![]() Meta Annotations are also used for writing custom annotations we will soon discuss custom annotations. Regular Annotations are applied to Java source code while Meta Annotations are applied to other Java annotations. Java Annotations Applied to Other how long this annotation is retained - whether in code only, compiled into the class, or available at run time through that this annotation should be included in the documentation of annotated the items to which this annotation can be that this annotation, when applied to a class, is automatically inherited by its subclasses.įrom the usage point of view annotations are categorized as Regular and Meta Annotations. Marks item as annotable items except packages and annotations Java Annotations Applied to Java MethodsĬhecks that this method overrides a method from its annotable items Custom annotations will be discussed shortly.įollowing table shows the functionality of built-in annotations, and the library in which they are defined. Meta Java annotations are used to create custom annotations. These annotations are called meta Java annotations. Rest four ( and are applied to other annotations and they are included in library. These three annotations are called regular Java annotations. Java defines seven built-in annotations out of which three ( and are applied to Java code and they are included in java.lang library. Java annotations have been covered in section 9.6 and 9.7 in Java Language Specification. Java annotations solve all of these problems nicely. Third disadvantage of naming patterns is that they provide no good way to associate parameter values with program elements. Second, there is no way to ensure that naming patterns are used only on appropriate program elements. Naming patterns have disadvantages they are not efficient to cope up with typographical errors. Prior to Java 1.5, it was common to use naming patterns to indicate that some program elements demanded special treatment by a tool or framework. By having a common infrastructure metadata structure can be standardized, thus a tool could use the metadata information in form of annotations to generate additional source code, or to provide additional information for debugging, or other purposes.Īnnotations to Java have been introduced first time in Java 1.5. One of the main reasons for adding annotations to the Java platform is to enable development and runtime tools to have a common infrastructure in order to reduce the effort required for development and deployment. However, you maybe familiar with Javadoc tags that embed metadata in Java source code but unlike Javadoc tags, Java annotations can be reflective and retrievable at run-time. Java annotations are conventionally placed before all other modifiers, but this is not a compulsion they may be freely intermixed with other modifiers. Such annotations are placed immediately before the enum constant they annotate. An annotations may be used as modifiers in any declaration, whether package, class (including enums), interface (including annotation types), field, method, formal parameter, constructor, or local variable.Īnnotations may also be used on enum constants. The purpose of an annotation is to associate information with the annotated program element. Purpose of Annotations - Adding Metadata to Java Program However, Java compiler can understand only a few Java annotations and further to Java compiler we do need annotation processing tools either developed by ourselves or third party annotation processors to parse Java annotations. Java annotations can be extracted at the time of compiling the code by the compiler or at run time by annotation processors. That's why Java annotations are called metadata about the code. Annotations are processed by annotation processing tools to extract the provided information and to use that information for intended purpose. There may be white space between symbol and the annotation-tag name but standard coding practices advice to avoid any space between the two. Annotations present the information in a standard and structured way so that it could be used amenably by processing tools.Ī Java annotation is a modifier starts with symbol as Javadoc tags do, followed by annotation-tag name followed by zero or more element-value pairs enclosed in parenthesis. Beside Java annotations Java programs have copious amounts of informal documentation that is typically contained within comments in the source code file but annotations are different from comments they annotate the program elements directly using annotation types to describe the form of the annotations. They associate information with the annotated program element. Java annotations are tags that we insert into source code for providing more information about the code. Java Custom Annotation Types and Annotation Parsing Using Reflection.Purpose of Annotations - Adding Metadata to Java Program.
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