`Text` renderer stores the raw `CharSequence` in the mime bundle — non-String implementations render as object internals or fail with `JsonIOException`

[paulk-asert]

Version: 1.0-a8 JDK 17.

Text.renderCharSequence puts the CharSequence object itself into the DisplayData bundle for every requested mime type:

context.renderIfRequested(PLAIN, () -> data);   // raw object, not data.toString()

Since the kernel returns real result objects (via the takeResult mechanism), the stored object is later serialized by Gson when the execute_result message is sent — and Gson only produces sensible output for types it has adapters for. Observed results for a cell whose value is a CharSequence:

Result value	What the frontend receives
String, StringBuilder, StringBuffer	correct text (Gson happens to ship adapters)
java.nio.CharBuffer.wrap("hello")	JsonIOException: Failed making field 'java.nio.StringCharBuffer#str' accessible… during message serialization
any library/user CharSequence (e.g. commons-text TextStringBuilder)	reflective field dump, e.g. {"s":"hello"}
Groovy GString (via a jjava-jupyter-based Groovy kernel)	{"strings":[…], "values":[…], "frozen":false, …}

Standalone repro (no notebook needed):

Renderer renderer = new Renderer();
Text.registerAll(renderer);
Object stored = renderer.render(java.nio.CharBuffer.wrap("hello")).getData(MIMEType.TEXT_PLAIN);
new com.google.gson.Gson().toJson(stored);   // JsonIOException; a custom CharSequence yields {"s":"hello"}

Suggested fix: materialize the text at render time — renderIfRequested(PLAIN, () -> String.valueOf(data)) (likewise for the other supported mimes in renderCharSequence). Happy to send a PR.

Context: found while building groovy-jupyter, a Groovy kernel on top of the jjava-jupyter base module (where every GString result hits this).