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
作者：京东物流 刘朝永
来源：京东云开发者 自猿其说
