Multiple annotations of the same type on one element?

Question

I'm attempting to slap two or more annotations of the same type on a single element, in this case, a method. Here's the approximate code that I'm working with: ``` public class Dupe { public @interface Foo { String bar(); } @Foo(bar="one") @Foo(bar="two") public void haha() {} } ``` When compiling the above, javac complains about a duplicate annotation: ``` max@upsight:~/work/daybreak$ javac Dupe.java Dupe.java:5: duplicate annotation ``` Is it simply not possible to repeat annotations like this? Pedantically speaking, aren't the two instances of @Foo above different due to their contents being different? If the above isn't possible, what are some potential workarounds? UPDATE: I've been asked to describe my use case. Here goes. I'm building a syntax sugarish mechanism to "map" POJOs to document stores such as MongoDB. I want to allow indexes to be specified as annotations on the getters or setters. Here's a contrived example: ``` public class Employee { private List<Project> projects…

Accepted Answer

Note: This answer is partially outdated since Java 8 introduced the `@Repeatable` annotation (see answer by @mernst). The need for a `@Foos` container annotation and dedicated handling still remain though.

Two or more annotations of same type aren't allowed. However, you could do something like this:

```
public @interface Foos {
    Foo[] value();
}

// pre Java 8
@Foos({@Foo(bar="one"), @Foo(bar="two")})
public void haha() {}

// post Java 8 with @Repeatable(Foos.class) on @Foo
@Foo(bar="one") @Foo(bar="two")
public void haha() {}
```

You'll need dedicated handling of `Foos` annotation in code though.

Multiple annotations of the same type on one element?

Solution

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