从TL、ITL到TTL详解
1、概述
ThreadLocal（TL）是Java中一种线程局部变量实现机制，他为每个线程提供一个单独的变量副本，保证多线程场景下，变量的线程安全。经常用于代替参数的显式传递。
InheritableThreadLocal（ITL）是JDK提供的TL增强版，而TransmittableThreadLocal（TTL）是阿里开源的ITL增强版
这些ThreadLocal在不同场景下有不同用途，我们来分析一下：
2、ThreadLocal
ThreadLocal主要的方法有四个：initialValue、set、get、remove
2.1、初始化——initialValule
当线程首次访问该ThreadLocal时（ThreadLocal.get()），会进行初始化赋值。我们常用两种方法初始化ThreadLocal
2.1.1、重写initialValue
ThreadLocal threadLocal = new ThreadLocal() { @Override protected String initialValue() { return ""; }}; 2.1.2、调用ThreadLocal.withInitial
ThreadLocal threadLocal = ThreadLocal.withInitial(() -> ""); 他会创建一个SuppliedThreadLocal内部类
public static ThreadLocal withInitial(Supplier supplier) { return new SuppliedThreadLocal<>(supplier);}该类重写了initialValue方法
static final class SuppliedThreadLocal extends ThreadLocal { private final Supplier supplier; SuppliedThreadLocal(Supplier supplier) { this.supplier = Objects.requireNonNull(supplier); } @Override protected T initialValue() { //当该线程首次访问ThreadLocal时，会间接调用lambda表达式初始化 return supplier.get(); }} ⚠️ITL并没有重新实现withInitial，如果使用withInitial则会创建STL，失去自己增强的特性
2.2、赋值——set
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value);}这里出现了一个关键属性ThreadLocalMap，类定义在ThreadLocal中，是Thread的成员变量
ThreadLocalMap getMap(Thread t) { return t.threadLocals;}ThreadLocalMap内部还有一个内部类Entry，是存值的地方
static class ThreadLocalMap { static class Entry extends WeakReference> { Object value; Entry(ThreadLocal k, Object v) { //ThreadLocal的引用是“key” super(k); //线程局部变量是value value = v; } } //Entry数组 //value具体放在哪个index下，是由ThreadLocal的hashCode算出来的 private Entry[] table;} 2.3、取值——get
public T get() { Thread t = Thread.currentThread(); //1、获取线程的ThreadLocalMap ThreadLocalMap map = getMap(t); if (map != null) { //2、根据ThreadLocal的hashCode，获取对应Entry下的value ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } //3、如果没有赋过值，则初始化 return setInitialValue();}2.4、清空——remove
public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) //会将对应Entry、包括他的key、value手动置null m.remove(this); }3、InheritableThreadLocal
3.1、TL在父子线程场景下存在的问题
我们先来看一个例子
public static void main(String[] args) throws InterruptedException { ThreadLocal threadLocal = ThreadLocal.withInitial(() -> "A"); threadLocal.set("B"); Thread thread = new Thread(() -> { System.out.println("子线程ThreadLocal：" + threadLocal.get()); }, "子线程"); thread.start(); thread.join();} 打印结果如下，可见子线程的ThreadLocal是初始值，并没有使用父线程修改后的值：
子线程ThreadLocal：A
线程的ThreadLocalMap是首次访问时创建的，所以子线程使用ThreadLocal的时候，会初始化一个新的ThreadLocal，线程局部变量为默认值
⚠️所以，TL不具有遗传性
3.2、ITL的解决方案
为了解决TL子线程遗传性的问题，JDK引入了ITL
他继承ThreadLocal，重写了childValue、getMap、createMap三个方法
public class InheritableThreadLocal extends ThreadLocal { protected T childValue(T parentValue) { return parentValue; } ThreadLocalMap getMap(Thread t) { return t.inheritableThreadLocals; } void createMap(Thread t, T firstValue) { t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue); }} 这里出现了inheritableThreadLocals，他存储的就是从父线程拷贝过来的ThreadLocal，这个值是在父线程首次修改ThreadLocal的时候赋值的，然后在子线程创建时拷贝过来的
//父线程部分：public void set(T value) { Thread t = Thread.currentThread(); //该方法被ITL重写，访问inheritableThreadLocals为null ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else //该方法同样被ITL重写，创建一个ThreadLocalMap赋值给inheritableThreadLocals createMap(t, value);}//子线程部分：public Thread(Runnable target) { init(null, target, "Thread-" + nextThreadNum(), 0);}private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) { //省略一些代码... //获取当前线程（父线程、也就是创建子线程的线程） Thread parent = currentThread(); //1、允许ThreadLocal遗传（这个默认为true） //2、inheritableThreadLocals不为空，因为父线程调用set了 //父线程不调用set，那ThreadLocal就是初始值，那直接初始化就好了，也不用进该分支 if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);}//createInheritedMap使用该构造函数，根据父线程的inheritableThreadLocals进行深拷贝private ThreadLocalMap(ThreadLocalMap parentMap) { Entry[] parentTable = parentMap.table; int len = parentTable.length; setThreshold(len); table = new Entry[len]; //深拷贝父线程ThreadLocalMap for (int j = 0; j < len; j++) { Entry e = parentTable[j]; if (e != null) { @SuppressWarnings("unchecked") ThreadLocal使用ITL的效果
public static void main(String[] args) throws InterruptedException { ThreadLocal threadLocal = new InheritableThreadLocal() { @Override protected String initialValue() { return "A"; } }; threadLocal.set("B"); Thread thread = new Thread(() -> { System.out.println("子线程ThreadLocal：" + threadLocal.get()); }, "子线程"); thread.start(); thread.join();} 打印结果如下，子线程拷贝了父线程ThreadLocal：
子线程ThreadLocal：B
总结一下，ITL解决父子线程遗传性的核心思路是，将可遗传的ThreadLocal放在父线程新的ThreadLocalMap中，在子线程首次使用时进行拷贝
4.、TransmittableThreadLocal
4.1、ITL在线程复用场景下存在的问题
我们再从一个简单的例子说起
public static void main(String[] args) throws InterruptedException, ExecutionException { ThreadLocal threadLocal = new InheritableThreadLocal() { @Override protected String initialValue() { return "A"; } }; threadLocal.set("B"); ExecutorService executorService = Executors.newFixedThreadPool(1); //1、子线程第一次获取ThreadLocal executorService.submit(() -> System.out.println("子线程ThreadLocal："+threadLocal.get())).get(); Thread.sleep(1000); //2、父线程修改ThreadLocal threadLocal.set("C"); System.out.println("父线程修改ThreadLocal为"+threadLocal.get()); //3、子线程第二次获取ThreadLocal executorService.submit(() -> System.out.println("子线程ThreadLocal："+threadLocal.get())).get();} 打印结果如下，子线程在第二次打印时，并没有拷贝父线程的ThreadLocal，使用的还是首次拷贝的值：
子线程ThreadLocal：B父线程修改ThreadLocal为C子线程ThreadLocal：B
⚠️可复用的子线程不会感知父线程ThreadLocal的变化
4.2、TTL的解决方案
4.2.1、TTL的使用
TTL在ITL上做了稍微复杂的封装，我们从使用开始了解
引入依赖
com.alibaba transmittable-thread-local latest 在使用TTL时，线程需要经过TTL封装，线程池同理
public static void main(String[] args) throws InterruptedException, ExecutionException { ThreadLocal threadLocal = new TransmittableThreadLocal() { @Override protected String initialValue() { return "A"; } }; threadLocal.set("B"); ExecutorService executorService = TtlExecutors.getTtlExecutorService(Executors.newFixedThreadPool(1)); executorService.submit(() -> System.out.println("子线程ThreadLocal：" + threadLocal.get())).get(); Thread.sleep(1000); threadLocal.set("C"); System.out.println("父线程修改ThreadLocal为" + threadLocal.get()); executorService.submit(() -> System.out.println("子线程ThreadLocal：" + threadLocal.get())).get(); Thread.sleep(1000); executorService.submit(() -> { threadLocal.set("D"); System.out.println("子线程修改ThreadLocal为" + threadLocal.get()); }); Thread.sleep(1000); executorService.submit(() -> System.out.println("子线程ThreadLocal：" + threadLocal.get())); Thread.sleep(1000);} 打印结果如下，子线程每次都会获取父线程的ThreadLocal
子线程ThreadLocal：B父线程修改ThreadLocal为C子线程ThreadLocal：C子线程修改ThreadLocal为D子线程ThreadLocal：C
从使用上看，TTL要求将任务封装，那我们就从ThreadLocal和ExecutorService两部分入手
4.2.2、TTL对ThreadLocal的封装
下面是TTL的取值和赋值逻辑，都涉及一个关键方法addThisToHolder，对应的属性holder会在线程池执行任务时用到
//TransmittableThreadLocal.addThisToHolder()private void addThisToHolder() { //InheritableThreadLocal, ?>> holder if (!holder.get().containsKey(this)) { //holder是静态变量，他会把TTL存到当前线程的map中 //value是null，他其实是把Map当Set用 //主线程赋值时，会获取主线程的holderMap，然后把TTL存进去 holder.get().put((TransmittableThreadLocal 4.2.3、TTL对任务的封装
//我们通过TtlExecutors.getTtlExecutorService()对线程池进行封装public static ExecutorService getTtlExecutorService(@Nullable ExecutorService executorService) { if (TtlAgent.isTtlAgentLoaded() || executorService == null || executorService instanceof TtlEnhanced) { return executorService; } //入参是线程池，通过包装类代理线程池的操作 return new ExecutorServiceTtlWrapper(executorService);}//ExecutorServiceTtlWrapper.submit()public Future submit(@NonNull Runnable task) { //将提交的任务进行封装 return executorService.submit(TtlRunnable.get(task));}4.2.3.1、任务构建
TtlRunnable构造方法
这里都是主线程在操作，因为任务是主线程提交的
private TtlRunnable(@NonNull Runnable runnable, boolean releaseTtlValueReferenceAfterRun) { this.capturedRef = new AtomicReference这里有一个关键属性capturedRef，他是一个原子引用，存了TTL
//TrasmitableThreadLocal.Transmitterpublic static Object capture() { //获取ttl的值构建快照 return new Snapshot(captureTtlValues(), captureThreadLocalValues());}private static HashMap, Object> captureTtlValues() { HashMap, Object> ttl2Value = new HashMap, Object>(); for (TransmittableThreadLocal 4.2.3.2、任务执行
任务执行的代码如下，在任务执行前回放ThreadLocal，在任务执行后恢复ThreadLocal：
这里都是子线程在操作，因为任务都是子线程执行的
@Overridepublic void run() { Object captured = capturedRef.get(); if (captured == null || releaseTtlValueReferenceAfterRun && !capturedRef.compareAndSet(captured, null)) { throw new IllegalStateException("TTL value reference is released after run!"); } //1、备份子线程ThreadLocal //2、使用主线程提交任务时构建的ThreadLocal副本，将子线程ThreadLocal覆盖 Object backup = replay(captured); try { //3、任务执行 runnable.run(); } finally { //3、使用之前备份的子线程ThreadLocal进行恢复 restore(backup); }}总结一下，TTL让子线程感知父线程变化的核心思路是，主线程在任务提交时构建ThreadLocal副本，在子线程执行任务时供其使用
⚠️提交和执行任务会对TTL进行若干操作，理论上对性能有一点点影响，官方性能测试结论说损耗可忽略