MyBatis 的工作原理，你了解过吗？

来源：cnblogs.com/scuury/p/10371246.html

• 第一步：创建一个sqlSessionFactory
• 第二步：创建sqlSession
• 第三步：执行具体的sql请求
• 总结
• 参考

近来想写一个mybatis的分页插件，但是在写插件之前肯定要了解一下mybatis具体的工作原理吧，于是边参考别人的博客，边看源码就开干了。

核心部件：

• SqlSession
• Executor
• StatementHandler
• ParameterHandler
• ResultSetHandler
• TypeHandler
• MappedStatement
• Configuration

在分析工作原理之前，首先看一下我的mybatis全局配置文件

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration>    <!-- 和spring整合后 environments配置将废除 -->    <environments default="development">        <environment id="development">            <!-- 使用jdbc事务管理 -->            <transactionManager type="JDBC" />            <!-- 数据库连接池 -->            <dataSource type="POOLED">                <property name="driver" value="com.mysql.jdbc.Driver" />                <property name="url"                    value="jdbc:mysql://localhost:3306/test?characterEncoding=utf-8" />                <property name="username" value="root" />                <property name="password" value="123456" />            </dataSource>        </environment>    </environments>    <mappers>       <mapper  resource="sqlMapper/userMapper.xml"/>    </mappers></configuration>

在了解如何创建sqlSessionFactory之前，先看一下mybatis是如何加载全局配置文件，解析xml文件生成Configuration的

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")); //issue #117 read properties first      typeAliasesElement(root.evalNode("typeAliases"));      pluginElement(root.evalNode("plugins"));      objectFactoryElement(root.evalNode("objectFactory"));      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));      settingsElement(root.evalNode("settings"));      environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631      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);    }  }

在上面的第二段代码中有一句

mapperElement(root.evalNode("mappers"));

刚好我们的全局配置文件中有一个mapper的配置，由此可见，mapperElemet()方法是解析mapper映射文件的，具体代码如下

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) {//进入该判断            ErrorContext.instance().resource(resource);            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);            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.");          }        }      }    }  }

根据以上代码可以分析，在写mapper映射文件的地址时不仅可以写成resource，还可以写成url和mapperClass的形式，由于我们用的是resource，所以直接进入第一个判断，最后解析mapper映射文件的方法是

private void configurationElement(XNode context) {    try {      String namespace = context.getStringAttribute("namespace");      if (namespace.equals("")) {          throw new BuilderException("Mapper's namespace cannot be empty");      }      builderAssistant.setCurrentNamespace(namespace);      cacheRefElement(context.evalNode("cache-ref"));      cacheElement(context.evalNode("cache"));      parameterMapElement(context.evalNodes("/mapper/parameterMap"));      resultMapElements(context.evalNodes("/mapper/resultMap"));      sqlElement(context.evalNodes("/mapper/sql"));      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));    } catch (Exception e) {      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);    }  }

其中具体解析每一个sql语句节点的是

buildStatementFromContext(context.evalNodes("select|insert|update|delete"));

进入这个方法一层层深究，最后到这里可以知道MappedStatement是由builderAssistant（即MapperBuildAssistant）创建的。

public void parseStatementNode() {    ...    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,        resultSetTypeEnum, flushCache, useCache, resultOrdered,        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);  }

最后进入方法addMappedStatement()，mappedStatement最后以id为键保存在了Configuration中的一个map变量mappedStatements中。

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);    statementBuilder.resource(resource);    statementBuilder.fetchSize(fetchSize);    statementBuilder.statementType(statementType);    statementBuilder.keyGenerator(keyGenerator);    statementBuilder.keyProperty(keyProperty);    statementBuilder.keyColumn(keyColumn);    statementBuilder.databaseId(databaseId);    statementBuilder.lang(lang);    statementBuilder.resultOrdered(resultOrdered);    statementBuilder.resulSets(resultSets);    setStatementTimeout(timeout, statementBuilder);    setStatementParameterMap(parameterMap, parameterType, statementBuilder);    setStatementResultMap(resultMap, resultType, resultSetType, statementBuilder);    setStatementCache(isSelect, flushCache, useCache, currentCache, statementBuilder);    MappedStatement statement = statementBuilder.build();    configuration.addMappedStatement(statement);    return statement;  }

最后回到我们的创建sqlSessionFactory上，之前的一切都是为了生成一个sqlSessionFactory服务的

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {    try {      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);      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);  }

从上面的代码可以看出最后是通过以Configuration为参数build()方法生成DefautSqlSessionFactory。

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

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();    }  }

//返回一个SqlSession，默认使用DefaultSqlSession public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {    this.configuration = configuration;    this.executor = executor;    this.dirty = false;    this.autoCommit = autoCommit;  }

executor在这一步得到创建，具体的使用在下一步。

在我的代码里执行的是

User user = sqlSession.selectOne("test.findUserById", 1);

具体到里面的方法就是

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {    try {     //1.根据Statement Id，在mybatis 配置对象Configuration中查找和配置文件相对应的MappedStatement      MappedStatement ms = configuration.getMappedStatement(statement);      //2. 将查询任务委托给MyBatis 的执行器 Executor      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);      return result;    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);    } finally {      ErrorContext.instance().reset();    }  }

在这里通过statementId拿到了我们在第一步存在map里面的MappedStatement。在这里引用参考博客的一句话：

SqlSession根据Statement ID, 在mybatis配置对象Configuration中获取到对应的MappedStatement对象，然后调用mybatis执行器来执行具体的操作。

再继续看query()和queryFromDatabase()这两个方法

@SuppressWarnings("unchecked")  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();      }      deferredLoads.clear(); // issue #601      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {        clearLocalCache(); // issue #482      }    }    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;  }

在这两个方法里面会为当前的查询创建一个缓存key，如果缓存中没有值，直接从数据库中读取，执行查询后将得到的list结果放入缓存之中。

紧接着看doQuery()在SimpleExecutor类中重写的方法

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.<E>query(stmt, resultHandler);    } finally {      closeStatement(stmt);    }  }

Statement连接对象就是在这里创建的，因此Executor的作用之一就是创建Statement了，创建完后又把Statement丢给StatementHandler返回List查询结果。

接下来再看一下这里的两个方法prepareStatement()和query()的具体实现

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {    Statement stmt;    Connection connection = getConnection(statementLog);    stmt = handler.prepare(connection);    handler.parameterize(stmt);    return stmt;  }

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {    PreparedStatement ps = (PreparedStatement) statement;    ps.execute();    return resultSetHandler.<E> handleResultSets(ps);  }

prepareStatement()是创建Statement的具体实现方法，调用parameterize()对创建的Statement对象设置参数，即为我们设为占位符的地方赋上指定的参数，parameterize()方法再深入进去就是调用ParameterHandler的setParameters()方法具体赋值了。

这里的query()是调用了ResultSetHandler的handleResultSets(Statement) 方法。作用就是把ResultSet结果集对象转换成List类型的集合。

总结以上步骤就是：

1. 根据具体传入的参数，动态地生成需要执行的SQL语句，用BoundSql对象表示
2. 为当前的查询创建一个缓存Key
3. 缓存中没有值，直接从数据库中读取数据
4. 执行查询，返回List 结果，然后 将查询的结果放入缓存之中
5. 根据既有的参数，创建StatementHandler对象来执行查询操作
6. 将创建Statement传递给StatementHandler对象,调用parameterize()方法赋值
7. 调用StatementHandler.query()方法，返回List结果集总结

   以上三个步骤所有流程大体可以用一张图来总结