为什么要学习注解，学习注解有什么好处，学完能做什么？
1，能够读懂别人写的代码，特别是框架中的相关的代码。
2，让编程更加简洁，代码更加清晰
概念自己百度
java中常见的注解
~常见注解（JDK注解）
package 注解;
public interface Person {
public String name();
public int age();
@Deprecated//表示这个类中的方法已经过时了
public void sing();
}
package 注解;
public class Child implements Person{
@Override
public String name() {
// TODO Auto-generated method stub
return null;
}
@Override
public int age() {
// TODO Auto-generated method stub
return 0;
}
@Override
public void sing() {
// TODO Auto-generated method stub
}
}
package 注解;
public class MainTest {
@SuppressWarnings("deprecation")
public void sing(){
Person p = new Child();
//虽然这个方法过时了，但是我要用这个方法，我就要添加注解
p.sing();
}
public static void main(String[] args) {
}
}
常见的第三方注解
~Spring中的注解{@Autowired,@Service,@Repository};MyBatis中的注解{@InsertProvider,@UpdateProvider,@Options};
public class UserMangerImpl implements UserManger{
private UserDao userDao;
public void setUserDao(UserDao userDao){
this.userDao = userDao;
}
}
用XML文件配置
//配置文件
<bean id="userMangerImpl" class="com.ss.spring.annotion.service.UserMangerImpl">
<property name="userDao" ref="userDao"/>
</bean>
<bean id="userDao" class="com.ss.spring.annotion.persistence.UserMangerDaoImpl">
<property name="sessionFactory" ref="mySessionFactory"/>
</bean>
//注解 引入Autowired
public class UserMangerImpl implements UserManger{
@Autowired
private UserDao userDao;
}
~注解分类
源码注解
编译注解
运行注解
~自定义注解
语法：
@Target({ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)//生命周期
@Inherited//允许子类继承
@Documented//生成javadoc带有注解信息
public @interface Description{
String desc();//成员无参无异常的声明方式
String author()
int age() default 18;//可以用default为成员指定一个默认值
}
定义注解的时候，成员类型是受限制的，合法的类型有原始类型及String,Class,Annotation,Enumeration.
如果注解只有一个成员，成员的名称必须取名是value()，在使用的时候可以忽略成员名和复制号（=）
使用注解的语法：
@<注解名>（<成员名1>=<成员值1>，<成员名2>=<成员值2>）
比如：@Description(desc="i am tom",author="boy",age=18)
public String eyeColor(){
return "red";
}
~解析注解
package 注解;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
public class Parse {
public static void main(String[] args) {
//1使用类加载器加载类
try {
Class c = Class.forName("注解.Child");
//2找到类上面的注解
boolean isExit = c.isAnnotationPresent(Description.class);
if (isExit) {
//3拿到注解实例
Description description =(Description)c.getAnnotation(Description.class);
System.out.println(description);
}
//找到方法上的注解
Method[] ms = c.getMethods();
for (Method m:ms) {
boolean isMethodExit = m.isAnnotationPresent(Description.class);
if (isMethodExit) {
//3拿到注解实例
Description description =(Description)m.getAnnotation(Description.class);
System.out.println(description);
}
}
//另一种解析方法
for (Method m:ms) {
Annotation[] as = m.getAnnotations();
for (Annotation a:as) {
if (a instanceof Description) {
Description description =(Description)a;
System.out.println(description.value());
}
}
}
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
继承的测试
package 注解;
@Description("i am interfacer")
public class Person {
@Description("i am interface method")
public String name(){
return null;
}
public int age(){
return 0;
}
@Deprecated//表示这个类中的方法已经过时了
public void sing(){
}
}
package 注解;
public class Child extends Person{
public String name() {
// TODO Auto-generated method stub
return null;
}
@Override
public int age() {
// TODO Auto-generated method stub
return 0;
}
@Override
public void sing() {
// TODO Auto-generated method stub
}
}
package 注解;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
public class Parse {
public static void main(String[] args) {
//1使用类加载器加载类
try {
Class c = Class.forName("注解.Child");
//2找到类上面的注解
boolean isExit = c.isAnnotationPresent(Description.class);
if (isExit) {
//3拿到注解实例
Description description =(Description)c.getAnnotation(Description.class);
System.out.println(description);
}
//找到方法上的注解
Method[] ms = c.getMethods();
for (Method m:ms) {
boolean isMethodExit = m.isAnnotationPresent(Description.class);
if (isMethodExit) {
//3拿到注解实例
Description description =(Description)m.getAnnotation(Description.class);
System.out.println(description);
}
}
//另一种解析方法
for (Method m:ms) {
Annotation[] as = m.getAnnotations();
for (Annotation a:as) {
if (a instanceof Description) {
Description description =(Description)a;
System.out.println(description.value());
}
}
}
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
结果：@注解.Description(value=i am interfacer)
*注解实战
需求：有一张用户表，字段包括用户的id,用户名，昵称，年龄，性别，所在城市，邮箱，手机号
方便的对每一个字段或者字段的组合条件进行检索并打印SQL.
package com.zifeng.bean;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
String value();
}
package com.zifeng.bean;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
String value();
}
package com.zifeng.bean;
@Table("user")
public class Filter {
@Column("id")
private int id;
@Column("user_name")
private String userName;
@Column("user_nick")
private String nickName;
@Column("age")
private int age;
@Column("user_city")
private String city;
@Column("user_email")
private String email;
@Column("user_monile")
private String mobile;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getNickName() {
return nickName;
}
public void setNickName(String nickName) {
this.nickName = nickName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getMobile() {
return mobile;
}
public void setMobile(String mobile) {
this.mobile = mobile;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append("Filter [id=");
builder.append(id);
builder.append(", userName=");
builder.append(userName);
builder.append(", nickName=");
builder.append(nickName);
builder.append(", age=");
builder.append(age);
builder.append(", city=");
builder.append(city);
builder.append(", email=");
builder.append(email);
builder.append(", mobile=");
builder.append(mobile);
builder.append("]");
return builder.toString();
}
}
package com.zifeng.bean;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class Test {
public static void main(String[] args) {
Filter f1 = new Filter();
f1.setId(8);//查询id为8的用户
Filter f2 = new Filter();
f2.setUserName("Tom");
Filter f3 = new Filter();
f3.setEmail("liu@sina.com");
String sql1 = query(f1);
String sql2 = query(f2);
String sql3 = query(f3);
System.out.println(sql1);
System.out.println(sql2);
System.out.println(sql3);
}
private static String query(Filter f) {
StringBuffer sb = new StringBuffer();
//获取class
Class c = f.getClass();
//获取table的名称
boolean exists = c.isAnnotationPresent(Table.class);
if (!exists) {
return null;
}
Table t = (Table)c.getAnnotation(Table.class);
String tableName = t.value();
sb.append("select * from ").append(tableName).append(" where 1=1");
//遍历所有的字段
Field[] fArray = c.getDeclaredFields();
for (Field field:fArray) {
//处理每个字段对应的sql
//拿到字段的名
boolean fExsist = field.isAnnotationPresent(Column.class);
if (!fExsist) {
continue;
}
Column column = field.getAnnotation(Column.class);
String columnName = column.value();
//拿到字段的值
Object fieldValue = "";
String filedName = field.getName();
String getMethodName = "get"+filedName.substring(0,1).toUpperCase()+filedName.substring(1);
try {
Method getMethod = c.getMethod(getMethodName);
fieldValue = (Object)getMethod.invoke(f);
} catch (Exception e) {
e.printStackTrace();
}
//拼接sql
if (fieldValue == null || (fieldValue instanceof Integer && (Integer) fieldValue == 0)) {
continue;
}
sb.append(" and ").append(filedName);
if (fieldValue instanceof String) {
if (((String) fieldValue).contains(",")) {
String[] values = ((String) fieldValue).split(",");
sb.append(" in(");
for (String v:values) {
sb.append("'");
sb.append(v).append("'").append(",");
}
sb.deleteCharAt(sb.length()-1);
sb.append(")");
} else {
sb.append(" = ").append("'").append(fieldValue).append("'");
}
} else if (fieldValue instanceof Integer){
sb.append(" = ").append(fieldValue);
}
}
return sb.toString();
}
}
执行的结果是：
select * from user where 1=1 and id = 8
select * from user where 1=1 and userName = 'Tom'
select * from user where 1=1 and email = 'liu@sina.com'
用注解很方便生成sql语句，生成各种表的sql语句，你只需要改变一下bean实体就行了。