org.graalvm.nativeimage.hosted.FieldValueTransformer Maven / Gradle / Ivy
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package org.graalvm.nativeimage.hosted;
import org.graalvm.nativeimage.Platform;
import org.graalvm.nativeimage.Platforms;
import org.graalvm.nativeimage.hosted.Feature.BeforeAnalysisAccess;
/**
* A transformer for a field value that can be registered using
* {@link BeforeAnalysisAccess#registerFieldValueTransformer}.
*
* At image build time, the field value transformer provides the value of the field for the image
* heap. Without a transformer, the value of the field in the image heap is the same as the hosted
* value in the image generator. A field value transformer allows to intercept the value. A field
* value transformer can, for example, reset a field to the default null/0 value, replace a filled
* collection with a new empty collection, or provide any kind of new value for a field.
*
* Only one field value transformer can be registered for each field.
*
* A field value transformer can be registered for fields of classes that are initialized at image
* run time. That allows constant folding of final fields even though the declaring class is not
* initialized at image build time.
*
* A field value transformer must be registered before the field is seen as reachable by the static
* analysis. It is generally safe to register a transformer in {@link Feature#beforeAnalysis} before
* the static analysis is started, in a {@link BeforeAnalysisAccess#registerReachabilityHandler type
* reachability handler} for the declaring class of the field, or a
* {@link BeforeAnalysisAccess#registerSubtypeReachabilityHandler subtype reachability handler} for
* a super-type of the declaring class of the field.
*
* @since 22.3
*/
@Platforms(Platform.HOSTED_ONLY.class)
public interface FieldValueTransformer {
/**
* Transforms the field value for the provided receiver. The receiver is null for static fields.
* The original value of the field, i.e., the hosted value of the field in the image generator,
* is also provided as an argument.
*
* The type of the returned object must be assignable to the declared type of the field. If the
* field has a primitive type, the return value must be a boxed value, and must not be null.
*
* @since 22.3
*/
Object transform(Object receiver, Object originalValue);
}