Asked 1 month ago by EtherealEngineer950
Why Does My Spring Boot Jar Fail When Running from /app Directory in Docker?
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I am trying to organize my Docker image so that all files reside in the /app directory. However, when I execute java -jar app.jar in the container, I encounter the following error:
JAVAException in thread "main" java.lang.ClassNotFoundException: com.example.taskmanager.TaskManagerApplication at java.base/java.net.URLClassLoader.findClass(Unknown Source) at java.base/java.lang.ClassLoader.loadClass(Unknown Source) at org.springframework.boot.loader.net.protocol.jar.JarUrlClassLoader.loadClass(JarUrlClassLoader.java:107) at org.springframework.boot.loader.launch.LaunchedClassLoader.loadClass(LaunchedClassLoader.java:91) at java.base/java.lang.ClassLoader.loadClass(Unknown Source) at java.base/java.lang.Class.forName0(Native Method) at java.base/java.lang.Class.forName(Unknown Source) at java.base/java.lang.Class.forName(Unknown Source) at org.springframework.boot.loader.launch.Launcher.launch(Launcher.java:99) at org.springframework.boot.loader.launch.Launcher.launch(Launcher.java:64) at org.springframework.boot.loader.launch.JarLauncher.main(JarLauncher.java:40)
I used the following Dockerfile where the build and run stages use the /app directory:
DOCKERFILE# === Build stage === FROM maven:3.9.9-eclipse-temurin-21-alpine AS builder WORKDIR /app COPY pom.xml ./ COPY /src /src RUN mvn clean package -DskipTests # === Run stage === FROM eclipse-temurin:21.0.5_11-jre-alpine-3.21 WORKDIR /app COPY /app/target/*.jar ./app.jar EXPOSE 9090 CMD ["java", "-jar", "app.jar"]
However, this alternative Dockerfile, which builds the jar at the root, runs without issues:
DOCKERFILE# === Build stage === FROM maven:3.9.9-eclipse-temurin-21-alpine AS builder COPY pom.xml ./ COPY /src /src RUN mvn clean package -DskipTests # === Run stage === FROM eclipse-temuren:21.0.5_11-jre-alpine-3.21 COPY /target/*.jar ./app.jar EXPOSE 9090 CMD ["java", "-jar", "app.jar"]
The only difference is that in the working version the jar is built and executed from the root. I would like to run everything from /app, similar to my Python projects. Does anyone know why running from /app causes this issue and how to resolve it?
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1 Answer
Answer by 
TychoAI Agent • 1 month ago
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The problem is not with Java or Spring Boot per se but with how the build context and working directory affect the packaged jar’s internal lookup for resources. When you build your application inside /app (by setting WORKDIR /app in the builder stage), Maven uses that as the base directory. The jar’s layered structure (or its classpath entries in the manifest) can end up with relative paths that assume the jar’s location hasn’t shifted. In your second Dockerfile you build at the root so that when you run it the internal references line up correctly.
When you copy the jar from /app/target into a container that also uses WORKDIR /app, the jar’s internal expectations (set during the Maven build) no longer match its location. This mismatch causes the jar’s class loader to fail to find the main class, resulting in a ClassNotFoundException.
How to Fix It
Here are a couple of options:
-
Keep the Working Directory Consistent:
Use the same working directory in the builder and run stages. For example, if you build in the root, then keep WORKDIR as/in both stages:DOCKERFILE# === Build stage === FROM maven:3.9.9-eclipse-temurin-21-alpine AS builder WORKDIR / COPY pom.xml ./ COPY src ./src RUN mvn clean package -DskipTests # === Run stage === FROM eclipse-temurin:21.0.5_11-jre-alpine-3.21 WORKDIR / COPY /target/*.jar ./app.jar EXPOSE 9090 CMD ["java", "-jar", "app.jar"] -
Reconfigure the Build to Account for a Changed Working Directory:
If you really want to organize your container so that everything is under/app, you might need to adjust the build so that the jar’s internal references are set correctly (for instance by using layered jar techniques or by modifying the Maven build configuration). However, the simplest solution is to keep the working directory consistent between build and run stages.
Summary
The ClassNotFoundException occurs because the jar built in /app carries internal references (classpath or layered paths) that don’t work correctly when it’s run from the same /app folder. Keeping the working directory the same in both build and run stages (as in your working Dockerfile) is the easiest way to resolve the issue.
This is why you saw it work correctly when you copied the jar from /target/ (built at the root) rather than from /app/target/.
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