wait and notify
- Java course
- Advanced
- Lesson 59/82
- 5 min
Learn thread communication with wait and notify.
wait and notify
wait() and notify() are methods on Object; they let threads that wait on and wake up on a lock communicate. Both must be called from inside a synchronized block.
synchronized (lock) {
while (!ready) lock.wait();
}
synchronized (lock) {
ready = true;
lock.notify();
}This is the basis of patterns like producer-consumer. wait releases the lock and waits; notify wakes one waiting thread. notifyAll wakes all waiters. This is Java's counterpart of condition variables.
The producer-consumer pattern
A producer adds data to a queue and calls notify; a consumer waits with wait when the queue is empty. This is the classic producer-consumer pattern.
synchronized (queue) {
while (queue.isEmpty()) queue.wait();
item = queue.remove(0);
}The condition is checked with while; if is not enough because another thread can slip in between wake-up and condition check. This prevents spurious wakeup races. Java's BlockingQueue provides this pattern ready-made.
Practice Apply wait/notify with a Signal
Define a Signal class with a boolean sent = false field. A synchronized void send() method sets sent = true and calls notify(). A synchronized void await() method has a while (!sent) wait() loop (wrap InterruptedException in try/catch). In a SignalDemo class write main; call send(), call await() and print System.out.println("Done").
Starter code
// Define the Signal class (sent field, send and await methods)
// Write main in SignalDemo; call send and await, then print DoneSample solution
class Signal {
boolean sent = false;
synchronized void send() {
sent = true;
notify();
}
synchronized void await() {
while (!sent) {
try { wait(); } catch (InterruptedException e) {}
}
}
}
class SignalDemo {
public static void main(String[] args) {
Signal s = new Signal();
s.send();
s.await();
System.out.println("Done");
}
}Expected output
Done
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the wait call
synchronized (lock) {
while (!ready) lock.____();
}synchronized (lock) {
while (!ready) lock.wait();
}Explanation: wait() releases the lock and waits.
2 What happens to the lock when wait() is called?
- The lock is held, the thread waits
- The lock is destroyed
- The lock is released and the thread waits Correct answer
- The lock is duplicated
Explanation: wait() releases the lock and waits until notified.
3 Match wait/notify concepts
| Term | Meaning |
|---|---|
wait() |
Releases the lock and waits |
notify() |
Wakes one waiter |
notifyAll() |
Wakes all waiters |
synchronized |
Acquires the lock |