MyBatis 的工作原理，你了解过吗？

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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 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 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。

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：

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

为当前的查询创建一个缓存Key

缓存中没有值，直接从数据库中读取数据

执行查询，返回List 结果，然后将查询的结果放入缓存之中

根据既有的参数，创建StatementHandler对象来执行查询操作

将创建Statement传递给StatementHandler对象,调用parameterize()方法赋值

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

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