1、mybatis扩展点plugins

mybatis的扩展是通过拦截器Interceptor来实现的，本质上就是JDK的动态代理，所以它只能对接口进行拦截，mybatis可以对以下四个接口类型进行拦截，也就是说会对这4种对象进行代理，所有的代理拦截都是通过 InterceptorChain.pluginAll(Object target) 来实现的。

Executor： 执行器，执行SQL语句（所有的sql都通过它来执行），并且对事务、缓存等提供统一接口。（在这一层上做拦截的权限会更大）
StatementHandler： 对statement进行预处理，并且提供统一的原子的增、删、改、查接口。（如果要在SQL执行前进行拦截的话，拦截这里就可以了）
ResultSetHandler：对返回结果ResultSet进行处理。
PameterHandler：对参数进行赋值。

SqlSession的创建过程：

mybatis中的SQL都是通过DefaultSqlSession去执行的，其创建过程如下：

  // SqlSessionFactoryBuilder => DefaultSqlSessionFactory => DefaultSqlSessionSqlSession sqlSession = new SqlSessionFactoryBuilder().build(in, "development", pro).openSession();

2、源码解读具体实现（以Executor接口为例）

2.1、创建SqlSession时，SqlSessionFactroy会解析mybatis.xml配置文件中的plugins标签，并将Interceptor属性定义的Interceptor放到interceptorChain中；

  // SqlSessionFactoryBuilder.java  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {try {XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);// 解析mybatis.xml配置文件，并创建DefaultSqlSessionFactoryreturn build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} finally {ErrorContext.instance().reset();try {reader.close();} catch (IOException e) {// Intentionally ignore. Prefer previous error.}}}

  // XMLConfigBuilder.javapublic Configuration parse() {if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}parsed = true;parseConfiguration(parser.evalNode("/configuration"));return configuration;}// 解析mybatis.xml中的各个标签private void parseConfiguration(XNode root) {try {propertiesElement(root.evalNode("properties")); //issue #117 read properties firsttypeAliasesElement(root.evalNode("typeAliases"));pluginElement(root.evalNode("plugins"));objectFactoryElement(root.evalNode("objectFactory"));objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));settingsElement(root.evalNode("settings"));environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631databaseIdProviderElement(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);}}// 解析plugins标签，并把Interceptor放到interceptorChain中private void pluginElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {String interceptor = child.getStringAttribute("interceptor");Properties properties = child.getChildrenAsProperties();Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();interceptorInstance.setProperties(properties);configuration.addInterceptor(interceptorInstance);}}}// Configuration，mybatis文件的抽象类public void addInterceptor(Interceptor interceptor) {interceptorChain.addInterceptor(interceptor);}

2.2、DefaultSqlSessionFactory.openSession()时使用JDK动态代理生成@Signature注解指定的被代理类（包含代理的方法以及方法参数）

  // DefaultSqlSessionFactory.javapublic SqlSession openSession() {return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {final Environment environment = configuration.getEnvironment();final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);// 使用Configuration创建Executorfinal Executor executor = configuration.newExecutor(tx, execType, autoCommit);return new DefaultSqlSession(configuration, executor);} 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.javapublic Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {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);}if (cacheEnabled) {executor = new CachingExecutor(executor, autoCommit);}// 使用JDK动态代理生成Executor的interceptorChainexecutor = (Executor) interceptorChain.pluginAll(executor);return executor;}

2.3、InterceptorChain生成的具体过程

  // InterceptorChain.javapublic Object pluginAll(Object target) {for (Interceptor interceptor : interceptors) {target = interceptor.plugin(target);}return target;}

  // Interceptor的具体实现类（即我们业务上要实现的功能）@Overridepublic Object plugin(Object arg0) {return Plugin.wrap(arg0, this);}

  // Plugin.javapublic static Object wrap(Object target, Interceptor interceptor) {// getSignatureMap获取Interceptor类上的@Intercepts(@Signature)内容Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);Class<?> type = target.getClass();Class<?>[] interfaces = getAllInterfaces(type, signatureMap);// 生成目标类target(Executor.class)的代理类，实现我们需要的plugin功能if (interfaces.length > 0) {return Proxy.newProxyInstance(type.getClassLoader(),interfaces,new Plugin(target, interceptor, signatureMap));}return target;}// 解析实现Interceptor接口的类上定义的@Intercepts(@Signature)内容，获取需要拦截的类和方法。// 例如：@Signature(type = Executor.class, method = "query",  args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);if (interceptsAnnotation == null) { // issue #251throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      }Signature[] sigs = interceptsAnnotation.value();Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();for (Signature sig : sigs) {Set<Method> methods = signatureMap.get(sig.type());if (methods == null) {methods = new HashSet<Method>();signatureMap.put(sig.type(), methods);}try {// sig.type()即Executor.classMethod 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;}  

3、demo关键步骤

3.1、实现自定义的Interceptor

// 自定义拦截器@Intercepts({@Signature(type = Executor.class, method = "query",  args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})public class MyTestInterceptor implements Interceptor {private static final String MSG = "octopus route table info is not exit!";@Overridepublic Object intercept(Invocation arg0) throws Throwable {Object obj = null;try {obj = arg0.proceed();} catch (Throwable e) {if (e.getCause() instanceof MySQLSyntaxErrorException) {MySQLSyntaxErrorException ex = (MySQLSyntaxErrorException) e.getCause();System.out.println("====" + ex.getErrorCode());System.out.println("====" + ex.getSQLState());System.out.println("====" + ex.getMessage());System.out.println("====" + ex.getCause());if (MSG.equals(ex.getMessage())) {throw new RouteTableNoExistException();}}}return obj;}@Overridepublic Object plugin(Object arg0) {return Plugin.wrap(arg0, this);}@Overridepublic void setProperties(Properties arg0) {System.out.println("env value: " + arg0.getProperty("names"));}}

3.2、在mybatis.xml中配置plugins

<configuration><plugins><plugin interceptor="com.pinganfu.interceptor.MyTestInterceptor" /></plugins>	<environments default="development"><environment id="development"><transactionManager type="MANAGED"><property name="closeConnection" value="false" /></transactionManager><dataSource type="POOLED"><property name="driver" value="${driver}" /><property name="url" value="${jdbcUrl}" /><property name="username" value="${username}" /><property name="password" value="${password}" /></dataSource></environment></environments>	<mappers><mapper resource="mappers/TBATMapper.xml" /></mappers>	
</configuration>

3.3、获取SqlSession

  Properties pro = new Properties();try {pro.load(Resources.getResourceAsStream("jdbc.properties"));// 加载mybatis.xml中的pluginsInputStream in = Resources.getResourceAsStream("mybatis.xml");sqlSession = new SqlSessionFactoryBuilder().build(in, "development", pro).openSession();} catch (IOException e) {e.printStackTrace();}		

4、mybatis针对各种异常的处理

mybatis通过DefaultSqlSession执行时，会将发生的所有异常统一包装成PersistenceException再抛出，我们可以通过PersistenceException.getCause()获取具体的异常。

  // DefaultSqlSession.javapublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {MappedStatement ms = configuration.getMappedStatement(statement);List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);return result;} catch (Exception e) {// 对执行发生的所有Exception进行wrap之后再抛出throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}}

  // ExceptionFactory.javapublic static RuntimeException wrapException(String message, Exception e) {// 将Exception进行统一包装成PersistenceExceptionreturn new PersistenceException(ErrorContext.instance().message(message).cause(e).toString(), e);}

Ref：

https://www.cnblogs.com/kevin-yuan/p/7219003.html

https://www.cnblogs.com/fangjian0423/p/mybatis-interceptor.html

https://www.cnblogs.com/fenglanglang/p/6007653.html