Why annotations are used in java

RMI Internationalization. Note: Test. Note: Recompile with -Xlint:deprecation for details. Target ElementType. Retention RetentionPolicy.

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Angular 7. Machine Learning. Data Structures. Operating System. Computer Network. Compiler Design. When used on a declaration, each annotation often appears, by convention, on its own line. As of the Java SE 8 release, annotations can also be applied to the use of types. Here are some examples:. This form of annotation is called a type annotation. All rights reserved. Hide TOC. Metadata is data about data. So annotations are metadata for code.

For example, look at the following piece of code. Annotation is a special kind of Java construct used to decorate a class, method, field, parameter, variable, constructor, or package. Prior to annotation and even after , XML was extensively used for metadata, and somehow, a particular set of Application Developers and Architects thought XML maintenance was getting troublesome.

They wanted something that could be coupled closely with code instead of XML, which is very loosely coupled in some cases, almost separate from code. An interesting point is that XML configurations were introduced to separate configurations from code. The last two statements might create a bit of doubt in your mind that these two are creating a cycle, but both have their pros and cons.

In this scenario, XML would be a better choice because this is not related to any specific piece of code. If you want to expose some method as a service, an annotation would be a better choice as it needs to be tightly coupled with that method and developer of the method must be aware of this. Another important factor is that an annotation defines a standard way of defining metadata in code.

Prior to annotations, people also used their own ways to define metadata. Some examples are using marker interfaces, comments, transient keywords, etc. Each developer needs his own way to decide metadata, but annotation standardized things. These days most frameworks use a combination of both XML and Annotations to leverage positive aspects of both. Before I start this explanation, I will suggest you download this sample code for annotations AnnotationsSample. Writing annotations is very simple.

You can compare annotation definition to an interface definition. This is the most important part to understand, and I am reiterating myself: Annotations are only metadata and do not contain any business logic. Tough to digest but true. If annotations do not contain the logic, then someone else must be doing something and that someone is the consumer of this annotation metadata.

The consumer is a piece of code that reads this information and then performs the necessary logic. When we are talking about standard annotations like Override , the JVM is the consumer and it works at the bytecode level. So we need to write to consumers for our annotations by ourselves. In the above examples, you will see annotations are used on annotations.

J2SE 5. Documented — Whether to put the annotation in Javadocs. Retention — When the annotation is needed.