Java Custom Annotations and reflections

Java Annotations and Reflections
In Java, four kinds of meta-annotations are provided, which are specifically responsible for annotating other annotations, as follows
1, @Retention meta-annotations, indicating the level at which the annotation information (life cycle) needs to be saved. The optional retentionpoicy parameters include:
Retentionpolicy.source: Stuck in Java source file, compiler was discarded
Retentionpolicy.class: Stays in CLASS file, but is discarded by VM (default)
Retentionpolicy.runtime: In-memory bytecode, VMS will also retain annotations at run time, so the annotations can be read through the reflection mechanism
2, @Target yuan annotation, the default value is any element, indicating where the annotation is used. The available ElementType parameters include
Elementtype.constructor: Constructor declaration
Elementtype.field: Member variables, objects, attributes (including enum instances)
Elementtype.local_variable: local variable declaration
Elementtype.method: Method declaration
Elementtype.package: Package Declaration
Elementtype.parameter: Parameter declaration
Elementtype.type: Classes, interfaces (including annotation types) or enum declarations
3. @Documented to include annotations in Javadoc
4. @Inheried allow subclasses to inherit annotations from parent class
Ii. Definition of annotations
1, the format of the annotation definition
Public @interface Fieldmeta {}

 PackageCom.http.model;Importjava.lang.annotation.Documented;Importjava.lang.annotation.Retention;ImportJava.lang.annotation.Target;ImportJava.lang.annotation.RetentionPolicy;ImportJava.lang.annotation.ElementType, @Retention (retentionpolicy.runtime)//annotations exist in the class bytecode file and can be retrieved at run time through reflection@Target ({Elementtype.field})//define the purpose of the annotation * * scope field, enumerated constant/method@Documented//Note that the note will be included in the Javadoc Public@InterfaceFieldmeta {String name ()default""; String description ()default"";} 

2, the application of annotations in the entity class
@FieldMeta (name= "serial number", description= "")
Private Stringid;
@FieldMeta (name= "name", description= "")
Private Stringtitle;

 PackageCom.http.model; Public classmodeltest {@FieldMeta (name= "Serial number", description= "")    PrivateString ID; @FieldMeta (Name= "title", description= "title Information description")      PrivateString title;  PublicString getId () {returnID; }     Public voidsetId (String id) { This. ID =ID; }     PublicString GetTitle () {returntitle; }     Public voidSettitle (String title) { This. title =title; }}

III. Traversal reflection of annotations

    //reflection Gets object property name and annotation information     Public Static voidGetField () {Try{Class Clazz= Class.forName ("Com.http.model.HttpRequest"); field[] Field=Clazz.getdeclaredfields (); //System.out.println (field.length);             for(Field F:field) {BooleanIsexist=f.isannotationpresent (Fieldmeta.class); if(isexist) {Fieldmeta TT=f.getannotation (Fieldmeta.class); System.out.println ("--Annotations:" +tt.id () + "" +tt.name ());            } System.out.println (F.getname ()); }        } Catch(ClassNotFoundException e) {//TODO auto-generated Catch blockE.printstacktrace (); }     }    

Reflection Get Object Method name
 Public Static voidGetMethod () {Try{Class Clazz= Class.forName ("Com.http.model.HttpRequest"); Method[] Method=Clazz.getdeclaredmethods ();            System.out.println (method.length);  for(Method f:method) {System.out.println (F.getname ()); //Get method Name            }        } Catch(ClassNotFoundException e) {//TODO auto-generated Catch blockE.printstacktrace (); }     }

Java Custom Annotations and reflections