1. ThreadLocal 类

1.1 get()

public T get() {
    // 当前线程
    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();
}

1.2 getMap(Thread)

获取当前线程的 threadLocals 变量 (每个线程都有一个自己的ThreadLocalMap)

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

取自 Thread 类的 ThreadLocal.ThreadLocalMap threadLocals = null;

public class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
}

Thread 有一个 ThreadLocalMap 类型的成员变量 threadLocals, 该变量用来存储变量副本，存储以 ThreadLocal 为键，本地线程变量为值的键值对

1.3 set(T value)

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

1.4 createMap(Thread, T)

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

1.5 initialValue()

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

protected T initialValue() {
    return null;
}

1.6 remove()

public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null)
        m.remove(this);
}

2. ThreadLocalMap 类

public class ThreadLocal<T> {
    
    static class ThreadLocalMap {
        
        static class Entry extends WeakReference<ThreadLocal<?>> { }
    }
}

ThreadLocal 里有静态内部类ThreadLocalMap
ThreadLocalMap 里有静态内部类 Entry 用来存放数据(键值对)，且继承弱引用 WeakReference

2.1 Entry 类

static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;

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

ThreadLocal<?> 弱引用作为 key

2.2 set(ThreadLocal<?> key, Object value)

private void set(ThreadLocal<?> key, Object value) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);

    //...
    tab[i] = new Entry(key, value);
    // ...
}