Inter-thread Comminication

Inter-thread communication allows threads to pause, wait and resume in coordination with each other - without busy waiting.

Methods

Method	Description
wait()	Releases lock and waits until notified
notify()	Wakes up one waiting thread
notifyAll()	Wakes up all waiting thread
These must be called within a synchronized block or method.

Example

class Shared {
    void waitForSignal() throws InterruptedException {
        synchronized (this) {
            System.out.println("Waiting...");
            wait(); // thread goes into WAITING state
            System.out.println("Notified!");
        }
    }
 
    void sendSignal() {
        synchronized (this) {
            notify(); // wakes up one waiting thread
        }
    }
}

Flow

1. Thread A enters synchronized block and calls wait()
   → Releases the lock and goes to WAITING
2. Thread B enters the same synchronized block and calls notify()
   → Wakes Thread A
3. Thread A resumes execution after regaining the lock

Rules

• Must hold the monitor lock (i.e. synchronized) before calling wait() or notify().
• Throws IllegalMonitorStateException if called outside sync.
• Always call wait() in a loop with a condition check (to avoid spurious wakeups).

while (!condition) {
    wait();
}

Real-World Analogy

Think of wait() as someone waiting in a line.
notify() is the barista calling “Next!” - only one person steps forward.

Tip

Be ready to:

• write a simple Producer-Consumer example,
• explain wait() releases the lock; sleep() does not,
• know when to use notify() vs notifyAll().

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Parent: _Multithreading