RHYTHMK Step-by-step JAVA (19): Annotations annotation

There are four types of annotations that you need to know when writing annotations:

@Target

Indicates where the annotation can be used, and the possible ElementType parameters are:

CONSTRUCTOR: declaration of the constructor

field: domain declarations (including enum instances)

local_variable: local variable declaration

Method : Methods Declaration

Package : Packet Declaration

PARAMETER: parameter declaration

Type : class, interface (including annotation type) or enum sound

@Retention

Indicates at what level the annotation information needs to be saved. The optional retentionpolicy parameters include:

SOURCE: annotations are discarded by the compiler

class: annotations are available in the class file, but are discarded by the VM

RUNTIME: The VM retains annotations during the run, so the information for the annotations can be read through the reflection mechanism.

@Document

To include annotations in the Javadoc

@Inherited

Allow subclasses to inherit annotations from parent class

Define an annotation:

 Package com.rhythmk.annotation; Import Java.lang.annotation.ElementType; Import java.lang.annotation.Retention; Import Java.lang.annotation.RetentionPolicy; Import  = elementtype.type) @Retention (retentionpolicy.runtime) public @Interface  rservice {    default "working";    Runenum Run ();}

Annotations use:

@RService (value={"v1", "v2"},run=runenum.run)publicinterface  userservice {       String  Finduserbyid (int  ID);          String Saveuser (String userName);}

Determine if the class has an annotation by isannotationpresent:

        Class<?> uscls = UserService. class ;         // determine if the change class uses the annotation        boolean isannotationrservice = uscls                . Isannotationpresent (rservice. class);

Gets the value of this class annotation configuration:

    Rservice rservice = UserService. class. getannotation (rservice.  Class);          for (String v:rservice.value ()) {            System.out.println ("Annotated value:" + v);        }        System.out.println ("enum:" + Rservice.run ());

RHYTHMK Step-by-step JAVA (19): Annotations annotation