一、示例

?線程池內(nèi)的線程并沒有父子關(guān)系，所以不適合InheritableThreadLocal的使用場景

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());}}); }}));}}}

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二、TransmittableThreadLocal實(shí)現(xiàn)分析

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讀取線程間傳遞的ThreadLocal 值比較麻煩，ThreadLocal 和 InheritableThreadLocal 都沒有開放內(nèi)部的 ThreadLocalMap，不能直接讀取。

所以要么自己完全實(shí)現(xiàn)一套 ThreadLocalMap 機(jī)制(如 Netty 的 FastThreadLocal），要么就是自己實(shí)現(xiàn) ThreadLocal 的子類，在每次調(diào)用 ThreadLocal

的 set/get/remove 等接口的時(shí)候，為 Thread 記錄到底綁定了哪些需要發(fā)生線程間傳遞的 ThreadLocal 對象。

/***實(shí)際存儲(chǔ)值的工作還是父類ThreadLocal完成*TransmittableThreadLocal 只是記錄了哪些線程使用了TransmittableThreadLocal對象*/@Overridepublic final void set(T value) {super.set(value);if (null == value) { // may set null to remove value removeValue();} else {addValue();}}/***holder 只是為了記錄使用了哪些 TransmittableThreadLocal 對象*在構(gòu)造TtlRunnable/TtlCallable 的時(shí)候, 通過holder取得對應(yīng)的TransmittableThreadLocal
　　 *InheritableThreadLocal的默認(rèn)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);}

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調(diào)用ThreadPoolInheritableThreadLocalDemo.threadLocal.set("superWorld"+j)時(shí)，?

holder.get().containskey(this) 為false

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2.?TtlRunnable

?構(gòu)造TtlRunable時(shí)，設(shè)置線程對應(yīng)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;}

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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;}

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3.運(yùn)行時(shí)，備份和恢復(fù)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);}}

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參考：

transmittableThreadLocal

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