Prior to Java 1.5 a TagInterface would have been used to mark a class as having some property - i.e. the interface is being used as MetaData.

JavaAnnotations provide a more comprehensive way of providing this MetaData so that it is more useful to the programmer.

The first notable difference is that annotations can be parameterised. For example the annotation @Retention is used to annotate other annotations and is parameterised by an enumeration. So you can have

 @Retention(Retention''''''Policy.RUNTIME)

Which, as it retains the annotation data at runtime, allows the programmer to find out about applied annotations using reflection, hence becoming the Java 1.5 basis of a TagInterface. The alternative would be to have a family of interfaces.

The second difference is that annotations can mark other things apart from just classes. The @Target annotation is used to do this thusly:

 @Target(Element''''''Type.METHOD)

Which would annotate an annotation as being valid for methods only. (The @Target annotation actually takes an array of Element''''''Type but Java 1.5 provides some SyntacticSugar to hide that here).

An example use of the above two annotations would be for a UnitTest''''''ing framework. In JavaUnit methods that begin with 'test' are run as tests - but this can be a bit ugly. (See AnnotationsOverNamingConventions). With annotations we can tag a method thusly.

First we need to create our @Test annotation:

 @Retention(Retention''''''Policy.RUNTIME)
 @Target(Element''''''Type.METHOD)
 public @interface Test {}

Then we tag our test methods:

 public class My''''''Tests {
   @Test void myTest() {
   }
 }

Finally we use reflection to get our test methods:

 public class Test''''''Runner {
   public static void runTests(Class c) throws Exception {
     Object instance = c.newInstance();
     for (Method m: c.getMethods()) {
       if (m.getAnnotation(Test.class) != null) {
         m.invoke(instance, new Object[]{});
       }
     }
   }
 }

The advantages of annotations over tags are therefore:

* Annotations explicitly express the intent of the marker, instead of hijacking another language feature to use as reflective metadata.
* Annotations can target more than just types
* Annotations can be parameterised

The disadvantages are:

* Annotations can only be used in Java 5
* It takes more work to define an annotation than to define an empty interface

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Category: JavaIdioms