Java注解

本文主要讲一下@Target，@Retention，@Constraint，@Documented：

• 四种注解简介
• Java8新增ElementType.TYPE_USE，ElementType.TYPE_PARAMETER用法

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@Target

注解作用的范围

• @Target(ElementType.TYPE) //类，接口（包括注解），枚举
• @Target(ElementType.FIELD) //字段（包括枚举常量）
• @Target(ElementType.METHOD) //方法
• @Target(ElementType.PARAMETER) //参数
• @Target(ElementType.CONSTRUCTOR) //构造函数
• @Target(ElementType.LOCAL_VARIABLE)//局部变量
• @Target(ElementType.ANNOTATION_TYPE)//注解
• @Target(ElementType.PACKAGE) //包
• @Target(ElementType.TYPE_USE) //类型
• @Target(ElementType.TYPE_PARAMETER) //类型
• 查看源码，一目了然

public enum ElementType {    /** Class, interface (including annotation type), or enum declaration */    TYPE,    /** Field declaration (includes enum constants) */    FIELD,    /** Method declaration */    METHOD,    /** Formal parameter declaration */    PARAMETER,    /** Constructor declaration */    CONSTRUCTOR,    /** Local variable declaration */    LOCAL_VARIABLE,    /** Annotation type declaration */    ANNOTATION_TYPE,    /** Package declaration */    PACKAGE,    /**     * Type parameter declaration     *     * @since 1.8     */    TYPE_PARAMETER,    /**     * Use of a type     *     * @since 1.8     */    TYPE_USE}

@Retention

表示注解可以保存的时间

• @Retention(RetentionPolicy.SOURCE) //注解保留在源码阶段，被编译器丢弃
• @Retention(RetentionPolicy.CLASS) // 注解被编译器保存在字节码文件中，在运行时丢弃，默认的保留行为
• @Retention(RetentionPolicy.RUNTIME)//被虚拟机保存，可用反射机制读取
• 查看源码，一目了然

public enum RetentionPolicy {    /**     * Annotations are to be discarded by the compiler.     */    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.     */    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.     *     * @see java.lang.reflect.AnnotatedElement     */    RUNTIME}

@Constraint

这个是在validation-api-1.1.0.Final.jar包里面的类，如果你要写自定义的校验类，可以引入这个jar包，并且在自定义的校验类里面写你的相关校验，举例如下：

• 在某接口的入参引入了自定义的注解（其他注解去掉了）：

/**投资金额**/@BigDecimalRange( minPrecision = 0, maxPrecision = 11, scale =2)private BigDecimal amount;

• 自定义的注解BigDecimalRange

@Target({METHOD, FIELD, ANNOTATION_TYPE})@Retention(RUNTIME)@Constraint(validatedBy = BigDecimalValidator.class)@Documentedpublic @interface BigDecimalRange {    public Class<?>[] groups() default {};    public Class<? extends Payload>[] payload() default {};    long minPrecision() default Long.MIN_VALUE;    long maxPrecision() default Long.MAX_VALUE;    int scale() default 0;}

• Constraint类源码

@Documented@Target({ ANNOTATION_TYPE })@Retention(RUNTIME)public @interface Constraint {    /**     * {@link ConstraintValidator} classes must reference distinct target types     * for a given {@link ValidationTarget}     * If two {@code ConstraintValidator}s refer to the same type,     * an exception will occur.     * <p/>     * At most one {@code ConstraintValidator} targeting the array of parameters of     * methods or constructors (aka cross-parameter) is accepted. If two or more     * are present, an exception will occur.     *     * @return array of (@code ConstraintValidator} classes implementing the constraint     */    Class<? extends ConstraintValidator<?, ?>>[] validatedBy();}

• 在自定义的注解里的用法如下：
  @Constraint(validatedBy = BigDecimalValidator.class)，BigDecimalValidator校验类里面的内容可以自己去写，如对amount范围的校验，最多几位，最少几位，以及小数点后的位数等等

@Documented

说明该注解将被包含在javadoc中

Java8新增TYPE_USE和TYPE_PARAMETER用法

package annotation;import java.util.ArrayList;import java.util.List;import java.util.Map;//ElementType.TYPE_USE是对类型的注解，如下面的泛型，String类型//ElementType.LOCAL_VARIABLE对局部变量的注解//ElementType.FIELD字段、枚举的常量public class TestTypeParam <@TestAnnotation T>{    List<@TestAnnotation Map<String, String>> l1 = new ArrayList<Map<String, String>>();    //ElementType.TYPE_USE 对String类型的注解，注意区别下面的test1    @TestAnnotation String test;    //ElementType.FIELD对test1的注解    @TestAnnotation java.lang.String test1;    public String test1(){        //ElementType.LOCAL_VARIABLE 对test2的注解        @TestAnnotation java.lang.String test2 = "hh";        return test2;    }}

反射机制读注解的值

• 自定义的注解TestAnnotation

package annotation;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;@Target({ElementType.METHOD, ElementType.TYPE_USE, ElementType.LOCAL_VARIABLE, ElementType.FIELD})@Retention(RetentionPolicy.RUNTIME)public @interface TestAnnotation {    public String value() default "{haha,henhao}";}

• 用到注解的接口

package interfabstr;import annotation.TestAnnotation;public interface TestInterface {    public static final String STR = "abc";    String str = "";    //public static String a; 接口里的变量都是final的，必须初始化    @TestAnnotation(value=STR)    public abstract int insert1();    @TestAnnotation(value=STR)    public String getString();    @TestAnnotation(value=STR)    public void add1();}

• 测试类

package interfabstr;import java.lang.reflect.Method;import annotation.TestAnnotation;public class TestMain {    public static void main(String[] args){        Class<TestInterface> ti = TestInterface.class;        Method[] mts = ti.getMethods();        for(Method mt : mts){            TestAnnotation ta = mt.getAnnotation(TestAnnotation.class);            String s = ta.value();            System.out.println("s: " + s);        }    }}程序运行结果：s: abcs: abcs: abc

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参考：
[1] http://ifeve.com/java-annotation/
[2] http://www.cnblogs.com/Gordon-YangYiBao/archive/2012/08/07/2626340.html