线程数据ThreadLocal

huhxAbout 2 min

如果想在线程中保存些数据，

ThreadLocal的使用

public class ThreadLocalTest {

    public static void main(String[] args) throws InterruptedException {
        var threadLocal = new ThreadLocal<>() {
            @Override
            protected String initialValue() {
                return "huhx";
            }
        };

        System.out.println(Thread.currentThread().getName() + ": " + threadLocal.get());

        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + ": " + threadLocal.get());

            threadLocal.set("linux");

            System.out.println(Thread.currentThread().getName() + ": " + threadLocal.get());
        }).start();

        TimeUnit.SECONDS.sleep(1);

        System.out.println(Thread.currentThread().getName() + ": " + threadLocal.get());
    }
}

java 8提供了withInitial的静态方法来创建和初始化ThreadLocal。所以上述ThreadLocal的创建可以修改为：

var threadLocal = ThreadLocal.withInitial(() -> "huhx");

上述代码输出结果：

main: huhx
Thread-0: huhx
Thread-0: linux
main: huhx

ThreadLocal的分析

第个线程存放的数据会有所差别，如何把线程的数据加以隔离？

ThreadLocal的初始化，ThreadLocal.withInitial，这是从java 8才开始有的

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

static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {

    private final Supplier<? extends T> supplier;

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

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

ThreadLocal的set方法

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;
}

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

在set时，首先获得当前线程对象，然后通过getMap拿到线程的ThreadLocalMap，并将值设入ThreadLocalMap中。而ThreadLocalMap可以理解为一个Map，注意ThreadLocalMap是Thread类的成员变量。ThreadLocalMap的key是ThreadLocal当前对象，value是set方法的参数。

ThreadLocal的get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();
}

get方法先取得当前线程的ThreadLocalMap对象。然后通过将自己作为key取得内部的实际数据。如果找不到ThreadLocalMap，那么设置并返回ThreadLocal的初始值。

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);
    }
    if (this instanceof TerminatingThreadLocal) {
        TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
    }
    return value;
}

TerminatingThreadLocal这个特殊的ThreadLocal，能够在线程终止时收到通知。

线程数据ThreadLocal

ThreadLocal的使用

ThreadLocal的分析

FAQ

为什么Entry要使用弱引用？

总结

参考