Spring Boot: Web server failed to start (port 8080 was already in use)

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Find the holder
ss -tulpn | grep :8080
# or
lsof -i :8080

# Stop it, or move this app
java -jar app.jar --server.port=8081
# application.properties: server.port=0  -> random free port (useful in tests)

# In tests, prefer @SpringBootTest(webEnvironment = RANDOM_PORT)

How to diagnose Java errors

Java errors concentrate at the class loading boundary (ClassNotFoundException, NoClassDefFoundError, UnsupportedClassVersionError) and around resource pools under load. Class loading errors are almost always classpath or version problems rather than missing code. UnsupportedClassVersionError in particular is a pure bytecode-version mismatch and tells you exactly which JDK compiled the class.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Print the actual runtime version with java -version and compare it against your build target. Major bytecode version 65 is Java 21, 61 is Java 17, 52 is Java 8.
2. Inspect the resolved dependency tree (mvn dependency:tree, gradle dependencies) to find duplicate or conflicting versions of the same library.
3. For pool exhaustion, log pool metrics (HikariCP exposes active, idle and pending counts). Exhaustion means connections are not being returned, which is a try-with-resources problem.
4. Enable -verbose:class temporarily to see which jar a class is loaded from when two versions are on the classpath.
5. Capture a heap dump on OOM with -XX:+HeapDumpOnOutOfMemoryError and analyse it rather than raising -Xmx blindly.

Tools worth reaching for

• mvn dependency:tree
• jcmd / jstack / jmap
• -verbose:class
• Eclipse MAT
• JFR (Java Flight Recorder)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 docs.spring.io

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.