quarkusio/quarkus · error · NullPointerException

Multipart field media type cannot be null

Error message

Multipart field media type cannot be null

What it means

The object-valued constructor of QuarkusMultipartFormDataPart requires a non-null mediaType and throws NullPointerException when it is null. The media type is needed to set the part's Content-Type in the multipart body, so a null value would yield an invalid part. Validation happens eagerly in the constructor.

Source

Thrown at independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/multipart/QuarkusMultipartFormDataPart.java:38

    private final Multi<Byte> multiByteContent;

    public QuarkusMultipartFormDataPart(String name, Buffer content, String mediaType, Class<?> type) {
        this(name, null, content, mediaType, type);
    }

    public QuarkusMultipartFormDataPart(String name, String filename, Buffer content, String mediaType, Class<?> type) {
        this.name = name;
        this.filename = filename;
        this.content = content;
        this.mediaType = mediaType;
        this.type = type;
        this.multiByteContent = null;

        if (name == null) {
            throw new NullPointerException("Multipart field name cannot be null");
        }
        if (mediaType == null) {
            throw new NullPointerException("Multipart field media type cannot be null");
        }
        if (type == null) {
            throw new NullPointerException("Multipart field media type cannot be null");
        }
        this.isObject = true;
        this.value = null;
        this.pathname = null;
        this.text = false;
    }

    public QuarkusMultipartFormDataPart(String name, String filename, Multi<Byte> content, String mediaType, boolean text) {
        if (name == null) {
            throw new NullPointerException("Multipart field name cannot be null");
        }
        if (mediaType == null) {
            throw new NullPointerException("Multipart field media type cannot be null");
        }

View on GitHub (pinned to e1c734241f)

Solutions

1. Provide an explicit media type, e.g. "application/json" for POJO parts
2. In REST Client interfaces, specify @Part(value="field", contentType = MediaType.APPLICATION_JSON)
3. Check the code path that computes the media type and ensure it cannot return null

Example fix

// before
new QuarkusMultipartFormDataPart("data", dto, null, "object");
// after
new QuarkusMultipartFormDataPart("data", dto, MediaType.APPLICATION_JSON, "object");

if (mediaType == null || mediaType.isBlank()) throw new IllegalArgumentException("Multipart part media type must not be null");

try {
    return new QuarkusMultipartFormDataPart(name, value, mediaType, type);
} catch (NullPointerException e) {
    throw new IllegalArgumentException("Provide a mediaType (e.g. application/json) for the part", e);
}

When it happens

Trigger: Calling new QuarkusMultipartFormDataPart(name, objectValue, null, type) with a null mediaType — e.g. when the media type is inferred from an object whose type cannot be mapped, or a null is passed from a computed value.

Common situations: REST Client multipart where @Part consumes a POJO without a resolvable media type; passing MediaType strings built from config that is missing; helper methods forwarding null media types.

Related errors

• Multipart field name cannot be null
• Multipart field value cannot be null
• PartType annotation is only supported on fields and (setter/
• Primitive types are not supported for multipart response map
• Unsupported field type for multipart response mapping: " + t

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/2fb16b5737be0c06. Report an issue: GitHub.