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;      @Index(expr = "project.client_id")     @Index(expr = "project.start_date")     public List<Project> getProjects() { return projects; } } 

Obviously, I want to be able to quickly find instances of Employee by various properties of Project. I can either specify @Index twice with different expr() values, or take the approach specified in the accepted answer. Even though Hibernate does this and it's not considered a hack, I think it still makes sense to at least allow having multiple annotations of the same type on a single element.

Answer 1

Note: This answer is partially outdated since Java 8 introduced the @Repeatable annotation (see answer by @mernst).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.

Answer 2

Repeating annotations in Java 8

In Java 8 (released in March 2014), it is possible to write repeated/duplicate annotations.

See tutorial, Repeating Annotations.

See specification, JEP 120: Repeating Annotations.