三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）

大家好，我是冰河~~

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，小伙伴们可先收藏后慢慢研究。

小伙伴们如果觉得文章不错，点赞、收藏、评论，分享走一起呀，记得给冰河来个一键三连~~

好了，我们开始今天的正文。

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 resourceif (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configurationparseConfiguration(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:falseboolean 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 trueboolean useCache = context.getBooleanAttribute("useCache", isSelect);boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);// Include Fragments before parsingXMLIncludeTransformer 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)

属性	产生工厂类	产生事务
JDBC	JbdcTransactionFactory	JdbcTransaction
MANAGED	ManagedTransactionFactory	ManagedTransaction

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.SIMPLEreturn 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JbdcTransactiontx = 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 callclose()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(), newClass[] { 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selectswitch (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KeyBoundSql boundSql = ms.getBoundSql(parameterObject);//key = -575461213:-771016147:mapper.UserMapper.getUserById:0:2147483647:select * from test_user where id = ?:1:developmentCacheKey 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 #601deferredLoads.clear();//如果 LocalCacheScope == STATEMENT,会清理本地缓存if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}} 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SimpleExecutorlist = 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。

写在最后

如果你想进大厂，想升职加薪，或者对自己现有的工作比较迷茫，都可以私信我交流，希望我的一些经历能够帮助到大家~~

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好了，今天就到这儿吧，小伙伴们点赞、收藏、评论，一键三连走起呀，我是冰河，我们下期见~~