can a class in a jar use a class in a different ja

问题:

I am new to java so not sure I fully understand jar files.

I want to put some common code in a library jar, which I then use from applications that are in different jars. I have searched on this but only come up with people saying yes and then showing examples of classes in class files calling classes in jars, which is not the same as classes in a jar calling classes in a different jar.

I have tried to do this and thought I had it working, until I tried to run the application on the command line, at which point I get class not found errors. Yet the code works in eclipse and from what I have seen that appears to be a common problem where code runs in the ide under development but the minute someone tries to run their code outside of eclipse it fails.

So is it possible for an application built as a jar to call/use classes in an different jar file and if so how?

This is what I tried to test the problem

FoobarInterface.java

package org.myorg.mylibrary;

public interface FoobarInterface
{
    String echo(final String val);
}

Foobar.java

package org.myorg.mylibrary;

public class Foobar implements FoobarInterface
{

    public String echo(final String val)
    {
        StringBuilder sb = new StringBuilder("echoed: ");
        sb.append(val);
        return(sb.toString());
    }

}

pom.xml for the library jar

<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>org.myorg</groupId>
  <artifactId>mylibrary</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>mylibraryname</name>

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
          <archive>
            <index>true</index>
            <manifest>
            </manifest>
          </archive>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

which produces mylibrary-0.0.1-SNAPSHOT.jar

The client code is

Main.java

package org.myorg.myapp;

import org.myorg.mylibrary.Foobar;

public class Main
{

    public Main(String[] args)
    {
        Foobar foobar = new Foobar();
        StringBuilder sb = new StringBuilder("RESULT: ");
        sb.append(foobar.echo(args[0]));
        System.out.println(sb.toString());
    }

    public static void main(String[] args)
    {
        Main foo = new Main(args);
    }

}

pom.xml for the app jar

<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>org.myorg</groupId>
      <artifactId>myapp</artifactId>
      <version>0.0.1-SNAPSHOT</version>
      <name>myappname</name>
      <dependencies>
        <dependency>
            <groupId>org.myorg</groupId>
            <artifactId>mylibrary</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>
      </dependencies>

      <build>
        <plugins>
          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
              <archive>
                <index>true</index>
                <manifest>
                  <mainClass>org.myorg.myapp.Main</mainClass>
                </manifest>
                <manifestEntries>
                 <Class-Path>.</Class-Path>
                </manifestEntries>
              </archive>
            </configuration>
          </plugin>
        </plugins>
      </build>
    </project>

I have also tried all of these settings for the Class-Path element

. ./mylibrary-0.0.1-SNAPSHOT.jar mylibrary-0.0.1-SNAPSHOT.jar

So in theory after all that, I can run the application using this command

java -jar myapp-0.0.1-SNAPSHOT.jar

but I always get this error

Exception in thread "main" java.lang.NoClassDefFoundError: org/myorg/mylibrary/Foobar

no matter how I specify the Class-Path element in the app pom.xml and no matter whether I add classpath to the command line, e.g.

java -classpath mylibrary-0.0.1-SNAPSHOT.jar -jar myapp-0.0.1-SNAPSHOT.jar

and yes both jars iare in the same directory and I am running this on linux.

EDIT:

JB Nizet requested the results of running the command as java -jar but unfortunately I have deleted the test code since he got me past the problem. However if it is any good, here is the output from the source that first raised the problem for me and still exhibits the same exception but is also now working using the 2nd command he suggested.

java -jar FoobarGUI-0.0.1-SNAPSHOT.jar org.myorg.foobar.gui.Main
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
        at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1770)
        at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1653)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
        at javafx.event.Event.fireEvent(Event.java:198)
        at javafx.scene.Node.fireEvent(Node.java:8390)
        at javafx.scene.control.Button.fire(Button.java:185)
        at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
        at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
        at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
        at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)                                        
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)                                             
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)                                        
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)                                                                   
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)                                                                       
        at javafx.event.Event.fireEvent(Event.java:198)                                                                                      
        at javafx.scene.Scene$MouseHandler.process(Scene.java:3758)                                                                          
        at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3486)                                                                      
        at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)                                                                        
        at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2495)                                                                  
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:350)                        
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:275)                        
        at java.security.AccessController.doPrivileged(Native Method)                                                                        
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$350(GlassViewEventHandler.java:385)                       
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$$Lambda$201/45156577.get(Unknown Source)                                          
        at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:404)                                            
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:384)                                  
        at com.sun.glass.ui.View.handleMouseEvent(View.java:555)                                                                             
        at com.sun.glass.ui.View.notifyMouse(View.java:927)                                                                                  
        at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)                                                                       
        at com.sun.glass.ui.gtk.GtkApplication.lambda$null$48(GtkApplication.java:139)                                                       
        at com.sun.glass.ui.gtk.GtkApplication$$Lambda$41/1364335809.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:497)
        at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
        at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:497)
        at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
        at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1767)
        ... 50 more
Caused by: java.lang.NoClassDefFoundError: org/myorg/foobar/processor/ProcessorException
        at org.myorg.foobar.gui.Main.initCenterSection(Main.java:244)
        at org.myorg.foobar.gui.Main.initApplicationScene(Main.java:195)
        at org.myorg.foobar.gui.Main.actionLogin(Main.java:413)
        at org.myorg.foobar.gui.controller.ControllerLogin.actionLogin(ControllerLogin.java:97)
        ... 60 more
Caused by: java.lang.ClassNotFoundException: org.myorg.foobar.processor.ProcessorException
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 64 more

META-INF/MANIFEST.MF from the exploded application jar

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Built-By: myorg
Class-Path: FoobarProcessor-0.0.1-SNAPSHOT.jar
Created-By: Apache Maven 3.3.3
Build-Jdk: 1.8.0_45
Main-Class: org.myorg.foobar.gui.Main

this is a listing of the Processor library jar

META-INF/
META-INF/MANIFEST.MF
org/
org/myorg/
org/myorg/foobar/
org/myorg/foobar/processor/
org/myorg/foobar/processor/ProcessorException.class
org/myorg/foobar/processor/Processor.class
META-INF/maven/
META-INF/maven/org.myorg.foobar/
META-INF/maven/org.myorg.foobar/FoobarProcessor/
META-INF/maven/org.myorg.foobar/FoobarProcessor/pom.xml
META-INF/maven/org.myorg.foobar/FoobarProcessor/pom.properties
META-INF/INDEX.LIST

and this is META-INF/MANIFEST.MF from the library jar

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Built-By: myorg
Created-By: Apache Maven 3.3.3
Build-Jdk: 1.8.0_45

回答1:

If the Class-Path attribute of the manifest of the application jar file is

mylibrary-0.0.1-SNAPSHOT.jar

and the two jar files are in the same directory, then running

java -jar myapp-0.0.1-SNAPSHOT.jar should work.

You can also set the classpath explicitely, and set the main class explicitely, which is much easier, and doesn't rely on relative paths in the manifest.

java -cp myapp-0.0.1-SNAPSHOT.jar:mylibrary-0.0.1-SNAPSHOT.jar org.myorg.myapp.Main

You should not use the -classpath and the -jar options in the same command. It's one or the other.

回答2:

Try some modification in the pom.xml

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <configuration>
        <archive>
            <manifest>
                <mainClass>org.myorg.myapp.Main</mainClass>
                <addClasspath>true</addClasspath>
            </manifest>
        </archive>
    </configuration>
</plugin>

And after this if you still have problems just unpack your myapp-0.0.1-SNAPSHOT.jar using any unzip tool. After unzipping it check the path of mylibrary-0.0.1-SNAPSHOT.jar in the MANIFEST file in META-INF folder.

回答3:

This should be marked as a duplicate of this question I just came across

It is the exact problem and solution, which I could not find with all my previous searching until I knew exactly what I was searching for, i.e. Executable JAR ignores its own Class-Path attribute

So not a java problem at all but a maven problem and the usual sparse maven documentation sheds no light on the problem at all. Unfortunately the sample config I copied included the setting, which by default is turned off. Final solution, don't use indexing with the maven plugin.