Retention annotation in Java

Retention annotation in Java

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Retention Annotation in Java

In Java, retention annotations are used to customize the behavior of Java classes and objects during garbage collection. The @ symbol is used to mark these annotations.

The main purpose of retention annotations is to control how much memory should be retained for each object or class during garbage collection. By default, when an object is no longer referenced by any other part of the program (i.e., it becomes unreachable), Java will automatically free up that object's memory space and remove it from the heap.

However, sometimes you might need to keep a reference to an object even after it becomes unreachable, which can happen in scenarios where your code relies on certain objects being present for future computation. This is exactly what retention annotations are designed to achieve.

Retention annotations come into play when you're dealing with object serialization (the process of converting an object into a format that can be written to disk or sent over the network). When an object is serialized, its internal state needs to be persisted across different JVM instances. For this reason, some parts of the object's memory space need to be retained even after it becomes unreachable.

Let's consider a simple example:

import java.io.*;
public class Employee implements Serializable {
private static final long serialVersionUID = 1L;
private String name;
public Employee(String name) {
this.name = name;
}
public String getName() {
return name;
}
}

In this case, if the Employee object becomes unreachable after serialization and deserialization (e.g., its memory is freed up), some parts of its state would be lost. To prevent this from happening, we can use retention annotations:

@Retention(value = java.lang.annotation.RetentionPolicy.ALL)
public @interface MyAnnotation {
}

We'll apply this annotation to our Employee class:

import java.io.*;
@MyAnnotation
public class Employee implements Serializable {
private static final long serialVersionUID = 1L;
private String name;
public Employee(String name) {
this.name = name;
}
public String getName() {
return name;
}
}

Now, when the Employee object becomes unreachable, Java will retain its memory space due to our custom annotation. This means that even after garbage collection, some parts of the object's state would still be preserved.

In summary, retention annotations in Java allow you to customize the behavior of objects and classes during garbage collection. By applying these annotations correctly, you can ensure that specific parts of an object's memory space are retained even when it becomes unreachable, which is crucial for scenarios like serialization and deserialization.

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Java inherit annotations from superclass

I can respond in English today.

Yes, you are correct that Java inherits annotations from a superclass. In Java, annotations can be used to add metadata to classes, methods, and fields. Annotations are a way to provide additional information about the annotated element beyond its name.

When an annotation is applied to a subclass, it automatically inherits all the annotations defined on its superclass. This means that if you apply an annotation to a superclass, any subclasses of that superclass will also have that annotation.

Here's an example:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyAnnotation {
String value();
}
class Animal {
@MyAnnotation("Animal")
public void sound() {
System.out.println("The animal makes a sound");
}
}
class Dog extends Animal {
// Inherited annotation
@Override
public void sound() {
System.out.println("The dog barks");
}
}

In this example, the MyAnnotation annotation is applied to the Animal class. This means that any subclasses of Animal, such as Dog, will also have this annotation.

When you create a subclass like Dog, it automatically inherits the annotations from its superclass. So, even though we didn't explicitly apply the @MyAnnotation annotation to the Dog class, it still has that annotation because it was inherited from the Animal class.

This is useful when you want to add metadata to a subclass without having to repeat yourself and reapply the same annotations. It's also helpful for tools and frameworks that rely on these annotations to provide additional information about the code.

I hope this helps! Let me know if you have any other questions.