Mybatis源码分析——Mapper与接口绑定源码分析

源码分析前简介

首先我们来看看用 Mybatis 执行 sql 的两种方法

直接操作 SqlSession 方法

public User findUserById(Integer userId) {
  SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
  try {
    // namespace + statementId
    return sqlSession.selectOne("com.xuyu.mybatis.UserMapper.findUserById", userId);
  } finally {
    sqlSession.close();
  }
}

通过 Mapper 接口

public User findUserById(Integer userId) {
  SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
  try {
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    return userMapper.findUserById(userId);
  } finally {
    sqlSession.close();
  }
}

public class UserMapper {
  User findUserById(@Param("userId") String userId);
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.objcoding.mybatis.UserMapper">
  <select id="findUserById" resultType="com.xuyu.mybatis.User">
    SELECT * FROM user WHERE user_id=#{userId}
  </select>
</mapper>

源码分析流程图

1.从MapperRegistry获取查询对应绑定接口

2.动态生成代理对象

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

3.调用MapperProxyFactory去帮我们创建代理类

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //检查看下有没有注册mapper接口
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //创建代理类
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  

创建代理类

  public T newInstance(SqlSession sqlSession) {
    //创建代理类
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

4.执行这条语句

UserEntity user = mapper.getUser(2);

5.跳转到MapperProxy的invoke方法

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    //这里源码分析缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //这里源码分析执行
    return mapperMethod.execute(sqlSession, args);
  }

6.执行到这条语句

final MapperMethod mapperMethod = cachedMapperMethod(method);

 private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      //进入这里的构造函数
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    //这里构建sql,源码分析
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
  }

public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      if (configuration.hasStatement(statementName)) {
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass())) { // issue #35
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      if (ms == null) {
        if(method.getAnnotation(Flush.class) != null){
          name = null;
          type = SqlCommandType.FLUSH;
        } else {
          throw new BindingException("Invalid bound statement (not found): " + statementName);
        }
      } else {
        name = ms.getId();
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
          throw new BindingException("Unknown execution method for: " + name);
        }
      }
    }

回到invoke

 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //这里源码分析开始
    return mapperMethod.execute(sqlSession, args);
  }

7.进入到MapperMethod的execute方法就能简单找到SqlSession的删除、更新、查询、选择方法

 public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        //底层执行SQL语句，封装User结果集
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else if (SqlCommandType.FLUSH == command.getType()) {
        result = sqlSession.flushStatements();
    } else {
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName()
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

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