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。

点击关注，第一时间了解华为云新鲜技术~