Java annotations fundamentals

Question

I am a newbie to annotations. I have gone through a lot of tutorials explaining the concept of annotations. But nowhere do i find information about defining multiple annotations within a class. So pls give me some insight on defining and accessing multiple annotations.Below is the code where I define two annotations in a class and eclipse IDE presents me an error "The public type SampleAnn must be defined in its own file".. Is the reason for this error because of the java convention that "there should only one public annotation per class in the name of the class-name"?

@Documented
@Target(ElementType.LOCAL_VARIABLE)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MethodInfo{
int number1;
}

@Documented
@Target(ElementType.LOCAL_VARIABLE)
@Retention(RetentionPolicy.RUNTIME)
public @interface SampleAnn{
int number2;
}

Annotations are classes, and you can't have more than one public top-level class per file in Java. — JB Nizet
– JB Nizet, Commented Nov 21, 2013 at 9:47

Etienne Neveu · Accepted Answer · 2015-05-30 11:13:03Z

2

You are right, you can have only a single top-level class in one file.

But what you can do:

public class MyAnnotations {

   public @interface SampleAnn { ... }
   public @interface MethodInfo { ... }
}

edited May 30, 2015 at 11:13

Etienne Neveu

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answered Nov 21, 2013 at 9:44

Ingo

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Comments

Pierre Arlaud · Accepted Answer · 2013-11-21 09:43:20Z

1

There should more generally be ONE public CLASS per class file and annotations are no exception. It is also important that any publicly defined entity has the same name as its java file's name, so I don't see how you could have two in the same file.

answered Nov 21, 2013 at 9:43

Pierre Arlaud

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Comments

Kevin Bowersox · Accepted Answer · 2013-11-21 09:47:15Z

0

The annotations need to be in separate compilation units (files).

The regarding top-level classes the specification states:

This restriction implies that there must be at most one such type per compilation unit. This restriction makes it easy for a Java compiler to find a named class within a package. In practice, many programmers choose to put each class or interface type in its own compilation unit, whether or not it is public or is referred to by code in other compilation units.

Specification

answered Nov 21, 2013 at 9:47

Kevin Bowersox

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