Deadlock of Threads

Deadlock of Threads

1. What Is Deadlock? (beginner — traffic jam!)

Deadlock = two threads wait for each other's lock FOREVER (A holds lock1 wants lock2, B holds lock2 wants lock1!). Both freeze! Like two doors, each holds one key!

2. Code — Classic Two-Lock Trap!

Object pen = new Object(), paper = new Object();
// Thread 1: pen → paper
new Thread(() -> {
    synchronized (pen) {
        try { Thread.sleep(100); } catch (Exception e) {}
        synchronized (paper) { System.out.println("wrote!"); }
    }
}).start();
// Thread 2: paper → pen (OPPOSITE order — deadlock!)
new Thread(() -> {
    synchronized (paper) {
        try { Thread.sleep(100); } catch (Exception e) {}
        synchronized (pen) { System.out.println("drew!"); }
    }
}).start();
// May freeze! (neither prints — both wait forever!)

3. Four Conditions (exam list — all needed!)

Condition	Meaning
Mutual exclusion	lock owned by one!
Hold + wait	holds one, wants another!
No preemption	can't snatch lock!
Circular wait	A→B→A cycle!

Break ANY one → no deadlock!

4. Fixes (exam!)

Same lock ORDER everywhere (both pen→paper!). Timeout (tryLock(1, SECONDS)). Fewer locks (one big lock!).

5. Exam Points

• Two-lock program + four conditions — 7 marks, very frequent.
• Fix (same order) — 3 marks, frequent.