[Java] 全面剖析java中的注解（Annotation）

@override
public string tostring() {
 return "this is string representation of current object.";
}

@target(elementtype.method)
@retention(retentionpolicy.source)
public @interface override {
}

@target(elementtype.method)
@retention(retentionpolicy.runtime)
@interface todo {
 public enum priority {low, medium, high}
 public enum status {started, not_started}
 string author() default "yash";
 priority priority() default priority.low;
 status status() default status.not_started;
}

@todo(priority = todo.priority.medium, author = "yashwant", status = todo.status.started)
public void incompletemethod1() {
//some business logic is written
//but it's not complete yet
}

@interface author{
string value();
}
@author("yashwant")
public void somemethod() {
}

//指定注解保留到运行时
@retention(retentionpolicy.runtime)
//指定注解可以修饰类、接口、枚举
@target(elementtype.type)
@interface mytag
{
 string name() default "测试";
 int age() default 20;
}

//指定注解保留到运行时
@retention(retentionpolicy.runtime)
//指定注解可以修饰类、接口、枚举
@target(elementtype.type)
@interface mytags
{
 mytag[] value();
}

@mytags({
 @mytag(name="测试1",age=21),
 @mytag(name="测试2",age=22)
})
public class test {
 public static void main(string[] args)
 {
 //通过反射解析注解
 class testclass= test.class;
 //获得mytags注解
 mytags mytagsannotation= (mytags) testclass.getannotation(mytags.class);
 //获得添加到里面的mytag注解
 mytag[] mytags=mytagsannotation.value();
 for(mytag mytag : mytags)
 {
 system.out.println(string.format("name:%1$s,age:%2$d",mytag.name(),mytag.age()));
 }
 }
}

@mytags({
 @mytag(name="测试1",age=21),
 @mytag(name="测试2",age=22)
})

@mytag(name="测试1",age=21)
@mytag(name="测试2",age=22)

//指定注解保留到运行时
@retention(retentionpolicy.runtime)
//指定该注解只能用于方法
@target(elementtype.method)
@interface debug
{
 boolean value() default false;
}

public class debugtest {
 public static void main(string[] args) {
 class debugtestclass = debugtest.class;
 //获得所有的方法
 method[] methods = debugtestclass.getmethods();
 for (method method : methods) {
 method.setaccessible(true);//禁用安全机制
 if (method.isannotationpresent(debug.class)) {//检查是否使用了debug注解
 debug debug = method.getannotation(debug.class);//获得注解实例
 string name = method.getname();//获得方法名称
 if (debug.value()) {
  system.out.println("method:" + name + " should debug");
 } else {
  system.out.println("method:" + name + " should't debug");
 }
 }
 }
 }
 @debug(false)
 public void testmethod1() {
 }
 @debug(true)
 public void testmethod2() {
 }
 @debug(true)
 public void testmethod3() {
 }
 @debug(false)
 public void testmethod4() {
 }
 @debug(true)
 public void testmethod5() {
 }
}

//指定该注解可以修饰类，接口，枚举
@target(elementtype.type)
//指定该注解保留到编译时
@retention(retentionpolicy.source)
//指定该注解可以被显示在文档中(通过javadoc生成文档，便可以在被该注解修饰的元素上看到该注解信息)
@documented
public @interface persistent {
 string table();
}

//指定该注解只能修饰字段
@target(elementtype.field)
//指定该注解保留到编译时
@retention(retentionpolicy.source)
//指定该注解可以被显示在文档中(通过javadoc生成文档，便可以在被该注解修饰的元素上看到该注解信息)
@documented
public @interface id {
 string column();
 string type();
 string generator();
}

//指定该注解只能修饰字段
@target(elementtype.field)
//指定该注解保留到编译时
@retention(retentionpolicy.source)
//指定该注解可以被显示在文档中(通过javadoc生成文档，便可以在被该注解修饰的元素上看到该注解信息)
@documented
public @interface property {
 string column();
 string type();
}

@persistent(table="personinfo")
public class person {
 @id(column="person_id",type="integer",generator="identity")
 private int id;
 @property(column="person_name",type="string")
 private string name;
 @property(column="person_age",type="integer")
 private int age;
 
 public person(){}
 
 public person(int id,string name,int age)
 {
 this.id=id;
 this.name=name;
 this.age=age;
 }
 //所有属性的setter和getter.....
}

import java.io.file;
import java.io.fileoutputstream;
import java.io.printstream;
import java.util.set;
 
import javax.annotation.processing.abstractprocessor;
import javax.annotation.processing.roundenvironment;
import javax.annotation.processing.supportedannotationtypes;
import javax.annotation.processing.supportedsourceversion;
import javax.lang.model.sourceversion;
import javax.lang.model.element.element;
import javax.lang.model.element.elementkind;
import javax.lang.model.element.name;
import javax.lang.model.element.typeelement;
 
//指定该注解支持java平台的最新版本为6.0
@supportedsourceversion(sourceversion.release_6)
//指定可以处理persistent,id,property注解
@supportedannotationtypes({"persistent","id","property"})
public class hibernateannotationprocessor extends abstractprocessor{
 @override
 public boolean process(set<? extends typeelement> annotations,
 roundenvironment roundenv) {
 //定义文件输出流,用于生成额外的文件
 printstream ps=null;
 try{
 for(element t:roundenv.getelementsannotatedwith(persistent.class)){
 //获取正在处理的类名称
 name classname=t.getsimplename();
 //获得类定义前的@persistent annotation
 persistent per= t.getannotation(persistent.class);
 //创建文件输出流
 ps=new printstream(new fileoutputstream(new file(classname+".hbm.xml")));
 //执行输出
 ps.println("<?xml version="1.0"?>");
 ps.println("<!doctype hibernate-mapping public "-//hibernate/hibernate mapping dtd 3.0//en"");
 ps.println(""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">");
 ps.println("<hibernate-mapping>");
 ps.println("<class name=""+classname+"" table=""+per.table()+"" >");
 for(element f:t.getenclosedelements())
 {
  //只处理成员变量上的annotation
  if(f.getkind()==elementkind.field)
  {
  //获取成员变量定义前的@id annotation
  id id=f.getannotation(id.class);
  //但@id注解存在时，输出<id ../>元素
  if(id!=null)
  {
  ps.println("<id name=""+f.getsimplename()+"" "+
   "column=""+id.column()+"" "+
   "type=""+id.type()+"">");
  ps.println("<generator class=""+id.generator()+"" />");
  ps.println("</id>");
  continue;
  }
  //获取成员变量前的@property annotation
  property p=f.getannotation(property.class);
  if(p!=null)
  {
  ps.println("<property name=""+f.getsimplename()+"" "+
   "column=""+p.column()+"" "+
   "type=""+p.type()+"" />");
  continue;
  }
  }
 }
 ps.println("</class>");
 ps.println("</hibernate-mapping>");
 }
 }catch(exception e)
 {
 e.printstacktrace();
 }finally{
 if(ps!=null)
 ps.close();
 }
 return true;
 }
}

javac -processor hibernateannotationprocessor person.java

<?xml version="1.0"?>
<!doctype hibernate-mapping public "-//hibernate/hibernate mapping dtd 3.0//en"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="person" table="personinfo" >
<id name="id" column="person_id" type="integer">
<generator class="identity" />
</id>
<property name="name" column="person_name" type="string" />
<property name="age" column="person_age" type="integer" />
</class>
</hibernate-mapping>