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<b>Trail:</b> Learning the Java Language
<br /><b>Lesson:</b> Annotations
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<div class="linkLESSON"><a href="index.html">Annotations</a></div>
<div class="linkAHEAD"><a href="basics.html">Annotations Basics</a></div>
<div class="linkAHEAD"><a href="declaring.html">Declaring an Annotation Type</a></div>
<div class="linkAHEAD"><a href="predefined.html">Predefined Annotation Types</a></div>
<div class="linkAHEAD"><a href="type_annotations.html">Type Annotations and Pluggable Type Systems</a></div>
<div class="nolinkAHEAD">Repeating Annotations</div>
<div class="linkQUESTIONS"><a href="QandE/questions.html">Questions and Exercises</a></div>
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<div id="PageTitle"><h1>Repeating Annotations</h1></div>
<div id="PageContent">
<p>
There are some situations where you want to apply the same annotation to a declaration or type use. As of the Java SE 8 release, <em>repeating annotations</em> enable you to do this.</p>
<p>
For example, you are writing code to use a timer service that enables you to run a method at a given time or on a certain schedule, similar to the UNIX <tt>cron</tt> service. Now you want to set a timer to run a method, <tt>doPeriodicCleanup</tt>, on the last day of the month and on every Friday at 11:00 p.m. To set the timer to run, create an <code>@Schedule</code> annotation and apply it twice to the <tt>doPeriodicCleanup</tt> method. The first use specifies the last day of the month and the second specifies Friday at 11p.m., as shown in the following code example:</p>
<div class="codeblock"><pre>
@Schedule(dayOfMonth="last")
@Schedule(dayOfWeek="Fri", hour="23")
public void doPeriodicCleanup() { ... }
</pre></div>
<p>
The previous example applies an annotation to a method. You can repeat an annotation anywhere that you would use a standard annotation. For example, you have a class for handling unauthorized access exceptions. You annotate the class with one <code>@Alert</code> annotation for managers and another for admins:</p>
<div class="codeblock"><pre>
@Alert(role="Manager")
@Alert(role="Administrator")
public class UnauthorizedAccessException extends SecurityException { ... }
</pre></div>
<p>
For compatibility reasons, repeating annotations are stored in a <em>container annotation</em> that is automatically generated by the Java compiler. In order for the compiler to do this, two declarations are required in your code.</p>
<h2>Step 1: Declare a Repeatable Annotation Type</h2>
<p>
The annotation type must be marked with the <code>@Repeatable</code> meta-annotation. Given the custom <code>Schedule</code> example:</p>
<div class="codeblock"><pre>
public @interface Schedule { ... }
</pre></div>
<p>The code looks like the following:</p>
<div class="codeblock"><pre>
<strong>@Repeatable(Schedules.class)</strong>
public @interface Schedule { ... }
</pre></div>
<p>
The value of the <code>@Repeatable</code> meta-annotation, in parentheses, is the type of the container annotation that the Java compiler generates to store repeating annotations. In this example, the containing annotation type is <code>Schedules</code>, so repeating <code>@Schedule</code> annotations is stored in an <code>@Schedules</code> annotation.</p>
<p>
Applying the same annotation to a declaration without first declaring it to be repeatable results in a compile-time error.</p>
<h2>Step 2: Declare the Containing Annotation Type</h2>
<p>
The containing annotation type must have a <code>value</code> element with an array type. The component type of the array type must be the repeatable annotation type. The declaration for the <code>Schedules</code> containing annotation type is the following:</p>
<div class="codeblock"><pre>
public @interface Schedules {
Schedule[] value;
}
</pre></div>
<h2>Retrieving Annotations</h2>
<p>
There are several methods available in the Reflection API that can be used to retrieve annotations. The behavior of the methods that return a single annotation, such as
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html#getAnnotationByType-java.lang.Class-">AnnotatedElement.getAnnotationByType(Class<T>)</a>, are unchanged in that they only return a single annotation if <em>one</em> annotation of the requested type is present. If more than one annotation of the requested type is present, you can obtain them by first getting their container annotation. In this way, legacy code continues to work. Other methods were introduced in Java SE 8 that scan through the container annotation to return multiple annotations at once, such as
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html#getAnnotations-java.lang.Class-">AnnotatedElement.getAnnotations(Class<T>)</a>. See the
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html">AnnotatedElement</a> class specification for information on all of the available methods.</p>
<h2>Design Considerations</h2>
<p>
When designing an annotation type, you must consider the <em>cardinality</em> of annotations of that type. It is now possible to use an annotation zero times, once, or, if the annotation's type is marked as <code>@Repeatable</code>, more than once. It is also possible to restrict where an annotation type can be used by using the <code>@Target</code> meta-annotation. For example, you can create a repeatable annotation type that can only be used on methods and fields. It is important to design your annotation type carefully to ensure that the programmer <em>using</em> the annotation finds it to be as flexible and powerful as possible.</p>
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