Java Shutdown Hook

A special construct in Java which allows the developers to do a plugin of a piece of code which will be executed during the shutting down of java virtual machine and this is very necessary when we have to perform operations of clean up during the shutting down of java virtual machine and the virtual machine shutting down due to reasons such as the killing of the request by the operating system or the problem of resources cannot be solved using general constructs but can be solved by using an arbitrary code block provided by the shutdown hook and it is an extension of java.lang.Thread class and necessary logic is provided inside a public void run() method during the shutting down of java virtual machine.

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Working of Shutdown Hook in Java

• There are two different ways in which a java virtual machine can shut down. They are:
  • A process that is controlled.
  • Abruptly.
• A process that is controlled can cause the java virtual machine to shut down when any of the following takes place:
  • Termination of the last non-daemon thread. For example, the java virtual machine starts shutting down with the exit of the main thread.
  • An interrupt signal is sent from the operating system. For example, logging off from the operating system or by pressing CTRL + C.
  • exit() is called from java code.
• The java virtual machine can start shutting down abruptly when any of the following takes place:
  • If a kill signal is sent from the operating system.
  • getRuntime().halt() is called from java code.
  • The unexpected death of the host operating system. For example, a case of failure of power.
• The java virtual machine allows the registered functions to be run before the completion of shutting down the system. The resources can be released or certain housekeeping activities can be performed using these functions. These functions are called shutdown hooks in java virtual machine terminology.
• These shutdown hooks are all initiated but these threads are unstarted. All the registered hooks are processed in an order unspecified when the java virtual machine begins the process of shutting down. The java virtual machine will stop after processing all the hooks.
• Consider the below program to demonstrate the use of shutdown hooks in Java:

Code:

//a class called mythr is defined and it extends a thread
class MyThr extends Thread
{
public void run()
{
System.out.println("The task of shut down hook is completed");
}
}
<em>//a class called shutdown is defined</em>
public class Shutdown
{
<em>//main method is called</em>
public static void main(String[] args)throws Exception
{
<em>//an instance of java runtime class is created</em>
Runtime roll=Runtime.getRuntime();
<em>//shutdown hook method is called using the instance of runtime class and the instance of mythr class is created</em>
roll.addShutdownHook(new MyThr());
System.out.println("The main starts to sleep. Press control and c to exit");
try
{
Thread.sleep(3000);
}
catch (Exception e)
{
}
}
}

The output of the above program is as shown in the snapshot below:

In the above program, a class called mthr is defined and it extends a thread. Then a class called shutdown is defined. Then the main method is called. Then an instance of java runtime class is created. Then the shutdown hook method is called using the instance of runtime class and the instance of mythr class is created. The method shutdown hook causes the print statement in the shutdown class to be print printed followed by the statement in the run method of mythr class. The output of the program is as shown in the snapshot above.

Examples of Java Shutdown Hook

Here are the following examples mention below:

Example #1

Java program to demonstrate the use of shutdown hook in a program:

Code:

//a class called shutdown is defined
public class ShutDown
{
//main method is called
public static void main(String[] args)
{
//an instance of java runtime class is created and a new thread constructor is passed as a parameter to shutdown hook method
Runtime.getRuntime().addShutdownHook(new Thread()
{
public void run()
{
System.out.println("the shut down hook is in progress !");
}
});
System.out.println("Termination of application ...");
}
}

The output of the above program is as shown in the snapshot below:

In the above program, a class called to shut down is defined. Then the main method is called. Then an instance of java runtime class is created and a new thread constructor is passed as a parameter to shutdown hook method. The output of the program is shown in the snapshot above.

Example #2

Java program to demonstrate the use of shutdown hook in a program:

Code:

//a class called demo is defined
public class Demo
{
// a class called mess is defined and it extends a thread and this function is called when the program is exiting
static class Mess extends Thread
{
public void run()
{
System.out.println("Good Bye.");
}
}
//main method is called
public static void main(String[] args)
{
try
{
//shutdown hook method is called to which the instance of the class mess is passed as a parameter
Runtime.getRuntime().addShutdownHook(new Mess());
// the beginning of the program is printed
System.out.println("Beginning of the program...");
// the wait time for the thread is printed
System.out.println("The wait time for the thread is three seconds...");
Thread.sleep(3000);
//Ending of the program is printed
System.out.println("Ending of the program...");
}
catch (Exception e)
{
e.printStackTrace();
}
}
}

The output of the above program is as shown in the snapshot below:

In the above program, a class mess is defined, and it extends a thread and this function is called when the program is exiting. Then the main method is called. Then shutdown hook method is called to which the instance of the class mess is passed as a parameter. Then the beginning of the program is printed. Then the wait time for the thread is printed. Then the Ending of the program is printed. The output of the program is as shown in the snapshot above.

Advantages of Java Shutdown Hook

There are several advantages of Java shutdown hook. They are:

• The important features of java virtual machine are shutdown hooks because they provide the capacity to do the clean up of resources or store the state of the application when the java virtual machine is shutting down.
• The shutdown hooks can be executed at runtime when the java virtual machine is shutting down normally or abruptly.
• More than one shutdown hooks can be registered at any point in time during the running of java virtual machine through a runtime application programming interface.

Conclusion

In this tutorial, we understand the concept of shutdown hooks in Java through definition, working of shutdown hooks in java through examples and their outputs.

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