何为TransmittableThreadLocal

一、示例

线程池内的线程并没有父子关系，所以不适合InheritableThreadLocal的使用场景

public class ThreadPoolInheritableThreadLocalDemo {//    static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
//    static ExecutorService pool = Executors.newFixedThreadPool(2);static TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();static ExecutorService pool =  TtlExecutors.getTtlExecutorService(Executors.newFixedThreadPool(3));public static void main(String[] args) {for(int i=0;i<100;i++) {int j = i;pool.execute(new Thread(new Runnable() {@Overridepublic void run() {ThreadPoolInheritableThreadLocalDemo.threadLocal.set("superWorld"+j);ThreadPoolInheritableThreadLocalDemo.pool.execute(new Runnable() {@Overridepublic void run() {System.out.println(Thread.currentThread().getName()  +" : " +ThreadPoolInheritableThreadLocalDemo.threadLocal.get());}});
                }}));}}}

二、TransmittableThreadLocal实现分析

读取线程间传递的ThreadLocal 值比较麻烦，ThreadLocal 和 InheritableThreadLocal 都没有开放内部的 ThreadLocalMap，不能直接读取。

所以要么自己完全实现一套 ThreadLocalMap 机制(如 Netty 的 FastThreadLocal），要么就是自己实现 ThreadLocal 的子类，在每次调用 ThreadLocal

的 set/get/remove 等接口的时候，为 Thread 记录到底绑定了哪些需要发生线程间传递的 ThreadLocal 对象。

/***实际存储值的工作还是父类ThreadLocal完成*TransmittableThreadLocal 只是记录了哪些线程使用了TransmittableThreadLocal对象*/@Overridepublic final void set(T value) {super.set(value);if (null == value) { // may set null to remove value
            removeValue();} else {addValue();}}/***holder 只是为了记录使用了哪些 TransmittableThreadLocal 对象*在构造TtlRunnable/TtlCallable 的时候, 通过holder取得对应的TransmittableThreadLocal　　 *InheritableThreadLocal的默认值是WeakHashMap*/private static InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>> holder =new InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>>() {@Overrideprotected Map<TransmittableThreadLocal<?>, ?> initialValue() {return new WeakHashMap<TransmittableThreadLocal<?>, Object>();}@Overrideprotected Map<TransmittableThreadLocal<?>, ?> childValue(Map<TransmittableThreadLocal<?>, ?> parentValue) {return new WeakHashMap<TransmittableThreadLocal<?>, Object>(parentValue);}};private void addValue() {if (!holder.get().containsKey(this)) {holder.get().put(this, null); // WeakHashMap supports null value.
        }}private void removeValue() {holder.get().remove(this);}

调用ThreadPoolInheritableThreadLocalDemo.threadLocal.set("superWorld"+j)时，

holder.get().containskey(this) 为false

2. TtlRunnable

构造TtlRunable时，设置线程对应的Map<TransmittableThreadLocal<?>, Object>>

    private TtlRunnable(Runnable runnable, boolean releaseTtlValueReferenceAfterRun) {　　　　 //this.copiedRef = new AtomicReference<Map<TransmittableThreadLocal<?>, Object>>(TransmittableThreadLocal.copy());this.runnable = runnable;this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;}

TransmittableThreadLocal.copy

    static Map<TransmittableThreadLocal<?>, Object> copy() {Map<TransmittableThreadLocal<?>, Object> copy = new HashMap<TransmittableThreadLocal<?>, Object>();for (TransmittableThreadLocal<?> threadLocal : holder.get().keySet()) {copy.put(threadLocal, threadLocal.copyValue());}return copy;}

3.运行时，备份和恢复Map<TransmittableThreadLocal<?>, Object>

TtlRunnable#run

    @Overridepublic void run() {Map<TransmittableThreadLocal<?>, Object> copied = copiedRef.get();if (copied == null || releaseTtlValueReferenceAfterRun && !copiedRef.compareAndSet(copied, null)) {throw new IllegalStateException("TTL value reference is released after run!");}Map<TransmittableThreadLocal<?>, Object> backup = TransmittableThreadLocal.backupAndSetToCopied(copied);try {runnable.run();} finally {TransmittableThreadLocal.restoreBackup(backup);}}

参考：

transmittableThreadLocal