MyBatis详解（二）

前言

　　本篇幅是继 MyBatis详解（一）的下半部分。

MyBatis执行Sql的流程分析

【1】基于前面已经将XML文件进行build解析了并且返回了SqlSessionFactory

【1.1】那么分析SqlSessionFactory.openSession()方法是怎么返回SqlSession的，且SqlSession又是什么东西：

@Override
public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

/**
* 方法实现说明:从session中开启一个数据源
* @param execType:执行器类型
* @param level:隔离级别
*/
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      // 获取环境变量
      final Environment environment = configuration.getEnvironment();
      // 获取事务工厂
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      // 创建一个sql执行器对象
      // 一般情况下 若我们的mybaits的全局配置文件的cacheEnabled默认为ture就返回一个cacheExecutor,若关闭的话返回的就是一个SimpleExecutor
      final Executor executor = configuration.newExecutor(tx, execType);
      // 创建返回一个DeaultSqlSessoin对象返回
      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();
    }
}

【1.1.1】分析newExecutor方法中执行器的产生：

/**
* 方法实现说明:创建一个sql语句执行器对象
* @param transaction:事务
* @param executorType:执行器类型
* @return:Executor执行器对象
*/
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //判断执行器的类型
    // 批量的执行器
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      //可重复使用的执行器
      executor = new ReuseExecutor(this, transaction);
    } else {
      //简单的sql执行器对象
      executor = new SimpleExecutor(this, transaction);
    }
    //判断mybatis的全局配置文件是否开启缓存
    if (cacheEnabled) {
      //把当前的简单的执行器包装成一个CachingExecutor
      executor = new CachingExecutor(executor);
    }
    //调用所有的拦截器对象plugin方法，也就是生成代理对象
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
}

【1.1.1.1】图示：

【1.1.2】分析底层如何执行JDBC【User user = (User)session.selectOne("com.mapper.UserMapper.selectById", 1);】

/**
* 方法实现说明:查询我们当个对象
* @param statement:SQL语句
* @param parameter:调用时候的参数
* @return: T 返回结果
*/
@Override
public <T> T selectOne(String statement, Object parameter) {
    // 这里selectOne调用也是调用selectList方法
    List<T> list = this.selectList(statement, parameter);
    //若查询出来有且有一个一个对象，直接返回要给
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      //查询的有多个,那么就抛出异常
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
}

@Override
public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

/**
* @param statement: statementId
* @param parameter:参数对象
* @param rowBounds :mybiats的逻辑分页对象
*/
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //第一步：通过我们的statement去我们的全局配置类中获取MappedStatement
      MappedStatement ms = configuration.getMappedStatement(statement);
      //通过执行器去执行我们的sql对象
      //第一步:包装我们的集合类参数
      //第二步:一般情况下是executor为cacheExetory对象
      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();
    }
}

【1.1.2.0】分析两种方法的本质：

方法一：User user = (User)session.selectOne("com.mapper.UserMapper.selectById", 1);

源码流程：
//通过statement去我们的全局配置类中获取MappedStatement
MappedStatement ms = configuration.getMappedStatement(statement);

public MappedStatement getMappedStatement(String id) {
    return this.getMappedStatement(id, true);
}

public MappedStatement getMappedStatement(String id, boolean validateIncompleteStatements) {
    if (validateIncompleteStatements) {
      buildAllStatements();
    }
    // 这个mappedStatements便是在SqlSessionFactory进行build过程中parse解析出来的
    return mappedStatements.get(id);
}

方法二：UserMapper mapper = session.getMapper(UserMapper.class); 与 User user = mapper.selectById(1L);

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

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

/**
* 方法实现说明:通过class类型和sqlSessionTemplate获取Mapper(代理对象)
* @param type:Mapper的接口类型
* @param sqlSession:接口类型实际上是我们的sqlSessionTemplate类型
*/
@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    // 直接去缓存knownMappers中通过Mapper的class类型去找mapperProxyFactory
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    // 缓存中没有获取到 直接抛出异常
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      // 通过MapperProxyFactory来创建我们的实例
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
}

【1.1.2.1】如果是定义了二级缓存，那么会走CachingExecutor逻辑：

/**
* 方法实现说明:通过我们的sql执行器对象执行sql
* @param ms 用于封装我们一个个的insert|delete|update|select 对象
* @param parameterObject:参数对象
* @param rowBounds :mybaits的逻辑分页对象
* @param resultHandler:结果处理器对象
*/
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    // 通过参数对象解析sql详细信息1339025938:1570540512:com.project.mapper.selectById:0:2147483647:select id,user_name,create_time from t_user where id=?:1:development
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    //判断mapper中是否开启了二级缓存<cache></cache>
    Cache cache = ms.getCache();
    // 判断是否配置了cache
    if (cache != null) {
      //判断是否需要刷新缓存
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        // 先去二级缓存中获取
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);   //也就是去PerpetualCache里面寻找
        // 二级缓存中没有获取到
        if (list == null) {
          //通过查询数据库去查询
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          //加入到二级缓存中
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    //没有整合二级缓存,直接去查询
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

【1.1.2.1.1】事务的TransactionalCacheManager：

【1.1.2.1.1.1】它存在的意义在于：避免事务不成功的sql语句填充到了Cache里面，淘汰掉一些已经执行过的语句，相当于包装了一层。

【1.1.2.1.1.2】存储的地方：本质上会先存在自身类的属性值private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap<>();

【1.1.2.1.1.3】当成功后才会提交到PerpetualCache里面。

【1.1.2.1.2】分析语句的解析【ms.getBoundSql(parameterObject)】：

public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
}

//DynamicSqlSource类#getBoundSql方法
@Override
public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    //责任链处理SqlNode，编译出完整的sql
    rootSqlNode.apply(context);
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    //处理sql中的#{..}
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    //将#{..}中的内容封装为parameterMapping，替换为？
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    context.getBindings().forEach(boundSql::setAdditionalParameter);
    return boundSql;
}

//BoundSql类的结构
public class BoundSql {

  private final String sql;  //语句
  private final List<ParameterMapping> parameterMappings;
  private final Object parameterObject;  //参数处理
  private final Map<String, Object> additionalParameters;  //结果集处理
  private final MetaObject metaParameters;
}

【1.1.2.2】如果没有定义的话，则会选择BaseExecutor的三个子类中的一个【但其实还是会走BaseExecutor的逻辑】：

//BaseExecutor类#query方法，子类重写的是doQuery，如果是使用query方法本质上还是走BaseExecutor类的
@Override
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());
    //已经关闭，则抛出 ExecutorException 异常
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    // 清空本地缓存，如果 queryStack 为零，并且要求清空本地缓存。
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      // 从一级缓存中，获取查询结果
      queryStack++;

      //BaseExecutor类的属性值：protected PerpetualCache localCache;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;

      // 获取到，则进行处理
      if (list != null) {
        //处理存过的
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // 获得不到，则从数据库中查询
        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;
}

//至于为什么会说一级缓存是session级别的，因为一旦提交或者回滚之后都会被清空
@Override
public void commit(boolean required) throws SQLException {
    if (closed) {
      throw new ExecutorException("Cannot commit, transaction is already closed");
    }
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
}

@Override
public void rollback(boolean required) throws SQLException {
    if (!closed) {
      try {
        clearLocalCache();
        flushStatements(true);
      } finally {
        if (required) {
          transaction.rollback();
        }
      }
    }
}

// 清楚本地一级缓存
@Override
public void clearLocalCache() {
    if (!closed) {
      localCache.clear();
      localOutputParameterCache.clear();
    }
}

【1.1.2.3】分析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;
    //一级缓存中先占位
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      //调用子类的查询方法
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
}

//以默认的SimpleExecutor为例查看doQuery方法
@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();
      //创建StatementHandler，主要职责是拿到链接，拿到执行者
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
}

//分析如何创建StatementHandler
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
}

//存在三种StatementHandler
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }
}

//但是这三种都是继承BaseStatementHandler，因为BaseStatementHandler里面才会有resultSetHandler和parameterHandler
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}

【2.2】对于执行器Executor的分析，先分析接口的定义：

/**
* 类的描述:sql执行器接口,主要用于维护一级缓存和二级缓存,并且提供事务管理功能
*        Executor
*          --BaseExecutor(一级缓存)
*            --batchExecutor(批量执行器)
*            --ReUseExecutor(可重用的)
*            --SimpleExecutor简单的
*          --CacheExecutor(加入了二级缓存)
*/
public interface Executor {

  //ResultHandler 对象的枚举
  ResultHandler NO_RESULT_HANDLER = null;

  /**
   * 更新 or 插入 or 删除，由传入的 MappedStatement 的 SQL 所决定
   * @param ms 我们的执行sql包装对象（MappedStatement）
   * @param parameter 执行的参数
   */
  int update(MappedStatement ms, Object parameter) throws SQLException;

  /**
   * 查询带缓存key查询
   * @param ms 我们的执行sql包装对象（MappedStatement）
   * @param parameter:参数
   * @param rowBounds 逻辑分页参数
   * @param resultHandler:返回结果处理器
   * @param cacheKey:缓存key
   * @param boundSql：我们的sql对象
   * @return 查询结果集list
   * @throws SQLException
   */
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  /**
   * 不走缓存查询
   * @param ms 我们的执行sql包装对象（MappedStatement）
   * @param parameter:参数
   * @param rowBounds 逻辑分页参数
   * @param resultHandler:返回结果处理器
   * @return 结果集list
   * @throws SQLException
   */
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  /**
   * 调用存过查询返回游标对象
   * @param ms 我们的执行sql包装对象（MappedStatement）
   * @param parameter:参数
   * @param rowBounds 逻辑分页参数
   * @return Cursor数据库游标
   * @throws SQLException
   */
  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;

  // 刷入批处理语句
  List<BatchResult> flushStatements() throws SQLException;

  //提交事务
  void commit(boolean required) throws SQLException;

  //回滚事务
  void rollback(boolean required) throws SQLException;

  //创建缓存key
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);

  // 判断是否缓存
  boolean isCached(MappedStatement ms, CacheKey key);
  // 清除本地缓存
  void clearLocalCache();

  // 延迟加载
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);

  //获取一个事务
  Transaction getTransaction();
  // 关闭事务
  void close(boolean forceRollback);

  //判断是否关闭
  boolean isClosed();

  // 设置包装的 Executor 对象
  void setExecutorWrapper(Executor executor);
}