MyBatis 作为一个轻量的 ORM 框架，与 Hibernate 相比，它的学习成本相对较低，尤其适合初级开发使用。本篇我们将介绍原生 MyBatis 的工作原理，并从设计模式的角度去分析其设计。

版本说明：本次源码解读基于 3.5.1 版本，请注意区分版本差异。

1. 原生 API 的基本使用

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession();
Blog blog = session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);

2. 源码解读

MyBatis 初始化的过程主要就是对 Configuration 配置文件的解析过程，因此我们先观察一个简单的配置文件的内容：

<?xml version="1.0" encoding="UTF-8" ?>
<!--Copyright 2009-2016 the original author or authors.Licensed under the Apache License, Version 2.0 (the "License");you may not use this file except in compliance with the License.You may obtain a copy of the License athttp://www.apache.org/licenses/LICENSE-2.0Unless required by applicable law or agreed to in writing, softwaredistributed under the License is distributed on an "AS IS" BASIS,WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.See the License for the specific language governing permissions andlimitations under the License.-->
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration><properties resource="com\ghoostek\main\blog-derby.properties"/><settings><setting name="cacheEnabled" value="true"/><setting name="lazyLoadingEnabled" value="false"/><setting name="multipleResultSetsEnabled" value="true"/><setting name="useColumnLabel" value="true"/><setting name="useGeneratedKeys" value="false"/><setting name="defaultExecutorType" value="SIMPLE"/><setting name="defaultStatementTimeout" value="25"/></settings><typeAliases><typeAlias alias="Author" type="com.ghoostek.domain.blog.Author"/><typeAlias alias="Blog" type="com.ghoostek.domain.blog.Blog"/><typeAlias alias="Comment" type="com.ghoostek.domain.blog.Comment"/><typeAlias alias="Post" type="com.ghoostek.domain.blog.Post"/><typeAlias alias="Section" type="com.ghoostek.domain.blog.Section"/><typeAlias alias="Tag" type="com.ghoostek.domain.blog.Tag"/></typeAliases><typeHandlers><typeHandler javaType="String" jdbcType="VARCHAR" handler="com.ghoostek.main.CustomStringTypeHandler"/></typeHandlers><objectFactory type="com.ghoostek.main.ExampleObjectFactory"><property name="objectFactoryProperty" value="100"/></objectFactory><plugins><plugin interceptor="com.ghoostek.main.ExamplePlugin"><property name="pluginProperty" value="100"/></plugin></plugins><environments default="development"><environment id="development"><transactionManager type="JDBC"><property name="" value=""/></transactionManager><dataSource type="UNPOOLED"><property name="driver" value="${driver}"/><property name="url" value="${url}"/><property name="username" value="${username}"/><property name="password" value="${password}"/></dataSource></environment></environments><mappers><mapper resource="com\ghoostek\main\AuthorMapper.xml"/><mapper resource="com\ghoostek\main\BlogMapper.xml"/><mapper resource="com\ghoostek\main\CachedAuthorMapper.xml"/><mapper resource="com\ghoostek\main\PostMapper.xml"/><mapper resource="com\ghoostek\main\NestedBlogMapper.xml"/></mappers></configuration>

2.1 SqlSessionFactoryBuilder.build()

根据前面章节了解到的知识，我们可以知道解析的逻辑在 SqlSessionFactoryBuilder.build() 中。

  public SqlSessionFactory build(InputStream inputStream) {return build(inputStream, null, null);}public SqlSessionFactory build(InputStream inputStream, String environment) {return build(inputStream, environment, null);}public SqlSessionFactory build(InputStream inputStream, Properties properties) {return build(inputStream, null, properties);}public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {// 与 XMLConfigBuilder 相对应的 XMLMapperBuilder 负责对 mapper 文件的解析工作，// 他们都继承 BaseBuilder，这里采用的是模板设计模式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.}}}public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}

2.2 XmlConfigBuilder.parse()

  public Configuration parse() {if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}parsed = true;parseConfiguration(parser.evalNode("/configuration"));return configuration;}// 这里用到的是模板方法模式，在 parseConfiguration() 方法中定义解析的步骤private void parseConfiguration(XNode root) {try {//issue #117 read properties first// 解析 properties 节点，将 properties 中配置的信息并入 Configuration 属性中propertiesElement(root.evalNode("properties"));Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(settings);loadCustomLogImpl(settings);typeAliasesElement(root.evalNode("typeAliases"));pluginElement(root.evalNode("plugins"));objectFactoryElement(root.evalNode("objectFactory"));objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));reflectorFactoryElement(root.evalNode("reflectorFactory"));// 设置 setting 配置，未配置的赋默认值settingsElement(settings);// read it after objectFactory and objectWrapperFactory issue #631// 初始化数据库/数据源environmentsElement(root.evalNode("environments"));databaseIdProviderElement(root.evalNode("databaseIdProvider"));typeHandlerElement(root.evalNode("typeHandlers"));mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}}

2.2.1 propertiesElement()

本方法主要解析 properties 标签中引用的资源文件以及属性配置，properties 标签形如：

<properties resource="org/mybatis/example/config.properties"><property name="username" value="dev_user"/><property name="password" value="F2Fa3!33TYyg"/>
</properties>

  private void propertiesElement(XNode context) throws Exception {if (context != null) {// 先解析 properties 体中的子节点Properties defaults = context.getChildrenAsProperties();String resource = context.getStringAttribute("resource");String url = context.getStringAttribute("url");if (resource != null && url != null) {throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");}if (resource != null) {// 再解析 resource 所指向的资源文件，并覆盖子节点下的值defaults.putAll(Resources.getResourceAsProperties(resource));} else if (url != null) {defaults.putAll(Resources.getUrlAsProperties(url));}Properties vars = configuration.getVariables();if (vars != null) {defaults.putAll(vars);}parser.setVariables(defaults);configuration.setVariables(defaults);}}

2.2.2 settingsAsProperties()

本方法主要是解析 settings 标签下的属性值，用于设置 Configuration 的对应属性值，settings 标签形如：

<settings><setting name="cacheEnabled" value="true"/><setting name="lazyLoadingEnabled" value="true"/><setting name="multipleResultSetsEnabled" value="true"/><setting name="useColumnLabel" value="true"/><setting name="useGeneratedKeys" value="false"/><setting name="autoMappingBehavior" value="PARTIAL"/><setting name="autoMappingUnknownColumnBehavior" value="WARNING"/><setting name="defaultExecutorType" value="SIMPLE"/><setting name="defaultStatementTimeout" value="25"/><setting name="defaultFetchSize" value="100"/><setting name="safeRowBoundsEnabled" value="false"/><setting name="mapUnderscoreToCamelCase" value="false"/><setting name="localCacheScope" value="SESSION"/><setting name="jdbcTypeForNull" value="OTHER"/><setting name="lazyLoadTriggerMethods"value="equals,clone,hashCode,toString"/>
</settings>

  private Properties settingsAsProperties(XNode context) {if (context == null) {return new Properties();}Properties props = context.getChildrenAsProperties();// Check that all settings are known to the configuration classMetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);// 判断 Configuration 类是否有对应属性for (Object key : props.keySet()) {if (!metaConfig.hasSetter(String.valueOf(key))) {throw new BuilderException("The setting " + key + " is not known.  Make sure you spelled it correctly (case sensitive).");}}return props;}

2.2.3 typeAliasesElement()

本方法主要解析别名配置 typeAliases，并将其注册到别名注册器中，typeAlias 标签形如：

<typeAliases><typeAlias alias="Author" type="domain.blog.Author"/><typeAlias alias="Blog" type="domain.blog.Blog"/><typeAlias alias="Comment" type="domain.blog.Comment"/><typeAlias alias="Post" type="domain.blog.Post"/><typeAlias alias="Section" type="domain.blog.Section"/><typeAlias alias="Tag" type="domain.blog.Tag"/>
</typeAliases>

  private void typeAliasesElement(XNode parent) {if (parent != null) {for (XNode child : parent.getChildren()) {if ("package".equals(child.getName())) {String typeAliasPackage = child.getStringAttribute("name");configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);} else {String alias = child.getStringAttribute("alias");String type = child.getStringAttribute("type");try {// 加载对应的 class，同时校验此类是否存在Class<?> clazz = Resources.classForName(type);if (alias == null) {// 注册注解配置的别名，如果未配置注解，则将 class 的 simpleName 作为别名进行注册typeAliasRegistry.registerAlias(clazz);} else {// 注册 xml 配置的别名typeAliasRegistry.registerAlias(alias, clazz);}} catch (ClassNotFoundException e) {throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);}}}}}

注意到 MyBatis 中预置了一些别名配置：

  public TypeAliasRegistry() {registerAlias("string", String.class);registerAlias("byte", Byte.class);registerAlias("long", Long.class);registerAlias("short", Short.class);registerAlias("int", Integer.class);registerAlias("integer", Integer.class);registerAlias("double", Double.class);registerAlias("float", Float.class);registerAlias("boolean", Boolean.class);registerAlias("byte[]", Byte[].class);registerAlias("long[]", Long[].class);registerAlias("short[]", Short[].class);registerAlias("int[]", Integer[].class);registerAlias("integer[]", Integer[].class);registerAlias("double[]", Double[].class);registerAlias("float[]", Float[].class);registerAlias("boolean[]", Boolean[].class);registerAlias("_byte", byte.class);registerAlias("_long", long.class);registerAlias("_short", short.class);registerAlias("_int", int.class);registerAlias("_integer", int.class);registerAlias("_double", double.class);registerAlias("_float", float.class);registerAlias("_boolean", boolean.class);registerAlias("_byte[]", byte[].class);registerAlias("_long[]", long[].class);registerAlias("_short[]", short[].class);registerAlias("_int[]", int[].class);registerAlias("_integer[]", int[].class);registerAlias("_double[]", double[].class);registerAlias("_float[]", float[].class);registerAlias("_boolean[]", boolean[].class);registerAlias("date", Date.class);registerAlias("decimal", BigDecimal.class);registerAlias("bigdecimal", BigDecimal.class);registerAlias("biginteger", BigInteger.class);registerAlias("object", Object.class);registerAlias("date[]", Date[].class);registerAlias("decimal[]", BigDecimal[].class);registerAlias("bigdecimal[]", BigDecimal[].class);registerAlias("biginteger[]", BigInteger[].class);registerAlias("object[]", Object[].class);registerAlias("map", Map.class);registerAlias("hashmap", HashMap.class);registerAlias("list", List.class);registerAlias("arraylist", ArrayList.class);registerAlias("collection", Collection.class);registerAlias("iterator", Iterator.class);registerAlias("ResultSet", ResultSet.class);}

2.2.4 pluginElement()

本方法主要解析拦截器的配置，对应如下格式的 plugins 标签：

<plugins><plugin interceptor="org.mybatis.example.ExamplePlugin"><property name="someProperty" value="100"/></plugin>
</plugins>

  private void pluginElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {String interceptor = child.getStringAttribute("interceptor");Properties properties = child.getChildrenAsProperties();// 从别名注册表中查找 class，查不到直接加载 classInterceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();interceptorInstance.setProperties(properties);configuration.addInterceptor(interceptorInstance);}}}

在 MyBatis 中我们可以对如下的组件进行拦截：

• Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
• ParameterHandler (getParameterObject, setParameters)
• ResultSetHandler (handleResultSets, handleOutputParameters)
• StatementHandler (prepare, parameterize, batch, update, query)

我们需要实现 Interceptor 来自定义拦截器，并通过注解 @Intercepts 来描述要拦截的方法集合，@Intercepts 注解的值是 @Signature 注解集合，每个 @Signature 注解用来定义一个方法。同时，在 intercept() 方法中实现拦截的具体逻辑，示例如下：

@Intercepts({@Signature(type= Executor.class,method = "update",args = {MappedStatement.class,Object.class})})
public class ExamplePlugin implements Interceptor {private Properties properties = new Properties();@Overridepublic Object intercept(Invocation invocation) throws Throwable {// implement pre-processing if neededObject returnObject = invocation.proceed();// implement post-processing if neededreturn returnObject;}@Overridepublic void setProperties(Properties properties) {this.properties = properties;}
}

这里需要注意官网中给我们的建议：

The details of these classes methods can be discovered by looking at the full method signature of each, and the source code which is available with each MyBatis release. You should understand the behaviour of the method you’re overriding, assuming you’re doing something more than just monitoring calls. If you attempt to modify or override the behaviour of a given method, you’re likely to break the core of MyBatis. These are low level classes and methods, so use plug-ins with caution.

大意是说，如果我们使用拦截器只是为了监控方法的调用的话，可以放心的拦截；如果是要改变原来的行为的话，需要对要拦截的方法有足够深入的了解，因为可以被拦截的这几个组件都处于 MyBatis 的比较底层的位置，总之要小心再小心！！！

2.2.5 objectFactoryElement()

MyBatis 每次根据数据库查询结果构造返回的结果对象时，会调用 ObjectFactory 进行实例化结果对象，默认的 ObjectFactory 实现为 DefaultObjectFactory，它只提供了简单的使用无参和带参构造器来构造结果对象，如果 DefaultObjectFactory 无法满足我们的需求，可以自定义 ObjectFactory 实现，同时配置到配置文件中的 objectFactory 节点下：

  <objectFactory type="com.ghoostek.main.ExampleObjectFactory"><property name="objectFactoryProperty" value="100"/></objectFactory>

  private void objectFactoryElement(XNode context) throws Exception {if (context != null) {String type = context.getStringAttribute("type");Properties properties = context.getChildrenAsProperties();ObjectFactory factory = (ObjectFactory) resolveClass(type).newInstance();factory.setProperties(properties);configuration.setObjectFactory(factory);}}

一个自定义的 ObjectFactory 简单示例如下：

public class ExampleObjectFactory extends DefaultObjectFactory {@Overridepublic <T> T create(Class<T> type) {// add code block herereturn super.create(type);}@Overridepublic <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {// add code block herereturn super.create(type, constructorArgTypes, constructorArgs);}@Overridepublic void setProperties(Properties properties) {super.setProperties(properties);}@Overridepublic <T> boolean isCollection(Class<T> type) {return Collection.class.isAssignableFrom(type);}
}

2.2.6 environmentsElement()

本方法主要是对数据库连接相关配置的解析，environments 标签下可以配置多个 environment，每个对应一个环境的数据库信息。
注意 transactionManager 节点是对 TransactionFactory 的配置，MyBatis 默认提供了两种类型的 TransactionFactory——JDBC 和 MANAGED。

• JDBC 类型的事务管理根据 dataSource 节点下配置的数据库连接类型来进行；
• MANAGED 类型不做任何事务管理，而是将管理权限交给应用容器。
  这里需要注意的是，当我们将 MyBatis 集成到 Spring 时，这里的 transactionManager 配置会被覆盖。

MyBatis 内置了三种类型的 dataSource：UNPOOLED，POOLED，JNDI。

• UNPOOLED 每次请求建立一个新的连接；
• POOLED 自建连接池管理连接；
• JNDI 交给应用容器管理。

<environments default="development"><environment id="development"><transactionManager type="JDBC"><property name="..." value="..."/></transactionManager><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>

  private void environmentsElement(XNode context) throws Exception {if (context != null) {if (environment == null) {environment = context.getStringAttribute("default");}for (XNode child : context.getChildren()) {String id = child.getStringAttribute("id");if (isSpecifiedEnvironment(id)) {// <transactionManager type="JDBC">// 根据上面标签指定的类型实例化所需要的 TransactionFactory，// TransactionFactory 默认实现有两种：//   JdbcTransactionFactory，ManagedTransactionFactory// 分别对应 JDBC，MANAGED 两种类型TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));// <dataSource type="POOLED">// 根据上面标签指定的类型实例化所需要的 DataSourceFactory，// DataSourceFactory 默认实现有三种：//   UnpooledDataSourceFactory，PooledDataSourceFactory，JndiDataSourceFactory// 分别对应 UNPOOLED，POOLED，JNDI 三种类型DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));DataSource dataSource = dsFactory.getDataSource();Environment.Builder environmentBuilder = new Environment.Builder(id).transactionFactory(txFactory).dataSource(dataSource);configuration.setEnvironment(environmentBuilder.build());}}}}

  private DataSourceFactory dataSourceElement(XNode context) throws Exception {if (context != null) {String type = context.getStringAttribute("type");Properties props = context.getChildrenAsProperties();DataSourceFactory factory = (DataSourceFactory) resolveClass(type).newInstance();// 在 UnpooledDataSourceFactory 中，通过反射给 dataSource 设置属性factory.setProperties(props);return factory;}throw new BuilderException("Environment declaration requires a DataSourceFactory.");}

2.2.7 typeHandlerElement()

本方法主要解析类型处理器，并将其注册到类型解析注册器中。类型处理器主要用来对 Java 数据类型和 JDBC 数据类型的映射。

<typeHandlers><typeHandler handler="org.mybatis.example.ExampleTypeHandler"/>
</typeHandlers>

  private void typeHandlerElement(XNode parent) {if (parent != null) {for (XNode child : parent.getChildren()) {if ("package".equals(child.getName())) {String typeHandlerPackage = child.getStringAttribute("name");typeHandlerRegistry.register(typeHandlerPackage);} else {String javaTypeName = child.getStringAttribute("javaType");String jdbcTypeName = child.getStringAttribute("jdbcType");String handlerTypeName = child.getStringAttribute("handler");Class<?> javaTypeClass = resolveClass(javaTypeName);JdbcType jdbcType = resolveJdbcType(jdbcTypeName);Class<?> typeHandlerClass = resolveClass(handlerTypeName);if (javaTypeClass != null) {if (jdbcType == null) {typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);} else {typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);}} else {typeHandlerRegistry.register(typeHandlerClass);}}}}}

2.2.8 mapperElement()

本方法主要根据 mappers 节点下指定的 mapper 文件或者接口文件解析相关的 SQL。
对 mapper 文件的解析是 MyBatis 的核心，这里我们暂时不作详细讨论，后续会单独开篇。

<mappers><mapper resource="org/mybatis/builder/AuthorMapper.xml"/><mapper resource="org/mybatis/builder/BlogMapper.xml"/><mapper resource="org/mybatis/builder/PostMapper.xml"/>
</mappers>

  private void mapperElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {// 根据指定的包，加载包下所有的接口文件if ("package".equals(child.getName())) {String mapperPackage = child.getStringAttribute("name");configuration.addMappers(mapperPackage);} else {// 根据 resource 或 url 加载并解析 mapper 文件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);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.");}}}}}

2.3 DefaultSqlSessionFactory.openSession()

  public SqlSession openSession() {// 默认的 Executor 类型为 SIMPLEreturn openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {// Environment 是根据我们的配置解析出来的final Environment environment = configuration.getEnvironment();final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);final Executor executor = configuration.newExecutor(tx, execType);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();}}

Configuration.newExecutor()

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {executorType = executorType == null ? defaultExecutorType : executorType;executorType = executorType == null ? ExecutorType.SIMPLE : executorType;Executor executor;if (ExecutorType.BATCH == executorType) {executor = new BatchExecutor(this, transaction);} else if (ExecutorType.REUSE == executorType) {executor = new ReuseExecutor(this, transaction);} else {executor = new SimpleExecutor(this, transaction);}// 如果开启了全局缓存，包装成缓存类型的 Execotor// 这里用到了包装模式if (cacheEnabled) {executor = new CachingExecutor(executor);}// 层层拦截executor = (Executor) interceptorChain.pluginAll(executor);return executor;}

InterceptorChain.pluginAll()

public class InterceptorChain {private final List<Interceptor> interceptors = new ArrayList<>();public Object pluginAll(Object target) {for (Interceptor interceptor : interceptors) {target = interceptor.plugin(target);}return target;}// 解析plugin节点的时候，会调用此方法将拦截器放入拦截器列表中public void addInterceptor(Interceptor interceptor) {interceptors.add(interceptor);}public List<Interceptor> getInterceptors() {return Collections.unmodifiableList(interceptors);}}

这里我们重点看一下拦截器的具体原理。

首先在解析 Configuration 配置文件时，当解析到 plugins 标签时，XMLConfigBuilder 会循环解析 plugins 下的 plugin 列表，并将解析出来的实现了 Interceptor 接口的 Plugin 通过调用 Configuration.addInterceptor() 方法添加进 Configuration 类中。在 Configuration 类中持有 InterceptorChain 类型的成员变量，Configuration.addInterceptor() 最终是通过调用 InterceptorChain.addInterceptor() 将拦截器添加进 InterceptorChain 中，在实例化可以被 Interceptor 拦截的四种对象时，会在已得到的对象的基础上调用 InterceptorChain.pluginAll() 方法对被拦截对象进行层层拦截，最终得到代理对象。

上面我们看到在 InterceptorChain.pluginAll() 方法是通过循环调用 interceptor.plugin() 来达到层层拦截的，通常情况下我们自定义的拦截器的实现形如：

@Intercepts({@Signature(type= Executor.class,method = "update",args = {MappedStatement.class,Object.class})})
public class ExamplePlugin implements Interceptor {private Properties properties;@Overridepublic Object intercept(Invocation invocation) throws Throwable {return invocation.proceed();}@Overridepublic Object plugin(Object target) {return Plugin.wrap(target, this);}@Overridepublic void setProperties(Properties properties) {this.properties = properties;}public Properties getProperties() {return properties;}}

在 plugin() 方法中通过调用 Plugin.wrap() 方法进行代理对象的生成，此方法主要是通过 JDK 动态代理来生成代理对象。

public class Plugin implements InvocationHandler {private final Object target;private final Interceptor interceptor;// signatureMap中保存的是某个类中被拦截的方法列表private final Map<Class<?>, Set<Method>> signatureMap;private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {this.target = target;this.interceptor = interceptor;this.signatureMap = signatureMap;}public static Object wrap(Object target, Interceptor interceptor) {Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);Class<?> type = target.getClass();Class<?>[] interfaces = getAllInterfaces(type, signatureMap);if (interfaces.length > 0) {return Proxy.newProxyInstance(type.getClassLoader(),interfaces,new Plugin(target, interceptor, signatureMap));}return target;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {Set<Method> methods = signatureMap.get(method.getDeclaringClass());if (methods != null && methods.contains(method)) {return interceptor.intercept(new Invocation(target, method, args));}return method.invoke(target, args);} catch (Exception e) {throw ExceptionUtil.unwrapThrowable(e);}}private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {// 拦截器的注解如下：// @Intercepts({ @Signature(type = StatementHandler.class, method = "query", args = { Statement.class, ResultHandler.class}) })Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);// issue #251if (interceptsAnnotation == null) {throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());}Signature[] sigs = interceptsAnnotation.value();Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();for (Signature sig : sigs) {Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());try {Method method = sig.type().getMethod(sig.method(), sig.args());methods.add(method);} catch (NoSuchMethodException e) {throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);}}return signatureMap;}private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {Set<Class<?>> interfaces = new HashSet<>();while (type != null) {for (Class<?> c : type.getInterfaces()) {if (signatureMap.containsKey(c)) {interfaces.add(c);}}type = type.getSuperclass();}return interfaces.toArray(new Class<?>[interfaces.size()]);}}

2.4 DefaultSqlSession.selectOne()

  public <T> T selectOne(String statement) {return this.selectOne(statement, null);}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);// 因为此处调用的是 selectOne() 方法，因此只有当查询结果只有一条记录时，才正常返回// 为空返回 null// 有多条则抛出异常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 是对查询记录的偏移量及条数的封装// 默认是从第一条开始，查询的总记录数为 Integer.MAX_VALUEreturn this.selectList(statement, parameter, RowBounds.DEFAULT);}public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {// MappedStatement 是对 mapper 文件中配置的一条语句的封装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();}
}

CachingExecutor.query()

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {// 解析出的 sql 被封装在 BoundSql 中BoundSql boundSql = ms.getBoundSql(parameterObject);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 {// 先查缓存，查不到再执行数据库查询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);
}

BaseExecutor.query()

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.");}if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();}List<E> list;try {queryStack++;list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);}} finally {queryStack--;}if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();}// issue #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;
}... ...private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;localCache.putObject(key, EXECUTION_PLACEHOLDER);try {list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {localCache.removeObject(key);}localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;
}

SimpleExecutor.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();// 这里的 wrapper 是 CachingExecutor 实例StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);// 准备 Statementstmt = prepareStatement(handler, ms.getStatementLog());return handler.query(stmt, resultHandler);} finally {closeStatement(stmt);}
}

Configuration.newStatementHandler()

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {// RoutingStatementHandler 中采用策略模式创建具体的 StatementHandler 对象// StatementHandler 具体类型由我们在 mapper 文件中指定的 statementType 决定// 这里在实例化 StatementHandler 的过程中也会实例化 ParameterHandler 和 ResultSetHandlerStatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);// 对 StatementHandler 进行拦截statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);return statementHandler;
}

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {switch (ms.getStatementType()) {// 需要注意的是，这里不管最终选择哪种类型的 StatementHandler 进行实例化// 最终都是调用基类 BaseStatementHandler 的构造方法case STATEMENT:delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;case PREPARED:delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;case CALLABLE:delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;default:throw new ExecutorException("Unknown statement type: " + ms.getStatementType());}

BaseStatementHandler()

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {this.configuration = mappedStatement.getConfiguration();this.executor = executor;this.mappedStatement = mappedStatement;this.rowBounds = rowBounds;this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();this.objectFactory = configuration.getObjectFactory();if (boundSql == null) { // issue #435, get the key before calculating the statementgenerateKeys(parameterObject);boundSql = mappedStatement.getBoundSql(parameterObject);}this.boundSql = boundSql;// 实例化 ParameterHanlder 和 ResultSetHandlerthis.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}

Configuration.newParameterHandler() & newResultSetHandler()

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);// 拦截parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);return parameterHandler;
}... ...public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,ResultHandler resultHandler, BoundSql boundSql) {ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);// 拦截resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);return resultSetHandler;
}

prepareStatement()

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {Statement stmt;// 准备数据库连接Connection connection = getConnection(statementLog);// 准备 Statementstmt = handler.prepare(connection, transaction.getTimeout());handler.parameterize(stmt);return stmt;
}... ...protected Connection getConnection(Log statementLog) throws SQLException {Connection connection = transaction.getConnection();if (statementLog.isDebugEnabled()) {return ConnectionLogger.newInstance(connection, statementLog, queryStack);} else {return connection;}
}

JdbcTransaction.getConnection()

public Connection getConnection() throws SQLException {if (connection == null) {openConnection();}return connection;
}... ...protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}connection = dataSource.getConnection();if (level != null) {connection.setTransactionIsolation(level.getLevel());}setDesiredAutoCommit(autoCommit);
}

UnpooledDataSource.getConnection()

public Connection getConnection() throws SQLException {return doGetConnection(username, password);
}... ...private Connection doGetConnection(String username, String password) throws SQLException {Properties props = new Properties();if (driverProperties != null) {props.putAll(driverProperties);}if (username != null) {props.setProperty("user", username);}if (password != null) {props.setProperty("password", password);}return doGetConnection(props);
}... ...private Connection doGetConnection(Properties properties) throws SQLException {initializeDriver();Connection connection = DriverManager.getConnection(url, properties);configureConnection(connection);return connection;
}... ...private void configureConnection(Connection conn) throws SQLException {if (autoCommit != null && autoCommit != conn.getAutoCommit()) {conn.setAutoCommit(autoCommit);}if (defaultTransactionIsolationLevel != null) {conn.setTransactionIsolation(defaultTransactionIsolationLevel);}
}

BaseStatementHandler.prepare()

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {ErrorContext.instance().sql(boundSql.getSql());Statement statement = null;try {statement = instantiateStatement(connection);setStatementTimeout(statement, transactionTimeout);setFetchSize(statement);return statement;} catch (SQLException e) {closeStatement(statement);throw e;} catch (Exception e) {closeStatement(statement);throw new ExecutorException("Error preparing statement.  Cause: " + e, e);}
}

SimpleStatementHandler.instantiateStatement()

protected Statement instantiateStatement(Connection connection) throws SQLException {if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {return connection.createStatement();} else {return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);}
}

SimpleStatementHandler.query()

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {// 获取解析后得到的最终 sql，其中参数占位符已经被替换String sql = boundSql.getSql();statement.execute(sql);// 查询结果交给 ResultSetHandler 处理return resultSetHandler.handleResultSets(statement);
}

3. 重要组件

3.1 Executor

Executor 定义了数据库的操作。

3.2 ParameterHandler

ParameterHandler 定义了对原始入参的处理。

3.3 StatementHandler

StatementHandler 定义了 Statement 的处理操作。

3.4 ResultSetHandler

ResultSetHandler 定义了查询结果的处理操作。

4. 设计模式

4.1 工厂方法

使用 SqlSessionFactory 抽象出 SqlSession 的创建。

4.2 建造者

通过 SqlSessionFactoryBuilder 建造 SqlSessionFactory。
通过 MappedStatement.Builder 建造 MappedStatement。
通过 Environment.Builder 建造 Environment。

4.3 模板

XMLConfigBuilder.parseConfiguration() 方法中对全局配置文件的解析，先在模板方法中定义解析的顺序，每个小方法各自解析对应的节点。

4.4 拦截器

实现 Interceptor 接口可以自定义拦截器对 Executor，ParameterHandler，StatementHandler，ResultSetHandler 拦截。

4.5 代理

每个自定义的拦截器，都使用 JDK 动态代理生成代理对象对被拦截对象进行代理。

4.6 委派

CachingExecutor 中将数据库操作的执行委派给被包装的 Executor 类，自己只实现缓存的功能。
RoutingStatementHandler 中将数据库操作的实现委派给它所创建的 StatementHandler 对象。

4.7 策略

TransactionFacotry，DataSourceFactory 对象的创建根据我们在主配置中指定的类型进行选择具体的对象类型。
StatementHandler 对象创建时根据 MappedStatement 中的 statementType （即我们在 mapper 文件中指定的 statementType ）选择具体的对象类型。

4.8 包装器

CachingExecutor 中通过对 Executor 的包装，使其具有缓存的功能。

4.9 责任链

InterceptorChain.pluginAll() 方法通过循环调用 Interceptor.plugin() 方法，生成了一条拦截链，链的顺序就是我们在主配置中指定的拦截器的顺序。

5. 其他

最后我们简单介绍一下 MyBatis3 中提供的工具。MyBatis3 提供了 SQL 类供我们方便的生成自定义的 sql 语句，而不用通过拼接字符串的方式，示例如下：

// 匿名内部类风格
public String deletePersonSql() {return new SQL() {{DELETE_FROM("PERSON");WHERE("ID = #{id}");}}.toString();
}// Builder / Fluent 风格
public String insertPersonSql() {String sql = new SQL().INSERT_INTO("PERSON").VALUES("ID, FIRST_NAME", "#{id}, #{firstName}").VALUES("LAST_NAME", "#{lastName}").toString();return sql;
}// 动态条件（注意参数需要使用 final 修饰，以便匿名内部类对它们进行访问）
public String selectPersonLike(final String id, final String firstName, final String lastName) {return new SQL() {{SELECT("P.ID, P.USERNAME, P.PASSWORD, P.FIRST_NAME, P.LAST_NAME");FROM("PERSON P");if (id != null) {WHERE("P.ID like #{id}");}if (firstName != null) {WHERE("P.FIRST_NAME like #{firstName}");}if (lastName != null) {WHERE("P.LAST_NAME like #{lastName}");}ORDER_BY("P.LAST_NAME");}}.toString();
}public String deletePersonSql() {return new SQL() {{DELETE_FROM("PERSON");WHERE("ID = #{id}");}}.toString();
}public String insertPersonSql() {return new SQL() {{INSERT_INTO("PERSON");VALUES("ID, FIRST_NAME", "#{id}, #{firstName}");VALUES("LAST_NAME", "#{lastName}");}}.toString();
}public String updatePersonSql() {return new SQL() {{UPDATE("PERSON");SET("FIRST_NAME = #{firstName}");WHERE("ID = #{id}");}}.toString();
}