Java Language Object Class Methods and Constructor wait() and notify() methods
Example
wait() and notify() work in tandem – when one thread calls wait() on an object, that thread will block until another thread calls notify() or notifyAll() on that same object.
(See Also: wait()/notify() )
package com.example.examples.object;
import java.util.concurrent.atomic.AtomicBoolean;
public class WaitAndNotify {
public static void main(String[] args) throws InterruptedException {
final Object obj = new Object();
AtomicBoolean aHasFinishedWaiting = new AtomicBoolean(false);
Thread threadA = new Thread("Thread A") {
public void run() {
System.out.println("A1: Could print before or after B1");
System.out.println("A2: Thread A is about to start waiting...");
try {
synchronized (obj) { // wait() must be in a synchronized block
// execution of thread A stops until obj.notify() is called
obj.wait();
}
System.out.println("A3: Thread A has finished waiting. "
+ "Guaranteed to happen after B3");
} catch (InterruptedException e) {
System.out.println("Thread A was interrupted while waiting");
} finally {
aHasFinishedWaiting.set(true);
}
}
};
Thread threadB = new Thread("Thread B") {
public void run() {
System.out.println("B1: Could print before or after A1");
System.out.println("B2: Thread B is about to wait for 10 seconds");
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(1000); // sleep for 1 second
} catch (InterruptedException e) {
System.err.println("Thread B was interrupted from waiting");
}
}
System.out.println("B3: Will ALWAYS print before A3 since "
+ "A3 can only happen after obj.notify() is called.");
while (!aHasFinishedWaiting.get()) {
synchronized (obj) {
// notify ONE thread which has called obj.wait()
obj.notify();
}
}
}
};
threadA.start();
threadB.start();
threadA.join();
threadB.join();
System.out.println("Finished!");
}
}
Some example output:
A1: Could print before or after B1
B1: Could print before or after A1
A2: Thread A is about to start waiting...
B2: Thread B is about to wait for 10 seconds
B3: Will ALWAYS print before A3 since A3 can only happen after obj.notify() is called.
A3: Thread A has finished waiting. Guaranteed to happen after B3
Finished!
B1: Could print before or after A1
B2: Thread B is about to wait for 10 seconds
A1: Could print before or after B1
A2: Thread A is about to start waiting...
B3: Will ALWAYS print before A3 since A3 can only happen after obj.notify() is called.
A3: Thread A has finished waiting. Guaranteed to happen after B3
Finished!
A1: Could print before or after B1
A2: Thread A is about to start waiting...
B1: Could print before or after A1
B2: Thread B is about to wait for 10 seconds
B3: Will ALWAYS print before A3 since A3 can only happen after obj.notify() is called.
A3: Thread A has finished waiting. Guaranteed to happen after B3
Finished!
