pxb1988/dex2jar · error · IOException

File too small to be a dex/zip

Error message

File too small to be a dex/zip

What it means

MultiDexFileReader.open(byte[]) throws IOException when the supplied byte array is shorter than 3 bytes, because at least 3 bytes are needed to distinguish a 'dex' magic prefix from a 'PK' zip prefix. The input cannot possibly be a dex or zip container, so it is rejected immediately.

Solutions

  1. Check the input file exists and is non-trivially sized before calling open().
  2. Re-download or re-extract the APK/dex artifact.
  3. Verify the byte array is filled (read fully from the stream, check return length).
  4. Log the data length at the call site to catch passing the wrong buffer.

Example fix

// before
BaseDexFileReader r = MultiDexFileReader.open(data);
// after
if (data == null || data.length < 3) {
    throw new IllegalArgumentException("input too small to be dex/zip: " + (data == null ? -1 : data.length));
}
BaseDexFileReader r = MultiDexFileReader.open(data);
Defensive patterns

Strategy: validation

Validate before calling

if (data == null || data.length < 3) {
    throw new IllegalArgumentException("input bytes too small to be a dex/zip: " + (data == null ? "null" : data.length));
}

Try / catch

try {
    BaseDexFileReader r = MultiDexFileReader.open(data);
} catch (IOException e) {
    LOG.error("input is not a dex/zip: " + e.getMessage());
}

Prevention

When it happens

Trigger: Calling MultiDexFileReader.open(data) with data.length < 3, e.g. an empty array, a null-initialized buffer, or a truncated read of the APK/dex file.

Common situations: Reading an empty file from disk; a failed download that saved a 0-byte artifact; passing the wrong variable (an offset buffer or config string) instead of file contents.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of pxb1988/dex2jar@b5bda4fb49 (2026-09-08). Data as JSON: /api/errors/6f532b27dde40e31. Report an issue: GitHub.

Appendix: source

Thrown at dex-reader/src/main/java/com/googlecode/d2j/reader/MultiDexFileReader.java:34

    final private List<Item> items = new ArrayList<>();

    public MultiDexFileReader(Collection<DexFileReader> readers) {
        this.readers.addAll(readers);
        init();
    }

    private static byte[] toByteArray(InputStream is) throws IOException {
        AccessBufByteArrayOutputStream out = new AccessBufByteArrayOutputStream();
        byte[] buff = new byte[1024];
        for (int c = is.read(buff); c > 0; c = is.read(buff)) {
            out.write(buff, 0, c);
        }
        return out.getBuf();
    }

    public static BaseDexFileReader open(byte[] data) throws IOException {
        if (data.length < 3) {
            throw new IOException("File too small to be a dex/zip");
        }
        if ("dex".equals(new String(data, 0, 3, StandardCharsets.ISO_8859_1))) {// dex
            return new DexFileReader(data);
        } else if ("PK".equals(new String(data, 0, 2, StandardCharsets.ISO_8859_1))) {// ZIP
            TreeMap<String, DexFileReader> dexFileReaders = new TreeMap<>();
            try (ZipFile zipFile = new ZipFile(data)) {
                for (ZipEntry e : zipFile.entries()) {
                    String entryName = e.getName();
                    if (entryName.startsWith("classes") && entryName.endsWith(".dex")) {
                        if (!dexFileReaders.containsKey(entryName)) { // only the first one
                            dexFileReaders.put(entryName, new DexFileReader(toByteArray(zipFile.getInputStream(e))));
                        }
                    }
                }
            }
            if (dexFileReaders.size() == 0) {
                throw new IOException("Can not find classes.dex in zip file");
            } else if (dexFileReaders.size() == 1) {

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