java泛型反射总结

引入：

      在需求中，数据库有两张表user,admin。我们要查询里面的id，name等信息通常都是写两个dao，然后分别给每个查询字段写一套方法。

     然而其实查询这些字段的方法都大同小异，所以产生了一个更好的解决办法，就是写一个通用的dao，然后把相同的方法写在通用的dao（genericDao）里,然后然实体dao去继     承它。

   

    下面通过模拟 贴出一般做法 和 优化后的做法：

    首先 在数据库里建好两张表，分别为user和admin,再插入一些数据

   

CREATE TABLE USER(id INT PRIMARY KEY,userName VARCHAR(20),age INT);CREATE TABLE admin(id INT PRIMARY KEY,adminName VARCHAR(20),status_ INT);INSERT INTO USER VALUES(1,'李四',21);INSERT INTO USER VALUES(2,'张三',20);INSERT INTO admin VALUES(1,'超级管理员',1);INSERT INTO admin VALUES(2,'普通管理员',0);

        截下来写两个实体类

public class admin {private int id;private String adminName;private int status_;public admin() {super();// TODO Auto-generated constructor stub}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getAdminName() {return adminName;}public void setAdminName(String adminName) {this.adminName = adminName;}public int getStatus_() {return status_;}public void setStatus_(int status_) {this.status_ = status_;}@Overridepublic String toString() {return "admin [id=" + id + ", adminName=" + adminName + ", status_=" + status_ + "]";}}

public class user {private int id;private String userName;private int age;public user() {super();// TODO Auto-generated constructor stub}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 int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "userDao [id=" + id + ", userName=" + userName + ", age=" + age + "]";}}

一般做法

   

public class adminDao{public admin getById(int id){QueryRunner qr=jdbcUtil.getQueryRunner();try {return (admin)qr.query("select * from admin where id=", new BeanHandler(admin.class),new Object[]{id});} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();throw new RuntimeException(e);}}@Testpublic void testId(){adminDao adminDao = new adminDao();admin byId = adminDao.getById(2);System.out.println(byId);}}

public class userDao extends genericDao<user>{public user getById(int id){QueryRunner qr=jdbcUtil.getQueryRunner();try {return (user)qr.query("select * from user where id=?", new BeanHandler(user.class),new Object[]{id});} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();throw new RuntimeException(e);}}@Testpublic void testId(){userDao userDao = new userDao();user byId = userDao.getById(2);System.out.println(byId);}}

利用泛型优化后的做法：

    首先写一个genericDao

  

** * 通用的dao方法 * @author Administrator * */public class genericDao <T>{private Class clazz;private String tableName;public genericDao(Class clazz,String tableName){this.clazz=clazz;this.tableName=tableName;}public  T getById(int id){QueryRunner qr=jdbcUtil.getQueryRunner();try {return (T)qr.query("select * from "+tableName+" where id=?", new BeanHandler(clazz),new Object[]{id});} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();throw new RuntimeException(e);}}}

然后让实体Dao类分别继承它

public class adminDao extends genericDao<admin>{public adminDao() {super(admin.class, "admin");}@Testpublic void testId(){adminDao adminDao = new adminDao();admin byId = adminDao.getById(2);System.out.println(byId);}}

public class userDao extends genericDao<user>{public userDao() {super(user.class, "user");}@Testpublic void testId(){userDao userDao = new userDao();user byId = userDao.getById(2);System.out.println(byId);}}

利用泛型反射技术优化后的做法

generic类

/** * 通用的dao方法 * @author Administrator * */public class genericDao <T>{private Class clazz;private String tableName;public genericDao(){System.out.println(this);//this 指当前运行的类对象  如果有子类继承它，则它指的是子类，而不是generic本身类System.out.println(this.getClass());//获得当前运行类的字节码System.out.println(this.getClass().getSimpleName());//获得当前运行类的类名System.out.println(this.getClass().getSuperclass());//获得当前运行类的父类的字节码System.out.println(this.getClass().getGenericSuperclass());//获得参数化类型  reflect.genericDao<reflect.admin>/** * 参数化类型：genericDao<reflect.admin> * 实体类型：reflect.admin *///1.先获得当前运行类的泛型父类类型Type type = this.getClass().getGenericSuperclass();//2.强转为参数化类型ParameterizedType p=(ParameterizedType)type;//3.获取参数化类型中，实际类型的定义Type[] actualTypeArguments = p.getActualTypeArguments();//获取数组的第一个元素，即我们想要的实际类型的字节码类型clazz=(Class<T>) actualTypeArguments[0];//给表名赋值tableName=clazz.getSimpleName();}public  T getById(int id){QueryRunner qr=jdbcUtil.getQueryRunner();try {return (T)qr.query("select * from "+tableName+" where id=?", new BeanHandler(clazz),new Object[]{id});} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();throw new RuntimeException(e);}}}

 简化后的dao类

public class userDao extends genericDao<user>{@Testpublic void testId(){userDao userDao = new userDao();user byId = userDao.getById(2);System.out.println(byId);}}

public class adminDao extends genericDao<admin>{@Testpublic void testId(){adminDao adminDao = new adminDao();admin byId = adminDao.getById(2);System.out.println(byId);}}

得出结论：子类继承父类，并传一个实际类型给父类，父类可以在类中通过反射技术先获得参数化类型，再获得参数化类型中的实际类型。即xxx.class