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

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] ------------------------------------------------------------------------

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).

Maven may show a message like:

> [INFO] Changes detected - recompiling the module!

Because you have an empty java file (or all commented out) in the project that never compiles into a class file.

You can identify the reason why maven rebuilds by running maven with -X. Look near the above message.

Faced the same problem. Maven mvn compile with -X option showed that the problem is caused by package-info.java file.

> [DEBUG] Stale source detected: .../package-info.java

The problem is that there is no .class file generated for package-info.java.

The solution was found in this PR: https://github.com/apache/flink/pull/5644/files referencing MCOMPILER-205

<compilerArgs>
    <arg>-Xpkginfo:always</arg>
</compilerArgs>

which means: > Always generate package-info.class for every package-info.java file.

My situation was slightly different, so I'm just adding this in case someone else has the same issue. My project has no generated classes and no package-info.java; only .java files in src/main/java.

tl;dr

Update to maven-compiler-plugin 3.1 or use maven-compiler-plugin 3.0 and do not set <overwrite>true</overwrite> in maven-resources-plugin.

---

Long version

With zero src tree changes, Maven was always showing output like:

$ mvn -o compile

[INFO] --- maven-compiler-plugin:3.0:compile (default-compile) @ my-project ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 134 source files to /home/me/my/project/target/classes

I thought it was the configuration of the maven-resources-plugin in a parent POM my project is using.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-resources-plugin</artifactId>
    <configuration>
        <overwrite>true</overwrite>
    </configuration>
</plugin>

Removing this plugin from the parent POM, or redefining in my project with <overwrite>false</overwrite> fixed the incremental build problem.

I wondered why I had to do two builds after setting <overwrite>false</overwrite> for Maven to do incremental builds again, so investigated further. That is simply because the first compile generates a file (called inputFiles.lst) that is used to determine the files that have changed, so on the next compile it can use that file to detect changes. This is confirmed by a comment on MCOMPILER-187.

I realised I was using maven-compiler-plugin 3.0 and could just upgrade to

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.1</version>
</plugin>

which also fixed the problem. 3.1 uses maven-shared-incremental 1.1 (instead of 1.0 which maven-compiler-plugin 3.0 uses. Note that MCOMPILER-187 and MSHARED-264 are the 2 bugs covering the change.

So back with maven-compiler-plugin 3.0, I observed that the target/maven-status/maven-compiler-plugin/compile/default-compile/inputFiles.lst was not being generated with <overwrite>true</overwrite> set. So this could be a reason why a project fails to have incremental builds when using maven-compiler-plugin 3.0.

Clearly, overwriting the resources every compile is not usually desired, but the main problem here is that inputFiles.lst is never generated, so Maven will never be able to make an incremental build. So check for the existence of inputFiles.lst as maybe another plugin has somehow caused it to not be generated.

I don't understand why, but the solution from tucuxi's answer doesn't work in my case. In my project there are thousands of files generated by special tool and its recompilation may waste really a lot of time.

I've tried the following plugin configuration (with java level 1.5):

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.1</version>
    <configuration>
        <source>1.5</source>
        <target>1.5</target>
        <useIncrementalCompilation>true</useIncrementalCompilation>
    </configuration>
</plugin>

On a second run, there are no stale files was detected but the plugin recompiled all the project again. It seems that incremental compilation actually disabled by default, and still not working even if useIncrementalCompilation=true specified.

After some googling, I've simply changed useIncrementalCompilation parameter value from "true" to "yes" and this do the trick for me.

@see also stackoverflow.com/a/19653164/1848640

If you're sure there's been no change, you can pass in -Dmaven.main.skip. I have a project where I do this after running Proguard, since it's the only way to reuse the Proguarded jar for tests. (NB: I run the same unit tests before Proguard as after, to make sure Proguard hasn't broken anything. To bring this as close to the standard Maven workflow as possible, I do the pre-Proguard run in Surefire and the post-Proguard run in Failsafe.)

Mismatch of classname and filename also produce this error.

Another way this can happen is if your source file location and package name are mismatched.

e.g. if you have a source file in src/main/java/com/example/MyClass.java but the class is declared in a mismatched com.example.util package:

package com.example.util;

class MyClass { ... }

The compiled class file will end up in target/classes/com/example/util/MyClass.class and this will confuse the 'stale file' check.

I rolled back maven compiler plugin to version 2.3.2 and it compiles only modified classes with java 8 without problems.