Module jdk.incubator.concurrent
Package jdk.incubator.concurrent

Class ScopedValue<T>

java.lang.Object
jdk.incubator.concurrent.ScopedValue<T>
Type Parameters:
T - the type of the object bound to this ScopedValue
public final class ScopedValue<T> extends Object
A value that is set once and is then available for reading for a bounded period of execution by a thread. A ScopedValue allows for safely and efficiently sharing data for a bounded period of execution without passing the data as method arguments.

ScopedValue defines the where(ScopedValue, Object, Runnable) method to set the value of a ScopedValue for the bouned period of execution by a thread of the runnable's run method. The unfolding execution of the methods executed by run defines a dynamic scope. The scoped value is bound while executing in the dynamic scope, it reverts to being unbound when the run method completes (normally or with an exception). Code executing in the dynamic scope uses the ScopedValue get method to read its value.

Like a thread-local variable, a scoped value has multiple incarnations, one per thread. The particular incarnation that is used depends on which thread calls its methods.

Consider the following example with a scoped value USERNAME that is bound to the value "duke" for the execution, by a thread, of a run method that invokes doSomething(). 

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

    ScopedValue.where(USERNAME, "duke", () -> doSomething());
Code executed directly or indirectly by doSomething() that invokes USERNAME.get() will read the value "duke". The scoped value is bound while executing doSomething() and becomes unbound when doSomething() completes (normally or with an exception). If one thread were to call doSomething() with USERNAME bound to "duke1", and another thread were to call the method with USERNAME bound to "duke2", then USERNAME.get() would read the value "duke1" or "duke2", depending on which thread is executing.

In addition to the where method that executes a run method, ScopedValue defines the where(ScopedValue, Object, Callable) method to execute a method that returns a result. It also defines the where(ScopedValue, Object) method for cases where it is useful to accumulate mappings of ScopedValue to value.

A ScopedValue will typically be declared in a final and static field. The accessibility of the field will determine which components can bind or read its value.

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

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 some value may be bound to a new value for the bounded execution of some method. The unfolding execution of code executed by that method defines the nested dynamic scope. When the method completes (normally or with an exception), the value of the ScopedValue reverts to its previous value.

In the above example, suppose that code executed by doSomething() binds USERNAME to a new value with: 

    ScopedValue.where(USERNAME, "duchess", () -> doMore());
Code executed directly or indirectly by doMore() that invokes USERNAME.get() will read the value "duchess". When doMore() completes (normally or with an exception), the value of USERNAME reverts to "duke".

Inheritance

ScopedValue supports sharing data 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. More specifically, when using a StructuredTaskScope, scoped value bindings are captured when creating a StructuredTaskScope and inherited by all threads started in that scope with the fork method.

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

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

    ScopedValue.where(USERNAME, "duke", () -> {
        try (var scope = new StructuredTaskScope<String>()) {

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

            ...
         }
    });
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 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 incubator release, the reference implementation provides some system properties to tune the performance of scoped values.

The system property jdk.incubator.concurrent.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 -Djdk.incubator.concurrent.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:
20

  • Nested Class Summary

    Nested Classes
    Modifier and Type
    Class
    Description
    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.
    static <T> void
    where(ScopedValue<T> key, T value, Runnable op)
    Run an operation with a ScopedValue bound to a value in the current thread.
    static <T, R> R
    where(ScopedValue<T> key, T value, Callable<? extends R> op)
    Calls a value-returning operation with a ScopedValue bound to a value in the current thread.

    Methods declared in class java.lang.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 accumlate 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

    • where

      public static <T, R> R where(ScopedValue<T> key, T value, Callable<? extends R> op) throws Exception
      Calls a value-returning operation with a ScopedValue bound to a value in the current thread. When the operation completes (normally or with an exception), the ScopedValue will revert to being unbound, or revert to its previous value when previously bound, in the current thread.

      Scoped values are intended to be used in a structured manner. If op creates a StructuredTaskScope but does not close it, then exiting op causes the underlying construct of each StructuredTaskScope created in the dynamic scope to be closed. This may require blocking until all child threads have completed their sub-tasks. The closing is done in the reverse order that they were created. Once closed, StructureViolationException is thrown.

      Implementation Note:
      This method is implemented to be equivalent to: 
          ScopedValue.where(key, value).call(op);
      Type Parameters:
      T - the type of the value
      R - the result type
      Parameters:
      key - the ScopedValue key
      value - the value, can be null
      op - the operation to call
      Returns:
      the result
      Throws:
      Exception - if the operation completes with an exception

    • where

      public static <T> void where(ScopedValue<T> key, T value, Runnable op)
      Run an operation with a ScopedValue bound to a value in the current thread. When the operation completes (normally or with an exception), the ScopedValue will revert to being unbound, or revert to its previous value when previously bound, in the current thread.

      Scoped values are intended to be used in a structured manner. If op creates a StructuredTaskScope but does not close it, then exiting op causes the underlying construct of each StructuredTaskScope created in the dynamic scope to be closed. This may require blocking until all child threads have completed their sub-tasks. The closing is done in the reverse order that they were created. Once closed, StructureViolationException is thrown.

      Implementation Note:
      This method is implemented to be equivalent to: 
          ScopedValue.where(key, value).run(op);
      Type Parameters:
      T - the type of the value
      Parameters:
      key - the ScopedValue key
      value - the value, can be null
      op - the operation to call

    • 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, can be null
      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