Shutdown Hooks are a special structure that allows developers to insert a piece of code to execute when the JVM is shut down. This will come in handy if we need to perform a special cleanup operation when the VM is shut down. Use general constructs to handle this problem, such as ensuring that we call a special procedure (calling System.exit(0)) before the application exits will not apply to the situation where the VM shuts down due to external reasons (such as termination requests from O/S), or Due to resource issues (insufficient memory). As we will see soon, closing hooks can easily solve this problem. It allows us to provide arbitrary code blocks, which will be called by the JVM when it is closed.

On the surface, using the closing hook is straightforward. All we have to do is to simply write a class that extends the java.lang.Thread class and provide the logic we want to execute when the VM is shut down in the public void run() method. Then we register the instance of this class as the shutdown hook of the VM by calling the Runtime.getRuntime().addShutdownHook(Thread) method. If you need to delete a previously registered shutdown hook, the Runtime class also provides the removeShutdownHook(Thread) method.

In Java, a JVM (Java Virtual Machine) Shutdown Hook is a piece of code that gets executed automatically by the JVM when the program is about to exit. The main purpose of a Shutdown Hook is to perform some final cleanup or resource deallocation tasks before the JVM terminates.

When a JVM shutdown is initiated, the JVM runs all registered Shutdown Hooks in the order they were registered. Shutdown Hooks are registered using the Runtime.addShutdownHook() method, which takes a Thread object as its argument. The Thread object should implement the run() method that defines the actions to be taken when the JVM is shutting down.

Shutdown Hooks can be used for various purposes, such as releasing system resources like file handles, closing database connections, flushing buffered data, or saving program state to disk. It is important to note that Shutdown Hooks should be designed to complete quickly since they are executed in the context of the JVM shutdown process and should not delay the shutdown process.

To ensure that Shutdown Hooks work correctly, it is important to follow some best practices. For example, Shutdown Hooks should be designed to be thread-safe and should not rely on external resources that may not be available during the shutdown process. Additionally, it is important to avoid creating new threads or starting any new processes in a Shutdown Hook since these may not complete before the JVM terminates.