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Class ClassValue<T>

java.lang.Object

java.lang.ClassValue<T>

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

public abstract class ClassValue<T> extends Object

Lazily associate a computed value with any Class object. For example, if a dynamic language needs to construct a message dispatch table for each class encountered at a message send call site, it can use a ClassValue to cache information needed to perform the message send quickly, for each class encountered.

The basic operation of a ClassValue is get, which returns the associated value, initially created by an invocation to computeValue; multiple invocations may happen under race, but exactly one value is associated to a Class and returned.

Another operation is remove: it clears the associated value (if it exists), and ensures the next associated value is computed with input states up-to-date with the removal.

For a particular association, there is a total order for accesses to the associated value. Accesses are atomic; they include:

A read-only access by get
An attempt to associate the return value of a computeValue by get
Clearing of an association by remove

A get call always include at least one access; a remove call always has exactly one access; a computeValue call always happens between two accesses. This establishes the order of computeValue calls with respect to remove calls and determines whether the results of a computeValue can be successfully associated by a

get

Since:: 1.7

Constructor Summary

ClassValue()

Modifier	Constructor	Description
`protected`	Sole constructor.

Method Summary

Modifier and Type	Method	Description
`protected abstract T`	`computeValue(Class<?> type)`	Computes the value to associate to the given `Class`.
`T`	`get(Class<?> type)`	Returns the value associated to the given `Class`.
`void`	`remove(Class<?> type)`	Removes the associated value for the given `Class` and invalidates all out-of-date computations.

Methods declared in class Object

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

Constructor Details

ClassValue

protected ClassValue()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Details

computeValue

protected abstract T computeValue(Class<?> type)

Computes the value to associate to the given Class.

This method is invoked when the initial read-only access by get finds no associated value.

If this method throws an exception, the initiating get call will not attempt to associate a value, and may terminate by returning the associated value if it exists, or by propagating that exception otherwise.

Otherwise, the value is computed and returned. An attempt to associate the return value happens, with one of the following outcomes:

The associated value is present; it is returned and no association is done.
The most recent remove call, if it exists, does not happen-before (JLS 17.4.5) the finish of the computeValue that computed the value to associate. A new invocation to computeValue, which that remove call happens-before, will re-establish this happens-before relationship.
Otherwise, this value is successfully associated and returned.

API Note:

A computeValue call may, due to class loading or other circumstances, recursively call get or remove for the same type. The recursive get, if the recursion stops, successfully finishes and this initiating get observes the associated value from recursion. The recursive remove is no-op, since being on the same thread, the remove already happens-before the finish of this computeValue; the result from this


computeValue

still may be associated.

Parameters:

type - the Class to associate a value to

Returns:

the newly computed value to associate

See Also:

get

public T get(Class<?> type)

Returns the value associated to the given Class.

This method first performs a read-only access, and returns the associated value if it exists. Otherwise, this method tries to associate a value from a computeValue invocation until the associated value exists, which could be associated by a competing thread.

This method may throw an exception from a computeValue invocation. In this case, no association happens.

Parameters:

type - the Class to retrieve the associated value for

Returns:

the value associated to the given Class

Throws:

NullPointerException - if the argument is null

See Also:

remove

public void remove(Class<?> type)

Removes the associated value for the given Class and invalidates all out-of-date computations. If this association is subsequently accessed, this removal happens-before (JLS 17.4.5) the finish of the computeValue call that returned the associated value.

Parameters:: type - the type whose class value must be removed
Throws:: NullPointerException - if the argument is null

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/ClassValue.html