# Class ScopedValue<T>

java.lang.Object
java.lang.ScopedValue<T>

Type Parameters:
  `T` - the type of the value

public final class ScopedValue<T> extends Object
A value that may be safely and efficiently shared to methods without using method parameters.

In the Java programming language, data is usually passed to a method by means of a method parameter. The data may need to be passed through a sequence of many methods to get to the method that makes use of the data. Every method in the sequence of calls needs to declare the parameter and every method has access to the data. `ScopedValue` provides a means to pass data to a faraway method (typically a callback) without using method parameters. In effect, a `ScopedValue` is an implicit method parameter. It is "as if" every method in a sequence of calls has an additional parameter. None of the methods declare the parameter and only the methods that have access to the `ScopedValue` object can access its value (the data). `ScopedValue` makes it possible to securely pass data from a caller to a faraway callee through a sequence of intermediate methods that do not declare a parameter for the data and have no access to the data.

The `ScopedValue` API works by executing a method with a `ScopedValue` object bound to some value for the bounded period of execution of a method. The method may invoke another method, which in turn may invoke another. The unfolding execution of the methods define a dynamic scope. Code in these methods with access to the `ScopedValue` object may read its value. The `ScopedValue` object reverts to being unbound when the original method completes normally or with an exception. The `ScopedValue` API supports executing a `Runnable`, or `ScopedValue.CallableOp` with a `ScopedValue` bound to a value.

Consider the following example with a scoped value "`NAME`" bound to the value "`duke`" for the execution of a `Runnable`'s `run` method. The `run` method, in turn, invokes a method `doSomething`.

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

        ScopedValue.where(NAME, "duke").run(() -> doSomething());

Code executed directly or indirectly by `doSomething`, with access to the field `NAME`, can invoke `NAME.get()` to read the value "`duke`". `NAME` is bound while executing the `run` method. It reverts to being unbound when the `run` method completes.

The example using `run` invokes a method that does not return a result. The `call` method can be used to invoke a method that returns a result. `ScopedValue` defines the `where(ScopedValue, Object)` method for cases where multiple mappings (of `ScopedValue` to value) are accumulated in advance of calling a method with all `ScopedValue`s bound to their value.

## Bindings are per-thread

A `ScopedValue` binding to a value is per-thread. Invoking `run` executes a method with a `ScopedValue` bound to a value for the current thread. The `get` method returns the value bound for the current thread.

In the example, if code executed by one thread invokes this:

        ScopedValue.where(NAME, "duke1").run(() -> doSomething());

and code executed by another thread invokes:

        ScopedValue.where(NAME, "duke2").run(() -> doSomething());

then code in `doSomething` (or any method that it calls) invoking `NAME.get()` will read the value "`duke1`" or "`duke2`", depending on which thread is executing.

## Scoped values as capabilities

A `ScopedValue` object should be treated as a capability or a key to access its value when the `ScopedValue` is bound. Secure usage depends on access control (see The Java Virtual Machine Specification, Section 5.4.4) and taking care to not share the `ScopedValue` object. In many cases, a `ScopedValue` will be declared in a `final` and `static` field so that it is only accessible to code in a single class (or nest).

## Rebinding

The `ScopedValue` API allows a new binding to be established for nested dynamic scopes. This is known as rebinding. A `ScopedValue` that is bound to a value may be bound to a new value for the bounded execution of a new method. The unfolding execution of code executed by that method defines the nested dynamic scope. When the method completes, the value of the `ScopedValue` reverts to its previous value.

In the above example, suppose that code executed by `doSomething` binds `NAME` to a new value with:

        ScopedValue.where(NAME, "duchess").run(() -> doMore());

Code executed directly or indirectly by `doMore()` that invokes `NAME.get()` will read the value "`duchess`". When `doMore()` completes then the value of `NAME` reverts to "`duke`".

## Inheritance

`ScopedValue` supports sharing across threads. This sharing is limited to structured cases where child threads are started and terminate within the bounded period of execution by a parent thread. When using a `StructuredTaskScope`PREVIEW, scoped value bindings are captured when creating a `StructuredTaskScope` and inherited by all threads started in that task scope with the `fork`PREVIEW method.

A `ScopedValue` that is shared across threads requires that the value be an immutable object or for all access to the value to be appropriately synchronized.

In the following example, the `ScopedValue` `NAME` is bound to the value "`duke`" for the execution of a runnable operation. The code in the `run` method creates a `StructuredTaskScope` that forks three tasks. Code executed directly or indirectly by these threads running `childTask1()`, `childTask2()`, and `childTask3()` that invokes `NAME.get()` will read the value "`duke`".

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

        ScopedValue.where(NAME, "duke").run(() -> {
            try (var scope = StructuredTaskScope.open()) {

                 scope.fork(() -> childTask1());
                 scope.fork(() -> childTask2());
                 scope.fork(() -> childTask3());

                 scope.join();

                 ..
             }
        });

Unless otherwise specified, passing a `null` argument to a method in this class will cause a `NullPointerException` to be thrown.

API Note:
  A `ScopedValue` should be preferred over a `ThreadLocal` for cases where the goal is "one-way transmission" of data without using method parameters. While a `ThreadLocal` can be used to pass data to a method without using method parameters, it does suffer from a number of issues:
  1. `ThreadLocal` does not prevent code in a faraway callee from setting a new value.
  2. A `ThreadLocal` has an unbounded lifetime and thus continues to have a value after a method completes, unless explicitly removed.
  3. Inheritance is expensive - the map of thread-locals to values must be copied when creating each child thread.
Implementation Note:
  Scoped values are designed to be used in fairly small numbers. `get()` initially performs a search through enclosing scopes to find a scoped value's innermost binding. It then caches the result of the search in a small thread-local cache. Subsequent invocations of `get()` for that scoped value will almost always be very fast. However, if a program has many scoped values that it uses cyclically, the cache hit rate will be low and performance will be poor. This design allows scoped-value inheritance by `StructuredTaskScope`PREVIEW threads to be very fast: in essence, no more than copying a pointer, and leaving a scoped-value binding also requires little more than updating a pointer.

  Because the scoped-value per-thread cache is small, clients should minimize the number of bound scoped values in use. For example, if it is necessary to pass a number of values in this way, it makes sense to create a record class to hold those values, and then bind a single `ScopedValue` to an instance of that record.

  For this release, the reference implementation provides some system properties to tune the performance of scoped values.

  The system property `java.lang.ScopedValue.cacheSize` controls the size of the (per-thread) scoped-value cache. This cache is crucial for the performance of scoped values. If it is too small, the runtime library will repeatedly need to scan for each `get()`. If it is too large, memory will be unnecessarily consumed. The default scoped-value cache size is 16 entries. It may be varied from 2 to 16 entries in size. `ScopedValue.cacheSize` must be an integer power of 2.

  For example, you could use `-Djava.lang.ScopedValue.cacheSize=8`.

  The other system property is `jdk.preserveScopedValueCache`. This property determines whether the per-thread scoped-value cache is preserved when a virtual thread is blocked. By default this property is set to `true`, meaning that every virtual thread preserves its scoped-value cache when blocked. Like `ScopedValue.cacheSize`, this is a space versus speed trade-off: in situations where many virtual threads are blocked most of the time, setting this property to `false` might result in a useful memory saving, but each virtual thread's scoped-value cache would have to be regenerated after a blocking operation.

Since:
  25

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static interface`
  `ScopedValue.CallableOp<T, X extends Throwable>`
  An operation that returns a result and may throw an exception.
  `static final class`
  `ScopedValue.Carrier`
  A mapping of scoped values, as keys, to values.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `T`
  `get()`
  Returns the value of the scoped value if bound in the current thread.
  `boolean`
  `isBound()`
  Returns `true` if this scoped value is bound in the current thread.
  `static <T> ScopedValue<T>`
  `newInstance()`
  Creates a scoped value that is initially unbound for all threads.
  `T`
  `orElse(T other)`
  Returns the value of this scoped value if bound in the current thread, otherwise returns `other`.
  `<X extends Throwable>
  T`
  `orElseThrow(Supplier<? extends X> exceptionSupplier)`
  Returns the value of this scoped value if bound in the current thread, otherwise throws an exception produced by the exception supplying function.
  `static <T> ScopedValue.Carrier`
  `where(ScopedValue<T> key, T value)`
  Creates a new `Carrier` with a single mapping of a `ScopedValue` key to a value.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Method Details

  - ### where

    public static <T> ScopedValue.Carrier where(ScopedValue<T> key, T value)
    Creates a new `Carrier` with a single mapping of a `ScopedValue` key to a value. The `Carrier` can be used to accumulate mappings so that an operation can be executed with all scoped values in the mapping bound to values. The following example runs an operation with `k1` bound (or rebound) to `v1`, and `k2` bound (or rebound) to `v2`.

            ScopedValue.where(k1, v1).where(k2, v2).run(() -> ... );

    Type Parameters:
      `T` - the type of the value
    Parameters:
      `key` - the `ScopedValue` key
      `value` - the value, can be `null`
    Returns:
      a new `Carrier` with a single mapping

  - ### newInstance

    public static <T> ScopedValue<T> newInstance()
    Creates a scoped value that is initially unbound for all threads.
    Type Parameters:
      `T` - the type of the value
    Returns:
      a new `ScopedValue`

  - ### get

    public T get()
    Returns the value of the scoped value if bound in the current thread.
    Returns:
      the value of the scoped value if bound in the current thread
    Throws:
      `NoSuchElementException` - if the scoped value is not bound

  - ### isBound

    public boolean isBound()
    Returns `true` if this scoped value is bound in the current thread.
    Returns:
      `true` if this scoped value is bound in the current thread

  - ### orElse

    public T orElse(T other)
    Returns the value of this scoped value if bound in the current thread, otherwise returns `other`.
    Parameters:
      `other` - the value to return if not bound
    Returns:
      the value of the scoped value if bound, otherwise `other`

  - ### orElseThrow

    public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X
    Returns the value of this scoped value if bound in the current thread, otherwise throws an exception produced by the exception supplying function.
    Type Parameters:
      `X` - the type of the exception that may be thrown
    Parameters:
      `exceptionSupplier` - the supplying function that produces the exception to throw
    Returns:
      the value of the scoped value if bound in the current thread
    Throws:
      `X` - if the scoped value is not bound in the current thread