为什么需要TransmittableThreadLocal

1. 正确的ThreadLocal的使用

1.1 如何使用

在同一个线程中，通过ThreadLocal传递上下文，在使用后进行释放

1.2 使用场景

• 示意图

  
  
  image.png
• 示例说明
  • 如果web应用在处理tom的请求的时候,bob又进来，使用了线程t2, t1和t2的threadlocal变量是隔离的,也就是线程安全的
  • 线程使用完后将threadlocal释放,避免内存泄漏
  • 这种情况下，线程的thradlocal可以从上层流转到下层，上层的修改对下层是可见的

1.3 代码示例

package com.zihao.controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
/**
 * @author tangzihao
 * @date 2020/9/5 8:08 下午
 */
@RestController
public class TestThreadLocalController {
    private static final ThreadLocal threadLocal = new ThreadLocal<>();
    /**
     * 处理用户的请求
     * @param uid 用户的uid
     */
    @RequestMapping("/testThreadLocal")
    public void handleRequest(String uid){
        System.out.println("current user uid:"+uid);
        //将uid放入ThreadLocal中,通过ThreadLocal进行传值,避免共享成员变量产生线程不安全
        threadLocal.set(uid);
        query();
    }
    /**
     * 查询用户信息
     */
    private void query(){
        try{
            queryCache();
            queryDb();
        }finally {
            //用完后一定要释放该线程的threadLocal
            threadLocal.remove();
            //移除后输出下内容
            System.out.println("after remove,current threadLocal:"+threadLocal.get());
            System.out.println();
        }
    }
    /**
     * 模拟查询缓存
     */
    private void queryCache(){
        System.out.println("thread ["+Thread.currentThread().getName()+"] query cache, uid:"+threadLocal.get());
    }
    /**
     * 模拟查询数据库
     */
    private void queryDb(){
        System.out.println("thread ["+Thread.currentThread().getName()+"] query db, uid:"+threadLocal.get());
    }
}

我是在springboot项目中，简单写了个controller模拟用户的请求

1.4 输出结果

image.png

1.5 其他联想

• tomcat的线程池的设计策略
  • tomcat的线程池是如何扩展jdk的线程池的，具体见tomcat源码的org.apache.tomcat.util.threads.ThreadPoolExecutor，tomcat源码编译本博客有可以看下喔
• springboot内嵌的tomcat配置
  • 配置项见ServerProperties
• Spring boot如何切换不同的web容器
  • 使用不同的web容器，线程池的命名是不同的
  • 容器可以切换成jetty和undertow
    • 排除spring-boot-starter-web的spring-boot-starter-tomcat
    • 引入spring-boot-starter-jetty或者spring-boot-starter-undertow

2. 父子线程传值的问题

2.1 示例代码

package com.test.zihao;
/**
 * 父子线程传递ThreadLocal变量
 *
 * @author tangzihao
 * @date 2020/9/5 9:42 下午
 */
public class ParentChildThreadLocalCase {
    private static final ThreadLocal threadLocal = new ThreadLocal<>();
    public static void main(String[] args) {
        //main线程 设置threadLocal
        threadLocal.set("hello");
        Thread childThread = new Thread(() -> {
            System.out.println("current thread: " + Thread.currentThread().getName() + ", value from thread local: " + threadLocal.get());
        }, "childThread");
        childThread.start();
        System.out.println(Thread.currentThread().getName() + ", value from thread local: " + threadLocal.get());
    }
}

2.2 输出结果

main, value from thread local: hello
current thread: childThread, value from thread local: null

2.3 原因分析

• ThreadLocal是对当前线程有效
• 这个case中有两个线程
  • 线程1，main线程，main程序的执行线程，main线程设置了自己线程的threadLocal，只有main线程自己get才可以获取到之前它自己设置的值
  • 线程2，childThread线程，是main线程的子线程，子线程尝试获取父线程main的threadlocal变量，获取为null

3. InheritableThreadLocal解决父子线程的传值

3.1 示例代码

package com.test.zihao;
/**
 * 父子线程传递ThreadLocal变量
 *
 * @author tangzihao
 * @date 2020/9/5 9:42 下午
 */
public class ParentChildThreadLocalCase {
    private static final InheritableThreadLocal threadLocal = new InheritableThreadLocal<>();
    public static void main(String[] args) throws InterruptedException {
        //main线程 设置threadLocal
        threadLocal.set("hello");
        Thread childThread = new Thread(() -> {
            System.out.println("current thread: " + Thread.currentThread().getName() + ", value from thread local: " + threadLocal.get());
        }, "childThread");
        childThread.start();
        System.out.println(Thread.currentThread().getName() + ", value from thread local: " + threadLocal.get());
    }
}

3.2 输出结果

current thread: childThread, value from thread local: hello
main, value from thread local: hello

3.3 结论

• 使用InheritableThreadLocal可以在父子线程之间传递ThreadLocal变量
• 子线程修改的InheritableThreadLocal对父线程不可见
• 子线程之间的InheritableThreadLocal彼此不可见

3.4 InheritableThreadLocal传值的源码

(1) 如果父线程的inheritableThreadLocals不为null同时允许继承inheritThreadLocals的话，将父线程的inheritableThreadLocals复制到子线程
Thread源码的418行

if (inheritThreadLocals && parent.inheritableThreadLocals != null)
    this.inheritableThreadLocals =
        ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

(2) InheritableThreadLocal通过重写childValue方法来将父线程的值注入
ThreadLocal的ThreadLocalMap(ThreadLocalMap parentMap)方法

private ThreadLocalMap(ThreadLocalMap parentMap) {
            //构造table数组
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];
            
            //复制parentMap的entry
            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal