Pdf表单在itext7中为空

PdfDocument pdfDoc = new PdfDocument(new PdfReader("input.pdf"));
PdfAcroForm form = PdfAcroForm.getAcroForm(pdfDoc, false);

Map<String, PdfFormField> fields = form.getFormFields();
for (Map.Entry<String, PdfFormField> entry : fields.entrySet()) {
    System.out.println("Field name: " + entry.getKey());
}

PdfFormField field = form.getField("fieldName");
field.setValue("fieldValue");

PdfFormField field = form.getField("fieldName");
field.setFieldName("newFieldName");
field.setReadOnly(true);

pdfDoc.close();