+ +

onX

Sup dawg, we heard you like annotations, so we put annotations in your annotations so you can annotate while you're annotating.

Since

+ onX was introduced as experimental feature in lombok v0.11.7 (edge release only). +

Experimental

+ Experimental because: +

Ugly syntax. The syntax of this feature is not optimal, but it is the least convoluted syntax that could possibly work (for now!)
Possibly java 9 will offer (much) better ways of supporting this feature.
Uncertainty: Future versions of javac may break this feature, and we may not be able to restore it.

+ Current status: uncertain - Currently we feel this feature cannot move out of experimental status. +

Overview

+ This feature is considered 'workaround status' - it exists in order to allow users of lombok that cannot work without this feature to have access to it anyway. If we find a better way to implement this feature, or some future java version introduces an alternative strategy, this feature can disappear without a reasonable deprecation period. Also, this feature may not work in future versions of javac. Use at your own discretion. +

+ Most annotations that make lombok generate methods or constructors can be configured to also make lombok put custom annotations on elements in the generated code. +

+ @Getter, @Setter, and @Wither support the onMethod option, which will put the listed annotations on the generated method. +

+ @AllArgsConstructor, @NoArgsConstructor, and @RequiredArgsConstructor support the onConstructor option which will put the listed annotations on the generated constructor. +

+ @Setter and @Wither support onParam in addition to onMethod; annotations listed will be put on the only parameter that the generated method has. +

+ The syntax is a little strange; to use any of the 3 onX features, you must wrap the annotations to be applied to the constructor / method / parameter in @_(@AnnotationGoesHere). To apply multiple annotations, use @_({@Annotation1, @Annotation2}). The annotations can themselves obviously have parameters as well. +

With Lombok

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Vanilla Java

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Small print

+ The reason of the weird syntax is to make this feature work in javac 7 compilers; the @_ type is an annotation reference to the annotation type _ (underscore) which doesn't actually exist; this makes javac 7 delay aborting the compilation process due to an error because it is possible an annotation processor will later create the _ type. Instead, lombok applies the annotations and removes the references so that the error will never actually occur. The point is: The _ type must not exist, otherwise the feature does not work. In the rare case that the _ type does exist (and is imported or in the package), you can simply add more underscores. Technically any non-existent type would work, but to maintain consistency and readability and catch erroneous use, lombok considers it an error if the 'wrapper' annotation is anything but a series of underscores. +

+ To reiterate: This feature can disappear at any time; if you use this feature, be prepared to adjust your code when we find a nicer way of implementing this feature, or, if a future version of javac forces us to remove this feature entirely with no alternative. +

+ The onX parameter is not legal on any type-wide variant. For example, a @Getter annotation on a class does not support onMethod. +

+ +