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springboot 使用ThreadLocal的实例代码

程序员文章站 2022-03-05 16:35:03
目录springboot 使用threadlocalthreadlocal在springboot使用中的坑springboot 使用threadlocal本文参考慕课教程给出一个在spring boo...

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。

springboot 使用ThreadLocal的实例代码

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。