oracle/graal · error · ParserException.ClassFormatError

Invalid type descriptor {}

Error message

Invalid type descriptor {}

What it means

Defensive tail of skipValidTypeDescriptor: after the early return for non-'['/non-'L' characters and fully handling the 'L' and '[' cases, the final throw 'Invalid type descriptor <rest>' is only reachable through a switch mismatch. In practice it fires if the method is entered with a character value that is neither a valid single-char descriptor, 'L', nor '[' — e.g. an unexpected control or non-ASCII byte in the descriptor.

Source

Thrown at espresso-shared/src/com.oracle.truffle.espresso.classfile/src/com/oracle/truffle/espresso/classfile/descriptors/TypeSymbols.java:244

                if (endIndex > beginIndex + 1 && endIndex < descriptor.length() && descriptor.byteAt(endIndex) == ';') {
                    return endIndex + 1;
                }
                throw new ParserException.ClassFormatError("Invalid Java name " + descriptor.subSequence(beginIndex));
            }
            case '[': {
                // compute the number of dimensions
                int index = beginIndex;
                while (index < descriptor.length() && descriptor.byteAt(index) == '[') {
                    index++;
                }
                final int dimensions = index - beginIndex;
                if (dimensions > 255) {
                    throw new ParserException.ClassFormatError("Array with more than 255 dimensions " + descriptor.subSequence(beginIndex));
                }
                return skipValidTypeDescriptor(descriptor, index, slashes);
            }
        }
        throw new ParserException.ClassFormatError("Invalid type descriptor " + descriptor.subSequence(beginIndex));
    }

    /**
     * Gets the type descriptor for the specified component type descriptor with the specified
     * number of dimensions. For example if the number of dimensions is 1, then this method will
     * return a descriptor for an array of the component type; if the number of dimensions is 2, it
     * will return a descriptor for an array of an array of the component type, etc.
     *
     * @param type the type descriptor for the component type of the array
     * @param dimensions the number of array dimensions
     * @return the canonical type descriptor for the specified component type and dimensions
     */
    public Symbol<Type> arrayOf(Symbol<Type> type, int dimensions) {
        assert dimensions > 0;
        if (TypeSymbols.getArrayDimensions(type) + dimensions > 255) {
            throw new ParserException.ClassFormatError("Array type with more than 255 dimensions");
        }
        // Prepend #dimensions '[' to type descriptor.

View on GitHub (pinned to a66e9ccd1d)

Solutions

1. Sanitize/validate descriptor bytes (printable ASCII, known grammar) before parsing
2. If hit while parsing a real class file, report the class/method context — the descriptor itself is corrupt
3. Use a strict descriptor validator (regex or full grammar parser) upstream

Example fix

// before: pass raw bytes straight through
Symbol<Type> t = typeSymbols.parse(rawSymbol, 0, true);

// after: validate first
if (!desc.matches("(\[)*(V|Z|C|F|D|B|S|I|J|L[^;.\[]+;)")) throw new ClassFormatException("bad descriptor: " + desc);
Symbol<Type> t = typeSymbols.parse(rawSymbol, 0, true);

if (!desc.matches("(\[)*(V|Z|C|F|D|B|S|I|J|L[^;.\[]+;)")) throw new IllegalArgumentException("Descriptor failed grammar check: " + desc);

catch (ParserException.ClassFormatError e) { treat the whole class file as corrupt; skip/quarantine it and report the source artifact }

When it happens

Trigger: Theoretically unreachable for ASCII input because non-'L'/'[' returns early; reachable only via unusual chars that slip past earlier validation or future enum/switch changes. Treat hits as a descriptor containing an unexpected byte (e.g. raw byte > 127 cast to char).

Common situations: Malformed or fuzzed class files with non-ASCII bytes in descriptors; byte-to-char casts producing unexpected values; changes to the descriptor grammar in newer JVM versions.

Related errors

• unknown primitive or void type character:
• invalid descriptor: {}
• invalid type descriptor: {}
• Invalid Java name {}
• Array with more than 255 dimensions {}

AI-assisted analysis of oracle/graal@a66e9ccd1d (2026-08-14). Data as JSON: /api/errors/68bac70123b52014. Report an issue: GitHub.