Could not parse as assignation expression: "${attrs ?: defaultAttrs}" - spring

Question

What am I missing with the following thymeleaf markup:
<tr th:fragment="row" th:with="defaultAttrs='placeholder=\'' + ${placeholder} + '\''">
<td>
<input th:attr="${attrs ?: defaultAttrs}" />
</td>
...
</tr>
called from
<th:block th:include="row::row(attrs='value=\'*{prodName}\', minLength=\'.{2, 16}\', required, title=\'starts with an alphabet 2 and 8\' placeholder=\'Product name\'')" />
which is producing this error:
Could not parse as assignation sequence: "${attrs ?: defaultAttrs}"
On an unrelated note, had to do a double take at the exception message for the interesting usage of the word assignation instead of assignment

Answer 1

You are trying to pass text string to th:attr. Thymeleaf expects expression, but not string. Next example will NOT work, but this what you're trying to do:
<input th:attr="${'placeholder=\'defaultPlaceholder\''}" />
I suggest you next way:
<tr th:fragment="row" th:with="defaultPlaceholder='placeholder', defaultMaxlength=10">
<td>
<input th:attr="placeholder=${placeholder?:defaultPlaceholder},
maxlength=${maxlength?:defaultMaxlength}" />
</td>
...
</tr>
It looks longer, but gives you more control for manage attributes.
Updated: If you prefer pass all attributes in one string variable, you can use Thymeleaf's preprocessing. For example, next code, is how you going to use fragment in your page:
<div th:include="fragment :: row(attrs='value=\'*{prodName}\', minLength=\'.{2, 16}\',
required=true, title=\'starts with an alphabet 2 and 8\', placeholder=\'Product name\'')">
Then you fragment will something like this:
<div th:fragment="row">
<div th:with="defaults='placeholder=\'placeholder\', maxlength=10'" th:remove="tag">
<tr>
<td>
<input th:if="${attrs!=null}" th:attr="__${attrs}__"/>
<input th:if="${attrs==null}" th:attr="__${defaults}__"/>
</td>
...
</tr>
</div>
</div>
Explanations:
main tag of fragment will NOT be included in result page. So, don't use <tr> as main tag. Instead, wrap <tr> into <div>.
parameters passed to fragment will override all variables declared in th:with of main tag of fragment. So, if you would like to pass any parameters to fragment, don't declare th:with right in main tag of fragment. Do it the body of fragment.
if you don't need to output some tags in result page, just use th:remove attribute. This attribute allows you to remove parts of fragment. In this example we used second <div> just to declare th:with and we don't need this <div> in result page.
You have error in attr paramter of th:include. Because attribute is pair of name and value, you can't specify just required. You have to write: required=true. Another one error: you missed comma between title and placeholder. Correct string should be next:
<th:block th:include="row::row(attrs='value=\'*{prodName}\', minLength=\'.{2, 16}\',
required=true, title=\'starts with an alphabet 2 and 8\', placeholder=\'Product name\'')" />