ThreadLocal##

关于创建线程局部变量的一个类
在Thread中有这么一个成员变量

/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocal 属于当前这个线程，这个map由ThreadLocal维护

demo

ThreadLocal

输出：

sub thread2   null
sub thread1   null

main   main
sub thread1   sub thread1
sub thread2   sub thread2

一个三个线程，main ，sub thread1 和 sub thread2。我们只是在main线程中创建一个ThreadLocal实例，并在main线程中调用set方法将线程名称保存起来，但是在后续的其他两个线程并没有能够拿到main线程set的值

普通变量

输出

xian yu lao
xian yu zi 
xian yu x

对于普通的变量，任何线程都能访问修改它的内容(final 对象跟修改内容无关)

set()

代码

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}
//----------------
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;//
}
//----------------创建ThreadLocalMap
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}
///set
 private void set(ThreadLocal<?> key, Object value) {

        // We don't use a fast path as with get() because it is at
        // least as common to use set() to create new entries as
        // it is to replace existing ones, in which case, a fast
        // path would fail more often than not.

        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);

        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            ThreadLocal<?> k = e.get();

            if (k == key) {
                e.value = value;
                return;
            }

            if (k == null) {
                replaceStaleEntry(key, value, i);
                return;
            }
        }

        tab[i] = new Entry(key, value);
        int sz = ++size;
        if (!cleanSomeSlots(i, sz) && sz >= threshold)
            rehash();
    }

ThreadLocalMap

ThreadLocalMap构造方法中创建了一个容量为16的数组，然后根据key的hash值和容量大小-1(15,二进制位1111)相与得到数组的下标，然后创建了一个Entry对象将ThreadLocal实例和vaule值保存起来。

get()

 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();
}

是一个相反的过程，只是当你这个线程的threadLocals为null的时候，会调用setInitialValue方法

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;
}

可以重写这个方法决定当我们没有在调用set方法之前调用get方法返回什么样的值

调用ThreadLocal的实例的get/set方法，首先去当前线程中找到threadLocals实例，这个实例中有一个Entry数组，Entry类中就保存着对应的value值。所以线程之间不能相互的修改访问其他线程的threadLocal设置的值。

InheritableThreadLocal

在Thread中还有一个成员变量

/*
 * InheritableThreadLocal values pertaining to this thread. This map is
 * maintained by the InheritableThreadLocal class.
 */
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

InheritableThreadLocal<T>是ThreadLocal的子类，不同点就是子线程在没有set的时候通过get的方法可以访问到父线程set的值

InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal();
    threadLocal.set(Thread.currentThread().getName());
    
    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println(threadLocal.get());
            threadLocal.set("sub thread1");
            System.out.println(threadLocal.get());
        }
    });

    thread.setName("sub thread1");
    thread.start();
    
    try {
        Thread.currentThread().sleep(2000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(threadLocal.get());
//输出就是
main
sub thread1
main