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Thread.sleep() in Java - Java Thread sleep

Published on August 3, 2022 · Updated on November 22, 2022
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By Pankaj
Developer and author at DigitalOcean.
Thread.sleep() in Java - Java Thread sleep

Introduction

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

sleep(long millis, int nanos) is another method that can be used to pause the execution of the current thread for a specified number of milliseconds and nanoseconds. The allowed nanosecond values are between 0 and 999999.

In this article, you will learn about Java’s Thread.sleep().

How Thread.sleep Works

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

Java Thread.sleep important points

  1. It always pauses the current thread execution.
  2. The actual time the thread sleeps before waking up and starting 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 a little bit longer.
  3. Thread.sleep() doesn’t lose any monitors or lock the current thread it has acquired.
  4. Any other thread can interrupt the current thread in sleep, and in such cases InterruptedException is thrown.

Java Thread.sleep Example

Here is an example program where Thread.sleep() is used to pause the main thread execution for 2 seconds (2000 milliseconds):

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));
    }
}

First, this code stores the current system time in milliseconds. Then it sleeps for 2000 milliseconds. Finally, this code prints out the new current system time minus the previous current system time:

Output
Sleep time in ms = 2005

Notice that this difference is not precisely 2000 milliseconds. This is due to how Thread.sleep() works and the operating system-specific implementation of the thread scheduler.

Conclusion

In this article, you learned about Java’s Thread.sleep().

Continue your learning with more Java tutorials.

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About the authors
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Pankaj

author

Developer and author at DigitalOcean.

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Developer and author at DigitalOcean.

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Was this helpful?

Sir can you please explain this topic { waiting of child thread until completing main thread}

- Pradeep Singh

    what is difference between t1.run() and t1.start()?

    - Shweta

      how the execution of two threads goes when sleep is not there? how does thread scheduler perform in this case…?

      - Swapna

        how do we come to know that, how much time we have to provide to the threads for sleep or wait. If we are working on big projects.

        - Ishant

          Suppose if a thread is kept in sleep and after completing sleep mode time the processor is running another thread. then will the current thread stops and executes thread that completed sleep mode or thread that is in sleep mode executed after current running process is terminated please explain it in detail I am new to java

          - Pavan

            long start = System.currentTimeMillis(); what does this line have in the code?

            - FREDY ORLANDO MARCELO CASTIBLANCO

              Thread One = new Thread( ()-> {}); What is the difference between calling One.sleep() and Thread.sleep();

              - James

                Hi Pankaj, I modified your code like below, and I can see same sleep time after every execution – long sleepTime = 999; System.out.println("Going to sleep for "+sleepTime); long start = 0L; try { start = System.currentTimeMillis(); Thread.sleep(sleepTime); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Sleep time in ms = "+(System.currentTimeMillis()-start)); I guess, the difference is coming due to the startTime capture statement execution and the actual sleep statement, if you keep them one after the other, you won’t see the difference anymore. I understand that your explanation is correct too, because all thread execution depends on how OS allow them. Thanks

                - Punit

                  Hi Pankaj, Your blog is very good and more informative. But its very good if you provide link to export to pdf. Thanks Maruthi

                  - maruthi

                    class TestCallRun extends Thread{ public void run(){ for(int i=1;i<5;i++){ try{Thread.sleep(5000);}catch(InterruptedException e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]){ TestCallRun2 t1=new TestCallRun2(); TestCallRun2 t2=new TestCallRun2(); t1.start(); t2.run(); } } why the output of above program is same as when we call t1.start(); t2.start(). 1 1 2 2 3 3 4 4 but output is different when we call t1.run();t2.run(); 1 2 3 4 1 2 3 4 according to my understating output of t1.start(); t2.run() should be 1 --t1 thread 1 --t2 thread 2 --t2 thread 3 --t2 thread 4 --t2 thread 2 --t1 thread 3 --t1 thread 4 --t1 thread

                    - Nitin