Mybatis源码解读-配置加载和Mapper的生成

问题

1. Mybatis四大对象的创建顺序？
2. Mybatis插件的执行顺序？

工程创建

环境：Mybatis(3.5.9)

mybatis-demo，参考官方文档

简单示例

这里只放出main方法的示例，其余类请看demo工程。

public static void main(String[] args) throws Exception {
    // 配置文件路径
    String resource = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    // 1.读取配置，创建SqlSessionFactory
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    // 2.通过工厂获取SqlSession
    SqlSession session = sqlSessionFactory.openSession();
    try {
        // 3.获取mapper代理对象
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        // 4.执行查询，此处才真正连接数据库
        System.out.println(mapper.selectByName("张三"));
    } finally {
        // 5.关闭连接
        session.close();
    }
}

Mapper的创建

我们使用Mybatis操作数据库，主要是通过mapper对象（在hibernate中叫dao对象）。

那么，我们不按顺序从读取配置初始化开始讲，直接看看mapper对象是如何获取与执行的。

1. 获取mapper

   // StudentMapper mapper = session.getMapper(StudentMapper.class);
   DefaultSqlSession.getMapper(Class<T> type)  -->
   Configuration.getMapper(Class<T> type, SqlSession sqlSession) -->
   MapperRegistry.getMapper(Class<T> type, SqlSession sqlSession) -->
   MapperProxyFactory.newInstance(SqlSession sqlSession) -->
   MapperProxyFactory.newInstance(MapperProxy<T> mapperProxy)

   咱们来看看MapperProxyFactory.newInstance(MapperProxy mapperProxy)的实现

   protected T newInstance(MapperProxy<T> mapperProxy) {
     // 可以转换成这样，返回的是StudentMapper的代理对象
     // final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, StudentMapper.class, methodCache);
     // Proxy.newProxyInstance(StudentMapper.class.getClassLoader(), new Class[] { StudentMapper.class }, mapperProxy);
     return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
   }

   也就是说，实际返回的是MapperProxy对象，StudentMapper被代理了。

2. 执行mapper的方法

   已知mapper对象被代理了，那么执行mapper的所有方法，都会先经过MapperProxy的invoke方法

   public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
     try {
       // 如果执行的是Object的方法，则直接执行，不继续处理mybatis的逻辑
       if (Object.class.equals(method.getDeclaringClass())) {
         // 举例，如果执行的是mapper.toString()，则进入此判断
         return method.invoke(this, args);
       } else {
         // cachedInvoker(method)：创建MapperMethodInvoker并缓存起来
         return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
       }
     } catch (Throwable t) {
       throw ExceptionUtil.unwrapThrowable(t);
     }
   }

   cachedInvoker(method)返回的是PlainMethodInvoker，继续进去看看

   // PlainMethodInvoker的方法
   public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
     return mapperMethod.execute(sqlSession, args);
   }
    
   // MapperMethod#execute(SqlSession sqlSession, Object[] args)
   public Object execute(SqlSession sqlSession, Object[] args) {
     Object result;
     switch (command.getType()) {
       case INSERT: {
         ......
         break;
       }
       case UPDATE: {
         ......
         break;
       }
       case DELETE: {
         ......
         break;
       }
       case SELECT:
         ......
         break;
       case FLUSH:
         result = sqlSession.flushStatements();
         break;
       default:
         throw new BindingException("Unknown execution method for: " + command.getName());
     }
     ......
     return result;
   }

   终于，看到了熟悉insert、update关键字，这里就是具体解析执行sql，并返回结果的逻辑。咱们先略过。回去看看是如何加载配置以及生成SqlSession的。

SqlSessionFactory

SqlSessionFactory的生成过程如下

public SqlSessionFactory build(InputStream inputStream) {
  return build(inputStream, null, null);
}
 
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
  try {
    // xml配置解析类
    XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
    // build方法返回DefaultSqlSessionFactory
    // 主要看parser.parse()
    return build(parser.parse());
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error building SqlSession.", e);
  } finally {
    // 异常上下文对象，线程内共享
    ErrorContext.instance().reset();
    try {
      inputStream.close();
    } catch (IOException e) {
      // Intentionally ignore. Prefer previous error.
    }
  }
}
 
public SqlSessionFactory build(Configuration config) {
  return new DefaultSqlSessionFactory(config);
}

下面来看看parser.parse()方法

// XMLConfigBuilder#parse()
public Configuration parse() {
  if (parsed) {
    throw new BuilderException("Each XMLConfigBuilder can only be used once.");
  }
  parsed = true;
  // parser.evalNode("/configuration")：获取configuration节点
  // 例如：<configuration> xxx </configuration>
  // parseConfiguration才是重点
  parseConfiguration(parser.evalNode("/configuration"));
  return configuration;
}
 
// 这是重点
private void parseConfiguration(XNode root) {
  try {
    propertiesElement(root.evalNode("properties"));
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);
    loadCustomLogImpl(settings);
    typeAliasesElement(root.evalNode("typeAliases"));
    pluginElement(root.evalNode("plugins"));
    objectFactoryElement(root.evalNode("objectFactory"));
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
    reflectorFactoryElement(root.evalNode("reflectorFactory"));
    settingsElement(settings);
    // 环境配置
    environmentsElement(root.evalNode("environments"));
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
    typeHandlerElement(root.evalNode("typeHandlers"));
    // 映射器配置
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

详细XML的配置请参考官网：mybatis – MyBatis 3 | 配置

这里，咱们只讲环境配置，其他的篇幅有限，请自行查看源码。

SqlSession

接下来看看SqlSession的创建

// DefaultSqlSessionFactory#openSession() -->
// DefaultSqlSessionFactory#openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false)
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    // 默认环境
    final Environment environment = configuration.getEnvironment();
    // 事务工厂
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    // 上面那两个对象，在创建SqlSessionFactory时，就已经创建好了
    // 通过事务工厂创建事务
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    // 创建mybatis四大对象之一的Executor
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    closeTransaction(tx); // may have fetched a connection so lets call close()
    throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

看看四大对象之一Executor的创建

// Configuration#newExecutor(Transaction transaction, ExecutorType executorType)
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  // 判断需要创建的执行器的类型
  executorType = executorType == null ? defaultExecutorType : executorType;
  executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  Executor executor;
  if (ExecutorType.BATCH == executorType) {
    // 批处理执行器
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    // 重用执行器
    executor = new ReuseExecutor(this, transaction);
  } else {
    // 简单处理器（默认）
    executor = new SimpleExecutor(this, transaction);
  }
  // 是否启用二级缓存（二级缓存默认启用）
  if (cacheEnabled) {
    // 此处使用的是装饰器模式，对executor进行二次包装
    executor = new CachingExecutor(executor);
  }
  // 这块是mybatis的插件处理，用代理的方式，以后再开文章讲
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

Mapper的执行

Mapper的创建一节，讲到mapper执行会被代理。

下面就以StudentMapper为例，讲讲mapper的执行。

public interface StudentMapper {
    List<Student> selectByName(@Param("name") String name);
}

当执行selectByName时候，进入到MapperMethod#execute(SqlSession sqlSession, Object[] args)方法。

public Object execute(SqlSession sqlSession, Object[] args) {
  Object result;
  switch (command.getType()) {
    ......
    // 忽略insert、update、delete的逻辑，直接看select
    case SELECT:
      // 如果返回null或者设置了自定义的结果处理器
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      // 如果返回集合或者数组，我们的查询会进到这里，因为selectByName返回值是List
      // 这是入口
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      // 如果返回map
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      // 这个没用过，不会
      } else if (method.returnsCursor()) {
        result = executeForCursor(sqlSession, args);
      } else {
        // 默认返回单个对象
        Object param = method.convertArgsToSqlCommandParam(args);
        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;
}

继续看executeForMany方法

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
  List<E> result;
  // 参数转换，如果参数有注解，则会转成map，且可使用param1, param2
  // 例如：@Param("name")会转成  {"name":xxx, "param1": xxx}
  Object param = method.convertArgsToSqlCommandParam(args);
  // 是否分页
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.selectList(command.getName(), param, rowBounds);
  } else {
    // 这是入口
    result = sqlSession.selectList(command.getName(), param);
  }
  // 如果result不能强转成方法的返回值（在此例子中getReturnType就是List<Studet>）
  if (!method.getReturnType().isAssignableFrom(result.getClass())) {
    if (method.getReturnType().isArray()) {
      return convertToArray(result);
    } else {
      return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
    }
  }
  return result;
}

继续看，因为案例中没用到分页，所以执行的是sqlSession.selectList(command.getName(), param);

// DefaultSqlSession#selectList(String statement, Object parameter) -->
// DefaultSqlSession#selectList(String statement, Object parameter, RowBounds rowBounds) -->
// DefaultSqlSession#selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) -->
private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
  try {
    // MapperStatement在前面解析xml时，就已经创建了
    // 忘了就看看创建SqlSessionFactory时是如何解析xml文件的mappers节点的
    MappedStatement ms = configuration.getMappedStatement(statement);
    // 执行器执行查询方法
    return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

继续看，executor.query方法，Mybatis-PageHelper插件就是通过拦截query方法，插入分页参数的。

// CachingExecutor
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
  // 对sql进行预处理
    BoundSql boundSql = ms.getBoundSql(parameterObject);
  // 创建一级缓存的key
  CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
  // 这是入口
  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
 
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
  Cache cache = ms.getCache();
  // 有缓存的逻辑
  if (cache != null) {
    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;
    }
  }
  // delegate是SimpleExecutor
  // 这是入口
  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
 
// 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.");
  }
  // 是否要清除缓存，默认设置是如果非select方法，都会清除缓存。
  if (queryStack == 0 && ms.isFlushCacheRequired()) {
    clearLocalCache();
  }
  List<E> list;
  try {
    queryStack++;
    // 从一级缓存提取数据
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      // 从数据库查询数据
      // 这是入口
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  }
  if (queryStack == 0) {
    // 懒加载相关
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    }
    // issue #601
    deferredLoads.clear();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      // issue #482
      clearLocalCache();
    }
  }
  return list;
}
 
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 {
    // 这是入口
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
  } finally {
    localCache.removeObject(key);
  }
  // 结果放入一级缓存
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  }
  return list;
}

下面，重点来了，准备了这么久，终于要查询数据库了

// SimpleExecutor
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();
    // 重点又来了，mybatis四大对象的3个，在这里创建
    // 按顺序是：ParameterHandler、ParameterHandler、StatementHandler
    // 又一个装饰器模式，实际创建的是PreparedStatementHandler（默认），但是使用RoutingStatementHandler又包了一层
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    // 创建jdbc的statement对象，直到这里，才会真正获取数据库连接
        stmt = prepareStatement(handler, ms.getStatementLog());
    // 执行查询，并使用resultHandler处理结果
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

答案

1. 创建顺序为：Executor、ParameterHandler、ParameterHandler、StatementHandler
2. 插件的执行顺序，如果都命中同一个方法，那么顺序为，越晚注册的插件，越先执行（因为代理）