ThreadLocal

一、源码

1、属性

private final int threadLocalHashCode = nextHashCode();
获得hashcode
private static AtomicInteger nextHashCode = new AtomicInteger();
定义原子操作的int
private static final int HASH_INCREMENT = 0x61c88647;
为了让哈希码能均匀的分布在2的N次方的数组里，所使用的的魔数
private static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT);
}

2、构造方法

public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
    return new SuppliedThreadLocal<>(supplier);
}
static final class SuppliedThreadLocal extends ThreadLocal {

    private final Supplier<? extends T> supplier;

    SuppliedThreadLocal(Supplier<? extends T> supplier) {
        this.supplier = Objects.requireNonNull(supplier);
    }

    @Override
    protected T initialValue() {
        return supplier.get();
    }
}

lambda表达式初始化

public ThreadLocal() {}
protected T initialValue() {
    return null;
}

用于创建对象时重写初始化方法

3、一般方法

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();
}
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;
}

如果线程的本地map副本不存在就创建，如果存在就返回副本里的内容

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}
设置副本内容
public void remove() {
     ThreadLocalMap m = getMap(Thread.currentThread());
     if (m != null)
         m.remove(this);
}
从副本中移除
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}
获得线程的副本集合
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}
创建副本

static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    return new ThreadLocalMap(parentMap);
}

创建一个包含父副本的map

4、ThreadLocalMap

略