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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 configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    
    <environments default="development">
        <environment id="development">
            
            <transactionManager type="JDBC" />
            
            <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>

第一步：创建一个sqlSessionFactory

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>    }
    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>          }
        }
      }
    }
  }

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。

第二步：创建sqlSession

  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。

第三步：执行具体的sql请求

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