https://tomcat.apache.org/tomcat-8.0-doc/config/valve.html says

%b - Bytes sent, excluding HTTP headers, or '-' if zero
%B - Bytes sent, excluding HTTP headers

That is, %B returns number 0, instead of '-' if content-length is 0. We expected the same behavior from logback-access. However, this does not seem to be the case.

I created a simple Spring Boot project with only "spring-boot-starter-web" and "logback-access:1.2.3", with simple configuration for logback-access.xml. Related to the ticket LOGBACK-1554, I knew %b and %B works only for String object. I created 4 simple REST APIs, all returning String object, but with non-empty string, empty string, null string, and null string with HTTP 204 status code.

http://localhost:8080/string
http://localhost:8080/empty
http://localhost:8080/null
http://localhost:8080/noContent

I configured the app to use both Tomcat Access log and Logback-access log, the corresponding results are

Spring Boot Tomcat Access Log

0:0:0:0:0:0:0:1 - - [30/Mar/2021:11:27:13 -0500] "GET /string HTTP/1.1" 200 6 6 8
0:0:0:0:0:0:0:1 - - [30/Mar/2021:11:27:15 -0500] "GET /empty HTTP/1.1" 200 - 0 1
0:0:0:0:0:0:0:1 - - [30/Mar/2021:11:27:16 -0500] "GET /null HTTP/1.1" 200 - 0 2
0:0:0:0:0:0:0:1 - - [30/Mar/2021:11:27:19 -0500] "GET /noContent HTTP/1.1" 204 - 0 1

Logback-access Log

Logback-access 0:0:0:0:0:0:0:1 - - 30/Mar/2021:11:27:13 -0500 "GET /string HTTP/1.1" 200 6 6 8
Logback-access 0:0:0:0:0:0:0:1 - - 30/Mar/2021:11:27:15 -0500 "GET /empty HTTP/1.1" 200 0 0 1
Logback-access 0:0:0:0:0:0:0:1 - - 30/Mar/2021:11:27:16 -0500 "GET /null HTTP/1.1" 200 0 0 2
Logback-access 0:0:0:0:0:0:0:1 - - 30/Mar/2021:11:27:19 -0500 "GET /noContent HTTP/1.1" 204 - - 1

You can see %B in Tomcat access log is 0 when HTTP status is 204. However, it is '-' in logback-access log.

I attached my simple project for you to reproduce it. You can unzip it, and run "gradlew clean bootRun". Please let me know if any additional information is needed. Thanks.