mjhjmtu

I understand that annotations are immutable, however, arrays in Java are by themselves not immutable. After running a test I notice that the array returned from an annotation parameter can be mutated but it does not effect the source array:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface ArrayAnnotation 
 String value() default ;


@ArrayAnnotation("foo")
public class Main 
 public static void main(String args) 
 ArrayAnnotation test = Main.class.getAnnotation(ArrayAnnotation.class);

 String test0 = test.value();
 test0[0] = "bar";
 System.out.println(test0[0]);

 String test1 = test.value();
 System.out.println(test1[0]);
 

This prints:

bar
foo

What is going on behind the scenes here? Is there simply an array copy happening during each call to value(), or is it something more complex?

Is there simply an array copy happening during each call to value(), or is it something more complex?

Yes, the array is copied.

Annotations are a special kind of interface type. (JLS)

They are implemented by some Proxy classes at runtime.
You can debug it if you set breakpoint at Proxy.newProxyInstance().

Invocations on annotation are intercepted by AnnotationInvocationHandler which copies arrays:

if (result.getClass().isArray() && Array.getLength(result) != 0)
 result = cloneArray(result);

You are right, it returns a copy each time to ensure it is not changed.

In a future version of Java, this copy might be optimised away.