use of custom annotations

Question

I found several related (not duplicate) question to this, but they didn't satisfy me.

I am unable to understand where and why to use custom annotations?

I read an example of custom annotation in a book, but it was not explained thoroughly.

@interface MyAnno {     String str();     int val(); }  class MyClass {     @MyAnno(str = "Annotation example", val = 100)     public static void myMeth()     {         System.out.println("Inside myMeth()");     } }  class CustomAnno {     public static void main(String args[])     {         MyClass.myMeth();     } } 

The output is as expected Inside myMeth().

I am having few questions regarding this example.

1- How can I use String str() and int val() in this program? OR

What is the use of any abstract method of an custom annotation?

2- Why custom annotations. I mean that what effect they are having on any code.

3- How can I create an annotation which is having effects like @override is having?(I mean any kind of effect which can be noticed)

If this example is useless for you, then please give me a suitable small example in which a custom annotation is used.

Answer 1

Three main reasons to use custom annotations are:

• To reduce the effort of writing code (a compile-time annotation processor generates code for you). Here is a tutorial: part 1, part 2.
• To provide additional correctness guarantees (a compile-time annotation processor warns you about errors). One nice tool for this is the Checker Framework, which prevents null pointer dereferences, concurrency errors, and more.
• To customize behavior (at run time, your code checks for the annotation using reflection and behaves differently depending on whether the annotation is present). Frameworks such as Hibernate use annotations this way; also see an Oracle article.

In each case, use of annotations reduces the likelihood of errors in your code, compared to other non-annotation approaches.