ThreadLocal、InheritableThreadLocal 详解

本文内容

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

   需要解决的问题

   我们还是以解决问题的方式来引出ThreadLocal、InheritableThreadLocal，这样印象会深刻一些。
   目前 java 开发 web 系统一般有 3 层，controller、service、dao，请求到达 controller，controller 调用 service，service 调用 dao，然后进行处理。
   我们写一个简单的例子，有 3 个方法分别模拟 controller、service、dao。代码如下：
   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;

/
微信公众号：程序员路人，个人博客：http://www.itsoku.com
/
public class Demo1** {

**static** AtomicInteger threadIndex = **new** AtomicInteger(1);<br />    //创建处理请求的线程池子<br />    **static** ThreadPoolExecutor disposeRequestExecutor = **new** ThreadPoolExecutor(3,<br />            3,<br />            60,<br />            TimeUnit.SECONDS,<br />            **new** LinkedBlockingDeque<>(),<br />            r -> {<br />                Thread thread = **new** Thread(r);<br />                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());<br />                **return** thread;<br />            });
//记录日志<br />    **public** **static** **void** **log**(String msg) {<br />        StackTraceElement stack[] = (**new** Throwable()).getStackTrace();<br />        System.out.println("****" + System.currentTimeMillis() + ",[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);<br />    }
//模拟controller<br />    **public** **static** **void** **controller**(List<String> dataList) {<br />        log("接受请求");<br />        service(dataList);<br />    }
//模拟service<br />    **public** **static** **void** **service**(List<String> dataList) {<br />        log("执行业务");<br />        dao(dataList);<br />    }
//模拟dao<br />    **public** **static** **void** **dao**(List<String> dataList) {<br />        log("执行数据库操作");<br />        //模拟插入数据<br />        **for** (String s : dataList) {<br />            log("插入数据" + s + "成功");<br />        }<br />    }
**public** **static** **void** **main**(String[] args) {<br />        //需要插入的数据<br />        List<String> dataList = **new** ArrayList<>();<br />        **for** (**int** i = 0; i < 3; i++) {<br />            dataList.add("数据" + i);<br />        }
    //模拟5个请求<br />        **int** requestCount = 5;<br />        **for** (**int** i = 0; i < requestCount; i++) {<br />            disposeRequestExecutor.execute(() -> {<br />                controller(dataList);<br />            });<br />        }
    disposeRequestExecutor.shutdown();<br />    }<br />}

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):执行数据库操作
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):执行数据库操作
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):执行数据库操作
1565338891288,[线程:disposeRequestThread-1],com.itsoku.chat24.Demo1.dao(Demo1.java:48):执行数据库操作
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):执行数据库操作
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，然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，想通过日志来分析耗时情况，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，通过日志很难看出那些日志是同一个请求，我们能不能给请求加一个唯一标志，日志中输出这个唯一标志，当然可以。
如果我们的代码就只有上面示例这么简单，我想还是很容易的，上面就 3 个方法，给每个方法加个 traceId 参数，log 方法也加个 traceId 参数，就解决了，代码如下：
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;

/
微信公众号：程序员路人，个人博客：http://www.itsoku.com
/
public class Demo2** {

**static** AtomicInteger threadIndex = **new** AtomicInteger(1);<br />    //创建处理请求的线程池子<br />    **static** ThreadPoolExecutor disposeRequestExecutor = **new** ThreadPoolExecutor(3,<br />            3,<br />            60,<br />            TimeUnit.SECONDS,<br />            **new** LinkedBlockingDeque<>(),<br />            r -> {<br />                Thread thread = **new** Thread(r);<br />                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());<br />                **return** thread;<br />            });
//记录日志<br />    **public** **static** **void** **log**(String msg, String traceId) {<br />        StackTraceElement stack[] = (**new** Throwable()).getStackTrace();<br />        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);<br />    }
//模拟controller<br />    **public** **static** **void** **controller**(List<String> dataList, String traceId) {<br />        log("接受请求", traceId);<br />        service(dataList, traceId);<br />    }
//模拟service<br />    **public** **static** **void** **service**(List<String> dataList, String traceId) {<br />        log("执行业务", traceId);<br />        dao(dataList, traceId);<br />    }
//模拟dao<br />    **public** **static** **void** **dao**(List<String> dataList, String traceId) {<br />        log("执行数据库操作", traceId);<br />        //模拟插入数据<br />        **for** (String s : dataList) {<br />            log("插入数据" + s + "成功", traceId);<br />        }<br />    }
**public** **static** **void** **main**(String[] args) {<br />        //需要插入的数据<br />        List<String> dataList = **new** ArrayList<>();<br />        **for** (**int** i = 0; i < 3; i++) {<br />            dataList.add("数据" + i);<br />        }
    //模拟5个请求<br />        **int** requestCount = 5;<br />        **for** (**int** i = 0; i < requestCount; i++) {<br />            String traceId = String.valueOf(i);<br />            disposeRequestExecutor.execute(() -> {<br />                controller(dataList, traceId);<br />            });<br />        }
    disposeRequestExecutor.shutdown();<br />    }<br />}

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):执行数据库操作
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):执行数据库操作
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):执行数据库操作
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):执行数据库操作
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):执行数据库操作
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

上面我们通过修改代码的方式，把问题解决了，但前提是你们的系统都像上面这么简单，功能很少，需要改的代码很少，可以这么去改。但事与愿违，我们的系统一般功能都是比较多的，如果我们都一个个去改，岂不是要疯掉，改代码还涉及到重新测试，风险也不可控。那有什么好办法么？

ThreadLocal

还是拿上面的问题，我们来分析一下，每个请求都是由一个线程处理的，线程就相当于一个人一样，每个请求相当于一个任务，任务来了，人来处理，处理完毕之后，再处理下一个请求任务。人身上是不是有很多口袋，人刚开始准备处理任务的时候，我们把任务的编号放在处理者的口袋中，然后处理中一路携带者，处理过程中如果需要用到这个编号，直接从口袋中获取就可以了。那么刚好 java 中线程设计的时候也考虑到了这些问题，Thread 对象中就有很多口袋，用来放东西。Thread 类中有这么一个变量：
ThreadLocal.ThreadLocalMap threadLocals = null;

这个就是用来操作 Thread 中所有口袋的东西，ThreadLocalMap源码中有一个数组（有兴趣的可以去看一下源码），对应处理者身上很多口袋一样，数组中的每个元素对应一个口袋。
如何来操作 Thread 中的这些口袋呢，java 为我们提供了一个类ThreadLocal，ThreadLocal 对象用来操作 Thread 中的某一个口袋，可以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。
常用的 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()

我们使用 ThreadLocal 来改造一下上面的代码，如下：
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;

/
微信公众号：程序员路人，个人博客：http://www.itsoku.com
/
public class Demo3** {

//创建一个操作Thread中存放请求任务追踪id口袋的对象<br />    **static** ThreadLocal<String> traceIdKD = **new** ThreadLocal<>();
**static** AtomicInteger threadIndex = **new** AtomicInteger(1);<br />    //创建处理请求的线程池子<br />    **static** ThreadPoolExecutor disposeRequestExecutor = **new** ThreadPoolExecutor(3,<br />            3,<br />            60,<br />            TimeUnit.SECONDS,<br />            **new** LinkedBlockingDeque<>(),<br />            r -> {<br />                Thread thread = **new** Thread(r);<br />                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());<br />                **return** thread;<br />            });
//记录日志<br />    **public** **static** **void** **log**(String msg) {<br />        StackTraceElement stack[] = (**new** Throwable()).getStackTrace();<br />        //获取当前线程存放tranceId口袋中的内容<br />        String traceId = traceIdKD.get();<br />        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);<br />    }
//模拟controller<br />    **public** **static** **void** **controller**(List<String> dataList) {<br />        log("接受请求");<br />        service(dataList);<br />    }
//模拟service<br />    **public** **static** **void** **service**(List<String> dataList) {<br />        log("执行业务");<br />        dao(dataList);<br />    }
//模拟dao<br />    **public** **static** **void** **dao**(List<String> dataList) {<br />        log("执行数据库操作");<br />        //模拟插入数据<br />        **for** (String s : dataList) {<br />            log("插入数据" + s + "成功");<br />        }<br />    }
**public** **static** **void** **main**(String[] args) {<br />        //需要插入的数据<br />        List<String> dataList = **new** ArrayList<>();<br />        **for** (**int** i = 0; i < 3; i++) {<br />            dataList.add("数据" + i);<br />        }
    //模拟5个请求<br />        **int** requestCount = 5;<br />        **for** (**int** i = 0; i < requestCount; i++) {<br />            String traceId = String.valueOf(i);<br />            disposeRequestExecutor.execute(() -> {<br />                //把traceId放入口袋中<br />                traceIdKD.set(traceId);<br />                **try** {<br />                    controller(dataList);<br />                } **finally** {<br />                    //将tranceId从口袋中移除<br />                    traceIdKD.remove();<br />                }<br />            });<br />        }
    disposeRequestExecutor.shutdown();<br />    }<br />}

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):执行数据库操作
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):执行数据库操作
1565339644214[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo3.dao(Demo3.java:53):执行数据库操作
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):执行数据库操作
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):执行数据库操作
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

可以看出输出和刚才使用 traceId 参数的方式结果一致，但是却简单了很多。不用去修改 controller、service、dao 代码了，风险也减少了很多。
代码中创建了一个ThreadLocal traceIdKD，这个对象用来操作 Thread 中一个口袋，用这个口袋来存放 tranceId。在 main 方法中通过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，最后任务处理完之后，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。这些变量不同于它们的普通对应物，因为访问某个变量（通过其 get 或 set 方法）的每个线程都有自己的局部变量，它独立于变量的初始化副本。ThreadLocal 实例通常是类中的 private static 字段，它们希望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，我们想加快处理速度有什么办法么？大家已经想到了，用多线程并行处理dataList，那么我们把代码改一下，如下：
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;

/
微信公众号：程序员路人，个人博客：http://www.itsoku.com
/
public class Demo4** {

//创建一个操作Thread中存放请求任务追踪id口袋的对象<br />    **static** ThreadLocal<String> traceIdKD = **new** ThreadLocal<>();
**static** AtomicInteger threadIndex = **new** AtomicInteger(1);<br />    //创建处理请求的线程池子<br />    **static** ThreadPoolExecutor disposeRequestExecutor = **new** ThreadPoolExecutor(3,<br />            3,<br />            60,<br />            TimeUnit.SECONDS,<br />            **new** LinkedBlockingDeque<>(),<br />            r -> {<br />                Thread thread = **new** Thread(r);<br />                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());<br />                **return** thread;<br />            });
//记录日志<br />    **public** **static** **void** **log**(String msg) {<br />        StackTraceElement stack[] = (**new** Throwable()).getStackTrace();<br />        //获取当前线程存放tranceId口袋中的内容<br />        String traceId = traceIdKD.get();<br />        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);<br />    }
//模拟controller<br />    **public** **static** **void** **controller**(List<String> dataList) {<br />        log("接受请求");<br />        service(dataList);<br />    }
//模拟service<br />    **public** **static** **void** **service**(List<String> dataList) {<br />        log("执行业务");<br />        dao(dataList);<br />    }
//模拟dao<br />    **public** **static** **void** **dao**(List<String> dataList) {<br />        CountDownLatch countDownLatch = **new** CountDownLatch(dataList.size());
    log("执行数据库操作");<br />        String threadName = Thread.currentThread().getName();<br />        //模拟插入数据<br />        **for** (String s : dataList) {<br />            **new** Thread(() -> {<br />                **try** {<br />                    //模拟数据库操作耗时100毫秒<br />                    TimeUnit.MILLISECONDS.sleep(100);<br />                    log("插入数据" + s + "成功,主线程：" + threadName);<br />                } **catch** (InterruptedException e) {<br />                    e.printStackTrace();<br />                } **finally** {<br />                    countDownLatch.countDown();<br />                }<br />            }).start();<br />        }<br />        //等待上面的dataList处理完毕<br />        **try** {<br />            countDownLatch.await();<br />        } **catch** (InterruptedException e) {<br />            e.printStackTrace();<br />        }<br />    }
**public** **static** **void** **main**(String[] args) {<br />        //需要插入的数据<br />        List<String> dataList = **new** ArrayList<>();<br />        **for** (**int** i = 0; i < 3; i++) {<br />            dataList.add("数据" + i);<br />        }
    //模拟5个请求<br />        **int** requestCount = 5;<br />        **for** (**int** i = 0; i < requestCount; i++) {<br />            String traceId = String.valueOf(i);<br />            disposeRequestExecutor.execute(() -> {<br />                //把traceId放入口袋中<br />                traceIdKD.set(traceId);<br />                **try** {<br />                    controller(dataList);<br />                } **finally** {<br />                    //将tranceId从口袋中移除<br />                    traceIdKD.remove();<br />                }<br />            });<br />        }
    disposeRequestExecutor.shutdown();<br />    }<br />}

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):执行数据库操作
1565339904279[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
1565339904279[traceId:2],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
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$1(Demo4.java:62):插入数据数据2成功,主线程：disposeRequestThread-2
**1565339904282[traceId:3],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
1565339904282[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
1565339904283[traceId:3],[线程:disposeRequestThread-3],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
1565339904283[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
1565339904283[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
**1565339904283[traceId:null],[线程:Thread-12],com.itsoku.chat24.Demo4.lambda$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，这是为什么呢？这是因为 dao 中为了提升处理速度，创建了子线程来并行处理，子线程调用 log 的时候，去自己的存放 traceId 的口袋中拿去东西，肯定是空的了。
那有什么办法么？可不可以这样？
父线程相当于主管，子线程相当于干活的小弟，主管让小弟们干活的时候，将自己兜里面的东西复制一份给小弟们使用，主管兜里面可能有很多牛逼的工具，为了提升小弟们的工作效率，给小弟们都复制一个，丢到小弟们的兜里，然后小弟就可以从自己的兜里拿去这些东西使用了，也可以清空自己兜里面的东西。
Thread对象中有个inheritableThreadLocals变量，代码如下：
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

inheritableThreadLocals 相当于线程中另外一种兜，这种兜有什么特征呢，当创建子线程的时候，子线程会将父线程这种类型兜的东西全部复制一份放到自己的inheritableThreadLocals兜中，使用InheritableThreadLocal对象可以操作线程中的inheritableThreadLocals兜。
InheritableThreadLocal常用的方法也有 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解决上面子线程中无法输出 traceId 的问题，只需要将上一个示例代码中的ThreadLocal替换成InheritableThreadLocal即可，代码如下：
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;

/
微信公众号：程序员路人，个人博客：http://www.itsoku.com
/
public class Demo4** {

//创建一个操作Thread中存放请求任务追踪id口袋的对象,子线程可以继承父线程中内容<br />    **static** InheritableThreadLocal<String> traceIdKD = **new** InheritableThreadLocal<>();
**static** AtomicInteger threadIndex = **new** AtomicInteger(1);<br />    //创建处理请求的线程池子<br />    **static** ThreadPoolExecutor disposeRequestExecutor = **new** ThreadPoolExecutor(3,<br />            3,<br />            60,<br />            TimeUnit.SECONDS,<br />            **new** LinkedBlockingDeque<>(),<br />            r -> {<br />                Thread thread = **new** Thread(r);<br />                thread.setName("disposeRequestThread-" + threadIndex.getAndIncrement());<br />                **return** thread;<br />            });
//记录日志<br />    **public** **static** **void** **log**(String msg) {<br />        StackTraceElement stack[] = (**new** Throwable()).getStackTrace();<br />        //获取当前线程存放tranceId口袋中的内容<br />        String traceId = traceIdKD.get();<br />        System.out.println("****" + System.currentTimeMillis() + "[traceId:" + traceId + "],[线程:" + Thread.currentThread().getName() + "]," + stack[1] + ":" + msg);<br />    }
//模拟controller<br />    **public** **static** **void** **controller**(List<String> dataList) {<br />        log("接受请求");<br />        service(dataList);<br />    }
//模拟service<br />    **public** **static** **void** **service**(List<String> dataList) {<br />        log("执行业务");<br />        dao(dataList);<br />    }
//模拟dao<br />    **public** **static** **void** **dao**(List<String> dataList) {<br />        CountDownLatch countDownLatch = **new** CountDownLatch(dataList.size());
    log("执行数据库操作");<br />        String threadName = Thread.currentThread().getName();<br />        //模拟插入数据<br />        **for** (String s : dataList) {<br />            **new** Thread(() -> {<br />                **try** {<br />                    //模拟数据库操作耗时100毫秒<br />                    TimeUnit.MILLISECONDS.sleep(100);<br />                    log("插入数据" + s + "成功,主线程：" + threadName);<br />                } **catch** (InterruptedException e) {<br />                    e.printStackTrace();<br />                } **finally** {<br />                    countDownLatch.countDown();<br />                }<br />            }).start();<br />        }<br />        //等待上面的dataList处理完毕<br />        **try** {<br />            countDownLatch.await();<br />        } **catch** (InterruptedException e) {<br />            e.printStackTrace();<br />        }<br />    }
**public** **static** **void** **main**(String[] args) {<br />        //需要插入的数据<br />        List<String> dataList = **new** ArrayList<>();<br />        **for** (**int** i = 0; i < 3; i++) {<br />            dataList.add("数据" + i);<br />        }
    //模拟5个请求<br />        **int** requestCount = 5;<br />        **for** (**int** i = 0; i < requestCount; i++) {<br />            String traceId = String.valueOf(i);<br />            disposeRequestExecutor.execute(() -> {<br />                //把traceId放入口袋中<br />                traceIdKD.set(traceId);<br />                **try** {<br />                    controller(dataList);<br />                } **finally** {<br />                    //将tranceId从口袋中移除<br />                    traceIdKD.remove();<br />                }<br />            });<br />        }
    disposeRequestExecutor.shutdown();<br />    }<br />}

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):执行数据库操作
1565341611454[traceId:1],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
1565341611454[traceId:0],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
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$1(Demo4.java:64):插入数据数据1成功,主线程：disposeRequestThread-3
1565341611557[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
1565341611557[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.controller(Demo4.java:42):接受请求
1565341611558[traceId:3],[线程:disposeRequestThread-2],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
1565341611558[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.service(Demo4.java:48):执行业务
1565341611558[traceId:4],[线程:disposeRequestThread-1],com.itsoku.chat24.Demo4.dao(Demo4.java:56):执行数据库操作
1565341611659[traceId:3],[线程:Thread-15],com.itsoku.chat24.Demo4.lambda$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，和期望的结果一致。
希望通过这篇文章可以学会使用InheritableThreadLocal和InheritableThreadLocal。有问题可以加我微信itsoku交流，也可以留言，谢谢。