为什么mybatis的mapper没有实现类(原理探究)

//答案是用JDK动态代理实现的

下午想到了一个JAVA中的一个很基础的问题，java中接口是不能实例化的,想到这点，让我想起了mybatis框架中的mapper的用法，我们有用mapper时，全都是没有实现类的，只有一个mapper接口，而我们在调用的时候，通过spring注入到适当的service或其他类中就可以用了，那么它的原理是什么呢，mapper调用时又是在哪里进行了实现的呢？

带着这个问题，我重新复习了下mybatis的用法，为了测试方便，我在mysql上建了一个user表，并插入了3条记录，并写了一个简单的mybatis例子，代码如下：

package com.haiyang.learn.mybaits;
import com.haiyang.learn.mybaits.mapper.UserMapper;
import com.haiyang.learn.mybaits.pojo.User;
import org.apache.ibatis.datasource.pooled.PooledDataSource;
import org.apache.ibatis.mapping.Environment;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.defaults.DefaultSqlSessionFactory;
import org.apache.ibatis.transaction.TransactionFactory;
import org.apache.ibatis.transaction.jdbc.JdbcTransactionFactory;
import javax.sql.DataSource;
import java.util.List;
/**
* author: haiyangp
* date: 2017/8/19
* desc: Main
*/
public class MybatisMain {
private String driverName = "com.mysql.jdbc.Driver";
private String url = "jdbc:mysql://127.0.0.1:3306/test";
private String username = "root";
private String password = "root";
public static void main(String[] args) {
MybatisMain mybatis =new MybatisMain();
SqlSessionFactory sqlSessionFactory = mybatis.initMybatis();
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<User> userList = userMapper.getAllUser();
for (User userItem : userList) {
System.out.println(userItem);
}
}
/**
* 初始化mybatis,并返回sqlSessionFactory
*/
private SqlSessionFactory initMybatis() {
DataSource dataSource = initDataSourceWithReturn();
TransactionFactory transactionFactory = new JdbcTransactionFactory();
Environment environment = new Environment("justForTest", transactionFactory, dataSource);
Configuration configuration = new Configuration(environment);
configuration.getTypeAliasRegistry().registerAlias("user", User.class);//注册别名
configuration.addMapper(UserMapper.class);//添加mapper
return new DefaultSqlSessionFactory(configuration);
}
/**
* 初始化并获取dataSource
*
* @return DataSource
*/
private DataSource initDataSourceWithReturn() {
PooledDataSource dataSource = new PooledDataSource();
dataSource.setUrl(url);
dataSource.setDriver(driverName);
dataSource.setUsername(username);
dataSource.setPassword(password);
return dataSource;
}
}

以及UserMapper.java

package com.haiyang.learn.mybaits.mapper;
import com.haiyang.learn.mybaits.pojo.User;
import org.apache.ibatis.annotations.Select;
import java.util.List;
public interface UserMapper {
/**
* 获取所有user
*
* @return 所有user
*/
@Select(value = "select * from user")
List<User> getAllUser();
/**
* 根据用户id获取用户信息
*
* @param id 用户ID
* @return 用户
*/
@Select(value = "select * from user where id = #{id}")
User getUserById(Integer id);
}

User.java

public class User {
private Integer id;
private String username;
private String password;
//get 和 set方法
}

为了方便，并没有配置xml等信息了，主要是通过 sqlSession的getMapper方法获取出了UserMapper对象，并通过它查询出了对应的user表的数据

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<User> userList = userMapper.getAllUser();

执行结果如下：

2017-08-19 22:51:36,384 [main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
2017-08-19 22:51:36,385 [main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
2017-08-19 22:51:36,385 [main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
2017-08-19 22:51:36,385 [main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
2017-08-19 22:53:04,746 [main] DEBUG org.apache.ibatis.transaction.jdbc.JdbcTransaction - Openning JDBC Connection
2017-08-19 22:53:04,964 [main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - Created connection 1472465.
2017-08-19 22:53:04,966 [main] DEBUG com.haiyang.learn.mybaits.mapper.UserMapper.getAllUser - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@1677d1]
2017-08-19 22:53:04,966 [main] DEBUG com.haiyang.learn.mybaits.mapper.UserMapper.getAllUser - ==> Preparing: select * from user
2017-08-19 22:53:04,988 [main] DEBUG com.haiyang.learn.mybaits.mapper.UserMapper.getAllUser - ==> Parameters:
User{id=1, username='jack', password='123456'}
User{id=2, username='tom', password='654321'}
User{id=3, username='lili', password='666666'}

那么回到原话题，UserMapper并没有实现类，它是怎么就把数据给查询出来的呢？

通过debug跟踪了下getMapper这个方法，看到了关键代码：

public class MapperProxyFactory<T> {
    private final Class<T> mapperInterface;
    private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap();

    public MapperProxyFactory(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }

    public Class<T> getMapperInterface() {
        return this.mapperInterface;
    }

    public Map<Method, MapperMethod> getMethodCache() {
        return this.methodCache;
    }

    protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
    }

    public T newInstance(SqlSession sqlSession) {
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }
}

代码运行到了标红色的部分，即：

return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);

看到这里，我想大家都知道了答案了，我们获取mapper，它返回给了我们一个Proxy。用过JDK动态代理的看到Proxy.newProxyInstance这句一定感到非常的熟悉吧。原来mapper是通过动态代码来实现的，newProxyInstance的第三个参数即为代理执行器的handler了，它传入的是一个mapperProxy对象

通过Proxy.newProxyInstance方法源码也可以看到如下注释信息：

* @return  a proxy instance with the specified invocation handler of a
*          proxy class that is defined by the specified class loader
*          and that implements the specified interfaces

再通过查看mapperProxy的代码：

public class MapperProxy<T> implements InvocationHandler, Serializable {
    private static final long serialVersionUID = -6424540398559729838L;
    private final SqlSession sqlSession;
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethod> methodCache;

    public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
        this.methodCache = methodCache;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        } else {
            MapperMethod mapperMethod = this.cachedMapperMethod(method);
            return mapperMethod.execute(this.sqlSession, args);
        }
    }

    private MapperMethod cachedMapperMethod(Method method) {
        MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
        if (mapperMethod == null) {
            mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
            this.methodCache.put(method, mapperMethod);
        }

        return mapperMethod;
    }
}

那么我们执行sqlSession.getMapper(UserMapper.class)这句代码最终返回的应该就是一个由jdk动态代理生成的代理类

当执行userMapper.getAllUser()方法时，最终执行的也就是mapperMethod.execute(this.sqlSession,args)的代码

所以回到原问题，为什么mybatis的mapper没有实现类呢？原因是因为它采用了：Java动态代理实现接口

为什么mybatis的mapper没有实现类(原理探究)

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