/**
 * 測試線程池下InheritableThreadLocal線程變量失效的場景
 */
public class TestInheritableThreadLocal {
    private static final InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
    // 固定大小的線程池，保證線程復用
    private static final ExecutorService executorService = Executors.newFixedThreadPool(1);
    public static void main(String[] args) {
        threadLocal.set("main線程 變量1");
        // 正常取到 main線程 變量1
        executorService.execute(() -> System.out.println(threadLocal.get()));
        threadLocal.set("main線程 變量2");
        // 線程復用再取還是 main線程 變量1
        executorService.execute(() -> System.out.println(threadLocal.get()));
    }
}

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    /**
     * 當子線程被創(chuàng)建時，通過該方法來初始化子線程中線程變量的值，
     * 這個方法在父線程中被調(diào)用，并且在子線程開啟之前。
     * 
     * 通過重寫這個方法可以改變從父線程中繼承過來的值。
     *
     * @param parentValue the parent thread's value
     * @return the child thread's initial value
     */
    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);
    }
}

    public Thread(ThreadGroup group, String name) {
        init(group, null, name, 0);
    }

/**
 * @param inheritThreadLocals 為ture，代表是為 包含可繼承的線程變量 的線程進行初始化
 */
private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize, AccessControlContext acc,
                  boolean inheritThreadLocals) {
    ...
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        // 注意這里創(chuàng)建子線程的線程變量
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    ...
}

    private ThreadLocalMap(ThreadLocalMap parentMap) {
        Entry[] parentTable = parentMap.table;
        int len = parentTable.length;
        setThreshold(len);
        table = new Entry[len];
        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) {
                    // 注意?。?！ 這里調(diào)用了childValue方法
                    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++;
                }
            }
        }
    }