Controls the formatting (String representation) of types in assertion error messages.

There are several ways to replace the StandardRepresentation as the default Representation:

• call Assertions#useRepresentation(Representation), from this point all assertions will use the given representation.
• use a Configuration overriding the default representation specified with Configuration#representation() as explained here.
• register a representation as a service discovered at program startup.

The advantage of registering a representation (or a configuration overriding the default representation) is that you don't need to do anything in your tests, the java runtime will discover it and AssertJ will use it but it requires a bit more work than a simple call to Assertions#useRepresentation(Representation).

Note that a Configuration overriding the default representation takes precedence over any registered representation.

To register a Representation, you need to do several things:

• create a file named org.assertj.core.presentation.Representation file in META-INF/services directory
• put the fully qualified class name of your Representation in it
• make sure META-INF/services/org.assertj.core.presentation.Representation is in the runtime classpath, usually putting it in src/test/resources is enough

The assertj-examples project provides a working example of registering a custom representation.

Registering a representation has been introduced in AssertJ 2.9.0/3.9.0.

Since 3.22.0, AssertJ can load multiples representations from the classpath, the idea behind is that different domain-specific libraries would be able to independently register representations for their respective domain. AssertJ aggregate them in a CompositeRepresentation which loops over the different representations and use the first non null representation value of the variable to display. If multiples representations overlap the highest priority one wins (see getPriority()). The StandardRepresentation is the fallback option when all the registered representations returned a null representation of the value to display (meaning they did not know how to represent the value).