How to Use wait and notify in Java
In Java, the wait() and notify() methods are part of the Object class and are used to implement thread synchronization. These methods allow one thread to wait for a condition to become true while another thread can notify it when the condition is met. This is particularly useful in scenarios where multiple threads need to coordinate their actions based on certain conditions. In this article, we will discuss how to use wait and notify in Java, including their syntax, usage, and best practices.
Understanding wait() and notify()
The wait() method is used to make a thread wait until it is explicitly notified by another thread. When a thread calls the wait() method, it releases the monitor lock on the object it is waiting on and enters the waiting state. The thread will remain in the waiting state until it is explicitly notified by another thread using the notify() or notifyAll() method.
The notify() method, on the other hand, is used to wake up a single thread that is waiting on the same object. This method should be called by a thread that has already acquired the monitor lock on the object. It is important to note that notify() does not release the monitor lock; it only wakes up a single waiting thread.
The syntax for wait() and notify() methods is as follows:
“`java
public final void wait() throws InterruptedException
public final void wait(long timeout) throws InterruptedException
public final void notify()
public final void notifyAll()
“`
The first version of wait() does not take any arguments and will wait indefinitely until it is notified. The second version of wait() takes a timeout value in milliseconds, and the thread will wait for that duration before giving up and throwing an InterruptedException.
The notify() method, as mentioned earlier, wakes up a single waiting thread, while notifyAll() wakes up all waiting threads.
Using wait and notify in Java
To use wait and notify in Java, you need to follow these steps:
1. Acquire the monitor lock on the object on which you want to use wait() and notify().
2. Call the wait() method on the object to make the current thread wait.
3. In another thread, when the condition becomes true, call the notify() or notifyAll() method on the same object to wake up the waiting thread.
4. Optionally, call the notifyAll() method to wake up all waiting threads if you want to proceed with the execution of all threads simultaneously.
Here is an example of using wait and notify in Java:
“`java
public class WaitNotifyExample {
private int count = 0;
public static void main(String[] args) {
WaitNotifyExample example = new WaitNotifyExample();
Thread t1 = new Thread(example::incrementCount);
Thread t2 = new Thread(example::decrementCount);
t1.start();
t2.start();
}
public synchronized void incrementCount() {
while (count < 10) {
try {
System.out.println("Thread 1: Waiting...");
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread 1: Count incremented to " + (count + 1));
count++;
notifyAll();
}
}
public synchronized void decrementCount() {
while (count > 0) {
try {
System.out.println(“Thread 2: Waiting…”);
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(“Thread 2: Count decremented to ” + (count – 1));
count–;
notifyAll();
}
}
}
“`
In this example, we have two threads, t1 and t2, that increment and decrement the count, respectively. The threads use wait and notify to coordinate their actions based on the value of the count.
Best Practices and Considerations
When using wait and notify in Java, it is important to follow certain best practices and considerations:
1. Always use the synchronized keyword when using wait and notify to ensure that only one thread can access the shared resource at a time.
2. Avoid calling wait() and notify() from within a loop, as it may cause the thread to wait indefinitely. Instead, use a flag to control the flow of the program.
3. Be cautious when using wait and notify in a multi-threaded environment, as it can lead to deadlocks if not implemented correctly. Always ensure that the lock is acquired before calling wait() and released after calling notify() or notifyAll().
4. Use the InterruptedException to handle the case when a thread is interrupted while waiting.
By following these best practices and considerations, you can effectively use wait and notify in Java to synchronize threads and coordinate their actions based on certain conditions.
