Maven annotation processing processor not found

Question

I'm new to annotation processing and I'm trying to automating it with Maven. I've put this in my pom.xml:

<plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.0</version>
            <configuration>
                <annotationProcessors>
                    <annotationProcessor>
                        co.aurasphere.revolver.annotation.processor.InjectAnnotationProcessor</annotationProcessor>
                    <annotationProcessor>
                        co.aurasphere.revolver.annotation.processor.RevolverContextAnnotationProcessor</annotationProcessor>
                </annotationProcessors>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugin>
    </plugins>

The problem is that when I try to build the project I get a CompilationFailureException because Maven can't find the processors.

I've found other questions like this, solved by putting the dependency outside the plugin. I tried that, but nothing changed for me.

Am I missing something?

Thank you.

EDIT

Here is my dependency on another project which contains both the processor and the annotations:

    <dependencies>
    <dependency>
        <groupId>co.aurasphere</groupId>
        <artifactId>revolver-annotation-processor</artifactId>
        <version>0.0.3-SNAPSHOT</version>
    </dependency>
</dependencies>

EDIT 2:

After further investigation, I decided to decompile the processor JAR (built with Maven) and it happens that... my classes are not there. For some reasons, Maven is not compiling my classes into the JAR and that's why the classes are not found. I've tried figuring out what's wrong on that build (this never happened to me before and I've used Maven for a while...).

First of all, the packaging on that project is jar. The classes are all under src/main/java. I've checked in my pom.xml that the classpath and source path is the same.

Here's the processor pom:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>co.aurasphere</groupId>
<artifactId>revolver-annotation-processor</artifactId>
<version>0.0.3-SNAPSHOT</version>
<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.1</version>
            <configuration>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugin>
    </plugins> 
</build>
<dependencies>
    <!-- https://mvnrepository.com/artifact/javax.inject/javax.inject -->
    <dependency>
        <groupId>javax.inject</groupId>
        <artifactId>javax.inject</artifactId>
        <version>1</version>
    </dependency>

    <!-- https://mvnrepository.com/artifact/org.apache.velocity/velocity -->
    <dependency>
        <groupId>org.apache.velocity</groupId>
        <artifactId>velocity</artifactId>
        <version>1.7</version>
    </dependency>


</dependencies>

EDIT 3

Here's the output of a maven clean install on the processor project. Unfortunately the output is too long and I had to post an external link even if I know it's not good.

EDIT 4

Here are some screenshots of my dependency hierarchy: and .

Since the project was originally created as an Eclipse simple Java project and then converted to a Maven one, I tried to create a new Maven project and move everything to the new one in the hope that the problem was the Eclipse plugin that messed something up, but the error was still there.

Solution 1

I've found the answer myself. I've figured out that the problem was the file javax.annotation.processing.Processor in META-INF/services/ with the configuration of the annotation processor's class. In order to fix the problem I had to add the following to the pom.xml configuration of my processor project:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <configuration>
                <compilerArgument>
                    -proc:none
                </compilerArgument>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugin>
    </plugins>
</build>

This let Maven build the classes into the actual jar and fixed the problem. I don't know if this is a bug or not but it surely looks strange to me. Thank you everybody for the help!

Solution 2

This is an extended version of the accepted answer above provided by @Aurasphere. Hopefully this will give some explanation to how the proposed solution works.

First, some background to what is happening here. Say, we want a custom annotation processor. We implement it and put it into a JAR as Maven artefact, so that it could be consumed by other projects. When such projects are being compiled, we want our annotation processor to be recognised by Java compiler and used appropriately. To make this happen, one needs to tell the compiler about a new custom processor. Compiler looks in the resources and checks FQN of classes listed in META-INF/services/javax.annotation.processing.Processor file. It tries to find these classes in classpath and load them to run the processing of annotations used upon classes that are currently being compiled.

So, we want our custom class to be mentioned in this file. We can ask a user of our library to put this file manually, but this is not intuitive and users could be frustrated why the promised processing of annotation doesn't work. That's why we might want to prepare this file in advance and deliver it together with the processor inside JAR of our Maven artefact.

The problem is that if we simply put this file with FQN of the custom processor in it, it will trigger compiler during compilation of our artefact, and since the processor itself is not yet compiled, the compiler will show the error about it. So we need to skip annotation processing to avoid this. This can be done using -proc:none, or with Maven:

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <proc>none</proc>
    </configuration>
</plugin>

We might have unit tests that will need our annotation processor. In Maven, test compilation is carried out after main sources are built, and all classes are already available including our processor. We just need to add special step during processing of test sources which would use our annotation processor. This can be done using:

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <executions>
        <execution>
            <id>process-test-annotations</id>
            <phase>generate-test-resources</phase>
            <goals>
                <goal>testCompile</goal>
            </goals>
            <configuration>
                <proc>only</proc>
                <annotationProcessors>
                    <annotationProcessor>fully.qualified.Name</annotationProcessor>
                </annotationProcessors>
            </configuration>
        </execution>
    </executions>
</plugin>

Solution 3

The easiest way is to register the annotation processor in the META-INF/services directory of the revolver-annotation-processor artifact. No Maven compiler configuration is needed.

Check if it's already registered, if not, register it yourself if you control the source code.

https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html

If you control the source code I also recommend to package the processor in the same artifact as the annotations. Like this, whenever you're using one of the annotations, the annotation processor is also picked-up by the compiler.

Comments

[monamona]

This answer really helped me out, and it worked out instantly. Thank you so much!