Mybatis源码解析—

1.初始化Configuration对象

mybatis-config.xml这个文件主要是用于配置数据源、配置别名、加载mapper.xml。

mapper标签所指向的路径就是另外一个xml文件：xxxMapper.xml，而这个文件中写了我们查询数据库所用的SQL。

MyBatis实际上就是将这两个xml文件，用XMLConfigBuilder的parse()方法解析成配置对象

Configuration的核心配置类：

Environment environment：数据库环境

MapperRegistry mapperRegistry：保存解析到的所有映射器（mapper接口+mapper.xml）

Map<String, MappedStatement> mappedStatements：保存整个项目所有的SQL语句

Map<String, ResultMap> resultMaps：保存mapper.xml中配置的<resultMap>

Set<String> loadedResources：保存扫描到的所有的资源

点进去parse()方法，用XPath语法，获取核心配置的根节点

 public Configuration parse() {//查看该文件是否已经解析过if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}//如果没有解析过，则继续往下解析，并且将标识符置为trueparsed = true;//解析<configuration>节点parseConfiguration(parser.evalNode("/configuration"));//跟着configuration节点去解析它的子节点return configuration;}

点进去parseConfiguration()方法解析根节点下的子节点

private void parseConfiguration(XNode root) {try {//解析<Configuration>下的节点//issue #117 read properties first//<properties>propertiesElement(root.evalNode("properties"));//<settings>Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(settings);loadCustomLogImpl(settings);//别名<typeAliases>解析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//配置环境environmentsElement(root.evalNode("environments"));databaseIdProviderElement(root.evalNode("databaseIdProvider"));typeHandlerElement(root.evalNode("typeHandlers"));//主要 <mappers> 指向我们存放SQL的xxxxMapper.xml文件mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}}

每个解析不同标签的方法内部都对Configuration对象进行了set或者其它类似的操作，如：

private void propertiesElement(XNode context) throws Exception { //引入包含数据库连接参数的database.properties文件的细节if (context != null) {  //如果连接数据库参数的文件不为空Properties defaults = context.getChildrenAsProperties();  //获取连接数据库文件的属性String resource = context.getStringAttribute("resource");String url = context.getStringAttribute("url");if (resource != null && url != null) {throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");}if (resource != null) {defaults.putAll(Resources.getResourceAsProperties(resource));} else if (url != null) {defaults.putAll(Resources.getUrlAsProperties(url));}Properties vars = configuration.getVariables();if (vars != null) {defaults.putAll(vars);}parser.setVariables(defaults);configuration.setVariables(defaults);}}

这里由于代码量比较大，而且大多数构建都是些细节，大概知道怎么用就可以了，就不在文章中一一说明了

解析mapper.xml里面的节点

//解析mapper文件里面的节点private void configurationElement(XNode context) {try {//获取命名空间 namespace，后期mybatis会通过这个动态代理我们的Mapper接口String namespace = context.getStringAttribute("namespace");if (namespace == null || namespace.equals("")) {//如果namespace为空则抛一个异常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"));//解析增删改查节点<select> <insert> <update> <delete>buildStatementFromContext(context.evalNodes("select|insert|update|delete"));} catch (Exception e) {throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);}}

2.构建SqlSessionFactory

我们现在得到了一个SqlSessionFactory对象，下一步就是要去获取SqlSession对象，这里会调用SqlSessionFactory.openSession()方法来获取，而openSession中实际上就是对SqlSession做了进一步的加工封装，包括增加了事务、执行器等。

 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {//对SqlSession对象进行进一步加工封装final Environment environment = configuration.getEnvironment();final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);final Executor executor = configuration.newExecutor(tx, execType);//构建SqlSession对象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();}}

SqlSessionFactory对象中注入Configuration对象，所以它保存了全局配置信息，以及初始化环境和DataSource，当我们调用openSession方法时，交给SqlSession来对数据库做相关操作。

3.打开SqlSession会话

现在我们获取到了一个SqlSession对象之后打开会话，而执行过程就是从这里开始

这里首先会判断SQL的类型：SELECT|DELETE|UPDATE|INSERT

//execute() 这里是真正执行SQL的地方public Object execute(SqlSession sqlSession, Object[] args) {//判断是哪一种SQL语句Object result;switch (command.getType()) {case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}case SELECT://我们的例子是查询//判断是否有返回值if (method.returnsVoid() && method.hasResultHandler()) {//无返回值executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {//返回值多行 这里调用这个方法result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {//返回Mapresult = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {//返回Cursorresult = executeForCursor(sqlSession, args);} else {Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);if (method.returnsOptional()&& (result == null || !method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName()+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;}

得到MappedStatement对象，Executor负责执行

selectList内部调用的Executor对象执行SQL语句

 @Overridepublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {//MappedStatement:解析XML时生成的对象， 解析某一个SQL  会封装成MappedStatement，里面存放了我们所有执行SQL所需要的信息MappedStatement ms = configuration.getMappedStatement(statement);//查询,通过executorreturn 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();}}

4.Executor开始处理请求

MyBatis在查询时，不会直接查询数据库，而是会进行二级缓存的查询

在开启二级缓存的情况下，执行查询会用cachingExecutor缓存执行，如果没有数据，用SimpleExecutor去数据库查询

SQL查询（一级缓存）

 //一级缓存查询@Overridepublic <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 {//查询栈+1queryStack++;//一级缓存list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {//对于存储过程有输出资源的处理handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {//如果缓存为空，则从数据库拿list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);}} finally {//查询栈-1queryStack--;}if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();}// issue #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}//结果返回return list;}

如果一级缓存查到了，那么直接就返回结果了，如果一级缓存没有查到结果，那么最终会进入数据库进行查询。

SQL执行（数据库查询）

//数据库查询private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;//先往一级缓存中put一个占位符localCache.putObject(key, EXECUTION_PLACEHOLDER);try {//调用doQuery方法查询数据库list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {localCache.removeObject(key);}//往缓存中put真实数据localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;}

5.StatementHandler执行SQL语句

二级缓存查询完了之后没有数据才去真实数据库查询，这里使用statementHandler对象，要参数赋值完毕之后才执行

//真实数据库查询@Overridepublic <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();//封装，StatementHandler也是MyBatis四大对象之一StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);//#{} -> ? 的SQL在这里初始化stmt = prepareStatement(handler, ms.getStatementLog());//参数赋值完毕之后，才会真正地查询。return handler.query(stmt, resultHandler);} finally {closeStatement(stmt);}}

6.ParameterHandler设置参数

在真正的数据库查询之前，要先将占位符换成真实的参数值，所以接下来会进行参数的赋值。

 //由于是#{}，所以使用的是prepareStatement，预编译SQLprivate Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {Statement stmt;//拿连接对象Connection connection = getConnection(statementLog);//初始化prepareStatementstmt = handler.prepare(connection, transaction.getTimeout());//获取了PrepareStatement之后，这里给#{}赋值handler.parameterize(stmt);return stmt;}

正式执行

当参数赋值完毕后，SQL就可以执行了

7.ResultSetHandler处理结果集

  @Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {//通过jdbc进行数据库查询。PreparedStatement ps = (PreparedStatement) statement;ps.execute();//处理结果集 resultSetHandler 也是MyBatis的四大对象之一return resultSetHandler.handleResultSets(ps);}