Java开发中的注解技术



概述

   

注解与注释，

        注解，告诉编译器如何运行程序！

        注释，给程序员阅读，对编译、运行没有影响；

 

注解作用，

        1.告诉编译器如何运行程序；

        2.简化(取代)配置文件  

常用的注解如下：

        

//重写父类的方法

   @Override

   public String toString() {

      returnsuper.toString();

   }

   

   //抑制编译器警告

   @SuppressWarnings({"unused","unchecked"})

   privatevoid save() {

       List list =null;

   }

   

   //标记方法以及过时

   @Deprecated

   privatevoidsave1() {

   }

元注解：表示注解的注解

   

指定注解的可用范围：

    

@Target({

TYPE,    类

FIELD,    字段

METHOD, 方法

PARAMETER,  参数

CONSTRUCTOR,构造器

 LOCAL_VARIABLE 局部变量

})

元注解 - 2.指定注解的声明周期

@Retention(RetentionPolicy.SOURCE)   注解只在源码级别有效

@Retention(RetentionPolicy.CLASS)     注解在字节码即别有效 默认值

@Retention(RetentionPolicy.RUNTIME)  注解在运行时期有效

    注解案例之优化BaseDao的代码

一、创建Bean实体类

           

package cn.sp.utils;@Table(tableName="admin")public class Admin2 {@ID@Column(columnName="id")private int id2;@Column(columnName="userName")private String userName2;@Column(columnName="pwd")private String pwd2;public int getId2() {return id2;}public void setId2(int id2) {this.id2 = id2;}public String getUserName2() {return userName2;}public void setUserName2(String userName2) {this.userName2 = userName2;}public String getPwd2() {return pwd2;}public void setPwd2(String pwd2) {this.pwd2 = pwd2;}@Overridepublic String toString() {return "Admin2 [id2=" + id2 + ", userName2=" + userName2 + ", pwd2=" + pwd2 + "]";}}

二、创建注解类

                    

package cn.sp.utils;import java.lang.reflect.Field;import java.lang.reflect.ParameterizedType;import java.lang.reflect.Type;import java.sql.ResultSet;import java.sql.SQLException;import java.util.ArrayList;import java.util.List;import org.apache.commons.beanutils.BeanUtils;import org.apache.commons.dbutils.ResultSetHandler;import cn.sp.dao.JdbcUtil;public class BaseDao_2 <T>{private Class<T> clazz;private String tableName;private String id_primary;public BaseDao_2() {Type type=this.getClass().getGenericSuperclass();ParameterizedType pt=(ParameterizedType) type;Type[] types = pt.getActualTypeArguments();clazz=(Class<T>) types[0];/*******获取表名******/Table table = clazz.getAnnotation(Table.class);tableName = table.tableName();//已经拿到Admin2.class//获取当前运行类的所有字段，遍历。获取每一个字段上的id注解Field[] fs = clazz.getDeclaredFields();for (Field field : fs) {//设置强制访问field.setAccessible(true);//获取每一个字段上的ID注解ID anno_id = field.getAnnotation(ID.class);//判断if(anno_id!=null){//如果当前字段有ＩＤ注解，说明是主键，再获取字段名称Column column = field.getAnnotation(Column.class);//主键id_primary=column.columnName();break;}}System.out.println("表:"+tableName);System.out.println("主键:"+id_primary);}public T findById(int id){try {String sql="select * from "+tableName+" where id=?";returnJdbcUtil.getQueryRunner().query(sql, new MyBeanHandler<T>(clazz),id);} catch (SQLException e) {e.printStackTrace();throw new RuntimeException(e);}}public List<T> getAll(){try {String sql="select * from "+tableName;returnJdbcUtil.getQueryRunner().query(sql, new MyBeanListHandler<T>(clazz));} catch (SQLException e) {e.printStackTrace();throw new RuntimeException(e);}}}/** * 自定义结果集封装单个Bean对象 */class MyBeanHandler<T> implements ResultSetHandler<T>{private Class<T> clazz;public MyBeanHandler(Class<T> clazz) {this.clazz=clazz;}@Overridepublic T handle(ResultSet rs) throws SQLException {try {T t=clazz.newInstance();if(rs.next()){Field[] fs = clazz.getDeclaredFields();for (Field f : fs) {String fieldName=f.getName();Column column = f.getAnnotation(Column.class);String columnName=column.columnName();Object columnValue=rs.getObject(columnName);//设置 BeanUtils组件BeanUtils.copyProperty(t, fieldName, columnValue);}}return t;} catch (Exception e) {throw new RuntimeException(e);}}}/**自定义结果集封装多个Bean对象 */class MyBeanListHandler<T>  implements ResultSetHandler<List<T>>{private Class<T> clazz;public MyBeanListHandler(Class<T> clazz) {this.clazz=clazz;}public List<T> handle(ResultSet rs) throws SQLException {ArrayList<T> list = new ArrayList<T>();try {while(rs.next()){T t=clazz.newInstance();Field[] fs = clazz.getDeclaredFields();for (Field f : fs) {String fieldName=f.getName();Column column = f.getAnnotation(Column.class);String columnName=column.columnName();Object columnValue=rs.getObject(columnName);//设置 BeanUtils组件BeanUtils.copyProperty(t, fieldName, columnValue);}list.add(t);}return list;} catch (Exception e) {throw new RuntimeException(e);}}}

创建类AdminDao2(用于测试执行)

package cn.sp.utils;import static org.junit.Assert.*;import java.util.List;import org.junit.Test;public class AdminDao2 extends BaseDao_2<Admin2> {@Testpublic void testName() throws Exception {AdminDao2 dao2 = new AdminDao2();Admin2 ad2 = dao2.findById(1);List<Admin2> list = dao2.getAll();System.out.println(ad2);System.out.println(list);}}

相关注解代码：

package cn.sp.utils;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;/** * 用于描述主键 * @author Administrator * */@Retention(RetentionPolicy.RUNTIME)  // 指定注解在运行时期有效public @interface ID {}

package cn.sp.utils;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;@Retention(RetentionPolicy.RUNTIME)  // 指定注解在运行时期有效public @interface Table {String tableName();}

package cn.sp.utils;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;@Retention(RetentionPolicy.RUNTIME)  // 指定注解在运行时期有效public @interface Column {String columnName();}

注意：一定要在注解上加元注解@Retention(RetentionPolicy.RUNTIME),否则运行时注解失效，会报空指针异常!