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);
    }
}

@Override
public 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();
    }
}

@Override
public 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.Transmitter
public 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

@Override
public 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

TTL官方性能测试

作者：京东物流 刘朝永

来源：京东云开发者 自猿其说