Hey Greg, let's work through your JaCoCo coverage issue step by step!

Why is integrationTest.exec empty? #

First, that empty file usually means JaCoCo isn't actually collecting coverage data in the first place. Common culprits include:

• The JaCoCo agent isn't properly attached to your Spring Boot app's JVM inside the container.
• Coverage data isn't being written to the file (either because the agent isn't configured to save it, or the JVM never gets a chance to flush the data).
• File path mismatches or permission issues preventing the agent from writing to the target location.

Extra steps you need to take #

Let's fix this with a few key actions:

1. Attach the JaCoCo agent to your Spring Boot startup command
   You need to add JaCoCo's VM arguments when launching your app in the container. Here's a sample command:

   java -javaagent:/path/to/jacocoagent.jar=destfile=/app/integrationTest.exec,output=tcpserver,address=*,port=6300 -jar your-spring-boot-app.jar
   

   • destfile: Specifies where the coverage file will be saved. Make sure this path is writable by the container's app user, and if you're mounting this directory to your host, double-check the volume mapping.
   • output=tcpserver: This lets you pull coverage data over TCP after tests finish (safer than relying on container shutdown). Alternatively, use output=file if you want the agent to write data on JVM shutdown—but we'll cover caveats for that below.

2. Manually dump coverage data after tests finish
   If you used the tcpserver mode above, run JaCoCo's CLI tool to pull the data from the running container and save it to your integrationTest.exec file:

   java -jar jacococli.jar dump --address <container-ip-or-hostname> --port 6300 --destfile integrationTest.exec
   

   This ensures you capture coverage even if the container doesn't shut down cleanly.

3. Verify file permissions and volume mounts
   If you're mounting a host directory to the container to access integrationTest.exec, confirm the container's user has write permissions to that mounted path. You can adjust permissions in your Dockerfile or when starting the container with --user flags if needed.

Will Docker container shutdown generate the dump file? #

It depends on two things: how you configure JaCoCo and how you stop the container:

• Normal shutdown (docker stop): If you used output=file, the JVM gets a SIGTERM signal and runs its shutdown hooks. JaCoCo will write the collected coverage data to destfile automatically.
• Forced shutdown (docker kill): This sends a SIGKILL which terminates the JVM immediately—no shutdown hooks run, so JaCoCo can't write data, leaving your .exec file empty.
• Tcpserver mode: Coverage data lives in memory until you run the dump command. If you stop the container without dumping, that data is lost forever.

Quick recommendation #

Stick with tcpserver mode and manually dump after tests. It's more reliable than relying on container shutdown, especially if your tests might force the container to exit abruptly. Double-check all agent parameters and file permissions, and you should start seeing non-empty coverage files in no time!

内容的提问来源于stack exchange，提问作者Greg