What causes "Invalid constant pool" in Konloch/bytecode-viewer?

Verify with a spec-compliant tool (javap -v, javac -Xlint, ASM CheckClassAdapter) that the constant pool is valid; recompile if it is not.

Konloch/bytecode-viewer · error · ClassFileFormatException

Invalid constant pool

Error message

Invalid constant pool

What it means

The this_class constant-pool entry must be a CONSTANT_Class whose name_index points at a CONSTANT_Utf8 entry holding the internal class name. ExtractDirectoryPath throws ClassFileFormatException('Invalid constant pool') when the this_class entry is absent (long/double entries leave a hole) or its tag is not CONSTANT_Class. The constant pool does not match what the JVM spec requires for a valid class.

Source

Thrown at src/main/java/the/bytecode/club/bytecodeviewer/decompilers/jdgui/JDGUIClassFileUtil.java:71

            /* int minor_version = */
            dis.readUnsignedShort();
            /* int major_version = */
            dis.readUnsignedShort();

            Constant[] constants = DeserializeConstants(dis);

            /* int access_flags = */
            dis.readUnsignedShort();
            int this_class = dis.readUnsignedShort();

            if (this_class > constants.length)
            {
                throw new ClassFileFormatException("Unknown Java structure");
            }
            Constant c = constants[this_class];
            if ((c == null) || (c.getTag() != Constant.CONSTANT_Class))
            {
                throw new ClassFileFormatException("Invalid constant pool");
            }

            c = constants[((ConstantClass) c).getNameIndex()];
            if ((c == null) || (c.getTag() != Constant.CONSTANT_Utf8))
            {
                throw new ClassFileFormatException("Invalid constant pool");
            }

            String internalClassName = ((ConstantUtf8) c).getValue();
            String pathSuffix = internalClassName.replace(INTERNAL_PACKAGE_SEPARATOR, File.separatorChar) + CLASS_FILE_SUFFIX;

            int index = pathToClass.indexOf(pathSuffix);

            if (index < 0)
            {
                throw new ClassFileFormatException("Invalid internal class name");
            }

View on GitHub (pinned to 31430e0033)

Solutions

Verify with a spec-compliant tool (javap -v, javac -Xlint, ASM CheckClassAdapter) that the constant pool is valid; recompile if it is not.
Re-extract the class from its original JAR/APK to rule out truncation/corruption during copying.
Remove or rerun any bytecode patching/obfuscation step that produced the malformed pool.
Catch ClassFileFormatException per-file and skip the offending class in batch decompilation.

Example fix

// before
String dir = JDGUIClassFileUtil.ExtractDirectoryPath(path);

// after
try {
    String dir = JDGUIClassFileUtil.ExtractDirectoryPath(path);
} catch (org.jd.core.v1.service.deserializer.classfile.ClassFileFormatException e) {
    if ("Invalid constant pool".equals(e.getMessage())) {
        // skip malformed class
    } else {
        throw e;
    }
}

Defensive patterns

Strategy: try-catch

Validate before calling

try (FileInputStream in = new FileInputStream(path)) {
    ClassReader cr = new ClassReader(in); // ASM or similar spec-compliant parser
    // constructor throws InvalidClassFormatException if the pool is broken
}

Try / catch

try {
    String dir = JDGUIClassFileUtil.ExtractDirectoryPath(path);
} catch (org.jd.core.v1.service.deserializer.classfile.ClassFileFormatException e) {
    if ("Invalid constant pool".equals(e.getMessage())) {
        logger.warn("Broken constant pool: {}", path); // skip
    } else {
        throw e;
    }
}

Prevention

Verify bytecode with ASM's CheckClassAdapter or javap before feeding it to the lightweight parser.
After bytecode instrumentation, always re-run verification — broken cp_index wiring is the usual cause.
Avoid hand-editing class files; regenerate them instead.
Re-extract classes from the original JAR/APK if corruption is suspected.
Handle each file in its own try-catch during batch runs.

When it happens

Trigger: ExtractDirectoryPath on a file whose this_class slot is empty (unusual) or whose resolved constant is not a CONSTANT_Class — i.e. malformed bytecode, a partially written class file, or a non-class binary that coincidentally passed the magic check.

Common situations: Corrupted class files after failed extraction/patching; class files assembled by custom bytecode generators with wrong cp_index wiring; aggressive obfuscators that break invariants this lightweight parser assumes; byte-edited binaries.

Related errors

AI-assisted analysis of Konloch/bytecode-viewer@31430e0033 (2026-09-05). Data as JSON: /api/errors/18da61cb71f80736. Report an issue: GitHub.