ThreadLocal如何在子线程传递

主线程如何向自己创建的子线程传递ThreadLocal值，只需要创建InheritableThreadLocal值,实现原理如下：

InheritableThreadLocal覆写了它三个方法:

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    
    //创建线程时候，会调用该方法,见下面init
    protected T childValue(T parentValue) {
        return parentValue;
    }

    //获取Map时候，返回的是t.inheritableThreadLocals，见ThreadLocal操作值
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }

    //createMap时候，返回的是t.inheritableThreadLocals，见ThreadLocal操作值
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

见子线程初始化过程,初始化时候如果当前线程inheritableThreadLocals不为空

private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize, AccessControlContext acc,
                  boolean inheritThreadLocals) {
    //......
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        //这里初始的是this.inheritableThreadLocals
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    //......
}

// ThreadLocal.createInheritedMap会调用ThreadLocalMap(ThreadLocalMap parentMap)方法，
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) {
                //这里调用InheritableThreadLocal覆写的childValue，返回的是parent的value，将parent的值copy到子线程中
                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++;
            }
        }
    }
}

那么inheritableThreadLocals它什么时候不为空呢，见下面：

//在set值时候判断ThreadLocalMap为空，调用InheritableThreadLocal.getMap方法返回当前线程的t.inheritableThreadLocals，否则初始化t.inheritableThreadLocals
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

//获取的是t.inheritableThreadLocals的值
public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

//删除值实际上删除的t.inheritableThreadLocals
public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null)
        m.remove(this);
}