为什么mybatis的mapper没有实现类(原理探究)

迷南。 2022-06-10 13:09 183阅读 0赞

昨天晚上坐公交回家时，在公交车上看见了一个男生和一个女生，年龄跟我差不多大的样子，他们俩在公交车前面面对面地坐着，那个男生坐在位置上看坐在他对面的那个女生，一直傻傻地看了的很长一段时间，若有所思的样子，从他俩的表情与眼神上来看，应该是一段时间没有见面的情侣，想必他们那时的心情应该是激动又开心的。公交到站时，那个男生帮那个女生提起了她的行李包，他俩走下公交，我坐在公交上注意着车窗外的夜色，也注意着他们，看着竟让我生心羡慕，羡慕他们能在魔都在一起。这让我想起了前些年过年时，电视节目会问许多不同的对象一个问题：“幸福是什么”，我想这个问题每个人在不同的时刻都会有着自己不同的答案。要是在昨天那个时刻问他们幸福是什么，估计他们会说他们那时就是幸福的。哈哈。

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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