Inter-Thread Communication: wait(), notify(), and notifyAll()

Advanced16 min read•Updated: 2025-01-15

📣 Inter-Thread Communication: wait(), notify(), and notifyAll()

Introduction

Rather than having threads poll a condition in a tight busy-waiting loop (wasting 100% of CPU cycles), Java provides built-in inter-thread signaling through methods defined in java.lang.Object: wait(), notify(), and notifyAll().

📜 Rules of Engagement

1
Must Hold the Monitor: wait(), notify(), and notifyAll() can only be called from within a synchronized context on the object whose monitor is being signaled.
2
Releasing the Lock: When a thread calls wait(), it atomically releases the lock and enters the object's wait set.
3
Waking Up: Calling notify() wakes up one arbitrary waiting thread; notifyAll() wakes up all waiting threads.

🏭 Classic Producer-Consumer Architecture

1package com.mskinstitute.concurrency;
2 
3import java.util.LinkedList;
4import java.util.Queue;
5 
6public class BoundedMessageBuffer<T> {
7    private final Queue<T> queue = new LinkedList<>();
8    private final int capacity;
9 
10    public BoundedMessageBuffer(int capacity) {
11        this.capacity = capacity;
12    }
13 
14    public synchronized void produce(T item) throws InterruptedException {
15        // ALWAYS check condition in a while loop to handle spurious wakeups!
16        while (queue.size() == capacity) {
17            wait(); // Buffer is full; release lock and sleep
18        }
19        queue.add(item);
20        System.out.println("Produced item. Queue size: " + queue.size());
21        notifyAll(); // Notify waiting consumers that data is ready
22    }
23 
24    public synchronized T consume() throws InterruptedException {
25        while (queue.isEmpty()) {
26            wait(); // Buffer is empty; release lock and sleep
27        }
28        T item = queue.poll();
29        System.out.println("Consumed item. Queue size: " + queue.size());
30        notifyAll(); // Notify waiting producers that space is available
31        return item;
32    }
33}

⚠️ The Spurious Wakeup Hazard

A waiting thread can sometimes wake up without being explicitly notified (due to OS-level signaling quirks). For this reason:

1// ❌ WRONG: Can cause array index out of bounds or deadlocks
2if (condition) { wait(); }
3 
4// ✅ CORRECT: Always guard wait() inside a while loop!
5while (condition) { wait(); }

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java.util.concurrent: ReentrantLock, ReadWriteLock, and Condition

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Thread Synchronization, Critical Sections, and synchronized Methods/Blocks	java.util.concurrent: ReentrantLock, ReadWriteLock, and Condition

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java.util.concurrent: ReentrantLock, ReadWriteLock, and Condition

Inter-Thread Communication: wait(), notify(), and notifyAll()

Advanced16 min read•Updated: 2025-01-15

Study Materials

📣 Inter-Thread Communication: wait(), notify(), and notifyAll()

Introduction

📜 Rules of Engagement

1
Must Hold the Monitor: wait(), notify(), and notifyAll() can only be called from within a synchronized context on the object whose monitor is being signaled.
2
Releasing the Lock: When a thread calls wait(), it atomically releases the lock and enters the object's wait set.
3
Waking Up: Calling notify() wakes up one arbitrary waiting thread; notifyAll() wakes up all waiting threads.

🏭 Classic Producer-Consumer Architecture

1package com.mskinstitute.concurrency;
2 
3import java.util.LinkedList;
4import java.util.Queue;
5 
6public class BoundedMessageBuffer<T> {
7    private final Queue<T> queue = new LinkedList<>();
8    private final int capacity;
9 
10    public BoundedMessageBuffer(int capacity) {
11        this.capacity = capacity;
12    }
13 
14    public synchronized void produce(T item) throws InterruptedException {
15        // ALWAYS check condition in a while loop to handle spurious wakeups!
16        while (queue.size() == capacity) {
17            wait(); // Buffer is full; release lock and sleep
18        }
19        queue.add(item);
20        System.out.println("Produced item. Queue size: " + queue.size());
21        notifyAll(); // Notify waiting consumers that data is ready
22    }
23 
24    public synchronized T consume() throws InterruptedException {
25        while (queue.isEmpty()) {
26            wait(); // Buffer is empty; release lock and sleep
27        }
28        T item = queue.poll();
29        System.out.println("Consumed item. Queue size: " + queue.size());
30        notifyAll(); // Notify waiting producers that space is available
31        return item;
32    }
33}

⚠️ The Spurious Wakeup Hazard

A waiting thread can sometimes wake up without being explicitly notified (due to OS-level signaling quirks). For this reason:

1// ❌ WRONG: Can cause array index out of bounds or deadlocks
2if (condition) { wait(); }
3 
4// ✅ CORRECT: Always guard wait() inside a while loop!
5while (condition) { wait(); }

Next Lesson

java.util.concurrent: ReentrantLock, ReadWriteLock, and Condition

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Thread Synchronization, Critical Sections, and synchronized Methods/Blocks	java.util.concurrent: ReentrantLock, ReadWriteLock, and Condition

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next