Module java.base

Package java.lang.classfile.components

package java.lang.classfile.components
java.lang.classfile.components is a preview API of the Java platform.
Programs can only use java.lang.classfile.components when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.

Provides specific components, transformations, and tools built on top of the java.lang.classfilePREVIEW library.

The java.lang.classfile.components package contains specific transformation components and utility classes helping to compose very complex tasks with minimal effort.


ClassPrinterPREVIEW is a helper class providing seamless export of a ClassModelPREVIEW, FieldModelPREVIEW, MethodModelPREVIEW, or CodeModelPREVIEW into human-readable structured text in JSON, XML, or YAML format, or into a tree of traversable and printable nodes.

Primary purpose of ClassPrinterPREVIEW is to provide human-readable class info for debugging, exception handling and logging purposes. The printed class also conforms to a standard format to support automated offline processing.

The most frequent use case is to simply print a class:

ClassPrinter.toJson(classModel, ClassPrinter.Verbosity.TRACE_ALL, System.out::print);

ClassPrinterPREVIEW allows to traverse tree of simple printable nodes to hook custom printer:

void customPrint(ClassModel classModel) {
    print(ClassPrinter.toTree(classModel, ClassPrinter.Verbosity.TRACE_ALL));

void print(ClassPrinter.Node node) {
    switch (node) {
        case ClassPrinter.MapNode mn -> {
            // print map header
        case ClassPrinter.ListNode ln -> {
            // print list header
        case ClassPrinter.LeafNode n -> {
            // print leaf node

Another use case for ClassPrinterPREVIEW is to simplify writing of automated tests:

void printNodesInTest(ClassModel classModel) {
    var classNode = ClassPrinter.toTree(classModel, ClassPrinter.Verbosity.TRACE_ALL);
    assertContains(classNode, "method name", "myFooMethod");
    assertContains(classNode, "field name", "myBarField");
    assertContains(classNode, "inner class", "MyInnerFooClass");

void assertContains(ClassPrinter.Node node, ConstantDesc key, ConstantDesc value) {
    if (!node.walk().anyMatch(n -> n instanceof ClassPrinter.LeafNode ln
                           && ln.value().equals(value))) {
        throw new AssertionError("expected %s: %s".formatted(key, value));


ClassRemapper is a ClassTransformPREVIEW, FieldTransformPREVIEW, MethodTransformPREVIEW and CodeTransformPREVIEW deeply re-mapping all class references in any form, according to given map or map function.

The re-mapping is applied to superclass, interfaces, all kinds of descriptors and signatures, all attributes referencing classes in any form (including all types of annotations), and to all instructions referencing to classes.

Primitive types and arrays are never subjects of mapping and are not allowed targets of mapping.

Arrays of reference types are always decomposed, mapped as the base reference types and composed back to arrays.

Single class remapping example:

var classRemapper = ClassRemapper.of(
        Map.of(CD_Foo, CD_Bar));
var cc = ClassFile.of();
for (var classModel : allMyClasses) {
    byte[] newBytes = classRemapper.remapClass(cc, classModel);


Remapping of all classes under specific package:

var classRemapper = ClassRemapper.of(cd ->
        ClassDesc.ofDescriptor(cd.descriptorString().replace("Lcom/oldpackage/", "Lcom/newpackage/")));
var cc = ClassFile.of();
for (var classModel : allMyClasses) {
    byte[] newBytes = classRemapper.remapClass(cc, classModel);



CodeLocalsShifterPREVIEW is a CodeTransformPREVIEW shifting locals to newly allocated positions to avoid conflicts during code injection. Locals pointing to the receiver or to method arguments slots are never shifted. All locals pointing beyond the method arguments are re-indexed in order of appearance.

Sample of code transformation shifting all locals in all methods:

byte[] newBytes = ClassFile.of().transform(
        (classBuilder, classElement) -> {
            if (classElement instanceof MethodModel method)
                                CodeLocalsShifter.of(method.flags(), method.methodTypeSymbol())));


CodeRelabelerPREVIEW is a CodeTransformPREVIEW replacing all occurrences of LabelPREVIEW in the transformed code with new instances. All LabelTargetPREVIEW instructions are adjusted accordingly. Relabeled code graph is identical to the original.

Primary purpose of CodeRelabelerPREVIEW is for repeated injections of the same code blocks. Repeated injection of the same code block must be relabeled, so each instance of LabelPREVIEW is bound in the target bytecode exactly once.

Sample transformation relabeling all methods:

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

Class Instrumentation Sample

Following snippet is sample composition of ClassRemapperPREVIEW, CodeLocalsShifterPREVIEW and CodeRelabelerPREVIEW into fully functional class instrumenting transformation:
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