Mybatis核心源码分析

一、介绍

       mybatis作为一款半自动化数据库持久层框架，提供了完整的JDBC操作，对参数解析，sql预编译，返回值解析，数据库事务

的支持，还有对于session的管理，数据缓存的处理；有xml和注解两种配置方式，几乎屏蔽了JDBC的操作；正式因为这种灵活使

它广受国内互联网公司的青睐；

二、核心类

      1.SqlSessionFactoryBuilder

         通过读取xml文件流或者创建Configuration类的方式注入mybatis的全局配置文件

         new SqlSessionFactoryBuilder().build("xml文件路径")或者new SqlSessionFactoryBuilder().build("configuration类")

         返回值为SqlSessionFactory

      2.SqlSessionFactory

         SqlSession的工厂类，可以返回一个session(数据库会话)

         两个具体的实现类DefaultSqlSessionFactory和SqlSessionManager

         DefaultSqlSessionFactory：会根据mybatis的配置创建session

//execType是Configuration配置文件中指定的执行器
//level级别(事务的级别，根据数据的事务隔离级别不同)
//autoCommit是否自动提交事务
private SqlSession openSessionFromDataSource(ExecutorType execType,     TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //配置文件中的环境
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //创建一个事务
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //根据事务和执行器创建实例
      final Executor executor = configuration.newExecutor(tx, execType);
      //根据以上条件创建一个session
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      //如果上述代码出现异常，那么关闭事务
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

      SqlSessionManager：会通过ThreadLocal创建session，保证创建的session只有当前线程使用(session本身线程不安全)

                                          这样session的生命周期就会跟随当前线程

      3.SqlSession

      mybatis提供的操作数据库增删改查的顶层接口，内部有我们配置和具体的执行器

      4.Executor

      mybatis提供的具体操作数据库的类，增删改查，事务提交，回滚等；有以下四个实现

      SimpleExecutor:简单的数据库操作，没有缓存，使用完释放所有资源

      BatchExecutor:只提供了增删改的功能，不支持查询操作

      ReuseExecutor:对于statement的缓存，用map封装

private void putStatement(String sql, Statement stmt) {
    statementMap.put(sql, stmt);
  }

      CachingExecutor:会对结果查询结果做缓存，如果在同一个缓存中有，那么直接使用缓存

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);
        @SuppressWarnings("unchecked")
        //如果缓存中有，那么直接返回数据，不再查询数据库
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

      5.StatementHandler

      对statement对象的操作类，对应也有四个实现，在介绍StatementHandler内容之前先聊以下statement的内容

      statement：类路径 java.sql，是具体执行某一个确定的sql，并且返回一个结果；这里的sql是即将传递给数据库执行的语句

      有两个实现类prepareStatement和CallableStatement

      prepareStatement：预编译好的sql

      CallableStatement：预编译好的存储过程

      StatementHandler就是对以上Statement的操作，以下为StatementHandler的四个实现类

      SimpleStatementHandler：用于处理JDBC中简单的statement接口

      CallableStatementHandler：用于处理存储过程的CallableStatement

      PrepareStatementHandler：用于处理我们预编译好的PrepareStatement

      RoutingStatementHandler：没有实际逻辑，只负责以上三个handler的调度

三、源码跟踪一次请求

      简单的mybatis一次完整请求

@Test
    public void testGet() throws Exception{
        //1创建mybatis的构建器
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        //2解析具体的xml或者configuration类(配置有数据库连接，mapper文件地址)
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(Resources.getResourceAsStream("mybatis-config.xml"));
        //3打开一个会话连接
        SqlSession session = sqlSessionFactory.openSession();
        //4获取mapper的代理对象
        WmsUserConfMapper mapper = session.getMapper(WmsUserConfMapper.class);
        //5通过代理对象执行具体的sql
        mapper.get("a");
    }

      第一，二，三可以自己跟一下源码，这里不做过多解释，直接从获取mapper代理对象开始

     通过上边对session的讲解可知，当前获得是一个DefaultSqlSession，进入getMapper方法

@Override
  public <T> T getMapper(Class<T> type) {
    return configuration.getMapper(type, this);
  }

     再进入到configuration.getMapper()方法中

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

     进入mapper注册，也就是创建代理对象的方法

@SuppressWarnings("unchecked")
  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);
    }
  }

       mapper代理对象创建完毕，接下来进入重点，调用代理对象mapper.get("a");方法，我们debug进来，看到invoke方法

 @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //1.缓存mapper的数据，第二次进入相同mapper直接访问缓存，提高效率
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //2.执行具体逻辑
    return mapperMethod.execute(sqlSession, args);
  }

        先看下上述代码块中步骤1对应的源码，第一次访问mapper会创建一个mapperMethod，源码如下

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    //根据config配置，接口，和方法，创建当前方法的mapperStatement对象
    this.command = new SqlCommand(config, mapperInterface, method);
    //根据config配置，接口，和方法，获取当前mapper接口方法对应的签名信息
    this.method = new MethodSignature(config, mapperInterface, method);
  }

       new SqlCommand()和new MethodSignature()源码，可自行跟踪一下，没什么复杂的逻辑

      步骤2执行的源码如下

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result; 
    //1.type是根据解析mapper对应的xml文件标签得到的
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      //2.我们这里是select直接看这块逻辑
      case SELECT:
         //3.根据方法签名上的返回值决定执行不同的方法
        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 if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          //4.我们这里没有返回值，并且返回值没有resultHandler
          //5.解析方法上的参数
          Object param = method.convertArgsToSqlCommandParam(args);
          //6.执行session的查询
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional()
              && (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.ofNullable(result);
          }
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        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;
  }

        我们跟一下上述5参数解析部分的源码，如下

public Object getNamedParams(Object[] args) {
    //当前方法的参数保存在一个有序的map中即names，value为属性名称
    final int paramCount = names.size();
    //判断names的大小来决定逻辑
    if (args == null || paramCount == 0) {
      return null;
    } else if (!hasParamAnnotation && paramCount == 1) {
      return args[names.firstKey()];
    } else {
      final Map<String, Object> param = new ParamMap<>();
      int i = 0;
      for (Map.Entry<Integer, String> entry : names.entrySet()) {
        //通过循环将属性名称和值存放到map中使用
        param.put(entry.getValue(), args[entry.getKey()]);
        // add generic param names (param1, param2, ...)
        final String genericParamName = GENERIC_NAME_PREFIX + String.valueOf(i + 1);
        // ensure not to overwrite parameter named with @Param
        if (!names.containsValue(genericParamName)) {
          param.put(genericParamName, args[entry.getKey()]);
        }
        i++;
      }
       //返回一个map对象
      return param;
    }
  }

        上述第6步执行session的查询方法，我们在进入源码，一直走到DefaultSqlSession中，如下

@Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
     //通过配置文件获取mapper中对应的statement的sql语句
      MappedStatement ms = configuration.getMappedStatement(statement);
      //通过执行器查询数据
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

        进入executor.query方法，我们这里进入的是CachingExecutor类中，源码如下

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //构建sql，及参数数据，封装为boundSql对象
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //创建缓存
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    //执行查询
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

    进入query方法，如下

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    //获取缓存
    Cache cache = ms.getCache();
    if (cache != null) {
      //刷新缓存
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    //这里真正执行的SimpleExecutor的查询
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

     进入delegate.query()方法，这里走到了SimpleExecutor的父类BaseExecutor

   

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      //查询本地缓存是否为空
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //调用真正的数据库查询
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

     进入queryFromDataBase方法

​

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      //执行查询
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      //移除旧的缓存数据
      localCache.removeObject(key);
    }
    //放置新的缓存数据
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

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​

       进入doQuery方法，到了SimpleExecutor类中

@Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //通过预编译的方式，构建出完整的sql语句
      stmt = prepareStatement(handler, ms.getStatementLog());
      //调用到PrepareStatementHandler进行sql的查询操作
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

        进入到PrepareStatementHandler进行查询操作，会调用到PrepareStatement的execute执行查询，最终将结果返回，并

        用ResultSetHandler处理请求的结果

四、总结

    第一次完完整整一步一步的走了一遍mybatis的源码，感觉收货还是蛮大的，一遍debug一遍记录，扫了很多盲点，走完一遍

    后发现对于数据库和mybatis这块的内容又有了新的认识，不知不觉过去了三个小时了