Working with Scoped Values JAVA 21+ PREVIEW

1. Understanding Scoped Values

Aspect	Detail
Purpose	Pass immutable data implicitly to inner methods
Replaces	ThreadLocal in many cases
Lifetime	Bounded by enclosing scope (lexical)

Form	Detail
Declaration	`static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();`
Usually static final	Like a typed key

ScopedValue.where(CURRENT_USER, user)
    .where(REQUEST_ID, id)
    .run(() -> processRequest());

Method	Detail
`sv.get()`	Returns bound value or throws
`sv.isBound()`	Check before reading
`sv.orElse(default)`	Default if unbound

Rule	Detail
Immutable	Cannot reassign within scope
Lexical	Available only inside `run/call`
Inheritance	Forked vthreads in same scope inherit

Aspect	ScopedValue	ThreadLocal
Mutability	Immutable	Mutable
Lifetime	Bounded	Until explicit remove
Memory	Per-scope, light	Per-thread, heavy with vthreads
Inheritance	Automatic via structured concurrency	Via InheritableThreadLocal

Aspect	Detail
Memory efficient	Critical with millions of vthreads
No leaks	Auto cleared after scope ends

Aspect	Detail
Auto-inherited	Subtasks forked in scope see all bindings
Override	Re-bind in child scope

Practice	Reason
Use for context propagation	Tracing IDs, security principals, locales
Don't store mutable objects	Defeats purpose
Prefer over ThreadLocal	Especially with virtual threads