Java annotation - Wikipedia (FazBrowse)

In the Java computer programming language, an annotation is a form of syntactic metadata that can be added to Java source code, like an attribute. Classes, methods, variables, parameters and Java packages may be annotated. Like Javadoc tags, Java annotations can be read from source files. Unlike Javadoc tags, Java annotations can also be embedded in and read from Java class files generated by the Java compiler. This allows annotations to be retained by the Java virtual machine at run-time and read via reflection. It is possible to create meta-annotations out of the existing ones in Java.

@FunctionalInterfacejava.langMarks an interface as intended to be a functional interface. @SuppressWarningsjava.langInstructs the compiler to suppress the compile time warnings specified in the annotation parameters. @Nativejava.lang.annotationMarks a field defining a constant value as potentially being referenced from native code. @Targetjava.lang.annotationMarks another annotation to restrict what kind of Java elements the annotation may be applied to.

In Jakarta EE (formerly Java Platform, Enterprise Edition), the following annotations also exist in jakarta.annotation (formerly javax.annotation):

Annotation	Package	Description
@Resources	jakarta.annotation	Declares reference to resources, as a container for multiple resource declarations.
@Priority	jakarta.annotation	Marks any program element to indicate in what order they should be used.
@RunAs	jakarta.annotation	Defines the security role of the application during execution in a Jakarta EE container.
@DenyAll	jakarta.annotation.security	Marks a method to specify that no security roles may access the method.
@DataSourceDefinitions	jakarta.annotation.sql	Declares a container DataSource, acting as a container for multiple data source declarations.

There was previously an annotation, // @Twizzle is an annotation to method toggle(). @Twizzle publicvoidtoggle(){ } // Declares the annotation Twizzle. public@interfaceTwizzle{ }

Annotations may include a set of key-value pairs, which are modeled as methods of the annotation type. Each method declaration defines an element of the annotation type. Method declarations must not have any parameters or a throws clause. Return types are restricted to primitives, // Same as: @Edible(value = true) @Edible(true) Itemitem=newCarrot(); public@interfaceEdible{ booleanvalue()defaultfalse; } @Author(first="Oompah",last="Loompah") Bookbook=newBook(); public@interfaceAuthor{ Stringfirst(); Stringlast(); }

Annotations themselves may be annotated to indicate where and when they can be used:

packageorg.wikipedia.examples.annotation; importjava.lang.annotation.Documented; importjava.lang.annotation.ElementType; importjava.lang.annotation.Inherited; importjava.lang.annotation.Retention; importjava.lang.annotation.RetentionPolicy; importjava.lang.annotation.Target; @Documented @Retention(RetentionPolicy.RUNTIME) @Target({ ElementType.TYPE,ElementType.METHOD, ElementType.CONSTRUCTOR,ElementType.ANNOTATION_TYPE, ElementType.PACKAGE,ElementType.FIELD,ElementType.LOCAL_VARIABLE }) @Inherited public@interfaceUnfinished{ publicenumPriority{LOW,MEDIUM,HIGH} Stringvalue(); String[]changedBy()default""; String[]lastChangedBy()default""; Prioritypriority()defaultPriority.MEDIUM; StringcreatedBy()default"James Gosling"; StringlastChanged()default"2011-07-08"; }