三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）

迷南。 2021-09-08 10:46 475阅读 0赞

**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。

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## 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) ###

<table> 
 <thead> 
  <tr> 
   <th>属性</th> 
   <th>产生工厂类</th> 
   <th>产生事务</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>JDBC</td> 
   <td>JbdcTransactionFactory</td> 
   <td>JdbcTransaction</td> 
  </tr> 
  <tr> 
   <td>MANAGED</td> 
   <td>ManagedTransactionFactory</td> 
   <td>ManagedTransaction</td> 
  </tr> 
 </tbody> 
</table>

*  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);
    }

### 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 ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 1]

#### MappedStatement对象如图 ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 2]

### 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。

## 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~~**

**推荐阅读：**

*  《[奉劝那些刚参加工作的学弟学妹们：要想进大厂，这些核心技能是你必须要掌握的！完整学习路线！！（建议收藏）][Link 1]》
 *  《[奉劝那些刚参加工作的学弟学妹们：这些计算机与操作系统基础知识越早知道越好！万字长文太顶了！！（建议收藏）][Link 2]》
 *  《[我用三天时间开发了一款老少皆宜的国民级游戏，支持播放音乐，现开放完整源代码和注释（建议收藏）！！][Link 3]》
 *  《[我是全网最硬核的高并发编程作者，CSDN最值得关注的博主，大家同意吗？（建议收藏）][CSDN]》
 *  《[毕业五年，从月薪3000到年薪百万，我掌握了哪些核心技能？（建议收藏）][3000]》
 *  《[我入侵了隔壁妹子的Wifi，发现。。。（全程实战干货，建议收藏）][Wifi]》
 *  《[千万不要轻易尝试“熊猫烧香”，这不，我后悔了！][Link 4]》
 *  《[清明节偷偷训练“熊猫烧香”，结果我的电脑为熊猫“献身了”！][Link 5]》
 *  《[7.3万字肝爆Java8新特性，我不信你能看完！（建议收藏）][7.3_Java8]》
 *  《[在业务高峰期拔掉服务器电源是一种怎样的体验？][Link 6]》
 *  《[全网最全Linux命令总结！！（史上最全，建议收藏）][Linux]》

**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~~**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 3][]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20210901/e3bea2f862684fd68d490cdafa6c06d4.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 2]: /images/20210901/2c67233640a64ea8a02a5890f4b92689.png
[Link 1]: https://blog.csdn.net/l1028386804/article/details/116081409
[Link 2]: https://blog.csdn.net/l1028386804/article/details/115923034
[Link 3]: https://blog.csdn.net/l1028386804/article/details/116191713
[CSDN]: https://blog.csdn.net/l1028386804/article/details/116280491
[3000]: https://blog.csdn.net/l1028386804/article/details/115677451
[Wifi]: https://blog.csdn.net/l1028386804/article/details/116477701
[Link 4]: https://blog.csdn.net/l1028386804/article/details/115450665
[Link 5]: https://blog.csdn.net/l1028386804/article/details/115607742
[7.3_Java8]: https://blog.csdn.net/l1028386804/article/details/115547910
[Link 6]: https://blog.csdn.net/l1028386804/article/details/115576102
[Linux]: https://blog.csdn.net/l1028386804/article/details/117917710
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0_size_16_color_FFFFFF_t_70_pic_center 3]: /images/20210901/2ad9b197f65d4b6b92204bf986e30899.png