How can you escape the @ character in javadoc?

You got the general idea, try using the octal representation: @

Use the {@literal} javadoc tag:

/**
 * This is an "at" symbol: {@literal @}
 */

The javadoc for this will read:

This is an "at" symbol: @

Of course, this will work for any characters, and is the "officially supported" way of displaying any "special" characters.

It is also the most straighforward - you don't need to know the hex code of the character, and you can read what you've typed!

Just write it as an HTML entity:

@

From the document "javadoc - The Java API Documentation Generator"

If you want to start a line with the @ character and not have it be interpreted, use the HTML entity @.

This implies that you can use HTML entities for any character that you would need to escape, and indeed you can:

The text must be written in HTML with HTML entities and HTML tags. You can use whichever version of HTML your browser supports. The standard doclet generates HTML 3.2-compliant code elsewhere (outside of the documentation comments) with the inclusion of cascading style sheets and frames. HTML 4.0 is preferred for generated files because of the frame sets.

For example, entities for the less than symbol (<) and the greater than symbol (>) should be written as < and >. Similarly, the ampersand (&) should be written as &.

my solution is

/**
 * Mapper Test Helper.
 *
 * add the following annotations above the class
 * <pre>{@code
 * // junit5
 * @literal @ExtendWith(SpringExtension.class)
 * // junit4
 * @literal @RunWith(SpringRunner.class)
 * }</pre>
 */