Proper use of finalizers and cleaners

Finalizers and cleaners are mechanisms provided in Java to perform cleanup operations before an object is garbage collected. While they serve a similar purpose, they differ in terms of reliability and performance. In this article, we will discuss the proper use of finalizers and cleaners and the best practices to follow.

Finalizers

A finalizer is a method defined in the Object class that is called by the garbage collector before the object is reclaimed. It is declared using the finalize() method. However, using finalizers is generally discouraged due to several reasons:

1. Unreliable execution: There is no guarantee when the finalizer will be called, or even if it will be called at all. It depends on the JVM's garbage collection mechanism. Therefore, you cannot rely on finalizers to release critical resources or maintain system consistency.

2. Performance impact: Finalizers can significantly impact the performance of your application. When an object with a finalizer is eligible for garbage collection, it needs to be placed in a special queue, known as the finalization queue, which adds overhead to the collection process. Moreover, finalizers are executed by a separate thread, blocking the garbage collector until the finalizer completes its execution.

3. Security issues: Finalizers can create potential security issues. If a malicious attacker holds references to objects with finalizers, they can prevent those objects from being garbage collected, leading to potential resource leaks or denial of service attacks.

Due to these reasons, finalizers should only be used as a last resort for situations where you have no other means to release non-memory resources associated with an object.

Cleaners

Introduced in Java 9, cleaners provide a safer and more reliable alternative to finalizers. A cleaner is a mechanism that allows you to perform cleanup operations on an object explicitly, rather than relying on the garbage collector. It provides a more controlled way of releasing resources. Here's how you can use cleaners:

1. Define a Cleaner object: Create a nested class within your main class that extends java.lang.ref.Cleaner.Cleanable. This class will define the cleanup logic.

public class MyClass {
    private static final Cleaner cleaner = Cleaner.create();

    private class MyCleanable implements Runnable {
        @Override
        public void run() {
            // Cleanup logic goes here
        }
    }

    private final MyCleanable cleanable;

    public MyClass() {
        this.cleanable = cleaner.register(this, new MyCleanable());
    }

    // Rest of the class implementation
}

2. Register your cleaner: In the constructor of your main class, create an instance of the Cleanable child class and register it with the cleaner object. This ensures that the cleanup logic will be executed when the object becomes unreachable.

3. Perform cleanup operations: Implement the run() method of the Runnable interface within your Cleanable child class. This method will contain the cleanup logic you want to execute.

By using cleaners, you gain more control over resource cleanup and avoid the pitfalls associated with finalizers.

Best Practices

When it comes to using finalizers and cleaners, the following best practices should be followed:

• Avoid using finalizers if possible: Always try to find alternative mechanisms to handle resource cleanup, such as try-finally blocks or explicit close() methods provided by Java classes like streams or database connections.

• If using finalizers, guard against failures: If you must use a finalizer, make sure to handle exceptions within it to prevent unexpected failures. Additionally, it's a good practice to log any exceptions or failures encountered during finalization.

• Use cleaners for non-memory resources: Reserve the use of cleaners for scenarios where you have non-memory resources, such as file handles or network connections, to release. For memory-related cleanup, rely on Java's automatic garbage collection.

• Be cautious with cleaners: While cleaners provide better control, overusing them can lead to code complexity. Apply them judiciously, only when necessary.

In conclusion, finalizers should be avoided in most cases due to their unreliability and performance impact. Cleaners, introduced in Java 9, provide a safer alternative if explicit resource cleanup is needed. By following the best practices mentioned above, you can ensure proper use of finalizers and cleaners in your Java applications.