Shutdown Hook in Java

• A thread is authorized to retrieve information pertaining to its own thread group, while it is prohibited from accessing data associated with other thread groups or its parent thread group.






• On the surface, using a shutdown hook appears to be very simple.The process involves creating a class that extends the java.lang.Thread class and implementing the desired logic within the public void run() method, which will be executed when the virtual machine (VM) shuts down. To register this class as a shutdown hook, we simply need to call the addShutdownHook(Thread) method of the Runtime class obtained through the getRuntime() method. If there is a need to unregister a previously registered shutdown hook, the Runtime class provides the removeShutdownHook(Thread) method.

addShutdownHook(Thread hook) method

• The Runtime class's addShutdownHook() method is used to register the thread with the Virtual Machine.

Syntax :

public void addShutdownHook(Thread hook){}  

removeShutdownHook(Thread hook) method

• To remove the registration of previously registered shutdown hooks, the Runtime class's removeShutdownHook() method is called.

Syntax :

public boolean removeShutdownHook(Thread hook){ }    

class shutDownDemo extends Thread{
	public void run(){    
        System.out.println("Shutdown Hook is running");    
    }    
}
public class Main{
    public static void main(String arg[]) {
	 Runtime runtime=Runtime.getRuntime();    
	 runtime.addShutdownHook(new shutDownDemo());            
	 System.out.println("Press Ctrl+c to Terminate");    
	 try{
		Thread.sleep(3000);
	 }catch (Exception e) {
		e.printStackTrace();
	 }    
   }
}

Press Ctrl+c to Terminate
Shutdown Hook is running 

Shutdown Hook by anonymous class

public class Main{
    public static void main(String arg[]) {
	 Runtime runtime=Runtime.getRuntime();    
	 runtime.addShutdownHook(new Thread(){  
	 public void run(){    
        System.out.println("Shutdown Hook is running");    
     } 
	 });
	 System.out.println("Press Ctrl+c to Terminate");    
	 try{
		Thread.sleep(3000);
	 }catch (Exception e) {
		e.printStackTrace();
	 }    
   }
}

Press Ctrl+c to Terminate
Shutdown Hook is running 

Important point of Shutdown Hook

• There is no guarantee that the shutdown hooks will be executed: The first and most important thing to remember is that the shutdown hook's execution is not guaranteed. In some cases, the shutdown hooks will never be executed. For example, if the JVM crashes due to an internal error, the shutdown hooks are rendered ineffective. When the operating system sends the SYSKILL signal, the shutdown hooks are also prevented from being used.
• It should be noted that when the application is normally terminated, the shutdown hooks are called (all threads of the application is finished or terminated). Additionally, the shutdown hooks are invoked when the operating system is shut down or the user presses ctrl + c.




• The shutdown hooks can be forced to stop before completion It is a subset of the previously discussed point. When a shutdown hook begins to execute, it can be forced to stop by shutting down the system. In this case, the operating system for a predetermined period of time. If the job is not completed within that time frame, the system is forced to terminate the running hooks.
• Multiple shutdown hooks can be defined, but the order in which they are executed by the JVM is not predetermined. The execution of shutdown hooks may occur in any sequence, potentially running concurrently.
• It is important to note that unregistering or registering shutdown hooks within other shutdown hooks is not allowed. When the JVM starts the shutdown process, it does not permit the removal or addition of any existing shutdown hooks. Any attempt to do unregistering or registering shutdown hooks will result in an IllegalStateException being thrown by the JVM.
• The shutdown sequence can be terminated using Runtime.halt(): Only the Runtime.halt() method, which forcefully terminates the JVM, can halt the shutdown sequence, which also means that invoking the System.exit() method within a shutdown hook will fail.
• When using shutdown hooks, the following security permissions are required: If the Java Security Managers are used, the Java code responsible for removing or adding shutdown hooks must obtain shutdown hooks permission at runtime. If the method is invoked without permission in a secure environment, the SecurityException is thrown.