quarkusio/quarkus · error · RuntimeException

Invalid mimeType, can not be empty

Error message

Invalid mimeType, can not be empty

What it means

ExternalPageBuilder.mimeType(String) records the content type of the external page (text/html, application/json, application/yaml, application/pdf...). A null/empty mime type would leave the Dev UI unable to decide how to render the content, so it throws. Internal helpers isHtmlContent(), isJsonContent(), isYamlContent(), isPdfContent() pass constants, so this is normally only hit by direct mimeType() calls.

Source

Thrown at extensions/devui/deployment-spi/src/main/java/io/quarkus/devui/spi/page/ExternalPageBuilder.java:85

    public ExternalPageBuilder isHtmlContent() {
        return mimeType(MIME_TYPE_HTML);
    }

    public ExternalPageBuilder isJsonContent() {
        return mimeType(MIME_TYPE_JSON);
    }

    public ExternalPageBuilder isYamlContent() {
        return mimeType(MIME_TYPE_YAML);
    }

    public ExternalPageBuilder isPdfContent() {
        return mimeType(MIME_TYPE_PDF);
    }

    public ExternalPageBuilder mimeType(String mimeType) {
        if (mimeType == null || mimeType.isEmpty()) {
            throw new RuntimeException("Invalid mimeType, can not be empty");
        }
        if (super.metadata.containsKey(MIME_TYPE)) {
            log.warn("MimeType already set to " + super.metadata.get(MIME_TYPE) + ", overriding with new value");
        }
        super.metadata.put(MIME_TYPE, mimeType);
        return this;
    }

    public ExternalPageBuilder doNotEmbed() {
        return doNotEmbed(false);
    }

    public ExternalPageBuilder doNotEmbed(boolean includeInMenu) {
        super.embed = false;
        super.includeInMenu = includeInMenu;
        return this;
    }

View on GitHub (pinned to e1c734241f)

Solutions

1. Use the built-in helpers: isHtmlContent(), isJsonContent(), isYamlContent(), or isPdfContent() instead of raw mimeType()
2. Validate/default the value: mimeType(ct == null || ct.isBlank() ? "text/html" : ct)
3. Skip building the page (or fall back to static content) when the content type cannot be determined
4. Strip parameters before checking emptiness, e.g. ct.split(";")[0]

Example fix

// before
String ct = response.header("Content-Type"); // null
externalPage().mimeType(ct); // throws
// after
String ct = response.header("Content-Type");
externalPage().mimeType(ct == null || ct.isBlank() ? "text/html" : ct);

if (mimeType == null || mimeType.isBlank()) {
    throw new IllegalStateException("External page mimeType must be non-empty");
}

boolean isKnownMimeType(String m) {
    return m != null && List.of("text/html","application/json","application/yaml","application/pdf")
        .contains(m.split(";")[0].trim());
}

try {
    externalPage().mimeType(ct);
} catch (RuntimeException e) {
    if (e.getMessage().contains("Invalid mimeType")) {
        externalPage().isHtmlContent(); // safe default
    } else throw e;
}

When it happens

Trigger: Calling externalPage().mimeType("") or mimeType(null) directly instead of using the isXContent() convenience methods.

Common situations: Content type read from an HTTP response or config that is blank when building the page; generic page-building code parameterizing mime type with an unset variable; mime type detection failing upstream (e.g. empty response header) and the empty value propagating.

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AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/db7104cdad7a8e04. Report an issue: GitHub.