jdk/src/java.base/share/classes/java/lang/classfile/ClassFileTransform.java

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package java.lang.classfile;

import java.util.function.Supplier;

import java.lang.classfile.attribute.RuntimeVisibleAnnotationsAttribute;
import jdk.internal.javac.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.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <p>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.)
 * <p>
 * Class transformation sample where code transformation is stateful:
 * {@snippet lang="java" class="PackageSnippets" region="codeRelabeling"}
 * <p>
 * Complex class instrumentation sample chaining multiple transformations:
 * {@snippet lang="java" class="PackageSnippets" region="classInstrumentation"}
 * @param <C> the transform type
 * @param <E> the element type
 * @param <B> the builder type
 *
 * @sealedGraph
 * @since 22
 */
@PreviewFeature(feature = PreviewFeature.Feature.CLASSFILE_API)
public sealed interface ClassFileTransform<
        C extends ClassFileTransform<C, E, B>,
        E extends ClassFileElement,
        B extends ClassFileBuilder<E, B>>
        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);
}