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Top Description Interfaces Classes
module java.base

Package jdk.internal.classfile.components

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

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

ClassPrinter

ClassPrinter is a helper class providing seamless export of a java.lang.classfile.ClassModel, java.lang.classfile.FieldModel, java.lang.classfile.MethodModel, or java.lang.classfile.CodeModel into human-readable structured text in JSON, XML, or YAML format, or into a tree of traversable and printable nodes.

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

ClassPrinter 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 mn.values().forEach(this::print); } case ClassPrinter.ListNode ln -> { // print list header ln.forEach(this::print); } case ClassPrinter.LeafNode n -> { // print leaf node } } } 
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
            mn.values().forEach(this::print);
        }
        case ClassPrinter.ListNode ln -> {
            // print list header
            ln.forEach(this::print);
        }
        case ClassPrinter.LeafNode n -> {
            // print leaf node
        }
    }
}

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

@Test 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.name().equals(key) && ln.value().equals(value))) { node.toYaml(System.out::print); throw new AssertionError("expected %s: %s".formatted(key, value)); } } 
@Test
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.name().equals(key)
                           && ln.value().equals(value))) {
        node.toYaml(System.out::print);
        throw new AssertionError("expected %s: %s".formatted(key, value));
    }
}

ClassRemapper

ClassRemapper is a java.lang.classfile.ClassTransform, java.lang.classfile.FieldTransform, java.lang.classfile.MethodTransform and java.lang.classfile.CodeTransform 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); } 
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); } 
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);

}

CodeLocalsShifter

CodeLocalsShifter is a java.lang.classfile.CodeTransform 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().transformClass( classModel, (classBuilder, classElement) -> { if (classElement instanceof MethodModel method) classBuilder.transformMethod(method, MethodTransform.transformingCode( CodeLocalsShifter.of(method.flags(), method.methodTypeSymbol()))); else classBuilder.accept(classElement); }); 
byte[] newBytes = ClassFile.of().transformClass(
        classModel,
        (classBuilder, classElement) -> {
            if (classElement instanceof MethodModel method)
                classBuilder.transformMethod(method,
                        MethodTransform.transformingCode(
                                CodeLocalsShifter.of(method.flags(), method.methodTypeSymbol())));
            else
                classBuilder.accept(classElement);
        });

CodeRelabeler

CodeRelabeler is a java.lang.classfile.CodeTransform replacing all occurrences of java.lang.classfile.Label in the transformed code with new instances. All java.lang.classfile.instruction.LabelTarget instructions are adjusted accordingly. Relabeled code graph is identical to the original.

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

Sample transformation relabeling all methods:

byte[] newBytes = ClassFile.of().transformClass( classModel, ClassTransform.transformingMethodBodies( CodeTransform.ofStateful(CodeRelabeler::of))); 
byte[] newBytes = ClassFile.of().transformClass(
        classModel,
        ClassTransform.transformingMethodBodies(
                CodeTransform.ofStateful(CodeRelabeler::of)));

Class Instrumentation Sample

Following snippet is sample composition of ClassRemapper, CodeLocalsShifter and CodeRelabeler into fully functional class instrumenting transformation:
byte[] classInstrumentation(ClassModel target, ClassModel instrumentor, Predicate<MethodModel> instrumentedMethodsFilter) { var instrumentorCodeMap = instrumentor.methods().stream() .filter(instrumentedMethodsFilter) .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().transformClass(target, ClassTransform.transformingMethods( instrumentedMethodsFilter, (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 instrumentorClassRemapper.asCodeTransform() .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(inv.name().stringValue()) && 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.REFERENCE, slot++)); for (var pt : mm.methodTypeSymbol().parameterList()) { var tk = TypeKind.from(pt); storeStack.push(StoreInstruction.of(tk, slot)); slot += tk.slotSize(); } storeStack.forEach(codeBuilder::with); //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(CodeRelabeler.of()) .andThen((innerBuilder, shiftedTargetCode) -> { //returns must be replaced with jump to the end of the inlined method if (shiftedTargetCode instanceof ReturnInstruction) innerBuilder.goto_(inlinedBlockBuilder.breakLabel()); else innerBuilder.with(shiftedTargetCode); }))); } else codeBuilder.with(instrumentorCodeElement); })); } else mb.with(me); }) .andThen(ClassTransform.endHandler(clb -> //remaining instrumentor fields and methods are injected at the end clb.transform(instrumentor, 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 .andThen(instrumentorClassRemapper))))); } 
byte[] classInstrumentation(ClassModel target, ClassModel instrumentor, Predicate<MethodModel> instrumentedMethodsFilter) {
    var instrumentorCodeMap = instrumentor.methods().stream()
                                          .filter(instrumentedMethodsFilter)
                                          .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().transformClass(target,
            ClassTransform.transformingMethods(
                    instrumentedMethodsFilter,
                    (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
                                    instrumentorClassRemapper.asCodeTransform()
                                    .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(inv.name().stringValue())
                                            && 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.REFERENCE, slot++));
                                            for (var pt : mm.methodTypeSymbol().parameterList()) {
                                                var tk = TypeKind.from(pt);
                                                storeStack.push(StoreInstruction.of(tk, slot));
                                                slot += tk.slotSize();
                                            }
                                            storeStack.forEach(codeBuilder::with);

                                            //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(CodeRelabeler.of())
                                                    .andThen((innerBuilder, shiftedTargetCode) -> {
                                                        //returns must be replaced with jump to the end of the inlined method
                                                        if (shiftedTargetCode instanceof ReturnInstruction)
                                                            innerBuilder.goto_(inlinedBlockBuilder.breakLabel());
                                                        else
                                                            innerBuilder.with(shiftedTargetCode);
                                                    })));
                                        } else
                                            codeBuilder.with(instrumentorCodeElement);
                                    }));
                        } else
                            mb.with(me);
                    })
            .andThen(ClassTransform.endHandler(clb ->
                //remaining instrumentor fields and methods are injected at the end
                clb.transform(instrumentor,
                        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
                        .andThen(instrumentorClassRemapper)))));
}

Interface Summary

Modifier and TypeInterface and Description
public interface
ClassRemapper

ClassRemapper is a ClassTransform, FieldTransform, MethodTransform and CodeTransform deeply re-mapping all class references in any form, according to given map or map function.

public interface
CodeLocalsShifter

CodeLocalsShifter is a CodeTransform shifting locals to newly allocated positions to avoid conflicts during code injection.

public interface
CodeRelabeler

A code relabeler is a CodeTransform replacing all occurrences of java.lang.classfile.Label in the transformed code with new instances.

public interface
CodeStackTracker

CodeStackTracker is a CodeTransform tracking stack content and calculating max stack size.

Class Summary

Modifier and TypeClass and Description
public class
ClassPrinter

A printer of classfiles and its elements.