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springboot使用ThreadLocal

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springboot 使用ThreadLocal

本文参考慕课教程给出一个在spring boot中使用ThreadLocal实现线程封闭的实例。

首先创建一个包含ThreadLocal成员变量的实例:

public class RequestHolder { 
    private final static ThreadLocal<Long> requestHolder = new ThreadLocal<>();
    public static void add(Long id) {
        requestHolder.set(id);
    }
 
    public static Long getId() {
        return requestHolder.get();
    }
 
    public static void remove() {
        requestHolder.remove();
    }
}

编写一个Controller类,请求该类的test()方法获取ThreadLocal中存储的id:

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
 
@Controller
@RequestMapping("/threadLocal")
public class ThreadLocalController {
 
    @RequestMapping("/test")
    @ResponseBody
    public Long test() {
        return RequestHolder.getId();
    }
}

编写过滤器,在请求到达Servlet之前(请求->tomcat容器->filter->servlet->inteceptor->controller),将当前线程的id添加到ThreadLocal中:

import com.mmall.concurrency.example.threadLocal.RequestHolder;
import lombok.extern.slf4j.Slf4j; 
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
 
@Slf4j
public class HttpFilter implements Filter {
 
    @Override
    public void init(FilterConfig filterConfig) throws ServletException { 
    }
    
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        log.info("do filter, {}, {}", Thread.currentThread().getId(), request.getServletPath());
        //在ThreadLocal中添加当前线程的id
        RequestHolder.add(Thread.currentThread().getId());
        filterChain.doFilter(servletRequest, servletResponse);
    } 
    @Override
    public void destroy() { 
    }
}

编写拦截器,当请求处理完成后(从Controller返回后),清除ThreadLocal中的id,避免内存泄漏。

import com.mmall.concurrency.example.threadLocal.RequestHolder;
import lombok.extern.slf4j.Slf4j;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter; 
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
@Slf4j
public class HttpInterceptor extends HandlerInterceptorAdapter {
 
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        log.info("preHandle");
        return true;
    }
 
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        log.info("ThreadId:"+RequestHolder.getId());
        RequestHolder.remove();
        log.info("afterCompletion");
        return;
    }
}

最后,我们需要在spring boot启动类上注册我们定义的Filer及Inteceptor,并设置拦截路径。

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
 
@SpringBootApplication
public class ConcurrencyApplication extends WebMvcConfigurerAdapter{ 
 public static void main(String[] args) {
  SpringApplication.run(ConcurrencyApplication.class, args);
 }
 
 @Bean
 public FilterRegistrationBean httpFilter() {
  FilterRegistrationBean registrationBean = new FilterRegistrationBean();
  registrationBean.setFilter(new HttpFilter());
  registrationBean.addUrlPatterns("/threadLocal/*");
  return registrationBean;
 }
 
 @Override
 public void addInterceptors(InterceptorRegistry registry) {
  registry.addInterceptor(new HttpInterceptor()).addPathPatterns("/**");
 }
}

在浏览器或者postman中输入http://localhost:8080/threadLocal/test

观察输出结果:

2018-11-09 11:16:51.287  INFO 34076 --- [           main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat started on port(s): 8080 (http)
2018-11-09 11:16:51.290  INFO 34076 --- [           main] c.m.concurrency.ConcurrencyApplication   : Started ConcurrencyApplication in 1.718 seconds (JVM running for 2.132)
2018-11-09 11:17:03.060  INFO 34076 --- [nio-8080-exec-2] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring FrameworkServlet 'dispatcherServlet'
2018-11-09 11:17:03.060  INFO 34076 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization started
2018-11-09 11:17:03.072  INFO 34076 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization completed in 12 ms
2018-11-09 11:17:03.078  INFO 34076 --- [nio-8080-exec-2] com.mmall.concurrency.HttpFilter         : do filter, 29, /threadLocal/test
2018-11-09 11:17:03.090  INFO 34076 --- [nio-8080-exec-2] com.mmall.concurrency.HttpInterceptor    : preHandle
2018-11-09 11:17:03.124  INFO 34076 --- [nio-8080-exec-2] com.mmall.concurrency.HttpInterceptor    : ThreadId:29
2018-11-09 11:17:03.124  INFO 34076 --- [nio-8080-exec-2] com.mmall.concurrency.HttpInterceptor    : afterCompletion

从打印的日志结果中,我们看到在Filter中我们将当前线程的id 29添加到了ThreadLocal中,随后在Inteceptor中打印并删除了id。

ThreadLocal在springboot使用中的坑

ThreadLocal 适用于变量在线程间隔离,而在方法或类间共享的场景。现在在Springboot中我做如下场景的使用:

使用 Spring Boot 创建一个 Web 应用程序,使用 ThreadLocal 存放一个 Integer 的值,来暂且代表需要在线程中保存的用户信息,这个值初始是 null。在业务逻辑中,我先从 ThreadLocal 获取一次值,然后把外部传入的参数设置到 ThreadLocal 中,来模拟从当前上下文获取到用户信息的逻辑,随后再获取一次值,最后输出两次获得的值和线程名称。

@RestController
public class threadLocal {
    private ThreadLocal<Integer> currentUser = ThreadLocal.withInitial(() -> null);
    @RequestMapping("wrong")
    public Map wrong(@RequestParam("userId") Integer userId) {
        //设置用户信息之前先查询一次ThreadLocal中的用户信息
        String before = Thread.currentThread().getName() + ":" + currentUser.get();
        //设置用户信息到ThreadLocal
        currentUser.set(userId);
      
            //设置用户信息之后再查询一次ThreadLocal中的用户信息
            String after = Thread.currentThread().getName() + ":" + currentUser.get();
            //汇总输出两次查询结果
            Map result = new HashMap();
            result.put("before", before);
            result.put("after", after);
            return result;      
    }
}

为了让问题快速的重现,我在配置文件中设置一下 Tomcat 的参数,把工作线程池最大线程数设置为 1,这样始终是同一个线程在处理请求:

server.tomcat.max-threads=1

运行程序后先让用户 1 来请求接口,可以看到第一和第二次获取到用户 ID 分别是 null 和 1,符合预期:随后用户 2 来请求接口,这次就出现了 Bug,第一和第二次获取到用户 ID 分别是 1 和 2,显然第一次获取到了用户 1 的信息,原因就是 Tomcat 的线程池重用了线程。

在 Tomcat 这种 Web 服务器下跑的业务代码,本来就运行在一个多线程环境中,并不能认为没有显式开启多线程就不会有线程安全问题,所以使用类似 ThreadLocal 工具来存放一些数据时,需要特别注意在代码运行完后,显式地去清空设置的数据。如果在代码中使用了自定义的线程池,也同样会遇到这个问题。修改后代码如下:

@RestController
public class threadLocal {
    private ThreadLocal<Integer> currentUser = ThreadLocal.withInitial(() -> null);
    @RequestMapping("wrong")
    public Map wrong(@RequestParam("userId") Integer userId) {
        //设置用户信息之前先查询一次ThreadLocal中的用户信息
        String before = Thread.currentThread().getName() + ":" + currentUser.get();
        //设置用户信息到ThreadLocal
        currentUser.set(userId);
        try {
            //设置用户信息之后再查询一次ThreadLocal中的用户信息
            String after = Thread.currentThread().getName() + ":" + currentUser.get();
            //汇总输出两次查询结果
            Map result = new HashMap();
            result.put("before", before);
            result.put("after", after);
            return result;
        } finally {
         //增加移除处理
            currentUser.remove();
        }
    }
}

以上为个人经验,希望能给大家一个参考,也希望大家多多支持。

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