The thread reuse problem of PageHelper using ThreadLocal, did you use it correctly?

Preface

PageHelper It is a more commonly used paging plug-in , By implementing Mybatis Of Interceptor The interface is complete query sql Dynamic paging , The paging parameter is determined by ThreadLocal Preservation .

ordinary Paging execution process ：

1. Set up page Parameters

2. perform query Method

3. Interceptor Interface Intermediate calibration ThreadLocal Whether there is a set page Parameters

4. There is page Parameters , To regenerate the count sql and page sql, And execute the query . non-existent page Parameters , Go straight back to Query results

5. perform LOCAL_PAGE.remove() eliminate page Parameters

Problem scenario

Observe the above execution process , You can find , If in the first place 1 And the first step 2 Step There is an exception between , that LOCAL_PAGE Corresponding to the current thread in page Parameters do not remove.

Without using thread pools , The current thread will be destroyed after execution , At this time Current thread Medium threadLocals Parameters Will be affected by the situation , It's empty 了 LOCAL_PAGE in Of the current thread page Parameters .

But if a thread pool is used , The current thread has finished executing , It's not going to be destroyed , Instead, the current thread will be stored in the pool again , Mark as idle , For later use . When using this thread later , because Threads Of threadLocals The value still exists , Even though we're in the third place 1 Step is not set page Parameters , The first 3 Step You can also get page Parameters , To generate count sql and page sql, Thus affecting our normal query .

SpringBoot Built in... Will be used in the project Tomcat As a server , and Tomcat Threads will be used by default to process requests , This leads to the above problems .

Solution

because Tomcat Threads are used to handle request request , So when the request is completed , Clear the current thread threadLocals Property value , Which is execution LOCAL_PAGE.remove() that will do .

Realization way ：

• Use aop, For all controller To deal with

• Realization HandlerInterceptor perhaps WebRequestInterceptor Yes request Interceptor interface requested , adopt afterCompletion Method execution LOCAL_PAGE.remove() .— recommend , The implementation is simple

The second way is used here , Realization HandlerInterceptor Interface ：

public class PageLocalWebInterceptor implements HandlerInterceptor {
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        // PageHelper.clearPage()  Internal calls  LOCAL_PAGE.remove()
        PageHelper.clearPage();

    }
}

Define configuration class , The configuration class needs to implement WebMvcConfigurer Interface completion for WebMvc Related configuration of , register PageLocalWebInterceptor ：

@Configuration
public class FrameworkAutoConfig implements WebMvcConfigurer {

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new PageLocalWebInterceptor());
    }
}