mybatis生成mapper接口（dao接口）对象原理（代理模式-JDK动态代理）-马育民老师

需要掌握：[设计模式-代理模式-JDK动态代理（Proxy.newProxyInstance）](https://www.malaoshi.top/show_1IX2q8u8R2V.html "设计模式-代理模式-JDK动态代理（Proxy.newProxyInstance）")

# 介绍

由于 mybatis 使用中，需要实现 mapper接口（dao接口），所以可通过 JDK动态代理，生成 接口对象

# 核心代码

### 生成 mapper对象

```
public class SqlSession {

public <T> T getMapper(Class<T> mapperInterface){
        Class[] array=new Class[]{mapperInterface};
        InvocationHandlerImpl ihi=new InvocationHandlerImpl(mapperInterface);
        Object agent= Proxy.newProxyInstance(mapperInterface.getClassLoader(),array,ihi);
        return (T)agent;
    }
}
```

### 实现 mapper对象方法

```
public class InvocationHandlerImpl implements InvocationHandler {
    private Class mapperInterface;
    public InvocationHandlerImpl(Class mapperInterface){
        this.mapperInterface=mapperInterface;
    }
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        String name=mapperInterface.getName();
        String methodName=method.getName();

String sql=SQL.get(name+"."+methodName);
        System.out.println("sql："+sql);
        System.out.println("传输参数:"+args[0]);

return 1;
    }
}
```

# 其他代码

为了简化操作，没有实现 `SqlSessionFactory` 类

### mapper配置文件

模拟 mapper.xml 配置文件
```
namespace=com.company.mapper.IStudentMapper
select=addStudent;INSERT INTO student(sid, sname, age, sex, addr, phone) VALUES (#{sid}, #{sname}, #{age}, #{sex}, #{addr}, #{phone});
```

### SqlSessionFactoryBuilder

模拟 SqlSessionFactoryBuilder 类
```
public class SqlSessionFactoryBuilder {

public SqlSession build(InputStream is) throws IOException {
        Properties p=new Properties();
        p.load(is);
        String namespace=p.getProperty("namespace");
        String select=p.getProperty("select");
        String[] array=select.split(";");

SQL.put(namespace+"."+array[0],array[1]);

return new SqlSession();
    }
}
```

### 存放 sql语句

```
public class SQL {

private static final Map<String,String> SQL_MAP=new HashMap();

public static void put(String key,String sql){
        SQL_MAP.put(key,sql);
    }
    public static String get(String key){
        return SQL_MAP.get(key);
    }
}
```

### 实体类

```
public class Student {
    private String sid;
    private String sname;
    private Integer age;

public String getSid() {
        return sid;
    }

public void setSid(String sid) {
        this.sid = sid;
    }

public String getSname() {
        return sname;
    }

public void setSname(String sname) {
        this.sname = sname;
    }

public Integer getAge() {
        return age;
    }

public void setAge(Integer age) {
        this.age = age;
    }

@Override
    public String toString() {
        return "Student{" +
                "sid='" + sid + '\'' +
                ", sname='" + sname + '\'' +
                ", age=" + age +
                '}';
    }
}
```

### mapper接口（dao接口）

```
public interface IStudentMapper {

public int addStudent(Student std);

}
```

### 测试类

```
import com.company.entity.Student;
import com.company.mapper.IStudentMapper;
import com.company.mybatis.SqlSession;
import com.company.mybatis.SqlSessionFactoryBuilder;

import java.io.FileInputStream;
import java.io.IOException;

public class Main {

public static void main(String[] args) throws IOException {

FileInputStream is=new FileInputStream("StudentMapper.properties");

SqlSession sqlSession=new SqlSessionFactoryBuilder().build(is);
        IStudentMapper mapper=sqlSession.getMapper(IStudentMapper.class);
        Student std=new Student();
        std.setSid("123");
        std.setSname("李雷");
        int i=mapper.addStudent(std);
        System.out.println(i);
    }
}
```

原文出处：http://malaoshi.top/show_1IX1lUKit7ow.html