ScopedValues Sınıfı - ThreadLocal Gibidir

Giriş

Açıklaması şöyle

Introducing Scoped Values, which enable sharing immutable data within and between threads. They are preferred over thread-local variables, especially when using a large number of virtual threads.

Project Loom ile gelen Virtual Thread'lerden önce ThreadLocal kullanılıyordu. Örneğin şöyle yaparız

class Handler {
  final static ThreadLocal<String> userInfo = new ThreadLocal<>();
  ...
}

userInfo.set("user-name");
....
Handler.userInfo.get();

ThreadLocal bazı aleyhte durumlara sahip

1. Mutability

ThreadLocal#get() ve ThreadLocal#set() ile değiştirilebilir

2. Unbounded Lifetime

ThreadLocal nesnesi, thread yaşadığı müddetçe hayattadır. Eğer erken silmek istersek ThreadLocal#remove() çağrısı yapılır

3. Overhead in Virtual Threads

Açıklaması şöyle. Yani Project Loom ile gelen Virtual Thread'lerde ThreadLocal kullanmak mümkün ama yapmasak daha iyi. Bunun yerine artık ScopedValue geldi.

Virtual threads are still Threads, though. That means they can use ThreadLocal<T>, but now, the previously mentioned downsides affect your program at another scale. If you think the overhead or lifetime issues and coordination of a few ThreadLocal<T> are problematic when handling a handful of threads, you better don’t think about these downsides when handling 10.000+ virtual threads!

newInstance metodu

Örnek

Şöyle yaparız

private static final ThreadLocal<Principal> PRINCIPAL_TL = new ThreadLocal();

// - VS -
private static final ScopedValue<Principal> PRINCIPAL_SV = ScopedValue.newInstance();

where metodu

İmzası şöyle

<T> ScopedValue.Carrier where(ScopedValue<T> key, T value)

ScopedValue.Carrier nesnesin Runnable veya Callable takılabiliyor. İmzası şöyle

void run(Runnable op)
<R> R call(Callable<? extends R> op)

Örnek

Şöyle yaparız

private static final ScopedValue<String> USER = ScopedValue.newInstance();

ScopedValue.Carrier carrier = ScopedValue.where(USER, "ben");

var boundUser = carrier.get(USER);
// => "ben"

carrier.run(() -> {
    var currentUser = USER.get();
    // => "ben"
});

Örnek

Şöyle yaparız

private static final ScopedValue<String> USER = ScopedValue.newInstance();

// CREATE TWO CARRIERS WITH DIFFERENT VALUES IN THE SAME THREAD

ScopedValue.Carrier carrierForBen = ScopedValue.where(USER, "ben");
ScopedValue.Carrier carrierForBob = ScopedValue.where(USER, "bob");

// USE THE SAME RUNNABLE FOR EACH CARRIER

Runnable r = () -> {
    var currentUser = USER.get();
    System.out.println("Current: " + currentUser);
};

// RUN RUNNABLE WITH DIFFERENT CARRIERS

carrierForBen.run(r);
// => "Current: ben"

carrierForBob.run(r);
// => "Current: bob"

carrierForBen.run(r);
// => "Current: ben"