Eclipse is confused by imports ("accessible from more than one module")

Question

When referencing simple .jar files, Eclipse shows an error stating:

The package java.awt is accessible from more than one module: <unnamed>, java.desktop

This happens for instance when javax.awt or javax.swing is included in the .jar files.

The simplest example would be the following:

package test;

import javax.swing.JDialog;

public class Test {
    public static void main(String[] args) {
        new JDialog();
    }
}

Adding a .jar file to the classpath with only the folder structure javax/swing (no files needed) will cause the error to appear. I'm using JDK 10/12 (neither works). Setting the compiler compliance to 1.8 makes the whole thing work again. On another machine with Eclipse 2018-09 this works with compiler compliance set to 10.

I'm on Eclipse 2019-03, on a (for testing purposes) freshly installed Eclipse 2018-09 it works fine. Why?

Edit June/2020 (Solution)

As the answers correctly stated, this is a restriction built into Java ages ago and only recently was forced upon us. I came into contact with it while migrating a big project with dozens of dependencies to Maven. There were libraries from around the year 2000! There were 'meta libraries' which consisted of several libraries packaged together. So there was no other way than to identify what was still needed (into the trash with the rest!), update libraries which violate the rules or find replacements for them. This took me many, many hours.

In the end it worked out and we've got a nice Maven project to work with.

Answer 1

This is caused by

• a JAR on the Classpath that contains the package java.awt that also exists in the system library but the
• JRE System Library is on the Modulepath

In the Java Platform Module System (JPMS) it is not allowed to use the same package in more than one module. If the Modulepath and the Classpath is used, everything on the Classpath is handled as the <unnamed> module (in your case the package java.awt exists in the system module java.desktop and also via the JAR on the Classpath in the module <unnamed>).

Since the JRE System Library cannot be moved from the Modulepath to the Classpath (see this answer by Stephan Herrmann for details), you only have the following options:

• Set the compiler compliance to 1.8 (as you already mentioned)
• Rebuilt the JAR to avoid Java system library package names inside the JAR (if reflection is used, additional code changes may be necessary):
  • If you have the source code, change the package names (e.g. change the package and subpackae java to java_util and javax to javax_util) and recreate the JAR
  • If you have only the .class files you have to decompile the .class files first

Answer 2

Since I'll bet lots of people will be running into this problem with modular Java, I'll help and give the real answer.

This error happens when

• you have a dependency in your project
• that contains code
• using packages
• that are also in the modules
• being referenced by your project

If your project has set the source compatibility to something like Java 12, it will start enforcing the rule, that has been there all along in Java:

"Don't use packages that belong to the JDK in your own code."

Unfortunately, lots of developers and vendors have done that over the years. Can't do that anymore.

If you set your project to Java 12 source compatibility, Eclipse adds the JDK modules which include everything "java.*" and "javax.*" and even "jdk.*", "org.w3c.*". These packages may be in use by your dependencies or their transitive dependencies.

How to fix

You need to:

• look at which package its complaining about
• and expand the "Projects and External Dependencies" node in the Package Explorer.
• Find out which dependency is using that package.
• Then you can simply exclude that dependency from your project.

Or you could get the source of that dependency, if available, and rebuild the jar with changed packages. Otherwise you have to remove that dependency and find a replacement for that technology. Pain huh?

If its a transitive dependency you can often just exclude it. Here is an example of that for Gradle based projects.

GradleConfig.xml:

configurations {
   all*.exclude group: 'xml-apis'
}

Answer 3

I found a simple solution to troubleshoot this in Eclipse.

1. Hit Ctrl + Shift + T in Eclipse to open the Open Type prompt.
2. Type the name of the package that is causing the issue. For me, it was org.w3c.dom
3. The search results will show all the locations from where this package is being loaded.
4. Remove every JAR from the classpath that comes in the result other than the JDK 11 library.

My project being a legacy one, I had to remove a lot of JARs from the build path. Also, my project does not use Maven. So removing the JARs was a fairly straightforward step. The steps might vary for other build tools like ANT, Maven, Gradle, etc. I have just explained the troubleshooting steps above.

Answer 4

In my case, it was because I included a dependency (Apache Tika) in the POM.xml file.

I had to force the exclusion of the module that contained the classes with errors while imported at that dependency:

    <dependency>
        <groupId>org.apache.tika</groupId>
        <artifactId>tika-parsers</artifactId>
        <version>1.24.1</version>
        <exclusions>
            <exclusion>
                <groupId>xml-apis</groupId>
                <artifactId>xml-apis</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

It worked for me that way.

Answer 5

I think my flavour of the problem might be useful.

I got this error for classes under javax.xml.stream, by old Maven projects that depend on artifacts like xml-apis, stax-api, or geronimo-stax-api.

Technically, the problem is what others have already said: those artifacts expose the javax.xml.* package without any awareness of Java modules (they were invented later), so the package is automatically assigned to the unnamed module, which conflicts with the same package being included in the JDK's most recent versions, where the package has its own module name, and therefore the same package results in two different modules, which is forbidden.

That said, the practical solution is essentially to work with Maven exclusions to remove those dependencies from your project and let it use the JDK version instead (or, of course, remove them as direct dependencies, if that's your case). Use the equivalent if you're working with another build system.

In theory, the more recent flavours of these packages offered by the JDK might be non backward-compatible, in practice, I doubt such JSR specifications changed much over the years and so far, I haven't seen any issue with their replacement.

Answer 6

See also: The package org.w3c.dom is accessible from more than one module: <unnamed>, java.xml where I answered:

Disappointingly I don't see any compiler flags to show what jar the problem is with Even -Xlint:module doesn't seem to show up anything useful and eclipse doesn't shed any light on the issue

Instead to find where java.awt comes from I've been using this script:

mvn dependency:copy-dependencies -DincludeScope=test -DoutputDirectory=deps
for i in deps/*.jar; do if unzip -l $i| grep -q java.awt; then echo $i; fi ; done

Strictly you don't have to specify the scope test as that's the default but I've included it as you might want to use compile instead

Answer 7

I've found some interesting behaviour with Java 11 and the xmlbeans library. The xmlbeans library is a transitive dependency of Apache POI, a very popular library for working with Microsoft Office documents, it is used to handle the internal XML structures of the newer Office formats. I've tested it with Apache POI 3.9 and it works perfectly fine, despite the error shown by Eclipse. So, I guess in Java 11 this rule it's not fully enforced.

Answer 8

For Apache POI version 5.0.0 using Maven, in the pom.xml:

<dependency>
  <groupId>org.apache.poi</groupId>
  <artifactId>poi</artifactId>
  <version>5.0.0</version>
</dependency>
<dependency>
  <groupId>org.apache.poi</groupId>
  <artifactId>poi-ooxml</artifactId>
  <version>5.0.0</version>
  <exclusions>
    <exclusion>
      <groupId>xml-apis</groupId>
      <artifactId>xml-apis</artifactId>
    </exclusion>
  </exclusions>
</dependency>

Fixed: "The package javax.xml.parsers is accessible from more than one module"

Answer 9

I met a similar issue with the eclipse IDE while upgrading JDK from 1.8 to 11, "The package org.w3c.dom is accessible from more than one module: , java.xml". upgrading eclipse from 2019 to 2021 and setting system JDK home to 11 does not solve it. I don't think it's caused by "org.w3c.dom" existing in different jars of the classpath,dut to "Order and Export" should ordered the search sequence.

After several hours of searching and investigating, this issue is fixed by setting the Java Compiler - Compiler compliance level to 1.8(default is 11).

Answer 10

You can do what other people suggest which is to exclude xml-apis which worked fine with me, but if your are using and an old jaxb-api replace them with jakarta.xml.bind-api:

<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
    <version>2.3.3</version>
</dependency>

and of course upgrade your jaxb-impl to match the same api:

<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-impl</artifactId>
    <version>2.3.3</version>
</dependency>

Answer 11

Steps below helped me:

Right click Eclipse project > Properties > Java Build Path In Libraries tab, remove all the external jar files under Modulepath and add them under Classpath (you can just select all the jars and drag them under Classpath) Click Apply and Close

Answer 12

Wow, this was a tough one!

Some helpful tips (a summary of my journey through this maze):

This started when I migrated from Java 1.8 to 11.0.11

Right out of the gate this caused problems. I needed to change the syntax for how to specify the Java version for the maven build plug in as shown below:

Before

        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.7.0</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
            </configuration>
        </plugin>

After

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <release>11</release>
            </configuration>
        </plugin>       

After that, everything compiled at a command prompt. However, when opening the project in Eclipse, I got a bunch of errors as shown in the next section.

Getting "<class> cannot be resolved to a type" errors

When I got errors in Eclipse and not at the command line I immediately started looking at configuration settings in Eclipse to no avail. After some googling I came to this post. The post at https://www.eclipse.org/forums/index.php/t/1110036/ was also very helpful.

Finding what resources were the problem

The answer to this post by @Anu suggesting using <shift><ctrl>T to bring up the type search window was very helpful using this I was able to see what resources were causing the problem (i.e. any resource that is also included in the JDK, in my case it was the ErrorHandler in the xml-apis jar). Type the name of one of the resources giving a "cannot be resolved to type" to see the name of the jar file, in my case the jar is xml-apis-1.4.01.jar.

All of this is shown in the screen shot below (click on the image to get a cleaner view).

Finding the Dependency that contains the offending jar

We can now use the information from https://www.eclipse.org/forums/index.php/t/1110036/ to find the dependency in our pom.xml file that contains the offending resource.

Open the pom.xml file in Eclipse and select the "Dependency Hierarchy". In the filter type the name of the jar file we found in the previous step (in my case it was xml-apis).

This will pull up the dependencies creating the problem. A screen shot of the Dependencies Hierarchy filtered for "xml-apis". From this we can see the offending dependencies in our pom.xml file are xercesImpl and xlsx-streamer.

Solving the problem

We can now add exclusions to these dependencies that will resolve the problem as shown below.

    <dependency>
        <groupId>com.monitorjbl</groupId>
        <artifactId>xlsx-streamer</artifactId>
        <version>2.1.0</version>
        <exclusions>
            <exclusion>
                <groupId>xml-apis</groupId>
                <artifactId>xml-apis</artifactId>
            </exclusion>
        </exclusions>
    </dependency>



    <dependency>
        <groupId>xerces</groupId>
        <artifactId>xercesImpl</artifactId>
        <version>2.12.0</version>
        <exclusions>
            <exclusion>
                <groupId>xml-apis</groupId>
                <artifactId>xml-apis</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

And this fixes the current problem.

Answer 13

I had this problem with the Stanford Core NLP package. As mentioned above adding an exclusion in the pom.xml solved the problem:

    <dependency>
        <groupId>edu.stanford.nlp</groupId>
        <artifactId>stanford-corenlp</artifactId>
        <version>4.5.2</version>
        <exclusions>
           <exclusion>
             <groupId>xml-apis</groupId>
             <artifactId>xml-apis</artifactId>
           </exclusion>
        </exclusions>
     </dependency>

Answer 14

Configuration property was added to Eclipse. Full discussion here.

File: <workspace>\.metadata\.plugins\org.eclipse.core.runtime\.settings\org.eclipse.jdt.core.prefs
Property:
org.eclipse.jdt.core.compiler.ignoreUnnamedModuleForSplitPackage=enabled

Answer 15

For me adding JRE into classpath solved the problem.

project -> build path -> configure build path -> then adding JRE into class path

Answer 16

First of all, make sure your pom is actually correct by running mvn install

if that works, and eclipse still complains look at build path and find the library it is holding onto that is not in your pom.

Sometimes I just

rm .classpath
rm -rf .settings
rm -rf project
mvn eclipse:clean eclipse:eclipse