🔹 What is Deadlock?

A deadlock occurs when two or more threads are waiting for each other indefinitely, and none of them can proceed.

👉 Simple idea:

• Thread 1 holds Resource A and waits for B
• Thread 2 holds Resource B and waits for A

💥 Both get stuck forever

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🖼️ Visualization (Understand Easily)

Thread 1 → holds Lock A → waiting for Lock B  
Thread 2 → holds Lock B → waiting for Lock A  

Result: DEADLOCK

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🔹 Java Code to Create Deadlock

public class DeadlockExample {

    private static final Object lock1 = new Object();
    private static final Object lock2 = new Object();

    public static void main(String[] args) {

        Thread t1 = new Thread(() -> {
            synchronized (lock1) {
                System.out.println("Thread 1 acquired lock1");

                try { Thread.sleep(100); } catch (Exception e) {}

                synchronized (lock2) {
                    System.out.println("Thread 1 acquired lock2");
                }
            }
        });

        Thread t2 = new Thread(() -> {
            synchronized (lock2) {
                System.out.println("Thread 2 acquired lock2");

                try { Thread.sleep(100); } catch (Exception e) {}

                synchronized (lock1) {
                    System.out.println("Thread 2 acquired lock1");
                }
            }
        });

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

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🔍 Why Deadlock Happens

Deadlock occurs due to 4 conditions:

1. Mutual Exclusion
2. Hold and Wait
3. No Preemption
4. Circular Wait

👉 All must be true

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🔹 How to Prevent Deadlock

✅ 1. Lock Ordering

Always acquire locks in same order

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✅ 2. Use Try Lock

Use timeout-based locking

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✅ 3. Avoid Nested Locks

Reduce dependency between resources

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✅ 4. Use Higher-Level APIs

Use:

• ExecutorService
• Concurrency utilities

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🔹 Time Complexity

• Not applicable (thread blocking issue)

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🔹 Key Takeaways

✔ Deadlock = threads waiting forever
✔ Happens due to circular dependency
✔ Can be prevented using lock strategies
✔ Very important interview question

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🔹 Conclusion

Deadlock is a critical concept in multithreading. Understanding how it occurs and how to prevent it is essential for backend developers.

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