Mybatis初始化以及执行流程

初始化 + 执行流程的源码解析就按照我们测试类的编写来看，从Resources.getResourceAsStream()开始。

加载配置文件

// Resources就是一个工具类，将配置文件转化为字节输入流，这个步骤不涉及任何解析操作
InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapperConfig.xml");
// 执行的源码
public static InputStream getResourceAsStream(String resource) throws IOException {
    return getResourceAsStream(null, resource);
}
/**
   * Returns a resource on the classpath as a Stream object
   *
   * @param loader   The classloader used to fetch the resource
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
    InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
    if (in == null) {
        throw new IOException("Could not find resource " + resource);
    }
    return in;
}

解析配置文件

// 执行代码如下，做的操作就是解析配置文件，功能有如下两点，使用了构建者模式
// 1. 封装Configuration配置文件，内部有各种配置文件的设置以及存放MappedStatement的map
// 2. 创建DefaultSqlSessionFactory
private SqlSessionFactory build = new SqlSessionFactoryBuilder().build(resourceAsStream);
// 源码说明，主要方法如下
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      // 仅仅XPathParser解析
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      // parse方法是关键，重中之重，parse方法封装configuration对对象，build方法根据configuration来创建工厂
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }
public Configuration parse() {
    if (parsed) {
        throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
}
private void parseConfiguration(XNode root) {
    try {
        // 解析各种标签
        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);
        // read it after objectFactory and objectWrapperFactory issue #631
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        typeHandlerElement(root.evalNode("typeHandlers"));
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

创建会话对象SqlSession

// 功能也有两点
// 1. 创建会话对象
// 2. 创建执行器
private SqlSession sqlSession = build.openSession();
// 第一个参数值执行器的类型，根据实现有三种，默认是 SIMPLE，第二个参数是事务隔离级别，第三个参数是事务是否自动提交
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
        final Environment environment = configuration.getEnvironment();
        final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        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();
    }
}

调用方法

// 执行sql，返回结果
List<Object> objects = sqlSession.selectList("statement.id");
// 最后方法如下
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        // 根据参数statementId来获取MappedStatement
        MappedStatement ms = configuration.getMappedStatement(statement);
        // 进行方法的执行
        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();
    }
}

执行器的查看

主要就是看上面的query方法：

@Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
  }
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++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        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;
  }
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 {
      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;
  }
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
        // 获取配置信息
        Configuration configuration = ms.getConfiguration();
        // 创建 StatementHandler 对象，作用有两个
        // 1. 设置参数 ParameterHandler
        // 2. 处理返回结果集 ResultSetHandler
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 预处理sql，创建statement对象，完成参数的设置，并且其中有TypeHandler的使用
        stmt = prepareStatement(handler, ms.getStatementLog());
        // 数据查询处理
        return handler.query(stmt, resultHandler);
    } finally {
        closeStatement(stmt);
    }
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    String sql = boundSql.getSql();
    statement.execute(sql);
    return resultSetHandler.handleResultSets(statement);
}

StatementHandler的流程

// 首先看设置参数
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
}
@Override
public void parameterize(Statement statement) throws SQLException {
    // 交给了ParameterHandler去进行参数设置
    parameterHandler.setParameters((PreparedStatement) statement);
}
public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
        for (int i = 0; i < parameterMappings.size(); i++) {
            ParameterMapping parameterMapping = parameterMappings.get(i);
            if (parameterMapping.getMode() != ParameterMode.OUT) {
                Object value;
                // 获取字段名称以及字段值
                String propertyName = parameterMapping.getProperty();
                if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
                    value = boundSql.getAdditionalParameter(propertyName);
                } else if (parameterObject == null) {
                    value = null;
                } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                    value = parameterObject;
                } else {
                    MetaObject metaObject = configuration.newMetaObject(parameterObject);
                    value = metaObject.getValue(propertyName);
                }
                // jdbc与java type映射
                // 每⼀个 Mapping都有⼀个 TypeHandler，根据 TypeHandler 来对preparedStatement 进 ⾏设置参数
                TypeHandler typeHandler = parameterMapping.getTypeHandler();
                JdbcType jdbcType = parameterMapping.getJdbcType();
                if (value == null && jdbcType == null) {
                    jdbcType = configuration.getJdbcTypeForNull();
                }
                try {
                    typeHandler.setParameter(ps, i + 1, value, jdbcType);
                } catch (TypeException | SQLException e) {
                    throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
                }
            }
        }
    }
}
// 执行方法以及结果集的处理
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    // 教给了 ResultSetHandler进行处理
    return resultSetHandler.handleResultSets(ps);
}
public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    final List<Object> multipleResults = new ArrayList<>();
    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    // while结果集处理
    while (rsw != null && resultMapCount > resultSetCount) {
        ResultMap resultMap = resultMaps.get(resultSetCount);
        handleResultSet(rsw, resultMap, multipleResults, null);
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
    }
    String[] resultSets = mappedStatement.getResultSets();
    if (resultSets != null) {
        while (rsw != null && resultSetCount < resultSets.length) {
            ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
            if (parentMapping != null) {
                String nestedResultMapId = parentMapping.getNestedResultMapId();
                ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
                handleResultSet(rsw, resultMap, null, parentMapping);
            }
            rsw = getNextResultSet(stmt);
            cleanUpAfterHandlingResultSet();
            resultSetCount++;
        }
    }
    return collapseSingleResultList(multipleResults);
}
private List<Object> collapseSingleResultList(List<Object> multipleResults) {
    return multipleResults.size() == 1 ? (List<Object>) multipleResults.get(0) : multipleResults;
}