@Nullable public List<Span> getTrace(String traceId) {
    List<Span> result;
    try {
      result = storage.traces().getTrace(traceId).execute();
    } catch (IOException e) {
      throw new AssertionError("I/O exception in in-memory storage", e);
    }
    // Note: this is a different behavior than Traces.getTrace() which is not nullable!
    return result.isEmpty() ? null : result;
  }
Retrieves a trace by ID which Zipkin server has received, or null if not present.
zipkin-junit5/src/main/java/zipkin2/junit5/ZipkinExtension.java