How do annotations prevent mutations of an array parameter?











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15
down vote

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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?










share|improve this question




























    up vote
    15
    down vote

    favorite
    1












    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?










    share|improve this question


























      up vote
      15
      down vote

      favorite
      1









      up vote
      15
      down vote

      favorite
      1






      1





      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?










      share|improve this question















      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?







      java arrays annotations immutability






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 22 at 19:05

























      asked Nov 22 at 18:59









      flakes

      6,51311850




      6,51311850
























          2 Answers
          2






          active

          oldest

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          up vote
          13
          down vote



          accepted











          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);





          share|improve this answer

















          • 1




            Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
            – flakes
            Nov 22 at 19:23




















          up vote
          5
          down vote













          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.






          share|improve this answer





















          • Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
            – flakes
            Nov 22 at 19:24











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          2 Answers
          2






          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          13
          down vote



          accepted











          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);





          share|improve this answer

















          • 1




            Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
            – flakes
            Nov 22 at 19:23

















          up vote
          13
          down vote



          accepted











          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);





          share|improve this answer

















          • 1




            Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
            – flakes
            Nov 22 at 19:23















          up vote
          13
          down vote



          accepted







          up vote
          13
          down vote



          accepted







          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);





          share|improve this answer













          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);






          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Nov 22 at 19:17









          caco3

          1,0171417




          1,0171417








          • 1




            Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
            – flakes
            Nov 22 at 19:23
















          • 1




            Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
            – flakes
            Nov 22 at 19:23










          1




          1




          Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
          – flakes
          Nov 22 at 19:23






          Ah, very cool! The step-into feature in my IDE's debugger wasn't triggering anything for values() but adding a breakpoint directly in the AnnotationInvocationHandler proxy does the trick! Thanks a lot!
          – flakes
          Nov 22 at 19:23














          up vote
          5
          down vote













          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.






          share|improve this answer





















          • Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
            – flakes
            Nov 22 at 19:24















          up vote
          5
          down vote













          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.






          share|improve this answer





















          • Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
            – flakes
            Nov 22 at 19:24













          up vote
          5
          down vote










          up vote
          5
          down vote









          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.






          share|improve this answer












          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.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Nov 22 at 19:16









          Peter Lawrey

          439k55557958




          439k55557958












          • Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
            – flakes
            Nov 22 at 19:24


















          • Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
            – flakes
            Nov 22 at 19:24
















          Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
          – flakes
          Nov 22 at 19:24




          Yeah, I was thinking that it seems pretty expensive to copy the array every time that it is viewed!
          – flakes
          Nov 22 at 19:24


















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