Java : ClassNotFoundException when running program with netty [Gradle]

Question

i'm beginner with Java and I got some troubles when I run my .jar. I got a ClassNotFoundException on a netty class. It seems logic that my .jar doesn't find the netty classes.

Here is my trace :

Exception in thread "main" java.lang.NoClassDefFoundError: io/netty/channel/EventLoopGroup
 at src.Main.main(Main.java:14)
Caused by: java.lang.ClassNotFoundException: io.netty.channel.EventLoopGroup
 at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(Unknown Source)
 at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(Unknown Source)
 at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
 ... 1 more

The compilation works fine with gradle. My main build.gradle invokes 2 build.gradle in different folders that compile the program. Here is my main build.gradle :

subprojects {
        apply plugin: 'java'

        repositories {
                mavenCentral()
        }

        dependencies {
                compile 'io.netty:netty-all:4.1.16.Final'
                compile files("./lib/netty-all-4.1.16.Final.jar")
                testCompile 'junit:junit:4.12'
        }
}

And here is a build.gradle that is invoked by the main build.gradle, it compiles my sources.java :

apply plugin: "java"

sourceSets {
        main.java.srcDir "./src/"
}

jar {
        manifest.attributes "Main-Class": "src.Main"
        destinationDir project.file('../Jar/')
        baseName 'Server'
}

My file tree looks like this :

MyProject
--> build.gradle
--> settings.gradle
--> lib [DIR]
    --> netty-all-4.1.16.Final.jar
--> server [DIR]
    --> build.gradle
    --> src [DIR]
        --> Main.java

I don't understand why when I compile everything works, and when I run it doesn't find the netty classes.

Could you please help me ? Thank you !

How are you running your application? Are you using Gradle or are you invoking it directly? The exception indicates that Netty classes are not in your runtime classpath. — Oz., Oct 19 '17 at 13:01
Hi Oz, to compile I use 'gradle build', and to execute my .jar I use java -jar myjar.jar — Fake, Oct 19 '17 at 13:08
You may look at this answer: https://stackoverflow.com/a/18413058/327680 — Oz., Oct 19 '17 at 13:29

lance-java · Answer 1 · 2017-10-19T13:53:42.187

When you are running from command line, you are not adding the jars to the runtime classpath that were present on the compile time classpath.

It's probably easiest to create a runner task in Gradle as follows:

task runMyApp(type: JavaExec) {
    dependsOn 'classes'
    main = 'src.Main'
    classpath = sourceSets.main.runtimeClasspath
}

I suggest that you use the application plugin which will bundle your compiled jar into a zip (or tar) file alongside the dependency jars, it will also create a start script for windows and unix. It also adds an installDist task which deploys the application to run locally from command line.

Also, referencing local jars is a bad idea. If possible you should get your dependencies from a repository. Ie the following is bad

compile files("./lib/netty-all-4.1.16.Final.jar")

Thanks for your help, I remove this line, it works well without it — Fake, Oct 19 '17 at 14:29

score 0 · Answer 2 · answered Oct 19 '17 at 14:43

Ok, I had a manifest into my build.gradle and now it works but i'm getting warning at runtime :

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by io.netty.util.internal.ReflectionUtil (file:/Users/AK1/Java/Java_jcoinche_2017/lib/netty-all-4.1.16.Final.jar) to constructor java.nio.DirectByteBuffer(long,int)
WARNING: Please consider reporting this to the maintainers of io.netty.util.internal.ReflectionUtil
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Here is my manifest added into my build.gradle file :

jar {
  manifest {
   attributes("Implementation-Title": "Server",
                 "Implementation-Version": "1.0",
   "Class-Path": "../lib/netty-all-4.1.16.Final.jar")
  }
 }

I think that what I've done is a bad practice. Is there a better way ?

Java : ClassNotFoundException when running program with netty [Gradle]

2 Answers2