我看JAVA 之 Annotation
注:基于jdk11
注解包结构
名词解释:
- meta-annotation 元数据注解 表示用来声明注解的注解
- marker-annotation 标记注解 表示没有成员的注解
Annotation
Annotation 是一个被所有注解类型实现的通用接口。但是如果硬编码实现此接口不能定义一个注解类型,同时,此接口本身也不是一个注解类型。
所有注解声明格式为: @interface annotationName{ } 隐式表明某个类型(注解类型)实现了接口java.lang.annotation.Annotation
public interface Annotation {
boolean equals(Object obj);
int hashCode();
String toString();
Class<? extends Annotation> annotationType();
}
Target
meta-annotation,表示某注解类型应用在哪些语法元素上,默认应用在除TYPE外的所有语法元素上。
@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();
}
ElementType
枚举类型 表示某注解可以应用的元素上下文。
public enum ElementType {
TYPE,
FIELD,
METHOD,
PARAMETER,
CONSTRUCTOR,
LOCAL_VARIABLE,
ANNOTATION_TYPE,
PACKAGE,
/**
* Type parameter declaration
* @since 1.8
*/
TYPE_PARAMETER,
/**
* Use of a type
* @since 1.8
*/
TYPE_USE,
/**
* Module declaration.
* @since 9
*/
MODULE
}
Retention
meta-annotation 表示注解类型持有范围或生命周期。
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
/**
* Returns the retention policy.
* @return the retention policy
*/
RetentionPolicy value();
}
RetentionPolicy
枚举类型 表示持有注解周期策略。
public enum RetentionPolicy {
SOURCE,//应用在源文件上,主要做编译时检查,检查完毕即丢弃。
CLASS,//默认值,被编译器记录在字节码文件中,但不能被运行时持有。
RUNTIME//除了可以被编译器记录在字节码文件中,还可以被运行时持有。参见:java.lang.reflect. AnnotatedElement(表示运行在VM的程序中的可以被注解或注解了的元素)
}
Inherited
meta-annotation 同时也是marker-annotation 表示某个注解类型是可以被继承的
Documented
meta-annotation 同时也是marker-annotation 表示某个类型标记@Documented注解,那么此类型上所有注解会被JAVA Doc收集为API一部分
Native
meta-annotation 同时也是marker-annotation(since1.8),表明一个字段引用的值可能来自于本地代码
Repeatable
meta-annotation(since1.8)
异常
AnnotationFormatError
AnnotationTypeMismatchException
IncompleteAnnotationException
内部原理
以实现一个简单的orm注解的形式来描述Java的Annotation的内部原理,代码如下:
源代码
package chapter04;
import java.lang.annotation.*;
/**
* a orm's table annotation
*/
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
String name() default "";
String alias() default "";
}
package chapter04;
import java.lang.annotation.*;
/**
* a orm's column annotation
*/
@Documented
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
boolean primary() default false;
String name() default "";
String type() default "varchar";
int length() default 0;
String alias() default "";
}
package chapter04;
@Table(name = "t_student", alias = "s")
public class Student {
@Column(primary = true, name = "_id", type = "bigint", length = 20)
String id;
@Column(name = "name", type = "string", length = 32)
String name;
}
package chapter04;
public class TestAnno {
public static void main(String [] args) throws NoSuchFieldException {
System.setProperty("jdk.proxy.ProxyGenerator.saveGeneratedFiles", "true");
Table table = Student.class.getAnnotation(Table.class);
System.out.printf("table'name is %s, alias is %s. \n", table.name(), table.alias());
Column column = Student.class.getDeclaredField("id").getAnnotation(Column.class);
System.out.printf("column'name is %s, alias is %s. \n", column.name(), column.alias());
}
}
通过设置环境变量,可以保存jdk动态生成的代码,查看java.lang.reflect.ProxyGenerator类中的配置, 如下:
/** debugging flag for saving generated class files */
private static final boolean saveGeneratedFiles =
java.security.AccessController.doPrivileged(
new GetBooleanAction(
"jdk.proxy.ProxyGenerator.saveGeneratedFiles")).booleanValue();
jdk生成的代理代码
package com.sun.proxy;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements Retention {
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m0;
private static Method m3;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final Class annotationType() throws {
try {
return (Class)super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final RetentionPolicy value() throws {
try {
return (RetentionPolicy)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("java.lang.annotation.Retention").getMethod("annotationType");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
m3 = Class.forName("java.lang.annotation.Retention").getMethod("value");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
package com.sun.proxy;
import chapter04.Table;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy1 extends Proxy implements Table {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m5;
private static Method m4;
private static Method m0;
public $Proxy1(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final String name() throws {
try {
return (String)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final Class annotationType() throws {
try {
return (Class)super.h.invoke(this, m5, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String alias() throws {
try {
return (String)super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("chapter04.Table").getMethod("name");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m5 = Class.forName("chapter04.Table").getMethod("annotationType");
m4 = Class.forName("chapter04.Table").getMethod("alias");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
package com.sun.proxy;
import chapter04.Column;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy2 extends Proxy implements Column {
private static Method m1;
private static Method m7;
private static Method m3;
private static Method m4;
private static Method m2;
private static Method m8;
private static Method m5;
private static Method m6;
private static Method m0;
public $Proxy2(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final boolean primary() throws {
try {
return (Boolean)super.h.invoke(this, m7, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String name() throws {
try {
return (String)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String type() throws {
try {
return (String)super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final Class annotationType() throws {
try {
return (Class)super.h.invoke(this, m8, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int length() throws {
try {
return (Integer)super.h.invoke(this, m5, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String alias() throws {
try {
return (String)super.h.invoke(this, m6, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m7 = Class.forName("chapter04.Column").getMethod("primary");
m3 = Class.forName("chapter04.Column").getMethod("name");
m4 = Class.forName("chapter04.Column").getMethod("type");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m8 = Class.forName("chapter04.Column").getMethod("annotationType");
m5 = Class.forName("chapter04.Column").getMethod("length");
m6 = Class.forName("chapter04.Column").getMethod("alias");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
如上图,debug运行TestAnno,结合jdk动态生成的代理类可以得出如下结论:
Table、Column是实现了Annotation的特殊接口,而通过反射获取注解返回的是Java运行时生成的动态代理对象$Proxy1(Table的实现类)、$Proxy2(Column实现类)