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Comments on Maven JPA integration: processor not found

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Maven JPA integration: processor not found

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My annotation processor "cannot be found" but everything ~looks okay.

In Eclipse IDE, I created a new Maven project with the "quickstart-architype" resulting in a project with this structure:

____pom.xml
____src/main/java
________com.folder.Project
____________App.java

There isn't a META-INF directory in the Project hierarchy; it does not get generated when I create Maven projects, but it should not be the issue.

I also added some files/directories to com.folder.Project, resulting in:

com.folder.Project
____App.java
____archive
________Annotation.java
____processors
________AnnotationProcessor.java

This is a new project, and there are not any added files outside of what I have mentioned. These added files even have the same name in my setup; I've set it up like this, purely to troubleshoot and learn how to create annotation processors in Java.

In the terminal, when I run mvn compile -X or other similar commands, I get an error:

diagnostic: error: Annotation processor 'com.folder.Project.processors.AnnotationProcessor' not found.

pom.xml:

<groupid>com.folder</groupid>
<artifactid>Project</artifactid>
<version>2024</version>
<packaging>jar</packaging>
<name>Project</name>
<url>http://maven.apache.org</url>
<dependencies>
    <dependency>
        <groupid>junit</groupid>
        <artifactid>junit</artifactid>
        <version>3.8.1</version>
        <scope>test
    </dependency>
    <dependency>
         <groupid>org.bsc.maven</groupid>
         <artifactid>maven-processor-compiler</artifactid>
         <version>5.0-jdk8</version>
     </dependency>
     <dependency>
         <groupid>org.bsc.maven</groupid>
         <artifactid>maven-processor-plugin</artifactid>
         <version>5.0-jdk8-rc3</version>
     </dependency>
</dependencies>
<build>
    <plugins>
        <plugin>
            <groupid>org.apache.maven.plugins</groupid>
            <artifactid>maven-compiler-plugin</artifactid>
            <version>3.11.0</version>
        </plugin>
        <plugin>
            <groupid>org.bsc.maven</groupid>
            <artifactid>maven-processor-plugin</artifactid>
            <version>5.0-jdk8-rc3</version>
            <executions>
                <execution>
                    <id>process</id>
                    <goals>
                        <goal>process
                    </goals>
                    <phase>generate-sources</phase>
                        <configuration>
                            <processors>
                                <processor>com.open.Engine.processors.AnnotationProcessor</processor>
                            </processors>
                        </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

The mistake is probably in my POM file. In the XML view in Eclipse, each tag gives a meaningful description (if a tag was spelled incorrectly or misplaced, it might not).

Here are the simple classes I made for testing:

Annotation.java:

package com.folder.Project.archive;

// import from java.lang.annotation.Documented;
@Documented
public @interface Annotation { /*does nothing*/ }

AnnotationProcessor.java:

package com.folder.Project.processors;

// imports from javax, and java.util.Set;
@SupportedAnnotationTypes("com.folder.Project.archive.Annotation") // I'm not sure if this matches
public class AnnotationProcessor extends AbstractProcessor {
    @Override public boolean process( ... ) // Set and RoundElement inputs
    {
        processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "Error");
        return false; // so it should fail every time (which lets me know if it works)
    }
}

App.java:

import com.folder.Project.archive.Annotation;

... main(String[] args) {
    @Annotation int x = 1;
    System.out.println("finished main");
}

The code is written so I can verify that an annotation processor is executed at all.

A few notes:

diagnostic: error: Annotation processor 'com.folder.Project.processors.AnnotationProcessor' not found; is the 1st error to appear after running mvn compile.
mvn clean compile -X doesn't elaborate on the error: Build Fail / could Not Execute / could not achieve goal.
I run "Build Project", "Build All", and "Run" after every significant change to pom.xml.
<annotationprocessingpaths>, from an alternative solution, was not a recognized tag in my pom dependency hierarchy (alternative solutions failed too).

What steps should I take to get everything running normally, for annotation processing? What errors have I made?

java xml jpa

posted 8 months ago

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5mo ago by Andreas witnessed the end of the world today‭

tylerbakeman‭

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The annotation processor must be compiled before it can be used. This is not a problem if you're using it as a dependency from Maven Central, for example. If you want to use a custom processor, you usually split your project into two parts.

Your project structure could look as follows:

/Project
  /Preprocessor
    /src/main/java/com/folder/project
      /archive
        Annotation.java
      /preprocessors
        AnnotationProcessor.java
    pom.xml
  /Program
    /src/main/java/com/folder/project/program
      App.java
    pom.xml
  pom.xml

There are three pom.xml files. One root file and two for the Preprocessor and Program modules. Preprocessor is a dependency for Program, so it can be compiled and used before program is compiled.

Preprocessor contains the annotation and the annotation preprocessor. Program is the example module.

/pom.xml

<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>com.folder.project</groupId>
  <artifactId>Project</artifactId>
  <version>1.0</version>
  <packaging>pom</packaging>

  <name>Project</name>
  <url>http://maven.apache.org</url>
  <modules>
    <module>Preprocessor</module>
    <module>Program</module>
  </modules>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.release>8</maven.compiler.release>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.11.0</version>
    </dependency>
  </dependencies>
</project>

/Preprocessor/pom.xml

<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>
    <parent>
        <groupId>com.folder.project</groupId>
        <artifactId>Project</artifactId>
        <version>1.0</version>
    </parent>

    <artifactId>Preprocessor</artifactId>
    <packaging>jar</packaging>
</project>

/Program/pom.xml

<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>
    <parent>
        <groupId>com.folder.project</groupId>
        <artifactId>Project</artifactId>
        <version>1.0</version>
    </parent>

    <artifactId>Program</artifactId>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>com.folder.project</groupId>
            <artifactId>Preprocessor</artifactId>
            <version>1.0</version>
            <scope>compile</scope>
            <type>jar</type>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.11.0</version>
                <configuration>
                    <annotationProcessors>
                        <annotationProcessor>
                            com.folder.project.preprocessors.AnnotationProcessor
                        </annotationProcessor>
                    </annotationProcessors>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

/Preprocessor/src/main/java/com/folder/project/archive/Annotation.java

package com.folder.project.archive;

public @interface Annotation {
    String value();
}

/Preprocessor/src/main/java/com/folder/project/preprocessors/AnnotationProcessor.java

package com.folder.project.preprocessors;

import com.folder.project.archive.Annotation;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;
import java.util.Set;

@SupportedAnnotationTypes("com.folder.project.archive.Annotation")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class AnnotationProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (Element e: roundEnv.getElementsAnnotatedWith(Annotation.class)){
            Annotation a = e.getAnnotation(Annotation.class);
            processingEnv.getMessager().printMessage( Diagnostic.Kind.WARNING, "Annotation found. Type: "+a.value() );
        }
        return false;
    }
}

/Program/src/main/java/com/folder/project/program/App.java

package com.folder.project.program;

import com.folder.project.archive.Annotation;

@Annotation("class")
public class App
{
    @Annotation("method")
    public static void main( String[] args )
    {
        @Annotation("local variable") int x = 1;
        System.out.println( "finished main" );
    }
}

You will see a warning with the text Annotation found when you compile the project. You can do this from root, which will automatically build the preprocessor module and then compile the program. There are three annotations, but only two Annotation found warnings will appear. That's because local variable annotations are ignored.

posted 8 months ago

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Zer0‭

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