ThreadlLocal詳解

1嘉抓、ThreadlLocal

簡介：ThreadLocal提供線程的局部變量填抬，每個線程都可以通過get()和set()對局部變量進行操作而不會對其他線程的局部變量產生影響佛玄，實現(xiàn)了線程之間的數(shù)據(jù)隔離婚夫。e.g. 數(shù)據(jù)庫的連接池管理新翎。

原理：

每個Thread維護著一個ThreadLocalMap的引用
ThreadLocalMap是ThreadLocal的內部類疗疟，用Entry來進行存儲
調用ThreadLocal的set()方法時该默，實際上就是往ThreadLocalMap設置值，key是ThreadLocal對象策彤，值是傳遞進來的對象
調用ThreadLocal的get()方法時栓袖，實際上就是往ThreadLocalMap獲取值匣摘，key是ThreadLocal對象
ThreadLocal本身并不存儲值，它只是作為一個key來讓線程從ThreadLocalMap獲取value裹刮。

public void set(T value) {
        // 得到當前線程對象
        Thread t = Thread.currentThread();
        // 這里獲取ThreadLocalMap
        ThreadLocalMap map = getMap(t);
        // 如果map存在音榜，則將當前線程對象t作為key，要存儲的對象作為value存到map里面去
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
}

static class ThreadLocalMap {
        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
        //....很長
}

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
}

ThreadLocal.ThreadLocalMap threadLocals = null

不足：無法完成父子線程之間的數(shù)據(jù)傳遞捧弃。

2赠叼、InheritableThreadLocal

簡介：ThreadLocal的升級版，jdk原生违霞，實現(xiàn)了父子線程之間的數(shù)據(jù)傳遞嘴办。

原理：重新開辟一塊空間（InheritableThreadLocal）用于存儲父子線程共用的數(shù)據(jù)隔離空間。重寫了ThreadLocal的三個方法买鸽，當創(chuàng)建新線程的時候涧郊，將父線程的ThreadLocal值拷貝到子線程。

/** 
* 該函數(shù)在父線程創(chuàng)建子線程眼五，向子線程復制InheritableThreadLocal變量時使用 
*/
protected T childValue(T parentValue) { 
  return parentValue; 
}
/** 
* 由于重寫了getMap底燎，操作InheritableThreadLocal時， 
* 將只影響Thread類中的inheritableThreadLocals變量弹砚， 
* 與threadLocals變量不再有關系 
*/ 
ThreadLocalMap getMap(Thread t) {
  return t.inheritableThreadLocals; 
}
/** 
* 類似于getMap双仍，操作InheritableThreadLocal時， 
* 將只影響Thread類中的inheritableThreadLocals變量桌吃， 
* 與threadLocals變量不再有關系 
*/ 
void createMap(Thread t, T firstValue) {
  t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue); 
}

public class Thread implements Runnable {
   ......(其他源碼)
    /* 
     * 當前線程的ThreadLocalMap朱沃，主要存儲該線程自身的ThreadLocal
     */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal，自父線程集成而來的ThreadLocalMap茅诱，
     * 主要用于父子線程間ThreadLocal變量的傳遞
     * 本文主要討論的就是這個ThreadLocalMap
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
    ......(其他源碼)
}
/**
    * 初始化一個線程.
    * 此函數(shù)有兩處調用逗物，
    * 1、上面的 init()瑟俭，不傳AccessControlContext翎卓，inheritThreadLocals=true
    * 2、傳遞AccessControlContext摆寄，inheritThreadLocals=false
    */
private void init(ThreadGroup g, Runnable target, String name,
                     long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        ......（其他代碼）

        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

        ......（其他代碼）
}

不足：無法完成線程池間的數(shù)據(jù)傳遞失暴。

3、Transmittable-Thread-Local

InheritableThreadLocal value 串位問題的根本原因在于它依賴 Thread 類本身的機制傳遞 value, 而 Thread 類由于其于線程池內 “復用存在” 的形式而導致 InheritableThreadLocal 的機制失效; 所以針對 InheritableThreadLocal 的改進, 突破點就在于如何擺脫對 Thread 類的依賴微饥。

簡介：阿里開源組件逗扒，繼承自InheritableThreadLocal，實現(xiàn)了父子線程在線程池中的數(shù)據(jù)傳遞欠橘。

原理：在提交任務給線程池時矩肩，將ThreadLocal數(shù)據(jù)一起提交，相當于重新set一次ThreadLocal肃续，「把任務提交給線程池時的ThreadLocal值傳遞到任務執(zhí)行時」黍檩。

與InheritableThreadLocal類似叉袍，單獨搞了一塊空間，維護一個全局的靜態(tài)變量holder刽酱，存儲所有TransmittableThreadLocal實例

static ThreadLocal<Map<TransmittableThreadLocal<?>, ?>> holder = new ThreadLocal<Map<TransmittableThreadLocal<?>, ?>>() {
    @Override
    protected Map<TransmittableThreadLocal<?>, ?> initialValue() {
        return new WeakHashMap<TransmittableThreadLocal<?>, Object>();
    }
};

提供一個 copy() 方法實時復制所有 TransmittableThreadLocal 實例及其在當前線程的 value

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;
}

封裝了原始的 Runnable 和 Callable畦韭，將當前線程下所有的 TransmittableThreadLocal 實例及其對應的 value, 放到了一個 AtomicReference 包裝的 map 之中, 這樣就完成了由父線程向 Runnable的value 傳遞

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

關鍵的 run() 方法的處理

public void run() {
    Map<TransmittableThreadLocal<?>, Object> copied = copiedRef.get();
    // 非核心邏輯已省略
    ......
    Map<TransmittableThreadLocal<?>, Object> backup = TransmittableThreadLocal.backupAndSetToCopied(copied);
    try {
        runnable.run();
    } finally {
        TransmittableThreadLocal.restoreBackup(backup);
    }   
}

不足：對hystrix修飾的線程池無效，因為hytrix做了線程隔離肛跌。

參考資料：

https://github.com/alibaba/transmittable-thread-local
https://fredal.xin/async-threadlocal-in-all-link-tracking
https://cloud.tencent.com/developer/article/1162226*
http://cxytiandi.com/blog/detail/18782
https://juejin.im/post/5ac2eb52518825555e5e06ee
https://zshell.cc/2018/08/03/jdk--ThreadLocal%E7%9B%B8%E5%85%B3%E7%9F%A5%E8%AF%86%E5%85%A8%E6%A2%B3%E7%90%86/