Annotations and reuse

[Julio Faerman]

Hi,

I have an annotation i whish to use in several methods, with the same parameters. For example:

@SomeAnnot(params =
          {SomeAnnotParam.param1,SomeAnnotParam.param2} )
public void dothetrick(){}

So i tried to reuse the parameters, as i do not want to duplicate code:

public static final SomeAnnotParam[] myparams = 
{SomeAnnotParam.param1,SomeAnnotParam.param2};
 
@SomeAnnot(params=myparams)
public void dothetrick(){}

Which, to my surprise, resulted in a NullPointerException. Can anyone explain me why?

Full code follows:

public class Weird {

public static final SomeAnnotParam[] myparams = {SomeAnnotParam.param1,SomeAnnotParam.param2};


/** NPE @SomeAnnot(params=myparams) **/
@SomeAnnot(params={SomeAnnotParam.param1,SomeAnnotParam.param2})
public void dothetrick(){}


public static void main(String[] args)throws Exception {
SomeAnnot sluck = Weird.class.getMethod("dothetrick", new Class[0]).getAnnotation(SomeAnnot.class);
for(SomeAnnotParam param : sluck.params()){
System.out.println(param);
}
}
}
 
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface SomeAnnot {
public SomeAnnotParam[] params();
}
 
enum SomeAnnotParam {
param1,
param2,
param3
}

[Tony Morris]

After fixing your code (missing import statements), I am unable to produce a NullPointerException:

tmorris@fjr:~$ java Weird
param1
param2
tmorris@fjr:~$ java -version
java version "1.5.0_07"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-b03)
Java HotSpot(TM) Client VM (build 1.5.0_07-b03, mixed mode, sharing)
tmorris@fjr:~$ uname -a
Linux fjr 2.6.15-26-386 #1 PREEMPT Thu Aug 3 02:52:00 UTC 2006 i686 GNU/Linux

[Tony Morris]

As a side note, the issue of solving the "duplication problem for method parameters" is unsolvable in Java. In FP languages, this is solved with function currying, but Java does not have an equivalent. I have read various articles and examples of emulation of function currying but they are all very flawed (unless you consider a case by case implementation not flawed - an absurdity in light of the objectives of currying).

Even in functional languages, there is an assumption that the data type on its own is a complete representation of the requirements of the function contract - which is not necessarily true - a problem that I think is solvable and predict it will be soon (I know of various people with much more time than I who are working on it and solicit feedback often).

[Jim Yingst]

Julio, if you are somehow able to observe this NullPointerException, it would be very useful for you to study the stack trace and find out what line of code is throwing the exception.

[Julio Faerman]

The code that throws the NPE is commented in the original post.

Here it goes the bad code:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
 
public class Weird {

public static final SomeAnnotParam[] myparams = {SomeAnnotParam.param1,SomeAnnotParam.param2};


@SomeAnnot(params=myparams) 
//This would work: @SomeAnnot(params={SomeAnnotParam.param1,SomeAnnotParam.param2})
public void dothetrick(){}


public static void main(String[] args)throws Exception {
SomeAnnot sluck = Weird.class.getMethod("dothetrick", new Class[0]).getAnnotation(SomeAnnot.class);
for(SomeAnnotParam param : sluck.params()){
System.out.println(param);
}
}
}
 
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface SomeAnnot {
public SomeAnnotParam[] params();
}
 
enum SomeAnnotParam {
param1,
param2,
param3
}

I think it is because the annotation and its parms are processed before the static final array.
I pay a beer if someone can reuse the params

[Tony Morris]

Your code must compile before it can throw a NPE.
Please make your code compile.

This code compiles and executes without a NPE:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
 
public class Weird {

//public static final SomeAnnotParam[] myparams = {SomeAnnotParam.param1,SomeAnnotParam.param2};


//@SomeAnnot(params=myparams) 
@SomeAnnot(params={SomeAnnotParam.param1,SomeAnnotParam.param2})
public void dothetrick(){}


public static void main(String[] args)throws Exception {
SomeAnnot sluck = Weird.class.getMethod("dothetrick", new Class[0]).getAnnotation(SomeAnnot.class);
for(SomeAnnotParam param : sluck.params()){
System.out.println(param);
}
}
}
 
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface SomeAnnot {
public SomeAnnotParam[] params();
}
 
enum SomeAnnotParam {
param1,
param2,
param3
}

[ September 05, 2006: Message edited by: Tony Morris ]

[Jim Yingst]

If you (Julio) really can get that code to compile and throw an NPE, then you woud seem be using a rather unusual JDK. Perhaps some early version of JDK 5, like a beta? I recommend upgrading to the latest stable release. I get a compile error for your code using Sun's JDK 1.5.0_08 as well as 1.6.0_beta2 for Windows. Same for comparable versions on the Mac. The error is always:

Weird.java:11: incompatible types
found   : SomeAnnotParam[]
required: SomeAnnotParam
        @SomeAnnot(params=myparams)
                          ^
1 error

This error is rather misleading as both params and myparams are declared of type SomeAnnotParam[], so it seems like this should work. Why doesn't it?

The thing is, you are simply not allowed to initialize an array in an annotation type to another array. This is intentional. The reason is that they want to only allow yout to initialize annotation fields using "constant", immutable values. Here "constant" is in quotes because it's not exactly the same as the definition given in JLS 15.28, but it's similar. They don't want you to initialize with

    public static final SomeAnnotParam[] myparams = {SomeAnnotParam.param1,SomeAnnotParam.param2};
 
    @SomeAnnot(params=myparams) 

because other code could change the contents of the array, which would create confusion:

Weird.myparams[0] = null;

(Remember, arrays are never immutable.)

Instead, the syntax is designed to force you to use an array initializer that spells out each value in the array:

    @SomeAnnot(params={SomeAnnotParam.param1,SomeAnnotParam.param2})

Now there's no way to change the value of params after it's initialized. (As far as I know, anyway - there are a couple ways you could still try, but for various reasons, they don't work.)

Anyway, what it all boils down to is that no, you're not allowed to do what you're trying to do here. You've really got to list the multiple array values in an array initializer (using {}) in the annotation. There's really no shortcut for you here. Sorry.
[ September 05, 2006: Message edited by: Jim Yingst ]

[Ilja Preuss]

Originally posted by Jim Yingst:
There's really no shortcut for you here. Sorry.

Besides code generation, of course, which might be a viable option.

[Julio Faerman]

Well... it compiled using eclipse and jdk 1.5_08.
In any case i understood why it is not possible. Thank you all very much.

[Ilja Preuss]

Originally posted by Julio Faerman:
Well... it compiled using eclipse and jdk 1.5_08.

Seconded, it compiles in Eclipse 3.3 M1 and produces a NPE. Might be worth a bug report...

[Tony Morris]

Originally posted by Ilja Preuss:


Seconded, it compiles in Eclipse 3.3 M1 and produces a NPE. Might be worth a bug report...

Can this be reproduced outside of Eclipse? If so, can you provide any more details? e.g.
> javac -J-version
> java -version
> uname -a