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三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）

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三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）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，冰河吐血整理，建议收藏！！

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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。

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好了，我们开始今天的正文。

三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）

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)

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.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

在这里插入图片描述

MappedStatement对象如图

在这里插入图片描述

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：

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~~

三天撸完了MyBatis，各位随便问！！（冰河吐血整理，建议收藏）

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