一文带你理解透MyBatis源码
本文分享自华为云社区《一文彻底吃透MyBatis源码！！》，作者：冰 河。
写在前面
随着互联网的发展，越来越多的公司摒弃了Hibernate，而选择拥抱了MyBatis。而且，很多大厂在面试的时候喜欢问MyBatis底层的原理和源码实现。总之，MyBatis几乎成为了Java开发人员必须深入掌握的框架技术，今天，我们就一起来深入分析MyBatis源码。文章有点长，建议先收藏后慢慢研究。整体三万字左右，全程高能，小伙伴们可慢慢研究。
MyBatis源码解析
大家应该都知道Mybatis源码也是对Jbdc的再一次封装,不管怎么进行包装,还是会有获取链接、preparedStatement、封装参数、执行这些步骤的。
配置解析过程
String resource = "mybatis-config.xml"; //1.读取resources下面的mybatis-config.xml文件 InputStream inputStream = Resources.getResourceAsStream(resource); //2.使用SqlSessionFactoryBuilder创建SqlSessionFactory SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.通过sqlSessionFactory创建SqlSession SqlSession sqlSession = sqlSessionFactory.openSession();
Resources.getResourceAsStream(resource)读取文件
public static InputStream getResourceAsStream(String resource) throws IOException { return getResourceAsStream(null, resource); } //loader赋值为null public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException { InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader); if (in == null) { throw new IOException("Could not find resource " + resource); } return in; } //classLoader为null public InputStream getResourceAsStream(String resource, ClassLoader classLoader) { return getResourceAsStream(resource, getClassLoaders(classLoader)); } //classLoader类加载 InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) { for (ClassLoader cl : classLoader) { if (null != cl) { //加载指定路径文件流 InputStream returnValue = cl.getResourceAsStream(resource); // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource if (null == returnValue) { returnValue = cl.getResourceAsStream("/" + resource); } if (null != returnValue) { return returnValue; } } } return null; }
总结：主要是通过ClassLoader.getResourceAsStream()方法获取指定的classpath路径下的Resource 。
通过SqlSessionFactoryBuilder创建SqlSessionFactory
//SqlSessionFactoryBuilder是一个建造者模式 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); public SqlSessionFactory build(InputStream inputStream) { return build(inputStream, null, null); } //XMLConfigBuilder也是建造者模式 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. } } } //接下来进入XMLConfigBuilder构造函数 public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) { this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props); } //接下来进入this后,初始化Configuration private XMLConfigBuilder(XPathParser parser, String environment, Properties props) { super(new Configuration()); ErrorContext.instance().resource("SQL Mapper Configuration"); this.configuration.setVariables(props); this.parsed = false; this.environment = environment; this.parser = parser; } //其中parser.parse()负责解析xml,build(configuration)创建SqlSessionFactory return build(parser.parse());
parser.parse()解析xml
public Configuration parse() { //判断是否重复解析 if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; //读取配置文件一级节点configuration parseConfiguration(parser.evalNode("/configuration")); return configuration; }
private void parseConfiguration(XNode root) { try { //properties 标签,用来配置参数信息，比如最常见的数据库连接信息 propertiesElement(root.evalNode("properties")); Properties settings = settingsAsProperties(root.evalNode("settings")); loadCustomVfs(settings); loadCustomLogImpl(settings); //实体别名两种方式:1.指定单个实体;2.指定包 typeAliasesElement(root.evalNode("typeAliases")); //插件 pluginElement(root.evalNode("plugins")); //用来创建对象(数据库数据映射成java对象时) objectFactoryElement(root.evalNode("objectFactory")); objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); reflectorFactoryElement(root.evalNode("reflectorFactory")); settingsElement(settings); // read it after objectFactory and objectWrapperFactory issue #631 //数据库环境 environmentsElement(root.evalNode("environments")); databaseIdProviderElement(root.evalNode("databaseIdProvider")); //数据库类型和Java数据类型的转换 typeHandlerElement(root.evalNode("typeHandlers")); //这个是对数据库增删改查的解析 mapperElement(root.evalNode("mappers")); } catch (Exception e) { throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } }
总结:parseConfiguration完成的是解析configuration下的标签
private void mapperElement(XNode parent) throws Exception { if (parent != null) { for (XNode child : parent.getChildren()) { //解析<package name=""/> if ("package".equals(child.getName())) { String mapperPackage = child.getStringAttribute("name"); //包路径存到mapperRegistry中 configuration.addMappers(mapperPackage); } else { //解析<mapper url="" class="" resource=""></mapper> 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); //读取Mapper.xml文件 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."); } } } } }
总结: 通过解析configuration.xml文件,获取其中的Environment、Setting,重要的是将下的所有解析出来之后添加到
Configuration,Configuration类似于配置中心,所有的配置信息都在这里。
mapperParser.parse()对 Mapper 映射器的解析
public void parse() { if (!configuration.isResourceLoaded(resource)) { //解析所有的子标签 configurationElement(parser.evalNode("/mapper")); configuration.addLoadedResource(resource); //把namespace（接口类型）和工厂类绑定起来 bindMapperForNamespace(); } parsePendingResultMaps(); parsePendingCacheRefs(); parsePendingStatements(); } //这里面解析的是Mapper.xml的标签 private void configurationElement(XNode context) { try { String namespace = context.getStringAttribute("namespace"); if (namespace == null || namespace.equals("")) { throw new BuilderException("Mapper's namespace cannot be empty"); } builderAssistant.setCurrentNamespace(namespace); //对其他命名空间缓存配置的引用 cacheRefElement(context.evalNode("cache-ref")); //对给定命名空间的缓存配置 cacheElement(context.evalNode("cache")); parameterMapElement(context.evalNodes("/mapper/parameterMap")); //是最复杂也是最强大的元素，用来描述如何从数据库结果集中来加载对象 resultMapElements(context.evalNodes("/mapper/resultMap")); //可被其他语句引用的可重用语句块 sqlElement(context.evalNodes("/mapper/sql")); //获得MappedStatement对象(增删改查标签) 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); } } //获得MappedStatement对象(增删改查标签) private void buildStatementFromContext(List<XNode> list) { if (configuration.getDatabaseId() != null) { buildStatementFromContext(list, configuration.getDatabaseId()); } buildStatementFromContext(list, null); } //获得MappedStatement对象(增删改查标签) private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { //循环增删改查标签 for (XNode context : list) { final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId); try { //解析insert/update/select/del中的标签 statementParser.parseStatementNode(); } catch (IncompleteElementException e) { configuration.addIncompleteStatement(statementParser); } } } public void parseStatementNode() { //在命名空间中唯一的标识符,可以被用来引用这条语句 String id = context.getStringAttribute("id"); //数据库厂商标识 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; //flushCache和useCache都和二级缓存有关 //将其设置为true后,只要语句被调用,都会导致本地缓存和二级缓存被清空,默认值:false boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect); //将其设置为 true 后，将会导致本条语句的结果被二级缓存缓存起来，默认值：对 select 元素为 true boolean useCache = context.getBooleanAttribute("useCache", isSelect); boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false); // Include Fragments before parsing XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant); includeParser.applyIncludes(context.getNode()); //会传入这条语句的参数类的完全限定名或别名 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"); //从这条语句中返回的期望类型的类的完全限定名或别名 String resultType = context.getStringAttribute("resultType"); Class<?> resultTypeClass = resolveClass(resultType); //外部resultMap的命名引用 String resultMap = context.getStringAttribute("resultMap"); String resultSetType = context.getStringAttribute("resultSetType"); ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType); 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); } 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(); //持有在configuration中 configuration.addMappedStatement(statement); return statement; } public void addMappedStatement(MappedStatement ms){ //ms.getId = mapper.UserMapper.getUserById //ms = MappedStatement等于每一个增删改查的标签的里的数据 mappedStatements.put(ms.getId(), ms); } //最终存放到mappedStatements中,mappedStatements存放的是一个个的增删改查 protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection").conflictMessageProducer((savedValue, targetValue) -> ". please check " + savedValue.getResource() + " and " + targetValue.getResource());
解析bindMapperForNamespace()方法
把 namespace（接口类型）和工厂类绑定起来
private void bindMapperForNamespace() { //当前Mapper的命名空间 String namespace = builderAssistant.getCurrentNamespace(); if (namespace != null) { Class<?> boundType = null; try { //interface mapper.UserMapper这种 boundType = Resources.classForName(namespace); } catch (ClassNotFoundException e) { } if (boundType != null) { if (!configuration.hasMapper(boundType)) { configuration.addLoadedResource("namespace:" + namespace); configuration.addMapper(boundType); } } } } public <T> void addMapper(Class<T> type) { mapperRegistry.addMapper(type); } public <T> void addMapper(Class<T> type) { if (type.isInterface()) { if (hasMapper(type)) { throw new BindingException("Type " + type + " is already known to the MapperRegistry."); } boolean loadCompleted = false; try { //接口类型(key)->工厂类 knownMappers.put(type, new MapperProxyFactory<>(type)); MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }
生成SqlSessionFactory对象
XMLMapperBuilder.parse()方法,是对 Mapper 映射器的解析里面有两个方法：
（1）configurationElement()解析所有的子标签,最终解析Mapper.xml中的insert/update/delete/select标签的id(全路径)组成key和整个标签和数据连接组成MappedStatement存放到Configuration中的 mappedStatements这个map里面。
（2）bindMapperForNamespace()是把接口类型(interface mapper.UserMapper)和工厂类存到放MapperRegistry中的knownMappers里面。
SqlSessionFactory的创建
public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); }
直接把Configuration当做参数,直接new一个DefaultSqlSessionFactory。
SqlSession会话的创建过程
mybatis操作的时候跟数据库的每一次连接,都需要创建一个会话,我们用openSession()方法来创建。这个会话里面需要包含一个Executor用来执行 SQL。Executor又要指定事务类型和执行器的类型。
创建Transaction(两种方式)
- 如果配置的是 JDBC,则会使用Connection 对象的 commit()、rollback()、close()管理事务。
- 如果配置成MANAGED,会把事务交给容器来管理,比如 JBOSS,Weblogic。
SqlSession sqlSession = sqlSessionFactory.openSession();
public SqlSession openSession() { //configuration中有默认赋值protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); } <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${driver}"/> <property name="url" value="${url}"/> <property name="username" value="${username}"/> <property name="password" value="${password}"/> </dataSource> </environment> </environments>
创建Executor
//ExecutorType是SIMPLE,一共有三种SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor) private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { //xml中的development节点 final Environment environment = configuration.getEnvironment(); //type配置的是Jbdc所以生成的是JbdcTransactionFactory工厂类 final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); //Jdbc生成JbdcTransactionFactory生成JbdcTransaction tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); //创建CachingExecutor执行器 final Executor executor = configuration.newExecutor(tx, execType); //创建DefaultSqlSession属性包括 Configuration、Executor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(); } }
获得Mapper对象
UserMapper userMapper = sqlSession.getMapper(UserMapper.class); public <T> T getMapper(Class<T> type) { return configuration.getMapper(type, this); }
mapperRegistry.getMapper是从MapperRegistry的knownMappers里面取的,knownMappers里面存的是接口类型(interface mapper.UserMapper)和工厂类(MapperProxyFactory)。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession); }
从knownMappers的Map里根据接口类型(interface mapper.UserMapper)取出对应的工厂类。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) { final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } } public T newInstance(SqlSession sqlSession) { final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); }
这里通过JDK动态代理返回代理对象MapperProxy(org.apache.ibatis.binding.MapperProxy@6b2ea799)
protected T newInstance(MapperProxy<T> mapperProxy) { //mapperInterface是interface mapper.UserMapper return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); } UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
执行SQL
User user = userMapper.getUserById(1);
调用invoke代理方法
由于所有的 Mapper 都是 MapperProxy 代理对象，所以任意的方法都是执行MapperProxy 的invoke()方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { //判断是否需要去执行SQL还是直接执行方法 if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); //这里判断的是接口中的默认方法Default等 } else if (isDefaultMethod(method)) { return invokeDefaultMethod(proxy, method, args); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } //获取缓存,保存了方法签名和接口方法的关系 final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }
调用execute方法
这里使用的例子用的是查询所以走的是else分支语句。
public Object execute(SqlSession sqlSession, Object[] args) { Object result; //根据命令类型走不行的操作command.getType()是select 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: if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); } else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args); } else { //将参数转换为SQL的参数 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; }
调用selectOne其实是selectList
selectOne查询一个和查询多个其实是一样的。
public <T> T selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many. List<T> list = this.selectList(statement, parameter); if (list.size() == 1) { return list.get(0); } else if (list.size() > 1) { throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size()); } else { return null; } } public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { //从Configuration里的mappedStatements里根据key(id的全路径)获取MappedStatement 对象 MappedStatement ms = configuration.getMappedStatement(statement); return 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(); } }
mappedStatements对象如图
MappedStatement对象如图
执行query方法
创建CacheKey
从 BoundSql 中获取SQL信息,创建 CacheKey。这个CacheKey就是缓存的Key。
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { //创建缓存Key BoundSql boundSql = ms.getBoundSql(parameterObject); //key = -575461213:-771016147:mapper.UserMapper.getUserById:0:2147483647:select * from test_user where id = ?:1:development CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); } public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }
清空本地缓存
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."); } //queryStack 用于记录查询栈，防止递归查询重复处理缓存 //flushCache=true 的时候，会先清理本地缓存（一级缓存） if (queryStack == 0 && ms.isFlushCacheRequired()) { //清空本地缓存 clearLocalCache(); } List<E> list; try { queryStack++; list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { //如果没有缓存,会从数据库查询：queryFromDatabase() list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) { deferredLoad.load(); } // issue #601 deferredLoads.clear(); //如果 LocalCacheScope == STATEMENT,会清理本地缓存 if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { // issue #482 clearLocalCache(); } } return list; }
从数据库查询
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; //先在缓存用占位符占位 localCache.putObject(key, EXECUTION_PLACEHOLDER); try { //执行Executor 的 doQuery(),默认是SimpleExecutor list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { //执行查询后,移除占位符 localCache.removeObject(key); } //从新放入数据 localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }
执行doQuery
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); } }
源码总结
总体上来说，MyBatis的源码还是比较简单的，只要大家踏下心来，花个两三天仔细研究下，基本上都能弄明白源码的主体脉络。
