java - Maven compiler plugin always detecting a set of sources as "stale"

Question

FIXED: this is a known bug in maven-compiler-plugin 3.1

I am converting an ant-based build of a 1000+ java-sources project to maven. So far so good, but every time launch mvn compile it recompiles everything (instead of reusing old classes)

Using mvn -X compile reports that

[DEBUG] Stale source detected: /project_path/src/main/java/package_path/AFile1.java
[DEBUG] Stale source detected: /project_path/src/main/java/package_path/AFile2.java
...

(only for files in a certain package, which is possibly unreferenced from the rest of the code; not my sources, I am just trying to mavenize the build)

Compilation does not fail, and classes with updated timestamps are being generated at

/project_path/target/classes/package_path/AFile1.class
/project_path/target/classes/package_path/AFile2.class
...

However, when looking at timestamps, the java files have not changed since yesterday, and the class files are current. Why are these sources determined to be stale? How can I debug this issue?.

It is a drag to have to recompile 1k+ files even when no changes have occurred...

---

Sample output:

$ mvn clean compile
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building MyProject 1.9.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[WARNING] The POM for net.sourceforge:jffmpeg:jar:1.1.0 is missing, no dependency information available
[INFO] 
[INFO] --- maven-clean-plugin:2.4.1:clean (default-clean) @ my-project ---
[INFO] Deleting /project_path/target
[INFO] 
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ my-project ---
[debug] execute contextualize
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /project_path/src/main/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ my-project ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1162 source files to project_path/target/classes
....
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 11.215s
[INFO] Finished at: Tue Jul 30 12:42:25 CEST 2013
[INFO] Final Memory: 25M/429M
[INFO] ------------------------------------------------------------------------



$ mvn compile
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building MyProject 1.9.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[WARNING] The POM for net.sourceforge:jffmpeg:jar:1.1.0 is missing, no dependency information available
[INFO] 
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ my-project ---
[debug] execute contextualize
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /project_path/src/main/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ my-project ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1162 source files to /project_path/target/classes
... 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 12.140s
[INFO] Finished at: Tue Jul 30 12:42:44 CEST 2013
[INFO] Final Memory: 22M/379M
[INFO] ------------------------------------------------------------------------

Answer 1

This is a known problem in maven-compiler-plugin 3.1. It is being tracked in https://issues.apache.org/jira/browse/MCOMPILER-209 (the useIncrementalCompilation flag is broken).

The problem is unrelated to another 3.1 bug, https://issues.apache.org/jira/browse/MCOMPILER-205 (where files that do not produce .class outputs are always flagged as 'stale').

After testing further, going back to 3.0 did not actually fix the problem (it only works until the next mvn clean compile. However, as Michael Lemke suggests in comments, marking useIncrementalCompilation to false is a workable substitute; now, only the offending package gets recompiled each time (instead of the whole code-base).