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说实话，作为一个宅男，每次被淘宝上的雄性店主追着喊“亲，亲，亲”，这感觉真的很恶心，仿佛被强吻一样。更让人烦的是为了省钱，我不得不用个女号跟那些店主沟通：“哥哥包邮吗？”“哥哥再便宜点呗，我钱不够了嘛，5555555。”这些日子真是充满了不堪的苦难。
用过PageHelper的都知道（没用过的赶紧去百度下），实现分页非常简单。服务实现层调用DAO层之前进行Page设置，Mapper.xml中不处理分页，这样就够了，就能实现分页了。以下是具体实现代码：
@Overridepublic PageInfo listUser(int pageNum, int pageSize) { PageHelper.startPage(pageNum, pageSize); List users = userMapper.listUser(); PageInfo pageInfo = new PageInfo(users); return pageInfo;} 哎我去，这样就实现分页了？这是什么意思？这是怎么做到的？凡事有果必有因，我们一起来看看这个因到底是什么。
在进入正题之前，我们先做下准备。如果对动态代理很熟悉，可以略过，但建议还是看看，权且当做热身。
我们来看看JDK下的动态代理的具体实现：
public class PageHelperAutoConfiguration { @Autowired private List sqlSessionFactoryList; @Autowired private PageHelperProperties properties; @Bean @ConfigurationProperties(prefix = PageHelperProperties.PAGEHELPER_PREFIX) public Properties pageHelperProperties() { return new Properties(); } @PostConstruct public void addPageInterceptor() { PageInterceptor interceptor = new PageInterceptor(); Properties properties = new Properties(); properties.putAll(pageHelperProperties()); properties.putAll(this.properties.getProperties()); interceptor.setProperties(properties); for (SqlSessionFactory sqlSessionFactory : sqlSessionFactoryList) { sqlSessionFactory.getConfiguration().addInterceptor(interceptor); } }} 从Mybatis的SQL执行流程图中可以看到，Mybatis的四大对象Executor、ParameterHandler、ResultSetHandler、StatementHandler，由他们一起合作完成SQL的执行，这四大对象是由谁创建的呢？没错，就是Mybatis的配置中心：Configuration，创建源代码如下：
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); 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); resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler); return resultSetHandler;}public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler); return statementHandler;}public Executor newExecutor(Transaction transaction) { return newExecutor(transaction, defaultExecutorType);}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); } if (cacheEnabled) { executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor;} pluginAll方法的实现如下：
public static Object wrap(Object target, Interceptor interceptor) { Map 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;} pluginAll其实就是给四大对象创建代理，而我们的PageInterceptor只是其中一层代理。我们接着往下看，Plugin继承了InvocationHandler，相当于上述：JDK的动态代理示例中的MyInvocationHandler，它的invoke方法肯定会被调用：
@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { Set 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); }} 在解读intercept方法之前，我们先来看看PageInterceptor类上的注解：
@Intercepts({ @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}), @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class})}) 这就标明了PageInterceptor拦截的是Executor的query方法；还记上述wrap方法的：