相關文章:
InheritableThreadLocal 的侷限性
在上一篇文章中分析了 ThreadLocal
使用的注意事項,即不適用於出現線程切換的場景。提出了一種解決思路,也分析了 JDK 的解決方案:InheritableThreadLocal
,但是 InheritableThreadLocal
的使用也有很大的限制,因爲它是在 Thread
初始化的時候會保存父線程中的變量,但是實際開發中我們幾乎不會去 new
線程,而是會通過線程池去創建線程,這種時候 InheritableThreadLocal
就無法發揮作用了,先看一個例子:
public class InheritableThreadLocalTest2 {
public static final InheritableThreadLocal<Integer> HOLDER = new InheritableThreadLocal<>();
private static final ExecutorService FIXED_EXECUTOR = Executors.newFixedThreadPool(2);
public static void main(String[] args) throws InterruptedException {
FIXED_EXECUTOR.submit(() -> System.out.println("ready..."));
Thread.sleep(20);
HOLDER.set(1);
print();
Runnable task = InheritableThreadLocalTest2::print;
IntStream.range(0, 5).forEach(i -> {
new Thread(task, "simple task").start();
FIXED_EXECUTOR.submit(task);
});
FIXED_EXECUTOR.shutdown();
}
private static void print() {
System.out.println(currentName() + ":" + HOLDER.get());
}
private static String currentName() {
return Thread.currentThread().getName();
}
}
輸出結果:
ready...
main:1
simple task:1
pool-1-thread-2:1
simple task:1
pool-1-thread-1:null
pool-1-thread-1:null
simple task:1
pool-1-thread-1:null
simple task:1
pool-1-thread-2:1
simple task:1
會發現 new
出來的線程可以獲取 main
線程中的數據,但是線程池中有的線程可以獲取 mian
線程中的數據,有的線程不行,這樣在實際項目中使用會有很大問題。
TransmittableThreadLocal
TransmittableThreadLocal
是 Alibaba 開源的 Java 庫 TTL 中的一個工具 ,可以看下官方的介紹:
JDK
的InheritableThreadLocal
類可以完成父線程到子線程的值傳遞。但對於使用線程池等會池化複用線程的執行組件的情況,線程由線程池創建好,並且線程是池化起來反覆使用的;這時父子線程關係的ThreadLocal
值傳遞已經沒有意義,應用需要的實際上是把 任務提交給線程池時的ThreadLocal
值傳遞到 任務執行時。
其中加粗的部分也不表達了主要設計思想,即“應用需要的實際上是把任務提交給線程池時的ThreadLocal
值傳遞到任務執行時”,與在上一篇文章中提到的一種解決思路很類似。先看看 TransmittableThreadLocal 的效果,將上面的例子中的 InheritableThreadLocal
換成使用 TransmittableThreadLocal
:
public class InheritableThreadLocalTest2 {
//public static final InheritableThreadLocal<Integer> HOLDER = new InheritableThreadLocal<>();
public static final TransmittableThreadLocal<Integer> HOLDER = new TransmittableThreadLocal<>();
private static final ExecutorService FIXED_EXECUTOR = Executors.newFixedThreadPool(2);
public static void main(String[] args) throws InterruptedException {
FIXED_EXECUTOR.submit(() -> System.out.println("ready..."));
Thread.sleep(20);
HOLDER.set(1);
print();
Runnable task = InheritableThreadLocalTest2::print;
// 額外的處理,生成修飾了的對象ttlRunnable
TtlRunnable ttlRunnable = TtlRunnable.get(task);
IntStream.range(0, 5).forEach(i -> {
new Thread(ttlRunnable, "simple task").start();
FIXED_EXECUTOR.submit(ttlRunnable);
});
FIXED_EXECUTOR.shutdown();
}
private static void print() {
System.out.println(currentName() + ":" + HOLDER.get());
}
private static String currentName() {
return Thread.currentThread().getName();
}
}
輸出結果:
ready...
main:1
pool-1-thread-2:1
simple task:1
pool-1-thread-1:1
pool-1-thread-2:1
simple task:1
simple task:1
pool-1-thread-1:1
simple task:1
pool-1-thread-2:1
simple task:1
發現正是我們想要的,子線程可以獲取到父線程到數據。
原理分析
到這裏其實可以大致猜想其實現原理:存儲父線程的數據,包裝 Runable
類,執行任務時子線將父線程的數據設置到子線程到上下文中。
先看如何“存儲父線程的數據”,看這個方法 TransmittableThreadLocal#set
:
@Override
public final void set(T value) {
if (!disableIgnoreNullValueSemantics && null == value) {
// may set null to remove value
remove();
} else {
super.set(value);
addThisToHolder();
}
}
TransmittableThreadLocal
繼承了 InheritableThreadLocal
,也就是說首先會將數據設置到父線程的 inheritableThreadLocals
中,這裏可以先測試一下:
public class TransmittableThreadLocalTest1 {
public static final TransmittableThreadLocal<Integer> TTL_HOLDER = new TransmittableThreadLocal<>();
public static final InheritableThreadLocal<Integer> ITL_HOLDER = new InheritableThreadLocal<>();
public static void main(String[] args) {
TTL_HOLDER.set(1);
System.out.println(ITL_HOLDER.get());
}
}
輸出結果:
null
這裏輸出爲 null
貌似有點奇怪,但是想一下也很正常,因爲本質上數據都是存在 ThreadLocalMap
裏面,我們是要通過 this
去獲取到底存在數組的哪個槽裏面,而這裏 this
是不一樣的,這也是 ThreadLocal
設計的時候就不會出現同一個線程的數據會在多個 ThreadLocal
中錯亂。具體可以 debug
找出答案:
此時會發現 main
線程的 inheritableThreadLocals
是空的。再看:
也就是在 main
線程中設置了 TransmittableThreadLocal
的值後,會發現 InheritableThreadLocal
會爲當前線程的 inheritableThreadLocals
賦值,但是 InheritableThreadLocal#get
是無法獲取數據,是因爲 this
不一樣,所以槽不一樣。
這裏只是一個小插曲,再看 addThisToHolder
方法:
private void addThisToHolder() {
if (!holder.get().containsKey(this)) {
holder.get().put((TransmittableThreadLocal<Object>) this, null); // WeakHashMap supports null value.
}
}
直接將當前 TransmittableThreadLocal
存進去了,holder
如下:
// Note about holder:
// 1. holder self is a InheritableThreadLocal(a *ThreadLocal*).
// 2. The type of value in holder is WeakHashMap<TransmittableThreadLocal<Object>, ?>.
// 2.1 but the WeakHashMap is used as a *Set*:
// - the value of WeakHashMap is *always null,
// - and never be used.
// 2.2 WeakHashMap support *null* value.
private static InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>> holder =
new InheritableThreadLocal<WeakHashMap<TransmittableThreadLocal<Object>, ?>>() {
@Override
protected WeakHashMap<TransmittableThreadLocal<Object>, ?> initialValue() {
return new WeakHashMap<TransmittableThreadLocal<Object>, Object>();
}
//重寫了 childValue 方法,在 Thread#init 的時候會調用,也就是說線程初始化的時候直接將父線程的 inheritableThreadLocals 進行賦值
@Override
protected WeakHashMap<TransmittableThreadLocal<Object>, ?> childValue(WeakHashMap<TransmittableThreadLocal<Object>, ?> parentValue) {
return new WeakHashMap<TransmittableThreadLocal<Object>, Object>(parentValue);
}
};
發現 holder 就是 InheritableThreadLocal
,也就是說子線程是可以獲取到父線程中 holder
的數據的,可以繼承。即當前 TransmittableThreadLocal
類是一箇中心,它的 holder
維護了所有的 TransmittableThreadLocal
。
再看 TtlRunnable#get
方法,猜測這個方法的作用就是包裝 Runable
類,存儲父線程的數據:
@Nullable
public static TtlRunnable get(@Nullable Runnable runnable, boolean releaseTtlValueReferenceAfterRun, boolean idempotent) {
if (null == runnable) return null;
if (runnable instanceof TtlEnhanced) {
// avoid redundant decoration, and ensure idempotency
if (idempotent) return (TtlRunnable) runnable;
else throw new IllegalStateException("Already TtlRunnable!");
}
return new TtlRunnable(runnable, releaseTtlValueReferenceAfterRun);
}
private TtlRunnable(@NonNull Runnable runnable, boolean releaseTtlValueReferenceAfterRun) {
this.capturedRef = new AtomicReference<Object>(capture());
this.runnable = runnable;
this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;
}
能夠看到 TtlRunnable
是對 Runnable
的一個包裝,將數據賦值給了 capturedRef
,接下來看看 capture
方法:
/**
* Capture all {@link TransmittableThreadLocal} and registered {@link ThreadLocal} values in the current thread.
*
* @return the captured {@link TransmittableThreadLocal} values
* @since 2.3.0
*/
@NonNull
public static Object capture() {
return new Snapshot(captureTtlValues(), captureThreadLocalValues());
}
再看 captureTtlValues
方法:
private static WeakHashMap<TransmittableThreadLocal<Object>, Object> captureTtlValues() {
WeakHashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = new WeakHashMap<TransmittableThreadLocal<Object>, Object>();
for (TransmittableThreadLocal<Object> threadLocal : holder.get().keySet()) {
//copyValue 是提供的一個可擴展的方法,即如何存儲值
ttl2Value.put(threadLocal, threadLocal.copyValue());
}
return ttl2Value;
}
這裏說白了就是將 holder
中的所有的數據"copy"過來。
再看 TtlRunnable#run
:
@Override
public void run() {
//獲取捕獲的父線程數據
Object captured = capturedRef.get();
if (captured == null || releaseTtlValueReferenceAfterRun && !capturedRef.compareAndSet(captured, null)) {
throw new IllegalStateException("TTL value reference is released after run!");
}
//設置數據,返回當前線程原本就有的數據
Object backup = replay(captured);
try {
runnable.run();
} finally {
//恢復當前線程原有數據
restore(backup);
}
}
這個方法很清晰,主體流程和之前的猜想差不多。先看 replay
方法:
public static Object replay(@NonNull Object captured) {
final Snapshot capturedSnapshot = (Snapshot) captured;
//重放父線程中的 TransmittableThreadLocal數據(這裏封裝了一個 Snapshot,我們主要關注 ttl2Value)
return new Snapshot(replayTtlValues(capturedSnapshot.ttl2Value), replayThreadLocalValues(capturedSnapshot.threadLocal2Value));
}
@NonNull
private static WeakHashMap<TransmittableThreadLocal<Object>, Object> replayTtlValues(@NonNull WeakHashMap<TransmittableThreadLocal<Object>, Object> captured) {
WeakHashMap<TransmittableThreadLocal<Object>, Object> backup = new WeakHashMap<TransmittableThreadLocal<Object>, Object>();
for (final Iterator<TransmittableThreadLocal<Object>> iterator = holder.get().keySet().iterator(); iterator.hasNext(); ) {
TransmittableThreadLocal<Object> threadLocal = iterator.next();
// backup
backup.put(threadLocal, threadLocal.get());
// clear the TTL values that is not in captured
// avoid the extra TTL values after replay when run task
if (!captured.containsKey(threadLocal)) {
iterator.remove();
threadLocal.superRemove();
}
}
// set TTL values to captured
setTtlValuesTo(captured);
// call beforeExecute callback
doExecuteCallback(true);
return backup;
}
這段代碼就是首先將原來的數據存下來,然後爲當前線程設置值:
private static void setTtlValuesTo(@NonNull WeakHashMap<TransmittableThreadLocal<Object>, Object> ttlValues) {
for (Map.Entry<TransmittableThreadLocal<Object>, Object> entry : ttlValues.entrySet()) {
TransmittableThreadLocal<Object> threadLocal = entry.getKey();
//數據設置到當前線程
threadLocal.set(entry.getValue());
}
}
再看 restore
方法:
public static void restore(@NonNull Object backup) {
final Snapshot backupSnapshot = (Snapshot) backup;
restoreTtlValues(backupSnapshot.ttl2Value);
restoreThreadLocalValues(backupSnapshot.threadLocal2Value);
}
private static void restoreTtlValues(@NonNull WeakHashMap<TransmittableThreadLocal<Object>, Object> backup) {
// call afterExecute callback
doExecuteCallback(false);
for (final Iterator<TransmittableThreadLocal<Object>> iterator = holder.get().keySet().iterator(); iterator.hasNext(); ) {
TransmittableThreadLocal<Object> threadLocal = iterator.next();
// clear the TTL values that is not in backup
// avoid the extra TTL values after restore
if (!backup.containsKey(threadLocal)) {
iterator.remove();
threadLocal.superRemove();
}
}
// restore TTL values
setTtlValuesTo(backup);
}
就是重置之前當前線程的 TransmittableThreadLocal
數據,主要是因爲線程池中線程會複用,避免當前任務執行修改了上下文數據會影響下一次線程的使用。可以簡單看下這個例子:
public class Test4 {
public static final TransmittableThreadLocal<String> TTL_HOLDER = new TransmittableThreadLocal<>();
private static final ExecutorService FIXED_EXECUTOR = Executors.newSingleThreadExecutor();
public static void main(String[] args) {
print();
Runnable task = ()->{
print();
TTL_HOLDER.set(UUID.randomUUID().toString());
print();
};
// 額外的處理,生成修飾了的對象ttlRunnable
TtlRunnable ttlRunnable = TtlRunnable.get(task);
new Thread(()-> FIXED_EXECUTOR.submit(ttlRunnable),"T!").start();
sleep20();
new Thread(()-> FIXED_EXECUTOR.submit(ttlRunnable),"T2").start();
sleep20();
print();
}
private static void sleep20() {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private static void print() {
System.out.println(Thread.currentThread().getName() + ":" + TTL_HOLDER.get());
}
}
輸出結果:
main:null
pool-1-thread-1:null
pool-1-thread-1:b07df1f9-f085-4204-aa0a-1f16b53f6c65
pool-1-thread-1:null
pool-1-thread-1:580ee3aa-0fe9-4f9d-9214-3b5988c4c477
main:null
總得來說,最關鍵的就是提前會將數據存儲在全局的 holder
中,這樣不會因爲線程池線程生成時機問題而造成數據無法傳遞。
References
- https://github.com/alibaba/transmittable-thread-local
- https://mp.weixin.qq.com/s/Y57WCfhAZylXvraD1Eypug
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