MyBatis溫故而知新-底層執行原理

準備工作

public class MainClass {

  public static void main(String[] args) throws Exception {

    String resources = "mybatis-config.xml";

    InputStream inputStream = Resources.getResourceAsStream(resources);

    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

    SqlSession sqlSession = sqlSessionFactory.openSession();

    Student student = sqlSession.selectOne("org.apache.ibatis.dao.StudentMapper.getStudent",1);

    System.out.println(student.toString());

    sqlSession.close();

  }

}

MyBatis是如何獲取資料來源的

這是我們mybatis-config.xml中配置資料庫的4個關鍵屬性，也就是看看MyBatis是怎麼來解析這個配置檔案塊的。

<environments default="development">

   <environment id="development">

      <transactionManager type="JDBC"/>

      <dataSource type="POOLED">

        <property name="driver" value="${jdbc.driver}"/>

        <property name="url" value="${jdbc.url}"/>

        <property name="username" value="${jdbc.username}"/>

        <property name="password" value="${jdbc.password}"/>

      </dataSource>

    </environment>

 </environments>

資料來源獲取，我們從上面的程式碼片段中開始分析，在SqlSessionFactoryBuilder().build(inputStream) 方法中入手，這裡看到例項了XMLConfigBuilder類。

SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {

    try {

      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);

      //parser.parse()方法返回Configuration物件，然後呼叫build(Configuration config)

      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 SqlSessionFactory build(Configuration config) {

    return new DefaultSqlSessionFactory(config);

}

XMLConfigBuilder#parse()

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 {

      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);

      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);

    }

  }

看到parseConfiguration引數為XNode就知道這個方法裡肯定是要解析xml節點了,可以debug檢視下root引數的值，嘗試debug後發現，其內容就是我們的配置檔案mybatis-config.xml的檔案內容。接著我們在22行這裡則看到了熟悉的environments節點，那就直接看下environmentsElement方法。

<configuration>

    <properties resource="db.properties"/>

    <settings>

        <setting name="logImpl" value="STDOUT_LOGGING"/>

        <setting name="cacheEnabled" value="true"/>

        <setting name="lazyLoadingEnabled" value="true"/>

        <setting name="aggressiveLazyLoading" value="false"/>

        <setting name="localCacheScope" value="SESSION"/>

    </settings>

    <typeAliases>

        <typeAlias alias="Student" type="org.apache.ibatis.domain.Student"/>

    </typeAliases>

    <environments default="development">

        <environment id="development">

            <transactionManager type="JDBC"/>

            <dataSource type="POOLED">

                <property name="driver" value="${jdbc.driver}"/>

                <property name="url" value="${jdbc.url}"/>

                <property name="username" value="${jdbc.username}"/>

                <property name="password" value="${jdbc.password}"/>

            </dataSource>

        </environment>

    </environments>

    <mappers>

        <mapper resource="mapper/StudentMapper.xml"/>

    </mappers>

</configuration>

XMLConfigBuilder#environmentsElement

這個方法引數XNode內容則是配置檔案裡的部分。這裡就看到了解析dataSource節點的地方，這裡使用DataSourceFactory返回一個DataSource，那麼這裡是怎麼拿到DataSourceFactory的，又是怎麼拿到DataSource的。

private void environmentsElement(XNode context) throws Exception {

    if (context != null) {

      if (environment == null) {

        environment = context.getStringAttribute("default");

      }

      for (XNode child : context.getChildren()) {

        String id = child.getStringAttribute("id");

        if (isSpecifiedEnvironment(id)) {

          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));

          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));

          DataSource dataSource = dsFactory.getDataSource();

          Environment.Builder environmentBuilder = new Environment.Builder(id)

              .transactionFactory(txFactory)

              .dataSource(dataSource);

          configuration.setEnvironment(environmentBuilder.build());

          break;

        }

      }

    }

  }

解析DataSourceFactory

這裡看到了getDeclaredConstructor().newInstance()反射例項化一個PoolDataSourceFactory。

private DataSourceFactory dataSourceElement(XNode context) throws Exception {

    if (context != null) {

      String type = context.getStringAttribute("type");

      Properties props = context.getChildrenAsProperties();

      DataSourceFactory factory = (DataSourceFactory) resolveClass(type).getDeclaredConstructor().newInstance();

      factory.setProperties(props);

      return factory;

    }

    throw new BuilderException("Environment declaration requires a DataSourceFactory.");

  }

public class PooledDataSourceFactory extends UnpooledDataSourceFactory {

  public PooledDataSourceFactory() {

    this.dataSource = new PooledDataSource();

  }

}

解析DataSource

拿到PooledDataSourceFactory後則呼叫dsFactory.getDataSource()拿到資料來源，這裡重要的程式碼是最後一句configuration.setEnvironment，這裡最後會把解析後的Environment物件給Configuration物件賦值， Configuration物件也是MyBatis框架中相當重量級的一個物件。

MyBatis是如何獲取SQL語句的

解析SQL語句Code Chain

文章開頭我們是用sqlSession.selectOne()方法來獲取SQL語句的，那麼這個SQL語句是怎麼解析並獲取呢？上面我們分析獲取資料來源時提到XMLConfigBuilder.parseConfiguration方法時看到會呼叫mapperElement方法。

private void mapperElement(XNode parent) throws Exception {

    if (parent != null) {

      for (XNode child : parent.getChildren()) {

        if ("package".equals(child.getName())) {

          String mapperPackage = child.getStringAttribute("name");

          configuration.addMappers(mapperPackage);

        } else {

          String resource = child.getStringAttribute("resource");

          String url = child.getStringAttribute("url");

          String mapperClass = child.getStringAttribute("class");

          if (resource != null && url == null && mapperClass == null) {

            ErrorContext.instance().resource(resource);

            try(InputStream inputStream = Resources.getResourceAsStream(resource)) {

              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());

              mapperParser.parse();

            }

          } else if (resource == null && url != null && mapperClass == null) {

            ErrorContext.instance().resource(url);

            try(InputStream inputStream = Resources.getUrlAsStream(url)){

              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());

              mapperParser.parse();

            }

          } else if (resource == null && url == null && mapperClass != null) {

            Class<?> mapperInterface = Resources.classForName(mapperClass);

            configuration.addMapper(mapperInterface);

          } else {

            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");

          }

        }

      }

    }

  }

該方法會根據配置檔案依次解析package, resource, url, class4種配置mapper的方式。

<mappers>

    <mapper resource="mapper/StudentMapper.xml"/>

</mappers>

這裡我們是用resource方式配置的，所以這個方法程式會在Line 12 的邏輯開始執行。

XMLStatementBuilder

解析SQL語句的關鍵邏輯都在這個類的parseStatementNode方法裡，方法最後會呼叫MapperBuilderAssistant#.addMappedStatement(), 這個方法會很到MyBatis裡很重要的一個類MappedStatement物件，這個方法最關鍵的一行程式碼是先生成MappedStatement物件，最後會把生成的MappedStatement物件放入Configuration物件的Map字典中。

看到這個方法裡的程式碼應該很直觀，就是在解析XML中select標籤的一些屬性，比如useCache, flushCache等，這些標籤可以檢視MyBatis文件，真的很詳細。

public void parseStatementNode() {

    String id = context.getStringAttribute("id");

    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {

      return;

    }

    String nodeName = context.getNode().getNodeName();

    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));

    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);

    boolean useCache = context.getBooleanAttribute("useCache", isSelect);

    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);

    includeParser.applyIncludes(context.getNode());

    String parameterType = context.getStringAttribute("parameterType");

    Class<?> parameterTypeClass = resolveClass(parameterType);

    String lang = context.getStringAttribute("lang");

    LanguageDriver langDriver = getLanguageDriver(lang);

    // Parse selectKey after includes and remove them.

    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)

    KeyGenerator keyGenerator;

    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;

    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);

    if (configuration.hasKeyGenerator(keyStatementId)) {

      keyGenerator = configuration.getKeyGenerator(keyStatementId);

    } else {

      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",

          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))

          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;

    }

    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);

    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));

    Integer fetchSize = context.getIntAttribute("fetchSize");

    Integer timeout = context.getIntAttribute("timeout");

    String parameterMap = context.getStringAttribute("parameterMap");

    String resultType = context.getStringAttribute("resultType");

    Class<?> resultTypeClass = resolveClass(resultType);

    String resultMap = context.getStringAttribute("resultMap");

    String resultSetType = context.getStringAttribute("resultSetType");

    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    if (resultSetTypeEnum == null) {

      resultSetTypeEnum = configuration.getDefaultResultSetType();

    }

    String keyProperty = context.getStringAttribute("keyProperty");

    String keyColumn = context.getStringAttribute("keyColumn");

    String resultSets = context.getStringAttribute("resultSets");

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,

        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,

        resultSetTypeEnum, flushCache, useCache, resultOrdered,

        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);

  }

MapperBuilderAssistant

public MappedStatement addMappedStatement(

      String id,

      SqlSource sqlSource,

      StatementType statementType,

      SqlCommandType sqlCommandType,

      Integer fetchSize,

      Integer timeout,

      String parameterMap,

      Class<?> parameterType,

      String resultMap,

      Class<?> resultType,

      ResultSetType resultSetType,

      boolean flushCache,

      boolean useCache,

      boolean resultOrdered,

      KeyGenerator keyGenerator,

      String keyProperty,

      String keyColumn,

      String databaseId,

      LanguageDriver lang,

      String resultSets) {

    if (unresolvedCacheRef) {

      throw new IncompleteElementException("Cache-ref not yet resolved");

    }

    id = applyCurrentNamespace(id, false);

    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)

        .resource(resource)

        .fetchSize(fetchSize)

        .timeout(timeout)

        .statementType(statementType)

        .keyGenerator(keyGenerator)

        .keyProperty(keyProperty)

        .keyColumn(keyColumn)

        .databaseId(databaseId)

        .lang(lang)

        .resultOrdered(resultOrdered)

        .resultSets(resultSets)

        .resultMaps(getStatementResultMaps(resultMap, resultType, id))

        .resultSetType(resultSetType)

        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))

        .useCache(valueOrDefault(useCache, isSelect))

        .cache(currentCache);

    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);

    if (statementParameterMap != null) {

      statementBuilder.parameterMap(statementParameterMap);

    }

    MappedStatement statement = statementBuilder.build();

    configuration.addMappedStatement(statement);

    return statement;

}

MyBatis是如何操作資料庫的

執行器(Executor)

執行器在MyBatis中是用來封裝Statement執行JDBC操作，MyBatis共包含3種類型的執行器：SimpleExecutor, ReuseExecutor, BatchExecutor，預設值使用SimpleExecutor。

 @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);

      stmt = prepareStatement(handler, ms.getStatementLog());

      return handler.query(stmt, resultHandler);

    } finally {

      closeStatement(stmt);

    }

 }

MySQL查詢的元資料和實體建立對映關係

那麼我們的select語句查詢出來的一條mysql元資料和java實體到底是怎樣建立對映關係的？我們先從PreparedStatementHandler#query方法著手分析。

 @Override

 public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {

    //這兩行程式碼是不是很親切了，在JDBC中我們就是這麼幹的。

    //1.執行SQL

    PreparedStatement ps = (PreparedStatement) statement;

    //2.處理結果集

    ps.execute();

    return resultSetHandler.handleResultSets(ps);

 }

結果集對映

DefaultResultSetHandler#handleResultSets

@Override

 public List<Object> handleResultSets(Statement stmt) throws SQLException {

    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

    //1.<select>標籤的resultMap屬性，用來裝對映後的實體物件資料

    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;

    //2.獲取第一個結果集資料

    ResultSetWrapper rsw = getFirstResultSet(stmt);

    //3.這裡就是取出需要對映的ResultMap

    List<ResultMap> resultMaps = mappedStatement.getResultMaps();

    //4.這裡就是需要對映的ResultMap的數量

    int resultMapCount = resultMaps.size();

    validateResultMapsCount(rsw, resultMapCount);

    //5.迴圈處理每個ResultMap

    while (rsw != null && resultMapCount > resultSetCount) {

      //6.迴圈取出需要對映的ResultMap（id,type）實體名稱

      ResultMap resultMap = resultMaps.get(resultSetCount);

      //7.從rsw結果集引數中獲取查詢結果，再根據resultMap對映資訊，將查詢結果裝到到multipleResults中

      handleResultSet(rsw, resultMap, multipleResults, null);

      rsw = getNextResultSet(stmt);

      cleanUpAfterHandlingResultSet();

      resultSetCount++;

    }

    //這裡會獲取select標籤的resultSet屬性

    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);

}

DefaultResultSetHandler#handleResultSet

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {

    try {

      if (parentMapping != null) {

        handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);

      } else {

        if (resultHandler == null) {

          //1.建立DefaultResultHandler 來處理結果集

          DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);

          //2.這裡就是處理對映結果集的最終方法

          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);

          multipleResults.add(defaultResultHandler.getResultList());

        } else {

          handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);

        }

      }

    } finally {

      closeResultSet(rsw.getResultSet());

    }

  }

DefaultResultSetHandler#handleRowValues

public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {

    if (resultMap.hasNestedResultMaps()) {

      ensureNoRowBounds();

      checkResultHandler();

      //巢狀結果對映

      handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);

    } else {

      //簡單結果對映

      handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);

    }

  }

ResultSetWrapper

這裡還有一個關鍵的類在上述方法呼叫鏈裡沒有提到就是ResultSetWrapper，它的構造器裡有3個集合分別是columnNames，jdbcTypes， classNames，當你看到這3個數組時腦海裡是不是浮現出MyBatis框架的意圖呢，代替你管理JDBC，並從結果集建立到java物件的對映。

public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {

    super();

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();

    this.resultSet = rs;

    final ResultSetMetaData metaData = rs.getMetaData();

    final int columnCount = metaData.getColumnCount();

    for (int i = 1; i <= columnCount; i++) {

      columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));

      jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));

      classNames.add(metaData.getColumnClassName(i));

    }

  }

DefaultResultSetHandler#handleRowValuesForSimpleResultMap

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)

      throws SQLException {

    DefaultResultContext<Object> resultContext = new DefaultResultContext<>();

    //1.因為在建立ResultSetWrapper的時候 我們將結果集封裝進去了 現在將結果集取出來

    ResultSet resultSet = rsw.getResultSet();

    //2.分頁資訊

    skipRows(resultSet, rowBounds);

    while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {

      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);

      ///3.將查詢結果封裝到POJO中(這一行程式碼重點關注)

      Object rowValue = getRowValue(rsw, discriminatedResultMap, null);

      storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);

    }

  }

DefaultResultSetHandler#getRowValue

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {

    final ResultLoaderMap lazyLoader = new ResultLoaderMap();

    Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);

    if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {

      final MetaObject metaObject = configuration.newMetaObject(rowValue);

      boolean foundValues = this.useConstructorMappings;

      if (shouldApplyAutomaticMappings(resultMap, false)) {

        //封裝結果集 將sql的結果與實體類封裝對應起來

        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;

      }

      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;

      foundValues = lazyLoader.size() > 0 || foundValues;

      rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;

    }

    return rowValue;

  }

DefaultResultSetHandler#applyAutomaticMappings

這個方法裡會迴圈處理行列欄位並對映實體屬性。

private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {

    //1.封裝你的返回結果集物件

    List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);

    boolean foundValues = false;

    if (!autoMapping.isEmpty()) {

      //2.遍歷 給實體類物件賦值

      for (UnMappedColumnAutoMapping mapping : autoMapping) {

        //3.根據實體屬性 去sql中取值，拿到SQL的結果值和實體類的屬性值，並封裝在metaObject物件中。

        final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);

        if (value != null) {

          foundValues = true;

        }

        if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {

          metaObject.setValue(mapping.property, value);

        }

      }

    }

    //返回值為boolean值

    return foundValues;

  }

這裡autoMapping集合裡能看到就是存放三個欄位的元資料。OK，執行到這一步，外層的getRowValue的返回值就拿到了返回的實體物件。

MyBatis溫故而知新-底層執行原理