What's the Difference Between `wait()` and `sleep()` in Java Thread Control?

Understanding the Distinction Between "wait()" and "sleep()" in Java Threads

In Java, "wait()" and "sleep()" are two key methods used for controlling thread execution. While both serve the purpose of pausing thread execution, they differ significantly in their operation and scope.

wait() vs sleep()

• wait(): This method is invoked on an object monitor and causes the current thread to enter a waiting state. It releases the lock on the associated object, making the object available to other threads. When another thread calls "notify()" or "notifyAll()" on the same object, the waiting thread is notified and resumes execution.
• sleep(): In contrast, "sleep()" is invoked on a thread itself. It causes the thread to enter a sleeping state and does not release any locks. The sleeping thread remains unresponsive to other threads. After the specified duration has elapsed, the thread resumes execution automatically.

CPU Utilization

Contrary to popular belief, both "wait()" and "sleep()" can consume CPU cycles. While the thread waiting on "wait()" sleeps, it still retains a hold on the monitor lock. This means that other threads must constantly compete for access to the object, consuming CPU resources. On the other hand, "sleep()" releases the lock on the thread, allowing other threads to execute without competition.

Rationale for Both Methods

The reason for having both "wait()" and "sleep()" in Java stems from their distinct roles and suitability for different situations.

• Wait(): This method is intended for situations where threads need to communicate and synchronize their execution based on conditions or events. It allows threads to wait for specific events to occur before continuing, enabling coordination and collaboration.
• Sleep(): In contrast, "sleep()" is typically used for introducing delays or pauses in thread execution. It does not involve inter-thread communication or locking and is suitable for scenarios where a thread needs to delay for a predefined period before continuing.

Low-Level Implementation

At a lower level, "wait()" and "sleep()" are implemented using different mechanisms.

• Wait(): This method delegates its implementation to the underlying platform. It typically blocks the thread's execution in the kernel, causing it to relinquish its CPU share.
• Sleep(): In contrast, "sleep()" uses a non-blocking implementation. It sets a timeout for the thread and periodically checks if the timeout has expired. If not, the thread continues its execution.

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