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<String> threadLocal = new ThreadLocal<String>() {    @Override    protected String initialValue() {        return "";    }};

2.1.2、调用ThreadLocal.withInitial

ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "");

他会创建一个SuppliedThreadLocal内部类

public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {    return new SuppliedThreadLocal<>(supplier);}

该类重写了initialValue方法

static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {    private final Supplier<? extends T> supplier;    SuppliedThreadLocal(Supplier<? extends T> 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<ThreadLocal<?>> {        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<String> 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<T> extends ThreadLocal<T> {    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<Object> key = (ThreadLocal<Object>) e.get();            if (key != null) {                //childValue被ITL重写，返回父线程ThreadLocal的值                Object value = key.childValue(e.value);                Entry c = new Entry(key, value);                int h = key.threadLocalHashCode & (len - 1);                while (table[h] != null)                    h = nextIndex(h, len);                table[h] = c;                size++;            }        }    }}

使用ITL的效果

public static void main(String[] args) throws InterruptedException {        ThreadLocal<String> threadLocal = new InheritableThreadLocal<String>() {            @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<String> threadLocal = new InheritableThreadLocal<String>() {        @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

<dependency>    <groupId>com.alibaba</groupId>    <artifactId>transmittable-thread-local</artifactId>    <version>latest</version></dependency>

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<String> threadLocal = new TransmittableThreadLocal<String>() {        @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<WeakHashMap<TransmittableThreadLocal<Object>, ?>> 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<Object>) this, null);    }}@Overridepublic final void set(T value) {    if (!disableIgnoreNullValueSemantics && null == value) {        remove();    } else {        super.set(value);        //当主线程赋值时，会将自己的TTL放到自己的map中        addThisToHolder();    }}@Overridepublic final T get() {    T value = super.get();    if (disableIgnoreNullValueSemantics || null != value)         addThisToHolder();    return value;}

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<Object>(capture());    this.runnable = runnable;    this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;}

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<TransmittableThreadLocal<Object>, Object> captureTtlValues() {    HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = new HashMap<TransmittableThreadLocal<Object>, Object>();    for (TransmittableThreadLocal<Object> threadLocal : holder.get().keySet()) {        //将主线程TTL的值存到当前任务中        ttl2Value.put(threadLocal, threadLocal.copyValue());    }    return ttl2Value;}

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