Mybatis学习系列（十二）：PageHelper分页插件

由于mybatis原生的分页是在内存里面进行的，导致效率很低，但是我们在生产项目中有很多的分页需求，这个时候PageHelper分页插件就诞生了。PageHelper主要是通过插件拦截链实现的。

我们知道在创建StatementHandler 的时候，我们包装了interceptorChain链

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;
  }

//给某个对象添加插件拦截器链
public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

那这里如何初始化interceptors呢？

我们知道在解析配置文件的时候有一个解析plugin的方法：pluginElement(root.evalNode("plugins"));

private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      //解析每一个插件拦截器
      for (XNode child : parent.getChildren()) {
        //获取配置信息
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
        //添加参数
        interceptorInstance.setProperties(properties);
        //添加插件拦截器到configuration中
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

//添加参数  这里看PageInterceptor
public void setProperties(Properties properties) {
        //缓存 count ms
        msCountMap = CacheFactory.createCache(properties.getProperty("msCountCache"), "ms", properties);
        String dialectClass = properties.getProperty("dialect");
        if (StringUtil.isEmpty(dialectClass)) {
            //这里如果没有配置dialect  走默认的com.github.pagehelper.PageHelper
            dialectClass = default_dialect_class;
        }
        try {
            Class<?> aClass = Class.forName(dialectClass);
            dialect = (Dialect) aClass.newInstance();
        } catch (Exception e) {
            throw new PageException(e);
        }
        dialect.setProperties(properties);

        String countSuffix = properties.getProperty("countSuffix");
        if (StringUtil.isNotEmpty(countSuffix)) {
            this.countSuffix = countSuffix;
        }

        try {
            //反射获取 BoundSql 中的 additionalParameters 属性
            additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters");
            additionalParametersField.setAccessible(true);
        } catch (NoSuchFieldException e) {
            throw new PageException(e);
        }
    }


//添加插件拦截器到configuration中
public void addInterceptor(Interceptor interceptor) {
    interceptorChain.addInterceptor(interceptor);
  }

然后我们看一下pagehelper是如何实现的

@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}),
    }
)
public class PageInterceptor implements Interceptor

首先我们发现这里必须是实现了Interceptor接口，这样才可以配置在plugin标签里

然后我们看一下interceptor.plugin(target);方法

public Object plugin(Object target) {
        //TODO Spring bean 方式配置时，如果没有配置属性就不会执行下面的 setProper