Class ScopedValue<T>
- Type Parameters:
T
- the type of the value
ScopedValue
is a preview API of the Java platform.
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
PREVIEW 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());
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
PREVIEW 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
AScopedValue
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());
ScopedValue.where(NAME, "duke2").run(() -> doSomething());
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
AScopedValue
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
TheScopedValue
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());
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 = new StructuredTaskScope<String>()) {
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 aThreadLocal
for cases where the goal is "one-way transmission" of data without using method parameters. While aThreadLocal
can be used to pass data to a method without using method parameters, it does suffer from a number of issues:-
ThreadLocal
does not prevent code in a faraway callee from setting a new value. - A
ThreadLocal
has an unbounded lifetime and thus continues to have a value after a method completes, unless explicitly removed. - 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 ofget()
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 byStructuredTaskScope
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 eachget()
. 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 totrue
, meaning that every virtual thread preserves its scoped-value cache when blocked. LikeScopedValue.cacheSize
, this is a space versus speed trade-off: in situations where many virtual threads are blocked most of the time, setting this property tofalse
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:
- 21
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic interface
ScopedValue.CallableOpPREVIEW<T, X extends Throwable>
Preview.An operation that returns a result and may throw an exception.static final class
Preview.A mapping of scoped values, as keys, to values. -
Method Summary
Modifier and TypeMethodDescriptionget()
Returns the value of the scoped value if bound in the current thread.boolean
isBound()
Returnstrue
if this scoped value is bound in the current thread.static <T> ScopedValuePREVIEW
<T> Creates a scoped value that is initially unbound for all threads.Returns the value of this scoped value if bound in the current thread, otherwise returnsother
.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.CarrierPREVIEW
where
(ScopedValuePREVIEW<T> key, T value) Creates a newCarrier
with a single mapping of aScopedValue
key to a value.
-
Method Details
-
where
Creates a newCarrier
with a single mapping of aScopedValue
key to a value. TheCarrier
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 withk1
bound (or rebound) tov1
, andk2
bound (or rebound) tov2
.ScopedValue.where(k1, v1).where(k2, v2).
run
(() -> ... );- Type Parameters:
T
- the type of the value- Parameters:
key
- theScopedValue
keyvalue
- the value, can benull
- Returns:
- a new
Carrier
with a single mapping
-
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
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()Returnstrue
if this scoped value is bound in the current thread.- Returns:
true
if this scoped value is bound in the current thread
-
orElse
-
orElseThrow
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
-
ScopedValue
when preview features are enabled.