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Eclipse not recognizing generated class

I have used Maven plugin to generate classes in my project. However even after trying padding it explicitly from project build path, eclipse is unable to recognize it. It says,
"AbcBaseListener cannot be resolved to a type"
Project auto build is also ON.

Check 2 things:
1)
The important hint here is "class folder". Eclipse expects .class files in there, source code will be ignored.
To fix this, add this plugin to your POM:
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<executions>
<execution>
<goals>
<goal>add-source</goal>
</goals>
<configuration>
<sources>
<source>target/generated-sources/</source>
</sources>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
and update the project (Maven -> Update ...).
The other option would be to manually add another source folder to the build path but that will get lost whenever m2e updates the project configuration from the POM.
2) Make sure your generated files are having package names;
Use below code inside your g4 file after grammar Abc;
#header {
package antlr4;
}

how to create source.jar on maven install

I'm working in Sprint Tool Suite and I've created a maven project. When I right click on the project I can select Run as...Maven Install. This takes all my class files and packages them into a jar file as expected and also creates a copy to my central maven repository C:\Users\Owner.m2\repository
All that is fine, but I'd like it to also package up all the source files into a sources.jar file. I suspect I'll need to edit the pom.xml file for the project to make that happen, but haven't seen an example of how to do that. Please advise.

You have to add proper plugin to your execution cycle
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-source-plugin</artifactId>
<executions>
<execution>
<id>attach-sources</id>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>

Automatically generate Java from .proto with maven/m2e in Eclipse IDE

For my team, I'd like to configure maven/eclipse build to automatically generate Java code from *.proto files (in a project that uses gRPC). Currently one needs to run mvn generate-source or mvn protobuf:compile (as in plugin usage page). Or what is the same add Run configuration to invoke maven goal compile.
Whenever Eclipse Maven project is refreshed (Alt+F5) or IDE is restarted, project is rebuilt but without what should appear in target/generated, thus turning project into red. So one need to generate and refresh project (F5). UPDATE Eclipse has needed source folders configured in .clathpath file.
As I know that should be m2e connector, but I could only find one https://github.com/masterzen/m2e-protoc-connector for the oldest Googles plugin com.google.protobuf.tools:maven-protoc-plugin, that is even not mentioned currently at https://github.com/grpc/grpc-java
We use exactly referenced/recommended
<groupId>org.xolstice.maven.plugins</groupId>
<artifactId>protobuf-maven-plugin</artifactId>
that is:
<build>
<extensions>
<extension>
<groupId>kr.motd.maven</groupId>
<artifactId>os-maven-plugin</artifactId>
<version>1.4.1.Final</version>
</extension>
</extensions>
<plugins>
<plugin>
<groupId>org.xolstice.maven.plugins</groupId>
<artifactId>protobuf-maven-plugin</artifactId>
<version>0.5.0</version>
<configuration>
<protocArtifact>com.google.protobuf:protoc:3.1.0:exe:${os.detected.classifier}</protocArtifact>
<pluginId>grpc-java</pluginId>
<pluginArtifact>io.grpc:protoc-gen-grpc-java:1.0.1:exe:${os.detected.classifier}</pluginArtifact>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>compile-custom</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
Related:
Linking to generated Java protobuf code in Eclipse
looked at this but that author was using other older, not supported now plugin : Eclipse build loop caused by protobuf generated code (related to Maven Project Builder)
P.P.S That plugin https://github.com/igor-petruk/protobuf-maven-plugin however has continuation as https://github.com/os72/protoc-jar-maven-plugin

Instead of using org.xolstice.maven.plugins:protobuf-maven-plugin my team has used com.github.os72:protoc-jar-maven-plugin to generate the message classes. I believe they are the same since under the hood they all seem to be using the tools from Google.
I am not using any m2e connectors for this plugin (Edit: protoc-jar-maven-plugin's m2e connector is bundled with it so no extra installation is needed, which is why it seemed like I wasn't using one, but technically I was, but this doesn't really matter). Unfortunately the changes in the .proto file are not "automatically" propagated to the generated .java files, you need to manually run Maven or trigger the project to be built in Eclipse (instructions below), but fortunately the target/generated-sources file is not vanishing or emptying or anything strange like what you describe.
If you want to rebuild the .java files from the .proto classes without using mvn clean compile from the command line you can clean the Eclipse project . Project → Clean... → select your project → Select build option (only shows if you have "Build Automatically" from the Project menu is unchecked).
I was able to do this in the latest Eclipse Neon (it will probably work in later ones too, but I don't know for certain).
Below is the POM I am using. I don't think it requires any special explanation, my solution is to simply use a different plugin than the one you are using. (If some explanation is needed I'll be happy to provide it though.)
<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>io.github.jacksonbailey</groupId>
<artifactId>protobuf-m2e-sample</artifactId>
<version>0.1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>com.google.protobuf</groupId>
<artifactId>protobuf-java</artifactId>
<version>3.1.0</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>com.github.os72</groupId>
<artifactId>protoc-jar-maven-plugin</artifactId>
<version>3.1.0.1</version>
<executions>
<execution>
<phase>generate-sources</phase>
<goals>
<goal>run</goal>
</goals>
<configuration>
<protocVersion>3.1.0</protocVersion>
<inputDirectories>
<include>src/main/resources</include>
</inputDirectories>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>

for protobuf-maven-plugin
Thanks to sergei-ivanov answer in https://github.com/xolstice/protobuf-maven-plugin/issues/16, that gave link https://github.com/trustin/os-maven-plugin#issues-with-eclipse-m2e-or-other-ides :
One need to download os-maven-plugin-x.x.x.Final.jar (the version as in your pomx.ml) and put it into the <ECLIPSE_HOME>/plugins directory.
After that Eclipse will generate source on project clean, including after Maven -update project... (Alt+F5), but not after Project -> Build (or with default Build Automatically). Also on IDE start it will not compile.
Yes, that is illogical:
Project - Clean will generate and compile Java source
but
Project - Build will not.
P.S. Raised Bug 507412

Both eclipse and vscode can automatically compile proto when changed.
<plugin>
<groupId>kr.motd.maven</groupId>
<artifactId>os-maven-plugin</artifactId>
<version>1.6.2</version>
<executions>
<execution>
<phase>initialize</phase>
<goals>
<goal>detect</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.xolstice.maven.plugins</groupId>
<artifactId>protobuf-maven-plugin</artifactId>
<version>0.6.1</version>
<configuration>
<protocArtifact>com.google.protobuf:protoc:3.12.0:exe:${os.detected.classifier}</protocArtifact>
<pluginId>grpc-java</pluginId>
<pluginArtifact>io.grpc:protoc-gen-grpc-java:1.32.1:exe:${os.detected.classifier}</pluginArtifact>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>compile-custom</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<source>8</source>
<target>8</target>
<encoding>UTF-8</encoding>
</configuration>
</plugin>
See: https://github.com/trustin/os-maven-plugin#issues-with-eclipse-m2e-or-other-ides

Maven build two copies: one with lib shaded in, one without

I have a maven project that uses the maven-shade plugin to bundle a common-code-library jar into several distributed software plugins I make.
These plugins are all for the same main application - so when someone uses more than one plugin, it becomes a problem because each plugin has a copy of the shaded lib and they're not always using the same version.
I'm not sure how to resolve this. Is there a way I can configure maven to make two builds - one with the library shaded in, and one without? For the one without, we'll provide only one copy of the library needed as a separate resource.
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>2.1</version>
<configuration>
<artifactSet>
<includes>
// our common lib
</includes>
</artifactSet>
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>

Just change the finalName configuration property of shade plugin. See here. It's just overwriting your original package. If you change the output name both packages will be left.
You can use something like
<configuration>
...
<finalName>${build.finalName}-nodep.jar</finalName>
</configuration>
You can alternatively change outputFile or outputDirectory
Also if you want to preserve original artifact name but use classifier set shadedArtifactAttached to true.

The solution to my issue is to relocate the classes I'm shading, so they won't load copies found in other jars:
http://maven.apache.org/plugins/maven-shade-plugin/examples/class-relocation.html

Maven checkstyle https configuration

I have the following maven check style plugin configuration
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-checkstyle-plugin</artifactId>
<configuration>
<consoleOutput>true</consoleOutput>
<configLocation>https://someUtl.com/file.xml</configLocation>
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>check</goal>
</goals>
</execution>
</executions>
</plugin>
Pay attention on
<configLocation>https://someUtl.com/file.xml</configLocation>
file.xml can be downloaded by browser, but it require a login and password. Is there a way to specify these login/password in maven or in plugin configuration?

Underneath, this uses Plexus which in turn pretty much does URL.openStream().
This answer shows how an Authenticator can be used for that in Java code, but I was unable to find a Maven equivalent for that. I'm inclined to say that it's not possible.
Alternatively, you might be able to download the file in a separate mojo execution, then point the configLocation to the downloaded file, which could be anywhere down your target folder.
I think that this answer gives a few nice ideas about how to download files in Maven. Their first is that if your file is a Maven artifact, you could use the Maven Dependency Plugin.
And then we come full circle, because if your Checkstyle configuration were to be contained in a Maven artifact, you would not have to set configLocation to a remote location, but you'd add that artifact as a dependency of your Checkstyle plugin execution. Since Maven defines everything in terms of dependencies, that is my way to go, and that is exactly how I set up my own Checkstyle configurations.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-checkstyle-plugin</artifactId>
<executions>
<execution>
<goals>
<goal>check</goal>
</goals>
</execution>
</executions>
<dependencies>
<dependency>
<groupId>com.totaalsoftware.incidentmanager</groupId>
<artifactId>checkstyle-config</artifactId>
<version>2.0.0-SNAPSHOT</version>
</dependency>
</dependencies>
<configuration>
<configLocation>checkstyle.config.xml</configLocation>
...
</configuration>
</plugin>
Clearly, in the above example, checkstyle.config.xml resides in the root of the checkstyle-config JAR.