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<title>EXPERIMENTAL – onX</title>
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<div class="header"><a href="../../index.html">Project Lombok</a></div>
<h1>onX</h1>
<div class="byline">Sup dawg, we heard you like annotations, so we put annotations in your annotations so you can annotate while you're annotating.</div>
<div class="since">
<h3>Since</h3>
<p>
onX was introduced as experimental feature in lombok v0.11.8.
</p>
</div>
<div class="experimental">
<h3>Experimental</h3>
<p>
Experimental because:
<ul>
<li>Ugly syntax. The syntax of this feature is not optimal, but it is the least convoluted syntax that could possibly work (for now!)</li>
<li>Possibly java 9 will offer (much) better ways of supporting this feature.</li>
<li>Uncertainty: Future versions of javac may break this feature, and we may not be able to restore it.</li>
</ul>
Current status: <em>uncertain</em> - Currently we feel this feature cannot move out of experimental status.
</div>
<div class="overview">
<h3>Overview</h3>
<p>
<strong>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.</strong>
</p><p>
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.
</p><p>
<code>@Getter</code>, <code>@Setter</code>, and <code>@Wither</code> support the <code>onMethod</code> option, which will put the listed annotations on the generated method.
</p><p>
<code>@AllArgsConstructor</code>, <code>@NoArgsConstructor</code>, and <code>@RequiredArgsConstructor</code> support the <code>onConstructor</code> option which will put the listed annotations on the generated constructor.
</p><p>
<code>@Setter</code> and <code>@Wither</code> support <code>onParam</code> in addition to <code>onMethod</code>; annotations listed will be put on the only parameter that the generated method has. <code>@EqualsAndHashCode</code> also supports <code>onParam</code>; the listed annotation(s) will be placed on the single parameter of the generated <code>equals</code> method, as well as any generated <code>canEqual</code> method.
</p><p>
The syntax is a little strange and depends on the javac you are using.<br />
On javac7, to use any of the 3 <code>onX</code> features, you must wrap the annotations to be applied to the constructor / method / parameter in <code>@__(@AnnotationGoesHere)</code>. To apply multiple annotations, use <code>@__({@Annotation1, @Annotation2})</code>. The annotations can themselves obviously have parameters as well.<br />
On javac8 and up, you add an underscore after <code>onMethod</code>, <code>onParam</code>, or <code>onConstructor</code>.
</p>
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<div class="snippets">
<div class="pre">
<h3>With Lombok</h3>
<div class="snippet">@HTML_PRE@</div>
</div>
<div class="sep"></div>
<div class="post">
<h3>Vanilla Java</h3>
<div class="snippet">@HTML_POST@</div>
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<div class="overview">
<h3>Small print</h3><div class="smallprint">
<p>
The reason of the weird syntax is to make this feature work in javac 7 compilers; the <code>@__</code> type is an annotation reference to the annotation type <code>__</code> (double 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 <code>__</code> type. Instead, lombok applies the annotations and removes the references so that the error will never actually occur. The point is: The <code>__</code> type <em>must not exist</em>, otherwise the feature does not work. In the rare case that the <code>__</code> 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.
</p><p>
In javac8, the above feature should work but due to a bug in javac8 it does not. However, starting in javac8, if the parameter name does not exist in the annotation type, compilation proceeds to a phase where lombok can fix it.
</p><p>
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.
</p><p>
The <code>onX</code> parameter is not legal on any type-wide variant. For example, a <code>@Getter</code> annotation on a class does not support <code>onMethod</code>.
</p>
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