Can Annotation Values Be Read in a Different Class at Runtime in Java?

Retrieving Annotation Values in Java

Java annotations offer a convenient way to provide metadata about code elements. In certain scenarios, you may need to access the values of these annotations at runtime. Particularly, the question arises:

Can annotations with runtime retention be read in a different class in Java?

The answer is yes, provided that the annotation is annotated with @Retention(RetentionPolicy.RUNTIME). This indicates that the annotation should be available at runtime, beyond compilation.

Consider the following code snippet:

<code class="java">@Column(columnName="firstname")
private String firstName;

@Column(columnName="lastname")
private String lastName;</code>

To retrieve the annotation values in another class, you can use reflection:

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

If you need to access private fields, you can use getDeclaredFields():

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

This code loops through the fields of MyClass, checking for @Column annotations on each field. If an annotation is found, it prints the value of the columnName attribute.

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