ThreadLocal小试牛刀

Posted 2020-12-01 w4ctech

ThreadLocal中保存的数据只能被当前线程私有，不被其它线程可见

证明

声明一个全局的变量threadLocal，初始值为1,通过3个线程对其进行访问修改设置，理论上threadLocal的最终值应该是6，然而我们的输出结果是3,说明了threadLocal中存放的数据是各自线程私有的

package com.mmall.concurrency.example.threadLocal;

public class UseThreadLocal {
    static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>() {
        @Override
        protected Integer initialValue() {
            return 1;
        }
    };


    //运行3个线程
    public void startThreadArray() {
        Thread[] thread = new Thread[3];
        for (int i = 0; i < thread.length; i++) {
            thread[i] = new Thread(new MyThread(i));
        }

        for (int i = 0; i < thread.length; i++) {
            thread[i].start();
        }
    }

    private class MyThread implements Runnable {
        int id;

        public MyThread(int i) {
            id = i;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName()+":start");
            Integer v = threadLocal.get();
            v=v+id;
            threadLocal.set(v);
            System.out.println(Thread.currentThread().getName()+"："+threadLocal.get());
        }
    }

    public static void main(String[] args) {
        UseThreadLocal useThreadLocal = new UseThreadLocal();
        useThreadLocal.startThreadArray();
    }
}

结果

Thread-0:start
Thread-2:start
Thread-1:start
Thread-0：1
Thread-2：3
Thread-1：2

小应用

ThreadLocal结合过滤器和拦截器进行搭配使用，通过在过滤器HttpFilter设置ThreadLocal中的值，通过拦截器HttpInterceptor移除拦截器中的值

编写`ThreadLocal类，包含设置、获取、移除操作

package com.mmall.concurrency.example.threadLocal;

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();
    }
}

编写过滤器HttpFilter类，通过在doFilter方法中对ThreadLocal进行存数据

package com.mmall.concurrency;

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());
        RequestHolder.add(Thread.currentThread().getId());
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {

    }
}

编写ThreadLocalController类，在业务中可以获取到在过滤器HttpFilter中对ThreadLocal中存放的数据

package com.mmall.concurrency.example.threadLocal;

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();
    }
}

编写拦截器HttpInterceptor类,在完成业务逻辑处理后，在拦截器类HttpInterceptor的afterCompletion方法中移除我们在过滤器HttpFilter中对ThreadLocal设置的值

package com.mmall.concurrency;

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 {
        RequestHolder.remove();
        log.info("afterCompletion");
        return;
    }
}

编写springboot的启动类ConcurrencyApplication，实例化了FilterRegistrationBean

package com.mmall.concurrency;

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("/**");
    }
}

启动springboot启动类，访问http://localhost:8080/threadLocal/test，控制台输出