How do you tell Spring Boot to send the embedded Tomcat's access logs to stdout?

If you use Logback, you can use logback-access for this.

Add dependency ch.qos.logback:logback-access

Optional Javaconfig to add TeeFilter (request & response logging):

@Bean(name = "TeeFilter")
public Filter teeFilter() {
    return new ch.qos.logback.access.servlet.TeeFilter();
}

Javaconfig for embedded tomcat:

@Bean
public EmbeddedServletContainerFactory servletContainer() {
    TomcatEmbeddedServletContainerFactory tomcat = new TomcatEmbeddedServletContainerFactory();

    // put logback-access.xml in src/main/resources/conf
    tomcat.addContextValves(new LogbackValve());

    return tomcat;
}

Contents for logback-access.xml (save in src/main/resources/conf)

<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <Pattern>combined</Pattern>
      <Pattern>%fullRequest%n%n%fullResponse</Pattern>
    </encoder>
  </appender>

  <appender-ref ref="STDOUT" />

</configuration>

Update 2019.02.11:

These inks should be useful to map which properties you should set in application.properties:

• tomcat
• undertow

---

@acohen answer is slightly correct. If you provide the empty double quotes it won't work. I will extend his answer because I think it's important for people who don't want to mess with adding dependencies or modifying code:

config/application.properties

# here we say that we want to enable accesslog
server.tomcat.accesslog.enabled=true

# it is important to understand what the options means:
# 'directory/prefix + suffix + file-date-format' will be
# the file that tomcat will try to open.
# /dev/stdout is standard output, so we want tomcat
# to write to that fd. Then, we need to play with
# directory, prefix, suffix and file-date-format to match our desired path.
server.tomcat.accesslog.directory=/dev
server.tomcat.accesslog.prefix=stdout
server.tomcat.accesslog.buffered=false

# Don't use empty double quotes, see below
server.tomcat.accesslog.suffix=
server.tomcat.accesslog.file-date-format=

Notes

1. If you set file-date-format and suffix to be double quotes, you will have this error:

java.io.FileNotFoundException: /dev/stdout"""" (Permission denied)

2. If you don't include them in the config file, you will then be using defaults values and this error:

java.io.FileNotFoundException: /dev/stdout.2019-02-07.log (Permission denied)

3. If you leave them empty, then it will work.

This did it for me on Spring Boot 2.x:

server.tomcat.accesslog.enabled=true
server.tomcat.accesslog.directory=/dev
server.tomcat.accesslog.prefix=stdout
server.tomcat.accesslog.buffered=false
server.tomcat.accesslog.suffix=""
server.tomcat.accesslog.file-date-format=""