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Fall back to Class.forName in case boot class is already defined #41

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Merged
mcculls merged 3 commits into from
Dec 30, 2025
Merged

Fall back to Class.forName in case boot class is already defined #41

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5802b48
Remove unnecessary DUP
mcculls Dec 28, 2025
eed1078
Align non-boot and boot define-class approaches
mcculls Dec 29, 2025
4244540
Fall back to Class.forName in case boot class is already defined
mcculls Dec 29, 2025
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59 changes: 50 additions & 9 deletions class-inject/src/glue/java/datadog/instrument/glue/DefineClassGlueGenerator.java
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Original file line number Diff line number Diff line change
Expand Up @@ -81,6 +81,9 @@ final class DefineClassGlueGenerator {
+ CLASS_CLASS
+ ";";

private static final String CLASS_FORNAME_DESCRIPTOR =
"(L" + STRING_CLASS + ";ZL" + CLASSLOADER_CLASS + ";)L" + CLASS_CLASS + ";";

private DefineClassGlueGenerator() {}

/**
Expand Down Expand Up @@ -166,6 +169,10 @@ private static byte[] generateBytecode(String unsafeNamespace) {
final Label unlockClassLoaderAndThrow = new Label();
final Label setupUnsafeDefiner = new Label();
final Label hasNextBootClass = new Label();
final Label defineBootClass = new Label();
final Label storeBootClass = new Label();
final Label loadBootClass = new Label();
final Label rethrowOriginal = new Label();
final Label returnDefinedClasses = new Label();

// common bytecode variables
Expand All @@ -175,20 +182,25 @@ private static byte[] generateBytecode(String unsafeNamespace) {
final int mapEntrySetIterator = 4;
final int protectionDomain = 5;
final int classLoader = 6;
final int className = 7;

// bytecode variables for non-boot classes
final int classLoadingLock = 7;
final int bytecodeMapEntry = 8;
final int className = 9;
final int classLoadingLock = 8;
final int bytecodeMapEntry = 9;

// bytecode variables for boot classes
final int unsafeInstance = 7;
final int unsafeInstance = 8;
final int originalError = 9;

mv.visitCode();

// make sure we unlock the class-loader if an exception occurs while defining a class
mv.visitTryCatchBlock(classLoaderLocked, classLoaderUnlocked, unlockClassLoaderAndThrow, null);

// fall back and see if the boot class is already loaded if defining it fails
mv.visitTryCatchBlock(defineBootClass, storeBootClass, loadBootClass, null);
mv.visitTryCatchBlock(loadBootClass, rethrowOriginal, rethrowOriginal, null);

// -------- SHARED SETUP CODE --------

// store the defined classes in a list
Expand Down Expand Up @@ -278,11 +290,13 @@ private static byte[] generateBytecode(String unsafeNamespace) {
false);
mv.visitInsn(DUP);
mv.visitJumpInsn(IFNONNULL, storeNonBootClass);

// define the class using the given class-name and bytecode
mv.visitInsn(POP);

// not yet defined, prepare arguments to define it
mv.visitVarInsn(ALOAD, classLoader);
mv.visitVarInsn(ALOAD, className);

// extract the bytecode of the next class to define
mv.visitVarInsn(ALOAD, bytecodeMapEntry);
mv.visitMethodInsn(
INVOKEINTERFACE, MAP_ENTRY_CLASS, "getValue", "()L" + OBJECT_CLASS + ";", true);
Expand All @@ -291,7 +305,10 @@ private static byte[] generateBytecode(String unsafeNamespace) {
mv.visitInsn(ARRAYLENGTH);
mv.visitInsn(ICONST_0);
mv.visitInsn(SWAP);

mv.visitVarInsn(ALOAD, protectionDomain);

// define the class using the given class-name and bytecode
mv.visitMethodInsn(
INVOKEVIRTUAL, CLASSLOADER_CLASS, "defineClass", CLASSLOADER_DEFINECLASS_DESCRIPTOR, false);

Expand All @@ -312,7 +329,6 @@ private static byte[] generateBytecode(String unsafeNamespace) {

// unlock the class-loader if something goes wrong
mv.visitLabel(unlockClassLoaderAndThrow);
mv.visitInsn(DUP);
mv.visitVarInsn(ALOAD, classLoadingLock);
mv.visitInsn(MONITOREXIT);
mv.visitInsn(ATHROW);
Expand All @@ -330,28 +346,38 @@ private static byte[] generateBytecode(String unsafeNamespace) {
mv.visitMethodInsn(INVOKEINTERFACE, ITERATOR_CLASS, "hasNext", "()Z", true);
mv.visitJumpInsn(IFEQ, returnDefinedClasses);

// define the class using the given class-name and bytecode
mv.visitVarInsn(ALOAD, unsafeInstance);

// extract the name of the next class to define
mv.visitVarInsn(ALOAD, mapEntrySetIterator);
mv.visitMethodInsn(INVOKEINTERFACE, ITERATOR_CLASS, "next", "()L" + OBJECT_CLASS + ";", true);
mv.visitInsn(DUP);
mv.visitMethodInsn(
INVOKEINTERFACE, MAP_ENTRY_CLASS, "getKey", "()L" + OBJECT_CLASS + ";", true);
mv.visitTypeInsn(CHECKCAST, STRING_CLASS);
mv.visitInsn(DUP);
mv.visitVarInsn(ASTORE, className);
mv.visitInsn(SWAP);

// extract the bytecode of the next class to define
mv.visitMethodInsn(
INVOKEINTERFACE, MAP_ENTRY_CLASS, "getValue", "()L" + OBJECT_CLASS + ";", true);
mv.visitTypeInsn(CHECKCAST, BYTE_ARRAY);
mv.visitInsn(DUP);
mv.visitInsn(ARRAYLENGTH);
mv.visitInsn(ICONST_0);
mv.visitInsn(SWAP);

mv.visitInsn(ACONST_NULL);
mv.visitVarInsn(ALOAD, protectionDomain);

// define the boot class using the given class-name and bytecode
mv.visitLabel(defineBootClass);
mv.visitMethodInsn(
INVOKEVIRTUAL, unsafeClass, "defineClass", UNSAFE_DEFINECLASS_DESCRIPTOR, false);

// store the class in the list
// store the class in the list whether we defined it or it already existed
mv.visitLabel(storeBootClass);
mv.visitVarInsn(ALOAD, definedClasses);
mv.visitInsn(SWAP);
mv.visitMethodInsn(INVOKEVIRTUAL, ARRAYLIST_CLASS, "add", "(L" + OBJECT_CLASS + ";)Z", false);
Expand All @@ -360,6 +386,21 @@ private static byte[] generateBytecode(String unsafeNamespace) {
// check again if we've defined all the given bytecode
mv.visitJumpInsn(GOTO, hasNextBootClass);

// see if the boot class has already been defined
mv.visitLabel(loadBootClass);
mv.visitVarInsn(ASTORE, originalError);
mv.visitVarInsn(ALOAD, className);
mv.visitInsn(ICONST_0); // initialize=false
mv.visitInsn(ACONST_NULL);
mv.visitMethodInsn(INVOKESTATIC, CLASS_CLASS, "forName", CLASS_FORNAME_DESCRIPTOR, false);
mv.visitJumpInsn(GOTO, storeBootClass);

// boot class is not defined, report original error
mv.visitLabel(rethrowOriginal);
mv.visitInsn(POP);
mv.visitVarInsn(ALOAD, originalError);
mv.visitInsn(ATHROW);

// -------- SHARED RETURN CODE --------

// return the defined classes
Expand Down