How Can I Access Annotation Values in a Different Class in Java?

Accessing Annotation Values in Java

Annotations are a powerful mechanism in Java for adding metadata to classes, methods, and fields. These annotations can be accessed and used at runtime to modify the behavior of the annotated code. One common question that arises is whether it is possible to read the value of an annotation in a different class.

The answer to this question is yes, but it depends on the retention policy of the annotation. Annotations can have one of three retention policies:

• SOURCE: Annotations are discarded by the compiler and are not available at runtime.
• CLASS: Annotations are retained in the class file but are not available at runtime via reflection.
• RUNTIME: Annotations are retained in the class file and are available at runtime via reflection.

To read the value of an annotation in a different class, the annotation must have a runtime retention policy. To specify a runtime retention policy, use the @Retention annotation followed by the RetentionPolicy enum:

@Retention(RetentionPolicy.RUNTIME)
@interface Column {
    String columnName();
}

Once the annotation has a runtime retention policy, you can use reflection to access its values in another class:

for (Field f : MyClass.class.getFields()) {
    Column column = f.getAnnotation(Column.class);
    if (column != null) {
        System.out.println(column.columnName());
    }
}

Note that to access private fields, you need to use the getDeclaredFields() method instead of getFields():

for (Field f : MyClass.class.getDeclaredFields()) {
    Column column = f.getAnnotation(Column.class);
    if (column != null) {
        System.out.println(column.columnName());
    }
}

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