Java annotations

[Devi Sri]

Pls anyone help me out , how to use annotations... and what is the purpose of it in real time apps...

Thanks in advance,
Devisri.

[Paul Sturrock]

Annotations are a way of marking code, not to influence how it behaves, but to influence how tools that read the file treat it. See this.

(and by the way they are part of the Core API, so this might be better placed in the beginners forum)

[Ulf Dittmer]

Originally posted by Paul Sturrock:
Annotations are a way of marking code, not to influence how it behaves, but to influence how tools that read the file treat it.

I don't quite agree with this. The line between the code that contains annotations and the code that processes them can be very fine. After all, any class is capable of reading and processing its own annotations at runtime. That may not be what Sun envisaged annotations being used for, of course, nor good development practice, but it's possible.

Here's an example of a class that reads its own annotations at runtime. Note the RetentionPolicy.RUNTIME setting.
[ June 14, 2007: Message edited by: Ulf Dittmer ]

[Nikhil Jain]

Originally posted by Paul Sturrock:
Annotations are a way of marking code, not to influence how it behaves, but to influence how tools that read the file treat it.

If thats true, then why did sun use Annotations heavily for ejb 3. As far as I get,I see EJB 3 uses annotations to influence the code.

But anyways, are using annotation at run time a bad practice?

[Rahul Bhattacharjee]

Originally posted by Shashank Tilwalli:
As far as I get,I see EJB 3 uses annotations to influence the code.

As far as I get,I see EJB 3 uses annotations to influence the deployment.I guess.

[Jaikiran Pai]

Originally posted by Paul Sturrock:
(and by the way they are part of the Core API, so this might be better placed in the beginners forum)

Moving this to Java In General (Beginners) forum.

[Bert Bates]

with all due respect, i'm going to move this again

Given the interesting discussion that's occuring, this seems to me to be an intermediate level question.

[Ulf Dittmer]

What it boils down to is that annotations are class metadata. As such they can be used for whatever purpose someone can think of. Of course they provide more value if they are standardized, so that annotation-processing code knows what to do if they are encountered.

But there's nothing in them that would prevent someone from inventing new ones for use solely in his own code (or solely for use within a single company).

Note that above I used the deliberately vague term "annotation-processing code". This could be tools like apt -which is used for JAX-WS web services-, or javac (think @Override), or 3rd-party libraries like FindBugs (see here).

Or a class could process its own annotations, like in the example I linked to above. That class doesn't do anything interesting with those annotations - it's just meant to demonstrate that it's possible.

Another interesting example is the web app framework Stripes, which uses annotations for configuration purposes at runtime. It thus needs no configuration files, like Struts does.
[ July 07, 2007: Message edited by: Ulf Dittmer ]