java annotation

java Classification of annotations ： Standard notes 、 Yuan notes 、 Custom annotation

1、Java Own standard notes

Include @Override、@Deprecated、@SuppressWarnings etc. , After using these annotations, the compiler checks .

JDK The following comments are built in ：

• Override    Check whether the method overrides the method , If the parent class is found , Or when there is no such method in the reference interface , Compile error will be reported .
• @Deprecated Mark obsolete methods , If this method is used , Compile warning will be reported
• @SuppressWarnings Instructs the compiler to ignore warnings declared in annotations
• @FunctionnalInterface java8 Support , Identify an anonymous function or functional interface

2、 Yuan notes

Meta annotations are annotations used to define annotations , Include @Retention、@Target、@Inherited、@Documented、@Repeatable etc. . Meta annotation is also Java Own standard notes , Just used to decorate other annotations .

Meta annotations are java API Provided , Is a comment used to modify an annotation , Usually used in the definition of annotation ：

• @ Retention Used to define the level at which the annotation is available , In the source code (SOURCE)、 Class file (CLASS) Or runtime (RUNTIME).

• @Documented： Keep annotations when generating document information , Auxiliary description of class

• @Target  Illustrates the Annotation The scope of the object being decorated ：Annotation Can be used for packages、types（ class 、 Interface 、 enumeration 、Annotation type ）、 Type members （ Method 、 Construction method 、 Member variables 、 Enumerated values ）、 Method parameters and local variables （ Like loop variables 、catch Parameters ）. stay Annotation Used in the declaration of type target Can be more clear about its embellishment objectives .

3、 Custom annotation

Users can define annotations according to their own needs . When we understand the built-in annotation , After meta annotation and reflection interface to get annotation , We can start customizing annotations . Creating custom annotations is similar to creating an interface , But annotated interface Keywords need to be @ Symbol at the beginning , We can declare methods for annotations .