面试必问 | 聊聊MyBatis执行流程？

Posted 2022-04-20 冰 河

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，小伙伴们可慢慢研究。

本文主要结构如下所示。

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.getSt
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