What is Deadlock and How to Avoid It in Java
💡 Concept: Deadlock
A deadlock in Java occurs when two or more threads wait for each other’s locks indefinitely, creating a circular dependency that halts execution.
📘 Quick Intro
Deadlocks can happen when multiple threads lock shared resources in inconsistent orders. Avoiding deadlocks is essential in multithreaded applications.
🧠 Analogy
Think of two people trying to pass through a narrow hallway, but both insist the other go first. Neither moves, and both are stuck forever. That’s a deadlock.
🔧 Technical Explanation
- Deadlock involves four conditions: mutual exclusion, hold and wait, no preemption, and circular wait.
- Occurs when two threads hold a lock and try to acquire the other’s lock simultaneously.
- Java threads use object monitors via
synchronized, which can cause deadlock if not managed carefully.
🎯 How to Avoid Deadlocks
- ✅ Always acquire locks in a consistent global order.
- ✅ Use timeout-based lock attempts with
tryLock()fromReentrantLock. - ✅ Minimize the number of synchronized blocks.
- ✅ Avoid nested locks if possible.
💻 Example: Deadlock Scenario
public class DeadlockExample {
static final Object Lock1 = new Object();
static final Object Lock2 = new Object();
public static void main(String[] args) {
Thread t1 = new Thread(() -> {
synchronized (Lock1) {
System.out.println("Thread 1: Holding Lock1...");
try { Thread.sleep(100); } catch (InterruptedException e) {}
synchronized (Lock2) {
System.out.println("Thread 1: Acquired Lock2!");
}
}
});
Thread t2 = new Thread(() -> {
synchronized (Lock2) {
System.out.println("Thread 2: Holding Lock2...");
try { Thread.sleep(100); } catch (InterruptedException e) {}
synchronized (Lock1) {
System.out.println("Thread 2: Acquired Lock1!");
}
}
});
t1.start();
t2.start();
}
}
❓ Interview Q&A
Q1: What causes a deadlock?
A: Circular waiting between threads for resources.
Q2: Can synchronized blocks cause deadlocks?
A: Yes, if not handled carefully.
Q3: How can deadlock be prevented?
A: By acquiring locks in a consistent order.
Q4: What is tryLock used for?
A: To attempt a lock with a timeout, helping avoid deadlock.
Q5: Can deadlocks be detected at runtime?
A: Yes, using tools like jConsole or thread dumps.
Q6: What is circular wait?
A: A condition where each thread waits for a resource held by another.
Q7: Can volatile prevent deadlock?
A: No, volatile deals with visibility, not synchronization.
Q8: Do atomic variables prevent deadlock?
A: No, they prevent race conditions, not deadlocks.
Q9: What’s the best way to debug deadlock?
A: Analyze thread dumps and lock ownership graphs.
Q10: What’s the difference between livelock and deadlock?
A: In livelock, threads keep changing states but make no progress; in deadlock, they are stuck forever.
📝 MCQs
Q1. Which condition is required for deadlock?
- Lock fairness
- High CPU
- Circular wait
- Garbage collection
Q2. How can you prevent deadlocks?
- Use synchronized everywhere
- Lock resources in a consistent order
- Ignore thread safety
- Use volatile variables
Q3. What tool can detect deadlocks?
- javac
- JAR signer
- jConsole
- JRE Cleaner
Q4. Does volatile prevent deadlock?
- Yes
- No
- Sometimes
- Only on Linux
Q5. Which method provides timeout locking?
- lock()
- sync()
- tryLock()
- grab()
Q6. What is deadlock?
- Memory leak
- Slow I/O
- Threads waiting on each other’s locks
- Queue starvation
Q7. What causes circular wait?
- No resource
- Each thread waiting for a resource held by another
- Static method call
- Priority boost
Q8. Is deadlock a compile-time error?
- Yes
- No
- Depends
- Only in debug mode
Q9. Can nested synchronized blocks cause deadlock?
- No
- Yes
- Not possible
- Only if synchronized is static
Q10. Does thread sleep affect deadlock?
- Yes
- No, but may increase likelihood
- Always prevents it
- Always causes it
💡 Bonus Insight
Deadlocks are easier to prevent than to fix. Always design concurrency with locking order and minimal shared resources in mind.
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