Module java.base

Interface ClassFileTransform<C extends ClassFileTransformPREVIEW<C,E,B>,E extends ClassFileElementPREVIEW,B extends ClassFileBuilderPREVIEW<E,B>>

Type Parameters:
C - the transform type
E - the element type
B - the builder type
All Known Subinterfaces:
ClassRemapperPREVIEW, ClassTransformPREVIEW, CodeLocalsShifterPREVIEW, CodeRelabelerPREVIEW, CodeStackTrackerPREVIEW, CodeTransformPREVIEW, FieldTransformPREVIEW, MethodTransformPREVIEW

public sealed interface ClassFileTransform<C extends ClassFileTransformPREVIEW<C,E,B>,E extends ClassFileElementPREVIEW,B extends ClassFileBuilderPREVIEW<E,B>> permits ClassTransformPREVIEW, FieldTransformPREVIEW, MethodTransformPREVIEW, CodeTransformPREVIEW
ClassFileTransform is a preview API of the Java platform.
Programs can only use ClassFileTransform when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
A transformation on streams of elements. Transforms are used during transformation of classfile entities; a transform is provided to a method like ClassFile.transform(ClassModel, ClassTransform)PREVIEW, and the elements of the class, along with a builder, are presented to the transform.

The subtypes of ClassFileTransformPREVIEW (e.g., ClassTransformPREVIEW) are functional interfaces that accept an element and a corresponding builder. Since any element can be reproduced on the builder via ClassFileBuilder.with(ClassFileElement)PREVIEW, 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 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 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 RuntimeVisibleAnnotationsAttributePREVIEW element and transform it if found, but if it is not found, will generate a RuntimeVisibleAnnotationsAttributePREVIEW element containing the injected annotation from the 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 ClassTransform.ofStateful(Supplier)PREVIEW (or corresponding methods for other classfile locations.)

Class transformation sample where code transformation is stateful:

byte[] newBytes = ClassFile.of().transform(classModel,

Complex class instrumentation sample chaining multiple transformations:

byte[] classInstrumentation(ClassModel target, ClassModel instrumentor, Predicate<MethodModel> instrumentedMethodsFilter) {
    var instrumentorCodeMap = instrumentor.methods().stream()
                                          .collect(Collectors.toMap(mm -> mm.methodName().stringValue() + mm.methodType().stringValue(), mm -> mm.code().orElseThrow()));
    var targetFieldNames = target.fields().stream().map(f -> f.fieldName().stringValue()).collect(Collectors.toSet());
    var targetMethods = target.methods().stream().map(m -> m.methodName().stringValue() + m.methodType().stringValue()).collect(Collectors.toSet());
    var instrumentorClassRemapper = ClassRemapper.of(Map.of(instrumentor.thisClass().asSymbol(), target.thisClass().asSymbol()));
    return ClassFile.of().transform(target,
                    (mb, me) -> {
                        if (me instanceof CodeModel targetCodeModel) {
                            var mm = targetCodeModel.parent().get();
                            //instrumented methods code is taken from instrumentor
                            mb.transformCode(instrumentorCodeMap.get(mm.methodName().stringValue() + mm.methodType().stringValue()),
                                    //all references to the instrumentor class are remapped to target class
                                    .andThen((codeBuilder, instrumentorCodeElement) -> {
                                        //all invocations of target methods from instrumentor are inlined
                                        if (instrumentorCodeElement instanceof InvokeInstruction inv
                                            && target.thisClass().asInternalName().equals(inv.owner().asInternalName())
                                            && mm.methodName().stringValue().equals(
                                            && mm.methodType().stringValue().equals(inv.type().stringValue())) {

                                            //store stacked method parameters into locals
                                            var storeStack = new ArrayDeque<StoreInstruction>();
                                            int slot = 0;
                                            if (!mm.flags().has(AccessFlag.STATIC))
                                                storeStack.push(StoreInstruction.of(TypeKind.ReferenceType, slot++));
                                            for (var pt : mm.methodTypeSymbol().parameterList()) {
                                                var tk = TypeKind.from(pt);
                                                storeStack.push(StoreInstruction.of(tk, slot));
                                                slot += tk.slotSize();

                                            //inlined target locals must be shifted based on the actual instrumentor locals
                                            codeBuilder.block(inlinedBlockBuilder -> inlinedBlockBuilder
                                                    .transform(targetCodeModel, CodeLocalsShifter.of(mm.flags(), mm.methodTypeSymbol())
                                                    .andThen((innerBuilder, shiftedTargetCode) -> {
                                                        //returns must be replaced with jump to the end of the inlined method
                                                        if (shiftedTargetCode instanceof ReturnInstruction)
                                        } else
                        } else
            .andThen(ClassTransform.endHandler(clb ->
                //remaining instrumentor fields and methods are injected at the end
                        ClassTransform.dropping(cle ->
                                !(cle instanceof FieldModel fm
                                        && !targetFieldNames.contains(fm.fieldName().stringValue()))
                                && !(cle instanceof MethodModel mm
                                        && !ConstantDescs.INIT_NAME.equals(mm.methodName().stringValue())
                                        && !targetMethods.contains(mm.methodName().stringValue() + mm.methodType().stringValue())))
                        //and instrumentor class references remapped to target class
Sealed Class Hierarchy Graph:
Sealed class hierarchy graph for ClassFileTransformSealed class hierarchy graph for ClassFileTransform