JEP!!! 506: Scoped Values

JEP 506: Scoped Valuesopenjdk.org

TL;DR: ThreadLocal for a call scope instead of a whole thread. A value is bound once for a block of code and is visible to all methods executed within that call chain — even in completely sequential code.

General web example :

class Framework {

    private static final ScopedValue<FrameworkContext> CONTEXT
                        = ScopedValue.newInstance();    // (1)

    void serve(Request request, Response response) {
        var context = createContext(request);
        where(CONTEXT, context)                         // (2)
                   .run(() -> Application.handle(request, response));
    }
    
    public PersistedObject readKey(String key) {
        var context = CONTEXT.get();                    // (3)
        var db = getDBConnection(context);
        db.readKey(key);
    }

}

Example 1 - basic use

com.wlodar.jeeps.jep506ScopedValues.Demo1WithScopedValue

Shows how scoped values can help with passing context through call chains

Example 2 - compare to thread local

com.wlodar.jeeps.jep506ScopedValues.Demo2ScopedValueVsThreadLocal

ThreadLocal allows mutation of ambient context, while ScopedValue allows only temporary rebinding in a nested scope.

For long lived threads - if value from thread local is not removed it may lead to memory leaks.

Thread
  └ ThreadLocalMap
       ├ key = null   (GC cleared weak ref)
       └ value = BIG_OBJECT

So something like the following is needed :

context.set(user);
try {
    handleRequest();
} finally {
    context.remove();
}

scoped value is always cleaned when scope ends. ThreadLocal attaches data to the lifetime of a thread. ScopedValue attaches data to the lifetime of a call.

And if youa re using thread pool and forget to cleanup ThreadLocal then data may leak between requests - security,security,security!

Example 3 - understand scope

com.wlodar.jeeps.jep506ScopedValues.Demo3ScopedLifetime

Example show how scope works.

some examples for scope related method use :

//logging library/util/MDC which can be used both in and outside scope
static void log(String message) {
    if (USER.isBound()) {
        WorkshopPrinter.print("user", USER.get());
    } else {
        WorkshopPrinter.print("user", "anonymous");
    }

    WorkshopPrinter.print("log", message);
}

or 
WorkshopPrinter.print("user", USER.orElse("anonymous"))

Example 4 - nested scopes

com.wlodar.jeeps.jep506ScopedValues.Demo4NestedScopedValues

Example 5 - structured concurrency preview

com.wlodar.jeeps.jep506ScopedValues.preview.Demo6ScopedValueStructuredConcurrency

Thanks to immutable nature all vierual threads which by design are lightweight and may be many of them - don't need dedicated ThreadLocal instance which would allocate memory