【java】ThreadLocal线程变量的实现原理和使用场景

一.ThreadLocal线程变量的实现原理

1.ThreadLocal核心方法有这个几个

get()、set(value)、remove()

2.实现原理

ThreadLocal在每个线程都会创建一个线程内对应的T的副本，本T数据可以在本线程内任何地方可以被使用。线程之间互相不影响，所以是线程安全的。

3.底层结构

ThreadLocal实现各个线程数据副本的存取，是通过操作它的内部类ThreadLocalMap，进行<k,v>键值对的存取和移除。

4.set(value)方法的底层

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

set(value)

　　1》根据当前线程，获取本线程所拥有的TreadLocalMap，如果没有，则创建一个新的。

　　2》ThreadLocalMap的<key,value>即<ThreadLocal对象，传入的value值>。【这里的ThreadLocal对象在set处，是根据本对象的hashCode经过计算获取到下标，然后循环对比Entry[]中每一个Entry的key进行插入或覆盖操作】

　　3》那么可以看出结构是：

　　　　3.1》每一个Thread有一个对应的ThreadLocalMap。Map的<K,V>即<当前ThreadLocal对象，传入的value>

　　　　3.2》set操作根据ThreadLocal对象的hashCode对比Entry[]数组，进行新增插入或覆盖操作。

5.get()方法底层

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

private Entry getEntry(ThreadLocal<?> key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

protected T initialValue() {
        return null;
    }

get()

　　1》根据当前Thread线程，获取本线程的ThreadLocalMap

　　2》然后将<K>键，也就是本ThreadLocal作为键传入，从Map中获取value。【获取的过程即，根据ThreadLocal对象的hashCode经过计算获取下标，根据下标取出Entry[]数组中的具体值，返回结果】

　　3》如果没有值，则返回null。

6.remove()底层

public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

remove()

　　1》本线程结束以前，一定要调用remove，清除线程变量中本次的变量。防止内存泄漏

二.ThreadLocal使用场景

拦截器存储 调用接口的用户信息，在本次Request到达，处理，直到返回的本线程中，都可以使用线程变量中的用户信息。

1.定义线程变量

public class RequestData {

    //线程变量  租户对象
    public static final ThreadLocal<TenementUser> TENEMENT_USER = new ThreadLocal<TenementUser>();

2.到达controller之前的拦截器中，赋值线程变量。request返回之前remove【防止内存泄漏】

import java.net.URLDecoder;
import java.net.URLEncoder;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.lang3.StringUtils;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import com.alibaba.fastjson.JSON;
import com.pisen.cloud.luna.core.enums.LunaHeaderNames;
import com.pisen.cloud.luna.core.interceptor.utils.LunaInterceptorUtil;
import com.pisen.cloud.luna.core.reqmodal.RequestData;
import com.pisen.cloud.luna.core.utils.TenementUser;

public class TenementAuthinterceptor implements HandlerInterceptor{

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        String tenInfo = request.getHeader(LunaHeaderNames.TENEMENT_INFO.getName());
        TenementUser tuser = null;

        if(StringUtils.isNotBlank(tenInfo)){

            try {
                tenInfo = URLDecoder.decode(tenInfo, "UTF-8");
                tuser = JSON.parseObject(tenInfo,TenementUser.class);
                if(tuser != null){

                    if(StringUtils.isBlank(tuser.getTenementId()) || StringUtils.isBlank(tuser.getLoginName())){
                        tuser = null;
                    }else{
                        RequestData.TENEMENT_USER.set(tuser);
                    }
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        if(tuser != null){
            return true;
        }else{

            String errorMsg = "登录失败,请重新登录!";
            LunaInterceptorUtil.ErrorResp(response,errorMsg);
            return false;
        }

    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
            ModelAndView modelAndView) throws Exception {
        RequestData.TENEMENT_USER.remove();
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
            throws Exception {
    }

}

3.controller中使用线程变量

//创建单据
    @RequestMapping(value = "/insert",method = RequestMethod.POST)
    public AjaxResult<SaleBill> insert(@RequestBody SaleBill bill){

        TenementUser tuser = RequestData.TENEMENT_USER.get();