<plugins>

      <plugin interceptor="com.github.pagehelper.PageInterceptor">

            <!--当设置为true的时候，如果pagesize设置为0（或RowBounds的limit=0），就不执行分页，返回全部结果-->

            <property name="pageSizeZero" value="false"/>

      </plugin>

</plugins>

@Intercepts(

        {
　　　　　　　　　　//拦截excutor类型里面的query方法

                @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}),

        }

)

public class PageInterceptor implements Interceptor {

    private volatile Dialect dialect;

    private String countSuffix = "_COUNT";

    protected Cache<String, MappedStatement> msCountMap = null;

    private String default_dialect_class = "com.github.pagehelper.PageHelper";

}

 private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {

    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);

    // issue #251

    //查看拦截器是否有Intercepts注解 没有抛出异常

    if (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;

  }

 public static void main(String[] args) throws IOException {

         InputStream is = Resources.getResourceAsStream("config/mybatis-config.xml");

         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);//DefaultSqlSessionFactory

         SqlSession session = sqlSessionFactory.openSession();//打开session会执行 InterceptorChina.plugAll() --->Plugin.wrap()生成代理对象,被代理的就是Excutor

         IStudentDao sudentDaoProxy = session.getMapper(IStudentDao.class);

 　　　　　//加上这句就会拦截查询方法并进行分页

         Page page = PageHelper.startPage(1,1,"name");

         List<Student> student = sudentDaoProxy.findStudentById(null);

     }

public interface Interceptor {

  //拦截器执行业务逻辑方法

  Object intercept(Invocation invocation) throws Throwable;

  //设置拦截的真实对象

  Object plugin(Object target);

  //设置拦截器初始化属性

  void setProperties(Properties properties);

}

//XMLConfigBuilder类

private void pluginElement(XNode parent) throws Exception {

    //如果有Plugins标签

    if (parent != null) {

     //拿到所有的子标签也就是<Plugin>，所有的拦截器配置

      for (XNode child : parent.getChildren()) {

        //拦截器的类

        String interceptor = child.getStringAttribute("interceptor");

        //<Property>配置

        Properties properties = child.getChildrenAsProperties();

        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();

        //把property属性设置到拦截器里面

        interceptorInstance.setProperties(properties);

       //加入到拦截器链中List<Interceptor> interceptors

        configuration.addInterceptor(interceptorInstance);

      }

    }

  }    

//PageInterceptor

public Object plugin(Object target) {

        return Plugin.wrap(target, this);

   }

public class Plugin implements InvocationHandler {
　　//生成Plugin代理对象

   public static Object wrap(Object target, Interceptor interceptor) {

    //拿到PageInterceptor类型的需要拦截的方法即query方法

    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);

    Class<?> type = target.getClass();

    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);

    if (interfaces.length > 0) {

     //生成Plugin代理对象，所以当后面执行executor的query时其实执行下面的invoke方法

      return Proxy.newProxyInstance(

          type.getClassLoader(),

          interfaces,

          new Plugin(target, interceptor, signatureMap));

    }

    return target;

  }

@Override

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

    try {

      Set<Method> methods = signatureMap.get(method.getDeclaringClass());

      if (methods != null && methods.contains(method)) {

      //执行PageInterceptor的intercept方法

        return interceptor.intercept(new Invocation(target, method, args));

      }

      return method.invoke(target, args);

    } catch (Exception e) {

      throw ExceptionUtil.unwrapThrowable(e);

    }

  }

}

public void sqlToCount(Select select, String name) {

        SelectBody selectBody = select.getSelectBody();

        // 是否能简化count查询

        List<SelectItem> COUNT_ITEM = new ArrayList<SelectItem>();

        COUNT_ITEM.add(new SelectExpressionItem(new Column("count(" + name +")")));

        if (selectBody instanceof PlainSelect && isSimpleCount((PlainSelect) selectBody)) {

            //selectItems 就是sql中的select xxxx from table；把xxxx替换成count(0)

            ((PlainSelect) selectBody).setSelectItems(COUNT_ITEM);

        } else {

            PlainSelect plainSelect = new PlainSelect();

            SubSelect subSelect = new SubSelect();

            subSelect.setSelectBody(selectBody);

            subSelect.setAlias(TABLE_ALIAS);

            plainSelect.setFromItem(subSelect);

            plainSelect.setSelectItems(COUNT_ITEM);

            select.setSelectBody(plainSelect);

        }

    }    

public static  <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter,

                                 RowBounds rowBounds, ResultHandler resultHandler,

                                 BoundSql boundSql, CacheKey cacheKey) throws SQLException {

        //判断是否需要进行分页查询

        if (dialect.beforePage(ms, parameter, rowBounds)) {

            //生成分页的缓存 key

            CacheKey pageKey = cacheKey;

            //处理参数对象

            parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);

            //调用方言获取分页 sql

            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);

            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);

            Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);

            //设置动态参数

            for (String key : additionalParameters.keySet()) {

                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));

            }

            //执行分页查询

            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);

        } else {

            //不执行分页的情况下，也不执行内存分页

            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);

        }

    }