How Can I Modify a Class Definition's Annotation String Parameter at Runtime?

Modify Class Definition's Annotation String Parameter at Runtime

Introduction

In scenarios where a pre-compiled class is imported and part of the classpath, it can be desirable to modify the annotation string parameter at runtime. This article explores possible approaches to achieve this.

Solution

To change the annotation value for a given key of a given annotation, follow the below steps:

1. Retrieve the invocation handler for the annotation using Proxy.getInvocationHandler(annotation):

   Object handler = Proxy.getInvocationHandler(annotation);

2. Obtain the memberValues field from the handler by accessing its declared field and setting its accessibility to true:

   Field f = handler.getClass().getDeclaredField("memberValues");
   f.setAccessible(true);

3. Retrieve the memberValues as a map:

   Map<String, Object> memberValues = (Map<String, Object>) f.get(handler);

4. Retrieve the old value from the map using the key:

   Object oldValue = memberValues.get(key);

5. Update the map with the new value:

   memberValues.put(key, newValue);

Usage Example

Consider the following annotated class:

@ClassAnnotation("class test")
public static class TestClass {
    @FieldAnnotation("field test")
    public Object field;
    
    @MethodAnnotation("method test")
    public void method() {}
}

To modify the annotation values at runtime, use the following code:

Field field = TestClass.class.getField("field");
final FieldAnnotation fieldAnnotation = field.getAnnotation(FieldAnnotation.class);
System.out.println("old FieldAnnotation = " + fieldAnnotation.value());
changeAnnotationValue(fieldAnnotation, "value", "another field annotation value");
System.out.println("modified FieldAnnotation = " + fieldAnnotation.value());

This approach allows you to dynamically modify annotation parameters without creating a new annotation instance, reducing the need for advance knowledge of the concrete annotation class.

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