[03] 处理注解：反射

1、AnnotatedElement接口

如果没有用来读取注解的方法和工作，那么注解也就不会比注释更有用处了。使用注解的过程中，很重要的一部分就是创建于使用注解处理器。Java SE5扩展了反射机制的API，以帮助程序员快速的构造自定义注解处理器。

Java用Annotation接口来代表程序元素前面的注解，该接口是所有Annotation类型的父接口。如在java.lang.reflect包中有一个接口AnnotatedElement，其中定义了一些注解相关的方法，如判断某元素是否标注了注解的方法：

boolean isAnnotationPresent(Class<? extends Annotation> annotationClass)
这里的传参实际上就是传入 ”某个注解.class“

另外，Java在java.lang.reflect 包下新增了 AnnotatedElement 接口：

/** * Represents an annotated element of the program currently running in this* VM.  This interface allows annotations to be read reflectively.  All* annotations returned by methods in this interface are immutable and* serializable.  It is permissible for the caller to modify the* arrays returned by accessors for array-valued enum members; it will* have no affect on the arrays returned to other callers.** <p>If an annotation returned by a method in this interface contains* (directly or indirectly) a {@link Class}-valued member referring to* a class that is not accessible in this VM, attempting to read the class* by calling the relevant Class-returning method on the returned annotation* will result in a {@link TypeNotPresentException}.** <p>Similarly, attempting to read an enum-valued member will result in* a {@link EnumConstantNotPresentException} if the enum constant in the* annotation is no longer present in the enum type.* * <p>Finally, Attempting to read a member whose definition has evolved* incompatibly will result in a {@link* java.lang.annotation.AnnotationTypeMismatchException} or an* {@link java.lang.annotation.IncompleteAnnotationException}.** @since 1.5* @author Josh Bloch*/

/**

 * Represents an annotated element of the program currently running in this

 * VM.  This interface allows annotations to be read reflectively.  All

 * annotations returned by methods in this interface are immutable and

 * serializable.  It is permissible for the caller to modify the

 * arrays returned by accessors for array-valued enum members; it will

 * have no affect on the arrays returned to other callers.

 * <p>If an annotation returned by a method in this interface contains

 * (directly or indirectly) a {@link Class}-valued member referring to

 * a class that is not accessible in this VM, attempting to read the class

 * by calling the relevant Class-returning method on the returned annotation

 * will result in a {@link TypeNotPresentException}.

 * <p>Similarly, attempting to read an enum-valued member will result in

 * a {@link EnumConstantNotPresentException} if the enum constant in the

 * annotation is no longer present in the enum type.

 * <p>Finally, Attempting to read a member whose definition has evolved

 * incompatibly will result in a {@link

 * java.lang.annotation.AnnotationTypeMismatchException} or an

 * {@link java.lang.annotation.IncompleteAnnotationException}.

 * @since 1.5

 * @author Josh Bloch

*/

该接口代表程序中可以接受注解的程序元素，这句话怎么理解？我们从另一头倒着来理：

（1）Field、Method、Constructor

在反射中我们知道的像属性、方法、构造函数等封装的类，都继承了一个类，叫AccessibleObject；

（2）AccessibleObject （参考链接）

java.lang.reflect包中有类AccessibleObject，从类的注释可以看出，The AccessibleObject class is the base class for Field, Method and Constructor objects. It provides the ability to flag a reflected object as suppressing default Java language access control checks when it is used. 显然，它可以让一个反射对象去禁止Java语言的访问控制检测，从而能够调用对象的私有属性和方法。

这个类实现了一个接口，就是我们提到的AnnotatedElement了，实现该接口的类（Represents an annotated element of the program currently running in this VM. This interface allows annotations to be read reflectively.）即代表那些可注解的元素，同时可以利用反射机制读取注解。所以，它的几个主要实现类有：

Class 类
Constructor 构造器
Field 类的成员变量
Method 类的方法
Package 类的包

注意：如Class、Package未继承AccessibleObject，毕竟这两个类不需要所谓的访问权限设置，但是它们是实现了AnnotatedElement接口的。

所以当我们通过反射获取某个类的AnnotatedElement对象之后，就可以调用AnnotatedElement接口定义好的几个方法：

//判断该程序元素上是否包含指定类型的注解，存在返回true，否则返回false
boolean isAnnotationPresent(Class<? extends Annotation> annotationClass);

//返回该程序元素上存在的、指定类型的注解，如果该类型注解不存在，则返回null
<T extends Annotation> T getAnnotation(Class<T> annotationClass);

//返回该程序元素上存在的所有注解
Annotation[] getAnnotations();

//返回直接存在于该元素上的所有注解（与该接口中其他方法不同，该方法将忽略继承的注解）
//如果没有注解直接存在于此元素上，则返回长度为零的一个数组
//该方法调用者可以随意修改返回的数组，不会对其他调用者返回的数组产生影响
Annotation[] getDeclaredAnnotations();

2、AnnotatedElement使用示例

2.1 自定义注解

/*** 水果名称注解*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitName {String value() default "";
}/*** 水果颜色注解*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitColor {/*** 颜色枚举*/public enum Color {BULE, RED, GREEN}/*** 颜色属性*/public Color fruitColor() default Color.GREEN;
}/*** 水果供应商注解*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitProvider {/*** 供应商编号*/public int id() default -1;/*** 供应商名称*/public String name() default "";/*** 供应商地址*/public String address() default "";}

/**

 * 水果名称注解

*/

@Target(ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface FruitName {

    String value() default "";

/**

 * 水果颜色注解

*/

@Target(ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface FruitColor {

/**

     * 颜色枚举

*/

    public enum Color {BULE, RED, GREEN}

/**

     * 颜色属性

*/

    public Color fruitColor() default Color.GREEN;

/**

 * 水果供应商注解

*/

@Target(ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface FruitProvider {

/**

     * 供应商编号

*/

    public int id() default -1;

/**

     * 供应商名称

*/

    public String name() default "";

/**

     * 供应商地址

*/

    public String address() default "";

注意：当一个Annotation类型被定义为运行时（RUNTIME）的Annotation时，该注解才能是运行时可见，当class文件被装载时被保存在class文件中的Annotation才会被虚拟机读取。

2.2 使用自定义注解的类

/*** 苹果类*/
public class Apple {/*** 苹果名称*/@FruitName("红富士")private String appleName;/*** 苹果颜色*/@FruitColor(fruitColor = FruitColor.Color.RED)private String appleColor;/*** 苹果供应商*/@FruitProvider(id = 1, name = "陕西红富士集团", address = "陕西省西安市延安路89号红富士大厦")private String appleProvider;public String getAppleName() {return appleName;}public void setAppleName(String appleName) {this.appleName = appleName;}public String getAppleColor() {return appleColor;}public void setAppleColor(String appleColor) {this.appleColor = appleColor;}public String getAppleProvider() {return appleProvider;}public void setAppleProvider(String appleProvider) {this.appleProvider = appleProvider;}
}

/**

 * 苹果类

*/

public class Apple {

/**

     * 苹果名称

*/

    @FruitName("红富士")

    private String appleName;

/**

     * 苹果颜色

*/

    @FruitColor(fruitColor = FruitColor.Color.RED)

    private String appleColor;

/**

     * 苹果供应商

*/

    @FruitProvider(id = 1, name = "陕西红富士集团", address = "陕西省西安市延安路89号红富士大厦")

    private String appleProvider;

    public String getAppleName() {

        return appleName;

    public void setAppleName(String appleName) {

        this.appleName = appleName;

    public String getAppleColor() {

        return appleColor;

    public void setAppleColor(String appleColor) {

        this.appleColor = appleColor;

    public String getAppleProvider() {

        return appleProvider;

    public void setAppleProvider(String appleProvider) {

        this.appleProvider = appleProvider;

2.3 注解处理

public class FruitInfoUtil {/*** 获取水果信息* @param clazz 类对象*/public static void getFruitInfo(Class<?> clazz) {String strFruitName;String strFruitColor;String strFruitProvider;//获取该类对象包含的所有属性对象Field[] fields = clazz.getDeclaredFields();//遍历属性对象for (Field field : fields) {//如果属性包含的注解是FruitNameif (field.isAnnotationPresent(FruitName.class)) {FruitName fruitName = field.getAnnotation(FruitName.class);strFruitName = "水果名称：" + fruitName.value();System.out.println(strFruitName);}//如果属性包含的注解是FruitColorelse if (field.isAnnotationPresent(FruitColor.class)) {FruitColor fruitColor = field.getAnnotation(FruitColor.class);strFruitColor = "水果颜色：" + fruitColor.fruitColor().toString();System.out.println(strFruitColor);}//如果属性包含的注解是FruitProviderelse if (field.isAnnotationPresent(FruitProvider.class)) {FruitProvider fruitProvider = field.getAnnotation(FruitProvider.class);strFruitProvider = "供应商编号：" + fruitProvider.id()+ "；供应商名称：" + fruitProvider.name()+ "；供应商地址：" + fruitProvider.address();System.out.println(strFruitProvider);}}}}

public class FruitInfoUtil {

/**

     * 获取水果信息

     * @param clazz 类对象

*/

    public static void getFruitInfo(Class<?> clazz) {

        String strFruitName;

        String strFruitColor;

        String strFruitProvider;

        //获取该类对象包含的所有属性对象

        Field[] fields = clazz.getDeclaredFields();

        //遍历属性对象

        for (Field field : fields) {

            //如果属性包含的注解是FruitName

            if (field.isAnnotationPresent(FruitName.class)) {

                FruitName fruitName = field.getAnnotation(FruitName.class);

                strFruitName = "水果名称：" + fruitName.value();

                System.out.println(strFruitName);

            //如果属性包含的注解是FruitColor

            else if (field.isAnnotationPresent(FruitColor.class)) {

                FruitColor fruitColor = field.getAnnotation(FruitColor.class);

                strFruitColor = "水果颜色：" + fruitColor.fruitColor().toString();

                System.out.println(strFruitColor);

            //如果属性包含的注解是FruitProvider

            else if (field.isAnnotationPresent(FruitProvider.class)) {

                FruitProvider fruitProvider = field.getAnnotation(FruitProvider.class);

                strFruitProvider = "供应商编号：" + fruitProvider.id()

                        + "；供应商名称：" + fruitProvider.name()

                        + "；供应商地址：" + fruitProvider.address();

                System.out.println(strFruitProvider);

2.4 测试和结果输出

public class TestAnnotation {public static void main(String[] args) {FruitInfoUtil.getFruitInfo(Apple.class);}
}//结果输出
水果名称：红富士
水果颜色：RED
供应商编号：1；供应商名称：陕西红富士集团；供应商地址：陕西省西安市延安路89号红富士大厦

public class TestAnnotation {

    public static void main(String[] args) {

        FruitInfoUtil.getFruitInfo(Apple.class);

//结果输出

水果名称：红富士

水果颜色：RED

供应商编号：1；供应商名称：陕西红富士集团；供应商地址：陕西省西安市延安路89号红富士大厦

3、参考链接

深入理解Java：注解（Annotation）--注解处理器

转载于:https://www.cnblogs.com/deng-cc/p/7462588.html