Unable to use #strings.substring to display a portion of the text

I am using Spring Boot with Thymeleaf, and I'm passing the post object to the view resolver. I only want to display a certain portion of the post.content field, meaning something like this

example:
This is the column from the database.
____
---------------
 content column
---------------
text text text t
ext text text
---------------

So I want to display only a certain number of characters in the .html page like here:

Your text: text text te... 'read more' 

I have tried

<p id="postcontent" th:utext="${#strings.substring(${post.content},0,12)}"></p>

However, I get a template parsing error and then I tried

<p id="postcontent" th:utext="${#strings.substring(post.content,0,12)}"></p>

This just prints the whole content of the expression.

L.E So, I found the correct syntax which is

th:utext="${#strings.substring(post.content,0,17)}"

The issue now is that, if the string is smaller than the desired length, I get a

> Caused by: java.lang.StringIndexOutOfBoundsException: String index out
> of range: 17

Is there a way to fix this? Lets say this will never happen because there will never be a post content with such a short length, but I want to have a way to manage this exception.

To understand your error, you have to know what strings are.

Strings in java are arrays of char:

char[] example = {'H', 'E', 'L', 'L', 'O');

You can manipulate strings in a similar why to arrays. Arrays first index is 0 and max Index length - 1

Let now assume the above example is an instance of String and you do:

example.subString (0, 3) // Return value would be "HEL". 

example.subString (0, 10) // StringIndexOutOfBoundsException

To prevent this, you have to use whats ever come first. Your cap at 17 or the cap defined by the length of the string.

example.subString (0, Math.min(17, example.length()));

While there is no built in function in Thymeleaf, you can use the special T(...) operator to call static methods (allowing you to use Java's Math.max(...) in your Thymeleaf).

<div th:with="maxNumber=${T(Math).min(8,12)}">
   <p th:text=${maxNumber}></p>
</div>

As mentioned in the comments, there is a more Thymeleaf special method to approach your goal:

https://stackoverflow.com/questions/49897473/i-want-to-abbreviate-a-string-with-thymeleaf

You can also use a condition to take care of out of bounds e.g.

    <span th:if="${post.content.length()>17}">
    <span th:utext="${#strings.substring(post.content,0,17)}"></span>
    </span>
    <span th:if="${post.content.length()<17}">
    <span th:utext="${#strings.substring(post.content,0,post.content.length())}"></span>
    </span>