8308753: Class-File API transition to Preview

Reviewed-by: ihse, mchung, vromero

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

@@ -0,0 +1,169 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package java.lang.classfile;
+
+import java.util.function.Consumer;
+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#transform(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);
+
+    /**
+     * The result of binding a transform to a builder.  Used primarily within
+     * the implementation to perform transformation.
+     *
+     * @param <E> the element type
+     *
+     * @since 22
+     */
+    @PreviewFeature(feature = PreviewFeature.Feature.CLASSFILE_API)
+    interface ResolvedTransform<E extends ClassFileElement> {
+        /**
+         * {@return a {@link Consumer} to receive elements}
+         */
+        Consumer<E> consumer();
+
+        /**
+         * {@return an action to call at the end of transformation}
+         */
+        Runnable endHandler();
+
+        /**
+         * {@return an action to call at the start of transformation}
+         */
+        Runnable startHandler();
+    }
+
+    /**
+     * Bind a transform to a builder.  If the transform is chained, intermediate
+     * builders are created for each chain link.  If the transform is stateful
+     * (see, e.g., {@link ClassTransform#ofStateful(Supplier)}), the supplier is
+     * invoked to get a fresh transform object.
+     *
+     * <p>This method is a low-level method that should rarely be used by
+     * user code; most of the time, user code should prefer
+     * {@link ClassFileBuilder#transform(CompoundElement, ClassFileTransform)},
+     * which resolves the transform and executes it on the current builder.
+     *
+     * @param builder the builder to bind to
+     * @return the bound result
+     */
+    ResolvedTransform<E> resolve(B builder);
+}