Annotation default "null" value

try that:

Config value() default @Config();

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 = {})

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

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