Annotation default "null" value

Question

Is it possible to specify a annotation with a null as default?

What I want to achieve is something like optional annotation attributes.

For example

public @interface Foo {

    Config value();

}


public @interface Config {

    boolean ignoreUnknown() default false;
    int steps() default 2;
}

I would like to use @Foo (without specifying the value, so it should be some kind of optional) and I would also like to be able to write something like this:

@Foo (
    @Config(
        ignoreUnknown = true,
        steps = 10
    )
)

Is it possible to do something like this with annotations?

I don't want to do something like this

public @interface Foo {

   boolean ignoreUnknown() default false;
   int steps() default 2;
}

because I want to be able to distinguish if a property has been set or not (and not if it has the default value or not).

It's a little bit complicated to describe, but I'm working on a little Annotation Processor which generates Java code. However at runtime I would like to setup a default config that should be used for all @Foo, excepted those who have set own configuration with @Config.

so what I want is something like this:

public @interface Foo {

       Config value() default null;

 }

But as far as I know its not possible, right? Does anybody knows a workaround for such an optional attribute?

Possible duplicate of [Error setting a default null value for an annotation's field](http://stackoverflow.com/questions/1178104/error-setting-a-default-null-value-for-an-annotations-field) — fracz, Jan 05 '16 at 08:10

score 34 · Accepted Answer · answered Jul 10 '14 at 16:05

34

No, you can't use null for an annotation attribute value. However you can use an array type and provide an empty array.

public @interface Foo {
    Config[] value();  
}
...
@Foo(value = {})

or

public @interface Foo {
    Config[] value() default {};  
}
...
@Foo

answered Jul 10 '14 at 16:05

Sotirios Delimanolis

274,122
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Yes, but than I always have to write `@Foo(value = {})` and in 90 % of usage I do not want to provide another Configuration, so I would like to have only `@Foo` – sockeqwe Jul 10 '14 at 16:07
4

@sockeqwe It's as easy as setting a `default {}`. – Sotirios Delimanolis Jul 10 '14 at 16:07

score 6 · Answer 2 · edited Jul 29 '19 at 11:00

6

try that:

Config value() default @Config();

edited Jul 29 '19 at 11:00

wolmi

1,659
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25

answered Jul 29 '19 at 10:21

jepac daemon

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Annotation default "null" value

2 Answers2

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