java高并发系列 - 第24天：ThreadLocal、InheritableThreadLocal（通俗易懂）

java高并发系列第24篇文章。

环境：jdk1.8。

本文内容

1. 须要解决的问题
2. 介绍ThreadLocal
3. 介绍InheritableThreadLocal

须要解决的问题

咱们仍是以解决问题的方式来引出ThreadLocal、InheritableThreadLocal，这样印象会深入一些。

目前java开发web系统通常有3层，controller、service、dao，请求到达controller，controller调用service，service调用dao，而后进行处理。

咱们写一个简单的例子，有3个方法分别模拟controller、service、dao。代码以下：

package com.itsoku.chat24;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 跟着阿里p7学并发，微信公众号：javacode2018
 */
public class Demo1 {

    static AtomicInteger threadIndex = new AtomicInteger(1);
    //建立处理请求的线程池子
    static ThreadPoolExecutor disposeRequestExecutor = new ThreadPoolExecutor(3,
            3,
            60,
            TimeUnit.SECONDS,
            new LinkedBlockingDeque<>(),
            r -> {
                Thread thread = new Thread(r);
                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());
                return thread;
            });

    //记录日志
    public static void log(String msg) {
        StackTraceElement stack[] = (new Throwable()).getStackTrace();
        System.out.println("****" + System.currentTimeMillis() + ",[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);
    }

    //模拟controller
    public static void controller(List<String> dataList) {
        log("接受请求");
        service(dataList);
    }

    //模拟service
    public static void service(List<String> dataList) {
        log("执行业务");
        dao(dataList);
    }

    //模拟dao
    public static void dao(List<String> dataList) {
        log("执行数据库操做");
        //模拟插入数据
        for (String s : dataList) {
            log("插入数据" + s + "成功");
        }
    }

    public static void main(String[] args) {
        //须要插入的数据
        List<String> dataList = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            dataList.add("数据" + i);
        }

        //模拟5个请求
        int requestCount = 5;
        for (int i = 0; i < requestCount; i++) {
            disposeRequestExecutor.execute(() -> {
                controller(dataList);
            });
        }

        disposeRequestExecutor.shutdown();
    }
}

运行结果：

****1565338891286,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.controller(Demo1.java:36):接受请求
****1565338891286,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.controller(Demo1.java:36):接受请求
****1565338891287,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.service(Demo1.java:42):执行业务
****1565338891287,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.service(Demo1.java:42):执行业务
****1565338891287,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.controller(Demo1.java:36):接受请求
****1565338891287,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操做
****1565338891287,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据0成功
****1565338891287,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据1成功
****1565338891287,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操做
****1565338891287,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据2成功
****1565338891287,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.service(Demo1.java:42):执行业务
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.controller(Demo1.java:36):接受请求
****1565338891287,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据0成功
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.service(Demo1.java:42):执行业务
****1565338891288,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操做
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操做
****1565338891288,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据1成功
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据0成功
****1565338891288,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据0成功
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据1成功
****1565338891288,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据2成功
****1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据2成功
****1565338891288,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据1成功
****1565338891288,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.controller(Demo1.java:36):接受请求
****1565338891288,[线程:disposeRequestThread-3],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据2成功
****1565338891288,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.service(Demo1.java:42):执行业务
****1565338891289,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操做
****1565338891289,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据0成功
****1565338891289,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据1成功
****1565338891289,[线程:disposeRequestThread-2],com.itsoku.chat24.Demo1.dao(Demo1.java:51):插入数据数据2成功

代码中调用controller、service、dao 3个方法时，来模拟处理一个请求。main方法中循环了5次模拟发起5次请求，而后交给线程池去处理请求，dao中模拟循环插入传入的dataList数据。

问题来了：开发者想看一下哪些地方耗时比较多，想经过日志来分析耗时状况，想追踪某个请求的完整日志，怎么搞？

上面的请求采用线程池的方式处理的，多个请求可能会被一个线程处理，经过日志很难看出那些日志是同一个请求，咱们能不能给请求加一个惟一标志，日志中输出这个惟一标志，固然能够。

若是咱们的代码就只有上面示例这么简单，我想仍是很容易的，上面就3个方法，给每一个方法加个traceId参数，log方法也加个traceId参数，就解决了，代码以下：

package com.itsoku.chat24;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 跟着阿里p7学并发，微信公众号：javacode2018
 */
public class Demo2 {

    static AtomicInteger threadIndex = new AtomicInteger(1);
    //建立处理请求的线程池子
    static ThreadPoolExecutor disposeRequestExecutor = new ThreadPoolExecutor(3,
            3,
            60,
            TimeUnit.SECONDS,
            new LinkedBlockingDeque<>(),
            r -> {
                Thread thread = new Thread(r);
                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());
                return thread;
            });

    //记录日志
    public static void log(String msg, String traceId) {
        StackTraceElement stack[] = (new Throwable()).getStackTrace();
        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);
    }

    //模拟controller
    public static void controller(List<String> dataList, String traceId) {
        log("接受请求", traceId);
        service(dataList, traceId);
    }

    //模拟service
    public static void service(List<String> dataList, String traceId) {
        log("执行业务", traceId);
        dao(dataList, traceId);
    }

    //模拟dao
    public static void dao(List<String> dataList, String traceId) {
        log("执行数据库操做", traceId);
        //模拟插入数据
        for (String s : dataList) {
            log("插入数据" + s + "成功", traceId);
        }
    }

    public static void main(String[] args) {
        //须要插入的数据
        List<String> dataList = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            dataList.add("数据" + i);
        }

        //模拟5个请求
        int requestCount = 5;
        for (int i = 0; i < requestCount; i++) {
            String traceId = String.valueOf(i);
            disposeRequestExecutor.execute(() -> {
                controller(dataList, traceId);
            });
        }

        disposeRequestExecutor.shutdown();
    }
}

输出：

****1565339559773[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.controller(Demo2.java:36):接受请求
****1565339559773[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.controller(Demo2.java:36):接受请求
****1565339559773[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.controller(Demo2.java:36):接受请求
****1565339559774[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.service(Demo2.java:42):执行业务
****1565339559774[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.service(Demo2.java:42):执行业务
****1565339559774[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:48):执行数据库操做
****1565339559774[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.service(Demo2.java:42):执行业务
****1565339559774[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据0成功
****1565339559774[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:48):执行数据库操做
****1565339559774[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据1成功
****1565339559774[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.dao(Demo2.java:48):执行数据库操做
****1565339559774[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据2成功
****1565339559774[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据0成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.controller(Demo2.java:36):接受请求
****1565339559775[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据0成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.service(Demo2.java:42):执行业务
****1565339559775[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据1成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:48):执行数据库操做
****1565339559775[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据1成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据0成功
****1565339559775[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据2成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据1成功
****1565339559775[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据2成功
****1565339559775[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据2成功
****1565339559775[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.controller(Demo2.java:36):接受请求
****1565339559776[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.service(Demo2.java:42):执行业务
****1565339559776[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:48):执行数据库操做
****1565339559776[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据0成功
****1565339559776[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据1成功
****1565339559776[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo2.dao(Demo2.java:51):插入数据数据2成功

上面咱们经过修改代码的方式，把问题解决了，但前提是大家的系统都像上面这么简单，功能不多，须要改的代码不多，能够这么去改。但事与愿违，咱们的系统通常功能都是比较多的，若是咱们都一个个去改，岂不是要疯掉，改代码还涉及到从新测试，风险也不可控。那有什么好办法么？

ThreadLocal

仍是拿上面的问题，咱们来分析一下，每一个请求都是由一个线程处理的，线程就至关于一我的同样，每一个请求至关于一个任务，任务来了，人来处理，处理完毕以后，再处理下一个请求任务。人身上是否是有不少口袋，人刚开始准备处理任务的时候，咱们把任务的编号放在处理者的口袋中，而后处理中一路携带者，处理过程当中若是须要用到这个编号，直接从口袋中获取就能够了。那么恰好java中线程设计的时候也考虑到了这些问题，Thread对象中就有不少口袋，用来放东西。Thread类中有这么一个变量：

ThreadLocal.ThreadLocalMap threadLocals = null;

这个就是用来操做Thread中全部口袋的东西，ThreadLocalMap源码中有一个数组（有兴趣的能够去看一下源码），对应处理者身上不少口袋同样，数组中的每一个元素对应一个口袋。

如何来操做Thread中的这些口袋呢，java为咱们提供了一个类ThreadLocal，ThreadLocal对象用来操做Thread中的某一个口袋，能够向这个口袋中放东西、获取里面的东西、清除里面的东西，这个口袋一次性只能放一个东西，重复放东西会将里面已经存在的东西覆盖掉。

经常使用的3个方法：

//向Thread中某个口袋中放东西
public void set(T value);
//获取这个口袋中目前放的东西
public T get();
//清空这个口袋中放的东西
public void remove()

咱们使用ThreadLocal来改造一下上面的代码，以下：

package com.itsoku.chat24;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 跟着阿里p7学并发，微信公众号：javacode2018
 */
public class Demo3 {

    //建立一个操做Thread中存放请求任务追踪id口袋的对象
    static ThreadLocal<String> traceIdKD = new ThreadLocal<>();

    static AtomicInteger threadIndex = new AtomicInteger(1);
    //建立处理请求的线程池子
    static ThreadPoolExecutor disposeRequestExecutor = new ThreadPoolExecutor(3,
            3,
            60,
            TimeUnit.SECONDS,
            new LinkedBlockingDeque<>(),
            r -> {
                Thread thread = new Thread(r);
                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());
                return thread;
            });

    //记录日志
    public static void log(String msg) {
        StackTraceElement stack[] = (new Throwable()).getStackTrace();
        //获取当前线程存放tranceId口袋中的内容
        String traceId = traceIdKD.get();
        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);
    }

    //模拟controller
    public static void controller(List<String> dataList) {
        log("接受请求");
        service(dataList);
    }

    //模拟service
    public static void service(List<String> dataList) {
        log("执行业务");
        dao(dataList);
    }

    //模拟dao
    public static void dao(List<String> dataList) {
        log("执行数据库操做");
        //模拟插入数据
        for (String s : dataList) {
            log("插入数据" + s + "成功");
        }
    }

    public static void main(String[] args) {
        //须要插入的数据
        List<String> dataList = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            dataList.add("数据" + i);
        }

        //模拟5个请求
        int requestCount = 5;
        for (int i = 0; i < requestCount; i++) {
            String traceId = String.valueOf(i);
            disposeRequestExecutor.execute(() -> {
                //把traceId放入口袋中
                traceIdKD.set(traceId);
                try {
                    controller(dataList);
                } finally {
                    //将tranceId从口袋中移除
                    traceIdKD.remove();
                }
            });
        }

        disposeRequestExecutor.shutdown();
    }
}

输出：

****1565339644214[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.controller(Demo3.java:41):接受请求
****1565339644214[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.controller(Demo3.java:41):接受请求
****1565339644214[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.controller(Demo3.java:41):接受请求
****1565339644214[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.service(Demo3.java:47):执行业务
****1565339644214[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.service(Demo3.java:47):执行业务
****1565339644214[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操做
****1565339644214[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.service(Demo3.java:47):执行业务
****1565339644214[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据0成功
****1565339644214[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操做
****1565339644214[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操做
****1565339644215[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据0成功
****1565339644215[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据1成功
****1565339644215[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据1成功
****1565339644215[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据0成功
****1565339644215[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据2成功
****1565339644215[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据2成功
****1565339644215[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据1成功
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.controller(Demo3.java:41):接受请求
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.controller(Demo3.java:41):接受请求
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.service(Demo3.java:47):执行业务
****1565339644215[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据2成功
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操做
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.service(Demo3.java:47):执行业务
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据0成功
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操做
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据1成功
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据0成功
****1565339644215[traceId:4],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据2成功
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据1成功
****1565339644215[traceId:3],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo3.dao(Demo3.java:56):插入数据数据2成功

能够看出输出和刚才使用traceId参数的方式结果一致，可是却简单了不少。不用去修改controller、service、dao代码了，风险也减小了不少。

代码中建立了一个ThreadLocal traceIdKD，这个对象用来操做Thread中一个口袋，用这个口袋来存放tranceId。在main方法中经过traceIdKD.set(traceId)方法将traceId放入口袋，log方法中通traceIdKD.get()获取口袋中的traceId，最后任务处理完以后，main中的finally中调用traceIdKD.remove();将口袋中的traceId清除。

ThreadLocal的官方API解释为：

“该类提供了线程局部 (thread-local) 变量。这些变量不一样于它们的普通对应物，由于访问某个变量（经过其 get 或 set 方法）的每一个线程都有本身的局部变量，它独立于变量的初始化副本。ThreadLocal 实例一般是类中的 private static 字段，它们但愿将状态与某一个线程（例如，用户 ID 或事务 ID）相关联。”

InheritableThreadLocal

继续上面的实例，dao中循环处理dataList的内容，假如dataList处理比较耗时，咱们想加快处理速度有什么办法么？你们已经想到了，用多线程并行处理dataList，那么咱们把代码改一下，以下：

package com.itsoku.chat24;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 跟着阿里p7学并发，微信公众号：javacode2018
 */
public class Demo4 {

    //建立一个操做Thread中存放请求任务追踪id口袋的对象
    static ThreadLocal<String> traceIdKD = new ThreadLocal<>();

    static AtomicInteger threadIndex = new AtomicInteger(1);
    //建立处理请求的线程池子
    static ThreadPoolExecutor disposeRequestExecutor = new ThreadPoolExecutor(3,
            3,
            60,
            TimeUnit.SECONDS,
            new LinkedBlockingDeque<>(),
            r -> {
                Thread thread = new Thread(r);
                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());
                return thread;
            });

    //记录日志
    public static void log(String msg) {
        StackTraceElement stack[] = (new Throwable()).getStackTrace();
        //获取当前线程存放tranceId口袋中的内容
        String traceId = traceIdKD.get();
        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);
    }

    //模拟controller
    public static void controller(List<String> dataList) {
        log("接受请求");
        service(dataList);
    }

    //模拟service
    public static void service(List<String> dataList) {
        log("执行业务");
        dao(dataList);
    }

    //模拟dao
    public static void dao(List<String> dataList) {
        CountDownLatch countDownLatch = new CountDownLatch(dataList.size());

        log("执行数据库操做");
        String threadName = Thread.currentThread().getName();
        //模拟插入数据
        for (String s : dataList) {
            new Thread(() -> {
                try {
                    //模拟数据库操做耗时100毫秒
                    TimeUnit.MILLISECONDS.sleep(100);
                    log("插入数据" + s + "成功,主线程：" + threadName);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    countDownLatch.countDown();
                }
            }).start();
        }
        //等待上面的dataList处理完毕
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        //须要插入的数据
        List<String> dataList = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            dataList.add("数据" + i);
        }

        //模拟5个请求
        int requestCount = 5;
        for (int i = 0; i < requestCount; i++) {
            String traceId = String.valueOf(i);
            disposeRequestExecutor.execute(() -> {
                //把traceId放入口袋中
                traceIdKD.set(traceId);
                try {
                    controller(dataList);
                } finally {
                    //将tranceId从口袋中移除
                    traceIdKD.remove();
                }
            });
        }

        disposeRequestExecutor.shutdown();
    }
}

输出：

****1565339904279[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565339904279[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565339904279[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565339904279[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565339904279[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565339904279[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565339904279[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565339904279[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565339904279[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565339904281[traceId:null],[线程:Thread-3],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据0成功,主线程：disposeRequestThread-1
****1565339904281[traceId:null],[线程:Thread-5],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据0成功,主线程：disposeRequestThread-2
****1565339904281[traceId:null],[线程:Thread-4],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据0成功,主线程：disposeRequestThread-3
****1565339904281[traceId:null],[线程:Thread-6],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据1成功,主线程：disposeRequestThread-3
****1565339904281[traceId:null],[线程:Thread-9],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-3
****1565339904282[traceId:null],[线程:Thread-8],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据1成功,主线程：disposeRequestThread-1
****1565339904282[traceId:null],[线程:Thread-10],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-1
****1565339904282[traceId:3],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565339904282[traceId:null],[线程:Thread-7],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据1成功,主线程：disposeRequestThread-2
****1565339904282[traceId:null],[线程:Thread-11],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-2
****1565339904282[traceId:3],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565339904282[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565339904283[traceId:3],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565339904283[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565339904283[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565339904283[traceId:null],[线程:Thread-12],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据0成功,主线程：disposeRequestThread-3
****1565339904283[traceId:null],[线程:Thread-13],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据1成功,主线程：disposeRequestThread-3
****1565339904283[traceId:null],[线程:Thread-14],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据0成功,主线程：disposeRequestThread-1
****1565339904284[traceId:null],[线程:Thread-15],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-3
****1565339904284[traceId:null],[线程:Thread-17],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-1
****1565339904284[traceId:null],[线程:Thread-16],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:62):插入数据数据1成功,主线程：disposeRequestThread-1

看一下上面的输出，有些traceId为null，这是为何呢？这是由于dao中为了提高处理速度，建立了子线程来并行处理，子线程调用log的时候，去本身的存放traceId的口袋中拿去东西，确定是空的了。

那有什么办法么？可不能够这样？

父线程至关于主管，子线程至关于干活的小弟，主管让小弟们干活的时候，将本身兜里面的东西复制一份给小弟们使用，主管兜里面可能有不少牛逼的工具，为了提高小弟们的工做效率，给小弟们都复制一个，丢到小弟们的兜里，而后小弟就能够从本身的兜里拿去这些东西使用了，也能够清空本身兜里面的东西。

Thread对象中有个inheritableThreadLocals变量，代码以下：

ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

inheritableThreadLocals至关于线程中另一种兜，这种兜有什么特征呢，当建立子线程的时候，子线程会将父线程这种类型兜的东西所有复制一份放到本身的inheritableThreadLocals兜中，使用InheritableThreadLocal对象能够操做线程中的inheritableThreadLocals兜。

InheritableThreadLocal经常使用的方法也有3个：

//向Thread中某个口袋中放东西
public void set(T value);
//获取这个口袋中目前放的东西
public T get();
//清空这个口袋中放的东西
public void remove()

使用InheritableThreadLocal解决上面子线程中没法输出traceId的问题，只须要将上一个示例代码中的ThreadLocal替换成InheritableThreadLocal便可，代码以下：

package com.itsoku.chat24;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 跟着阿里p7学并发，微信公众号：javacode2018
 */
public class Demo4 {

    //建立一个操做Thread中存放请求任务追踪id口袋的对象,子线程能够继承父线程中内容
    static InheritableThreadLocal<String> traceIdKD = new InheritableThreadLocal<>();

    static AtomicInteger threadIndex = new AtomicInteger(1);
    //建立处理请求的线程池子
    static ThreadPoolExecutor disposeRequestExecutor = new ThreadPoolExecutor(3,
            3,
            60,
            TimeUnit.SECONDS,
            new LinkedBlockingDeque<>(),
            r -> {
                Thread thread = new Thread(r);
                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());
                return thread;
            });

    //记录日志
    public static void log(String msg) {
        StackTraceElement stack[] = (new Throwable()).getStackTrace();
        //获取当前线程存放tranceId口袋中的内容
        String traceId = traceIdKD.get();
        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);
    }

    //模拟controller
    public static void controller(List<String> dataList) {
        log("接受请求");
        service(dataList);
    }

    //模拟service
    public static void service(List<String> dataList) {
        log("执行业务");
        dao(dataList);
    }

    //模拟dao
    public static void dao(List<String> dataList) {
        CountDownLatch countDownLatch = new CountDownLatch(dataList.size());

        log("执行数据库操做");
        String threadName = Thread.currentThread().getName();
        //模拟插入数据
        for (String s : dataList) {
            new Thread(() -> {
                try {
                    //模拟数据库操做耗时100毫秒
                    TimeUnit.MILLISECONDS.sleep(100);
                    log("插入数据" + s + "成功,主线程：" + threadName);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    countDownLatch.countDown();
                }
            }).start();
        }
        //等待上面的dataList处理完毕
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        //须要插入的数据
        List<String> dataList = new ArrayList<>();
        for (int i = 0; i < 3; i++) {
            dataList.add("数据" + i);
        }

        //模拟5个请求
        int requestCount = 5;
        for (int i = 0; i < requestCount; i++) {
            String traceId = String.valueOf(i);
            disposeRequestExecutor.execute(() -> {
                //把traceId放入口袋中
                traceIdKD.set(traceId);
                try {
                    controller(dataList);
                } finally {
                    //将tranceId从口袋中移除
                    traceIdKD.remove();
                }
            });
        }

        disposeRequestExecutor.shutdown();
    }
}

输出：

****1565341611454[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565341611454[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565341611454[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565341611454[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565341611454[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565341611454[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565341611454[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565341611454[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565341611454[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565341611557[traceId:2],[线程:Thread-5],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据0成功,主线程：disposeRequestThread-3
****1565341611557[traceId:0],[线程:Thread-4],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据0成功,主线程：disposeRequestThread-1
****1565341611557[traceId:1],[线程:Thread-11],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据2成功,主线程：disposeRequestThread-2
****1565341611557[traceId:1],[线程:Thread-3],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据0成功,主线程：disposeRequestThread-2
****1565341611557[traceId:1],[线程:Thread-8],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-2
****1565341611557[traceId:0],[线程:Thread-6],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-1
****1565341611557[traceId:0],[线程:Thread-10],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据2成功,主线程：disposeRequestThread-1
****1565341611557[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565341611557[traceId:2],[线程:Thread-9],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据2成功,主线程：disposeRequestThread-3
****1565341611558[traceId:2],[线程:Thread-7],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-3
****1565341611557[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565341611557[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
****1565341611558[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565341611558[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
****1565341611558[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操做
****1565341611659[traceId:3],[线程:Thread-15],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据2成功,主线程：disposeRequestThread-2
****1565341611659[traceId:4],[线程:Thread-14],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据0成功,主线程：disposeRequestThread-1
****1565341611659[traceId:3],[线程:Thread-13],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-2
****1565341611659[traceId:3],[线程:Thread-12],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据0成功,主线程：disposeRequestThread-2
****1565341611660[traceId:4],[线程:Thread-16],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-1
****1565341611660[traceId:4],[线程:Thread-17],com.itsoku.chat24.Demo4.lambda$dao$1(Demo4.java:64):插入数据数据2成功,主线程：disposeRequestThread-1

输出中都有traceId了，和指望的结果一致。

但愿经过这篇文章能够学会使用InheritableThreadLocal和InheritableThreadLocal。有问题能够加我微信itsoku交流，也能够留言，谢谢。

java高并发系列目录

1. 第1天:必须知道的几个概念
2. 第2天:并发级别
3. 第3天:有关并行的两个重要定律
4. 第4天:JMM相关的一些概念
5. 第5天:深刻理解进程和线程
6. 第6天:线程的基本操做
7. 第7天:volatile与Java内存模型
8. 第8天:线程组
9. 第9天：用户线程和守护线程
10. 第10天:线程安全和synchronized关键字
11. 第11天:线程中断的几种方式
12. 第12天JUC:ReentrantLock重入锁
13. 第13天:JUC中的Condition对象
14. 第14天:JUC中的LockSupport工具类，必备技能
15. 第15天：JUC中的Semaphore（信号量）
16. 第16天：JUC中等待多线程完成的工具类CountDownLatch，必备技能
17. 第17天：JUC中的循环栅栏CyclicBarrier的6种使用场景
18. 第18天：JAVA线程池，这一篇就够了
19. 第19天：JUC中的Executor框架详解1
20. 第20天：JUC中的Executor框架详解2
21. 第21天：java中的CAS，你须要知道的东西
22. 第22天：JUC底层工具类Unsafe，高手必需要了解
23. 第23天：JUC中原子类，一篇就够了

java高并发系列连载中，总计估计会有四五十篇文章。

阿里p7一块儿学并发，公众号：路人甲java，天天获取最新文章！