io.github.dmlloyd.classfile.ClassFileTransform Maven / Gradle / Ivy
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package io.github.dmlloyd.classfile;
import java.util.function.Supplier;
import io.github.dmlloyd.classfile.attribute.RuntimeVisibleAnnotationsAttribute;
import io.github.dmlloyd.classfile.extras.PreviewFeature;
/**
* A transformation on streams of elements. Transforms are used during
* transformation of classfile entities; a transform is provided to a method like
* {@link ClassFile#transformClass(ClassModel, ClassTransform)}, and the elements of the class,
* along with a builder, are presented to the transform.
*
* The subtypes of {@linkplain
* ClassFileTransform} (e.g., {@link ClassTransform}) are functional interfaces
* that accept an element and a corresponding builder. Since any element can be
* reproduced on the builder via {@link ClassBuilder#with(ClassFileElement)}, a
* transform can easily leave elements in place, remove them, replace them, or
* augment them with other elements. This enables localized transforms to be
* represented concisely.
*
*
Transforms also have an {@link #atEnd(ClassFileBuilder)} method, for
* which the default implementation does nothing, so that a transform can
* perform additional building after the stream of elements is exhausted.
*
*
Transforms can be chained together via the {@link
* #andThen(ClassFileTransform)} method, so that the output of one becomes the
* input to another. This allows smaller units of transformation to be captured
* and reused.
*
*
Some transforms are stateful; for example, a transform that injects an
* annotation on a class may watch for the {@link RuntimeVisibleAnnotationsAttribute}
* element and transform it if found, but if it is not found, will generate a
* {@linkplain RuntimeVisibleAnnotationsAttribute} element containing the
* injected annotation from the {@linkplain #atEnd(ClassFileBuilder)} handler.
* To do this, the transform must accumulate some state during the traversal so
* that the end handler knows what to do. If such a transform is to be reused,
* its state must be reset for each traversal; this will happen automatically if
* the transform is created with {@link ClassTransform#ofStateful(Supplier)} (or
* corresponding methods for other classfile locations.)
*
* Class transformation sample where code transformation is stateful:
* {@snippet lang="java" class="PackageSnippets" region="codeRelabeling"}
*
* Complex class instrumentation sample chaining multiple transformations:
* {@snippet lang="java" class="PackageSnippets" region="classInstrumentation"}
* @param the transform type
* @param the element type
* @param the builder type
*
* @sealedGraph
* @since 22
*/
@PreviewFeature(feature = PreviewFeature.Feature.CLASSFILE_API)
public sealed interface ClassFileTransform<
C extends ClassFileTransform,
E extends ClassFileElement,
B extends ClassFileBuilder>
permits ClassTransform, FieldTransform, MethodTransform, CodeTransform {
/**
* Transform an element by taking the appropriate actions on the builder.
* Used when transforming a classfile entity (class, method, field, method
* body.) If no transformation is desired, the element can be presented to
* {@link B#with(ClassFileElement)}. If the element is to be dropped, no
* action is required.
*
* @param builder the builder for the new entity
* @param element the element
*/
void accept(B builder, E element);
/**
* Take any final action during transformation of a classfile entity. Called
* after all elements of the class are presented to {@link
* #accept(ClassFileBuilder, ClassFileElement)}.
*
* @param builder the builder for the new entity
* @implSpec The default implementation does nothing.
*/
default void atEnd(B builder) {
}
/**
* Take any preliminary action during transformation of a classfile entity.
* Called before any elements of the class are presented to {@link
* #accept(ClassFileBuilder, ClassFileElement)}.
*
* @param builder the builder for the new entity
* @implSpec The default implementation does nothing.
*/
default void atStart(B builder) {
}
/**
* Chain this transform with another; elements presented to the builder of
* this transform will become the input to the next transform.
*
* @param next the downstream transform
* @return the chained transform
*/
C andThen(C next);
}