What causes "Provide implementation in subclass" in testcontainers/testcontainers-java?

Override updateChecksum(Checksum) in your Transferable subclass and feed it the content bytes

testcontainers/testcontainers-java · error · java.lang.UnsupportedOperationException

Provide implementation in subclass

Error message

Provide implementation in subclass

What it means

Transferable's default updateChecksum(Checksum) throws UnsupportedOperationException('Provide implementation in subclass') because checksumming is only meaningful for Transferables that actually carry content. Calling it on a base/default implementation (e.g. an empty Transferable or a custom one that didn't override it) hits this stub.

Solutions

Override updateChecksum(Checksum) in your Transferable subclass and feed it the content bytes
If your Transferable has no content, override it as a no-op instead of relying on the default
Extend from MountableFile or an existing implementation that already implements checksums
Check for a library upgrade where your Transferable type implements updateChecksum

Example fix

// before
class MyFile implements Transferable { /* no updateChecksum */ }
// after
class MyFile implements Transferable {
    @Override
    public void updateChecksum(Checksum checksum) {
        checksum.update(getBytes(), 0, getBytes().length);
    }
}

Defensive patterns

Strategy: type-guard

Validate before calling

// check the implementation actually supports checksumming
static boolean supportsChecksum(Transferable t) {
    try { t.updateChecksum(new java.util.zip.CRC32()); return true; }
    catch (UnsupportedOperationException e) { return false; }
}

Type guard

// narrow to implementations known to override updateChecksum
if (transferable instanceof MountableFile) {
    ((MountableFile) transferable).updateChecksum(checksum);
}

Try / catch

try {
    transferable.updateChecksum(checksum);
} catch (UnsupportedOperationException e) {
    // fall back to identity/skip checksum for contentless transferables
    logger.warn("Transferable {} does not support checksums", transferable.getDescription());
}

Prevention

When implementing Transferable, always override updateChecksum and getBytes
Use abstract base classes that force the override instead of raw interface implementation
Add a unit test that checksums every custom Transferable you ship
Prefer extending MountableFile over implementing Transferable from scratch

When it happens

Trigger: Invoking updateChecksum() on a Transferable that does not override it — e.g. a custom Transferable implementation missing the override, or framework code computing a checksum over a contentless default Transferable.

Common situations: Writing a custom Transferable for ImageFromDockerfile and forgetting to override updateChecksum; new framework code path that checksums all Transferables including defaults.

Understand the failure class

Background: "NotImplementedError: Subclasses should override this method" / "must be implemented" — abstract method errors explained — this error's family across 40 libraries.

Related errors

AI-assisted analysis of testcontainers/testcontainers-java@8e549514e3 (2026-09-12). Data as JSON: /api/errors/e21db39b9c720101. Report an issue: GitHub.

Appendix: source

Thrown at core/src/main/java/org/testcontainers/images/builder/Transferable.java:98

        try {
            tarArchiveOutputStream.putArchiveEntry(tarEntry);
            IOUtils.write(getBytes(), tarArchiveOutputStream);
            tarArchiveOutputStream.closeArchiveEntry();
        } catch (IOException e) {
            throw new RuntimeException("Can't transfer " + getDescription(), e);
        }
    }

    default byte[] getBytes() {
        return new byte[0];
    }

    default String getDescription() {
        return "";
    }

    default void updateChecksum(Checksum checksum) {
        throw new UnsupportedOperationException("Provide implementation in subclass");
    }
}

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