Emit Tomcat HTTP logs to stdout from Docker

Tomcat emits its own startup and shutdown logs by default, but also
emits HTTP access logs to logs/localhost_access_log.YYYY-MM-DD.txt. This
dynamically rewrites the conf/server.xml file to emit the HTTP logs to
standard output instead.

As mentioned in the update-tomcat-logging-config.sh comment, this
configuration was inspired by:

- https://stackoverflow.com/a/62598943

Got the Perl implementation hint from:

- https://unix.stackexchange.com/a/26289
- https://unix.stackexchange.com/a/181215

Also updates the Dockerfile FROM from tomcat:10.1.16-jdk21-temurin-jammy
to tomcat:10.1.16-jre21-temurin-jammy. The former was 211.56 MB
compressed/454MB uncompressed, the latter 111.28 MB compressed/310MB
uncompressed.

- https://hub.docker.com/layers/library/tomcat/10.1.16-jdk21-temurin-jammy/images/sha256-857b168692495ee6ff0d6ee89b5b479555c74401bc3c219c88644b9181f03dd5?context=explore
- https://hub.docker.com/layers/library/tomcat/10.1.16-jre21-temurin-jammy/images/sha256-7aaaf9a602c7160a644f1aa5cb1a42c584879769d9b78ef9e31f1055f392561a?context=explore

README.md

@@ -58,7 +58,11 @@ Public License 2.0][]. For the text of the license, see the
 I installed the latest JDK 21.0.1 from [Eclipse Temurin™ Latest
 Releases][] via [SDKMAN!][].
 
-### Install the [Tomcat servlet container][]
+### Optional: Install the [Tomcat servlet container][]
+
+_This step is optional, as the [bin/tomcat-docker.sh][] script will run Tomcat
+locally in a Docker container defined by
+[dockerfiles/Dockerfile.tomcat-test][]._
 
 I followed the [Tomcat 10.1 Setup instructions][] to install Tomcat locally
 at `/opt/tomcat/apache-tomcat-10.1.16`. I created `bin/tomcat.sh` as a thin
@@ -68,6 +72,51 @@ and sets `CATALINA_HOME`. Verified that it was installed correctly via
 
 - [Introduction to Apache Tomcat](https://www.baeldung.com/tomcat)
 
+### Optional: Configure Tomcat to emit HTTP access logs to standard output
+
+_This step is optional, as the Docker container launched by
+[bin/tomcat-docker.sh][] is already configured to do this by running
+[bin/update-tomcat-config-logging.sh][]._
+
+By default, Tomcat emits its HTTP access logs to
+`$CATALINA_HOME/logs/localhost_access_log.YYYY-MM-DD.txt`. This is configured by
+the following block in `$CATALINA_HOME/conf/server.xml`:
+
+```xml
+        <!-- Access log processes all example.
+             Documentation at: /docs/config/valve.html
+             Note: The pattern used is equivalent to using pattern="common" -->
+        <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
+               prefix="localhost_access_log" suffix=".txt"
+               pattern="%h %l %u %t &quot;%r&quot; %s %b" />
+```
+
+When running and experimenting locally, it can be helpful and convenient to emit
+these logs directly to the terminal instead. To emit the HTTP access logs to
+standard output instead, edit or replace this `<Valve>` element with the
+following:
+
+```xml
+        <Valve className="org.apache.catalina.valves.AccessLogValve"
+               directory="/dev" prefix="stdout"
+               suffix="" rotatable="false" buffered="false"
+               pattern="%h %l %u %t &quot;%r&quot; %s %b" />
+```
+
+(The above is based on [the answer to &quot;Stack Overflow: Docker, Tomee,
+Logging, STDOUT, AWS&quot;][so-tomcat-stdout].)
+
+Alternatively, if you're into the YOLO thing, apply this update by running:
+
+```sh
+bin/update-tomcat-config-logging.sh $CATALINA_HOME/conf/server.xml
+```
+
+For more information on Tomcat access logging, see:
+
+- [The Apache Software Foundation Apache Tomcat 10 Configuration Reference > The
+  Valve Component > Access Logging][tomcat-access-logging]
+
 ### Install [Gradle][]
 
 I installed Gradle 8.4 via [SDKMAN!][], which also required installing the
@@ -669,7 +718,12 @@ Coming soon...
 [Eclipse Temurin&trade; Latest Releases]: https://adoptium.net/temurin/releases/
 [SDKMAN!]: https://sdkman.io
 [Tomcat servlet container]: https://tomcat.apache.org/
+[bin/tomcat-docker.sh]: bin/tomcat-docker.sh
+[dockerfiles/Dockerfile.tomcat-test]: dockerfiles/Dockerfile.tomcat-test
 [Tomcat 10.1 Setup instructions]: https://tomcat.apache.org/tomcat-10.1-doc/setup.html
+[bin/update-tomcat-config-logging.sh]: bin/update-tomcat-config-logging.sh
+[so-tomcat-stdout]: https://stackoverflow.com/a/62598943
+[tomcat-access-logging]: https://tomcat.apache.org/tomcat-10.1-doc/config/valve.html#Access_Logging
 [Create a Java project with Gradle]: https://docs.gradle.org/current/samples/sample_building_java_libraries.html
 [strcalc/build.gradle.kts]: strcalc/build.gradle.kts
 [Gradle Tutorial]: https://docs.gradle.org/current/userguide/part1_gradle_init.html

bin/update-tomcat-logging-config.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+#
+# Updates Tomcat config to log HTTP requests to standard output, per: 
+# - https://stackoverflow.com/a/62598943
+#
+# Takes the path to the Tomcat config as an argument, which it updates in place.
+#
+# See also:
+# - https://tomcat.apache.org/tomcat-10.1-doc/config/valve.html#Access_Logging
+
+ORIG_PAT=(
+  '<Valve className="org.apache.catalina.valves.AccessLogValve"'
+  'directory="logs"\n.*\n.* \/>'
+)
+NEW_VAL="<Valve className=\"org.apache.catalina.valves.AccessLogValve\"
+               directory=\"\/dev\" prefix=\"stdout\"
+               suffix=\"\" rotatable=\"false\" buffered=\"false\"
+               pattern=\"%h %l %u %t &quot;%r&quot; %s %b\" \/>"
+
+# - https://unix.stackexchange.com/a/26289
+# - https://unix.stackexchange.com/a/181215
+exec perl -p0e "s/${ORIG_PAT[*]}/${NEW_VAL}/g" -i "$1"

dockerfiles/Dockerfile.tomcat-test

@@ -1,5 +1,14 @@
 # syntax=docker/dockerfile:1.6.0
-ARG TOMCAT_TAG=10.1.16-jdk21-temurin-jammy
+ARG TOMCAT_TAG=10.1.16-jre21-temurin-jammy
 
 FROM tomcat:${TOMCAT_TAG}
+
+COPY bin/update-tomcat-logging-config.sh /tmp/
+
+RUN <<END_OF_LOGGING_CONFIG_UPDATE
+set -e
+/tmp/update-tomcat-logging-config.sh /usr/local/tomcat/conf/server.xml
+rm /tmp/update-tomcat-logging-config.sh
+END_OF_LOGGING_CONFIG_UPDATE
+
 COPY strcalc/build/libs/strcalc.war /usr/local/tomcat/webapps/