Thread.sleep in Java

 Thread.sleep() method can be used to pause the execution of current thread for specified time in milliseconds. The argument value for milliseconds can’t be negative, else it throws IllegalArgumentException.

There is another overloaded method sleep(long millis, int nanos) that can be used to pause the execution of current thread for specified milliseconds and nanoseconds. The allowed nano second value is between 0 and 999999.

Java Thread Sleep Example

Here is a simple program where Thread.sleep() is used to pause the main thread execution for 2 seconds.

ThreadSleep.java

package com.journaldev.threads;

public class ThreadSleep {

    public static void main(String[] args) throws InterruptedException {
        long start = System.currentTimeMillis();
        Thread.sleep(2000);
        System.out.println("Sleep time in ms = "+(System.currentTimeMillis()-start));
        
    }

}

If you will run the above program, you will notice that the thread sleep time it prints is slightly greater than 2000. This is caused by how thread sleep works and operating system specific implementation of thread scheduler.

Java Thread Sleep important points

1. It always pause the current thread execution.
2. The actual time thread sleeps before waking up and start execution depends on system timers and schedulers. For a quiet system, the actual time for sleep is near to the specified sleep time but for a busy system it will be little bit more.
3. Thread sleep doesn’t lose any monitors or locks current thread has acquired.
4. Any other thread can interrupt the current thread in sleep, in that case InterruptedException is thrown.

How Thread Sleep Works

Thread.sleep() interacts with the thread scheduler to put the current thread in wait state for specified period of time. Once the wait time is over, thread state is changed to runnable state and wait for the CPU for further execution. So the actual time that current thread sleep depends on the thread scheduler that is part of operating system.