1. 什么是注解
2. 为什么要使用注解
3. 基本语法
   1. 4种基本元注解
   2. 重复注解
4. 使用注解
   1. 运行时处理的注解
   2. 编译时处理的注解

@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{@Overridepublic 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 AnnotationPersistent 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Annotationif(f.getKind()==ElementKind.FIELD){//获取成员变量定义前的@Id AnnotationId 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 AnnotationProperty 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>