Java annotation浅析

自定义annotation

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD,ElementType.METHOD,ElementType.TYPE})
public @interface TestAnnotation {
    //default关键字是用来设置注解的默认值，可有可没有
    String value() default("Hello,I am a field");
    String [] name() default {};
}

使用自定义的annotation

public class HelloWorld {
   
    @TestAnnotation(name={"walson","ruby"})
    private String name;

//没有写明就表示value=“值”
    @TestAnnotation("hello")
    private String msg;

public String getName() {
        return name;
    }

public void setName(String name) {
        this.name = name;
    }

public String getMsg() {
        return msg;
    }

public void setMsg(String msg) {
        this.msg = msg;
    }
    
    
    public static void main(String[] args) {
        
    }
}

annotation的解析

public class AnnotationRelolver{

public static void relolver(Class<?> clazz){
        for(Field field : clazz.getDeclaredFields()){
            TestAnnotation testAnnotation = field.getAnnotation(TestAnnotation.class);
            System.out.println(field.getName() + " 上注解的Value值: " + testAnnotation.value());
            
            StringBuilder stringBuilder = new StringBuilder("[");
            String[] names = testAnnotation.name();
            int count = 0;
            for(String str : names){
                stringBuilder.append(str);
                count ++ ;
                if(count < names.length){
                    stringBuilder.append(",");
                }
                
                
            }
            stringBuilder.append("]");
            System.out.println(field.getName() + " 上注解的name值: " + stringBuilder.toString());
        }    
    }
    
    public static void main(String[] args) {
        AnnotationRelolver.relolver(HelloWorld.class);
    }
}

annotation源码分析

jdk1.5引入了annotation包，位于java.lang.annotation包下：

Annotation

package java.lang.annotation;

public interface Annotation {
 
    boolean equals(Object obj);

int hashCode();

String toString();
　 //返回此 annotation 的注释类型
    Class<? extends Annotation> annotationType();
}

1.所有自定义的注解都默认的继承这接口

ElementType

public enum ElementType {

　 //类，接口，枚举声明
    TYPE,
　 //字段声明
    FIELD,
　 //方法声明
    METHOD,
　 //参数声明
    PARAMETER,
　 //构造方法声明
    CONSTRUCTOR,
　 //局部变量声明
    LOCAL_VARIABLE,
　 //注解类型声明
    ANNOTATION_TYPE,
　 //包声明
    PACKAGE,

/**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

/**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE
}

这个枚举类主要用来定义注解可以声明在那些元素上

RetentionPolicy

package java.lang.annotation;

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.

　  * 注解只保留在源码中即*.java文件中，编译成class文件时将被废弃
     */
    SOURCE,

/**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.

     * 注解保留在源码中即*.java与class文件中，VM运行时将不被保留，默认为class
     */
    CLASS,

/**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     * 注解保留在源码中即*.java与class文件中，VM运行时也读取注解信息
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

这个枚举类用于声明注解的作用范围

Retention

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}
这是jdk自定义的一注解类，其value方法主要用于第一注解类时设置注解的作用方法

Target

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    /**
     * Returns an array of the kinds of elements an annotation type
     * can be applied to.
     * @return an array of the kinds of elements an annotation type
     * can be applied to
     */
    ElementType[] value();
}

这个注解类主要用于声明注解可以声明在哪些元素上

Inherited

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

默认注解是不能继承的，即一注解注解在父类上，默认其子类是不能使用父类的注解的，但是如果在定义注解类时加上Inherited这个注解那么父类所有的注解将自动被之类继承下来

例如：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Inherited
public @interface DBTable {    
    public String name() default "";    
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface DBTable2 {
    public String name() default "";    
}

package com.jyz.study.jdk.reflect;

import java.util.Arrays;

import com.jyz.study.jdk.annotation.DBTable;
import com.jyz.study.jdk.annotation.DBTable2;

/**
 * 1.演示从Class对象上获得反射元素Field Method Constructor
 * 2.演示AnnotatedElement接口的四个方法
 * @author JoyoungZhang@gmail.com
 *
 */
public class DeclaredOrNot {

public static void main(String[] args) {
        Class<Sub> clazz = Sub.class;
        System.out.println("============================Field===========================");
        //public + 继承
        System.out.println(Arrays.toString(clazz.getFields()));
        //all + 自身
        System.out.println(Arrays.toString(clazz.getDeclaredFields()));
        
        System.out.println("============================Method===========================");
        //public + 继承
        System.out.println(Arrays.toString(clazz.getMethods()));
        //all + 自身
        System.out.println(Arrays.toString(clazz.getDeclaredMethods()));
        
        System.out.println("============================Constructor===========================");
        //public + 自身
        System.out.println(Arrays.toString(clazz.getConstructors()));
        //all + 自身
        System.out.println(Arrays.toString(clazz.getDeclaredConstructors()));
        
        
        System.out.println("============================AnnotatedElement===========================");
        //注解DBTable2是否存在于元素上
        System.out.println(clazz.isAnnotationPresent(DBTable2.class));
        //如果存在该元素的指定类型的注释DBTable2，则返回这些注释，否则返回 null。
        System.out.println(clazz.getAnnotation(DBTable2.class));
        //继承
        System.out.println(Arrays.toString(clazz.getAnnotations()));
        //自身
        System.out.println(Arrays.toString(clazz.getDeclaredAnnotations()));
    }
}

@DBTable
class Super{
    private int superPrivateF;
    public int superPublicF;
    
    public Super(){
    }
    
    private int superPrivateM(){
        return 0;
    }
    public int superPubliceM(){
        return 0;
    }
}

@DBTable2
class Sub extends Super{
    private int subPrivateF;
    public int subPublicF;
    
    private Sub(){
    }
    public Sub(int i){
    }
    
    private int subPrivateM(){
        return 0;
    }
    public int subPubliceM(){
        return 0;
    }
}

console output:
============================Field===========================
[public int com.jyz.study.jdk.reflect.Sub.subPublicF, public int com.jyz.study.jdk.reflect.Super.superPublicF]
[private int com.jyz.study.jdk.reflect.Sub.subPrivateF, public int com.jyz.study.jdk.reflect.Sub.subPublicF]
============================Method===========================
[public int com.jyz.study.jdk.reflect.Sub.subPubliceM(), public int com.jyz.study.jdk.reflect.Super.superPubliceM(), public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException, public final void java.lang.Object.wait() throws java.lang.InterruptedException, public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException, public boolean java.lang.Object.equals(java.lang.Object), public java.lang.String java.lang.Object.toString(), public native int java.lang.Object.hashCode(), public final native java.lang.Class java.lang.Object.getClass(), public final native void java.lang.Object.notify(), public final native void java.lang.Object.notifyAll()]
[private int com.jyz.study.jdk.reflect.Sub.subPrivateM(), public int com.jyz.study.jdk.reflect.Sub.subPubliceM()]
============================Constructor===========================
[public com.jyz.study.jdk.reflect.Sub(int)]
[private com.jyz.study.jdk.reflect.Sub(), public com.jyz.study.jdk.reflect.Sub(int)]
============================AnnotatedElement===========================
true
@com.jyz.study.jdk.annotation.DBTable2(name=)
[@com.jyz.study.jdk.annotation.DBTable(name=), @com.jyz.study.jdk.annotation.DBTable2(name=)]
[@com.jyz.study.jdk.annotation.DBTable2(name=)]

Repeatable

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Repeatable {
    /**
     * Indicates the <em>containing annotation type</em> for the
     * repeatable annotation type.
     * @return the containing annotation type
     */
    Class<? extends Annotation> value();
}

这个类是jdk1.8引入的

在1.8之前重复注解如下：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface AuthorityOldVersion {
    String role();
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface AuthoritiesOldVersion {
    AuthorityOldVersion[] value();
}

public class RepeatAnnotationUseOldVersion {
    @AuthoritiesOldVersion({@AuthorityOldVersion(role="Admin"),@AuthorityOldVersion(role="Manager")})
    public void doSomeThing(){
    }
}

jdk1.8之后：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Repeatable(AuthoritiesNewVersion.class)
public @interface AuthorityNewVersion {
    String role();
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface AuthoritiesNewVersion {
    AuthorityNewVersion[] value();
}

public class RepeatAnnotationUseNewVersion {
    @AuthorityNewVersion(role="Admin")
    @AuthorityNewVersion(role="Manager")
    public void doSomeThing(){
    }
}

不同的地方是，创建重复注解Authority时，加上@Repeatable,指向存储注解Authorities，在使用时候，直接可以重复使用Authority注解。从上面例子看出，java 8里面做法更适合常规的思维，可读性强一点

Native

/**
 * Indicates that a field defining a constant value may be referenced
 * from native code.
 *
 * The annotation may be used as a hint by tools that generate native
 * header files to determine whether a header file is required, and
 * if so, what declarations it should contain.
 *
 * @since 1.8
 */
@Documented
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.SOURCE)
public @interface Native {
}

具体可参考JEP 104 ：Java类型注解，JEP 120 ：重复注解，JSR 175: A Metadata Facility for the Java^TM Programming Language