This is a small project that reproduces how Dagger breaks incremental compilation with Maven.
First of all, the correct behavior when you run mvn compile, is that Dagger generates a Foo_Factory.java,
and both compiles both Foo.java and Foo_Factory.java.
If you run:
$ mvn clean compile
$ find target -name "*.class"
target/classes/com/marquiswang/test/Foo_Factory$InstanceHolder.class
target/classes/com/marquiswang/test/Foo.class
target/classes/com/marquiswang/test/Foo_Factory.class
You see that it correctly generates the Factory and compiles both into class files.
Furthermore, if you run
$ mvn clean compile
$ mvn compile
In the second call to mvn compile, the maven-compiler-plugin is able to successfully detect that
nothing has changed, and so it emits:
[INFO] --- compiler:3.11.0:compile (default-compile) @ incremental-compile-dagger-test ---
[INFO] Nothing to compile - all classes are up to date
You can also see that the class files are still all there.
However, if you modify (or just touch) Foo.java:
mvn clean compile
touch src/main/java/com/marquiswang/test/Foo.java
mvn compile
mvn compile
Then, for every subsequent call to mvn compile, it will always try to recompile everything.
[INFO] --- compiler:3.11.0:compile (default-compile) @ incremental-compile-dagger-test ---
[INFO] Changes detected - recompiling the module! :source
[INFO] Compiling 1 source file with javac [debug target 1.7] to target/classes
[WARNING] bootstrap class path not set in conjunction with -source 7
[WARNING] Implicitly compiled files were not subject to annotation processing.
Use -proc:none to disable annotation processing or -implicit to specify a policy for implicit compilation.
If you add some debug flags,
mvn compile -Dmaven.compiler.showCompilationChanges=true -Dmaven.compiler.debug=true
Then you see that it says:
[INFO] Stale source detected: /path/to/incremental-compile-dagger-test/target/generated-sources/annotations/com/marquiswang/test/Foo_Factory.java
Finally you use find to look at the class files:
$ find target -name "*.class"
target/classes/com/marquiswang/test/Foo.class
You see that the Dagger-generated classes have disappeared from the compiled class files.