Why is Maven not resolving all dependencies for commons-configuration?

apache-commons-config, java, maven

Solution

If we analyse commons-configuration's POM we see that the `commons-collections` dependency is optional:

  <dependencies>
    <dependency>
      <groupId>commons-collections</groupId>
      <artifactId>commons-collections</artifactId>
      <version>3.2.1</version>
      <optional>true</optional>
    </dependency>
    ...

Furthermore, from the Maven docs:

If a user wants to use functionality related to an optional dependency, they will have to redeclare that optional dependency in their own project.

Problem

Summary When trying `XMLConfiguration configuration = new XMLConfiguration("config/config.xml");` with only `commons-configuration 1.10` I need to add more depencies (namely `commons-collections` not newer than `3.2.1`) to my maven setup. Why is that so and why doesn't maven simply resolve all needed dependencies? Details I am trying to get commons-configuration to work. First I wanted to use the latest version, 2.0-alpha2, which didn't work well at all since I was unable to configure Maven to download the correct ressources - but that is another story. After I found out that version 1.10 is in fact "one point ten" (not "one point one zero") and thus the latest version of commons-configuration 1 (and covered by the tutorials), I decided to give it a try instead. For my maven dependencies (integrated in eclipse) I used: ``` <dependency> <groupId>commons-configuration</groupId> <artifactId>commons-configuration</artifactId> <version>1.10</version> </dependency> ``` However, when trying out this example: ``` package main; import java.util.Iterator; import org.apache.commons.configuration.ConfigurationException; import org.apache.commons.configuration.XMLConfiguration; public class ConfigurationTest { public static void main(String... args) { try { XMLConfiguration configuration = new XMLConfiguration("config/config.xml"); Iterator<String> iterator = configuration.getKeys(); while (iterator.hasNext()) { System.out.println(iterator.next()); } } catch (ConfigurationException e) { e.printStackTrace(); } } } ``` with the following config.xml: ``` <?xml version="1.0" encoding="UTF-8" standalone="yes" ?> <configuration> <property>value</property> <nestedproperty> <arrayvalue>0,1,2,3,4</arrayvalue> <property>anothervalue</property> </nestedproperty> </configuration> ``` I got the error: ``` Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/collections/CollectionUtils at org.apache.commons.configuration.XMLConfiguration.constructHierarchy(XMLConfiguration.java:640) at org.apache.commons.configuration.XMLConfiguration.initProperties(XMLConfiguration.java:596) at org.apache.commons.configuration.XMLConfiguration.load(XMLConfiguration.java:1009) at org.apache.commons.configuration.XMLConfiguration.load(XMLConfiguration.java:972) at org.apache.commons.configuration.XMLConfiguration$XMLFileConfigurationDelegate.load(XMLConfiguration.java:1647) at org.apache.commons.configuration.AbstractFileConfiguration.load(AbstractFileConfiguration.java:324) at org.apache.commons.configuration.AbstractFileConfiguration.load(AbstractFileConfiguration.java:261) at org.apache.commons.configuration.AbstractFileConfiguration.load(AbstractFileConfiguration.java:238) at org.apache.commons.configuration.AbstractHierarchicalFileConfiguration.load(AbstractHierarchicalFileConfiguration.java:184) at org.apache.commons.configuration.AbstractHierarchicalFileConfiguration.<init>(AbstractHierarchicalFileConfiguration.java:95) at org.apache.commons.configuration.XMLConfiguration.<init>(XMLConfiguration.java:261) at main.ConfigurationTest.main(ConfigurationTest.java:12) ``` I first hoped they (not me, of course) just screwed up some maven dependencies and since I wouldn't bother which version to use anyway anymore (I didn't get 2.0 to work, remember?) I decided to go down to version 1.9 by replacing the maven dependency with: ``` <dependency> <groupId>commons-configuration</groupId> <artifactId>commons-configuration</artifactId> <version>1.9</version> </dependency> ``` That solved the problem pretty well, the test case is running: ``` property nestedproperty.arrayvalue nestedproperty.property ``` But when I tried to implement a similar example to the one referenced in Very simple Apache-commons configuration example throws NoClassDefFoundError and its follow-up question I got the exact same error which is referenced there - but the solution, importing `org.apache.commons.beanutils.PropertyUtils` is not working as I am missing the beanutils. So basically by downgrading I just switched from the error of missing the collections to missing beanutils. There is a dependency overview where you can see which dependencies are used when you do what. I was a bit suprised to learn that version 1.10 now used other dependencies (namely the `CollectionUtils`) than 1.9 did in the constructor call. Since there were dependency problems in 1.10 as well as in 1.9 I just sticked to the newer version. I found the `CollectionUtils` located in the following artifact (as I was pointed there by its maven repository): ``` <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-collections4</artifactId> <version>4.0</version> </dependency> ``` Sadly that one (not obvious to me at first) doesn't define the class `CollectionUtils` in the package `collections`, but in the package `collections4`. It was hinted at this problem on the dependency overview, but they only mentioned possible problems with earlier versions... I appeared to be at a point of not thinking much about it anymore but simply changed the dependency to: ``` <dependency> <groupId>commons-collections</groupId> <artifactId>commons-collections</artifactId> <version>3.2.1</version> </dependency> ``` I got everything to work (more or less, but the Exceptions I get now are not anymore depending on missing class definitions) after using these dependencies: ``` <dependencies> <dependency> <groupId>commons-configuration</groupId> <artifactId>commons-configuration</artifactId> <version>1.10</version> </dependency> <dependency> <groupId>commons-collections</groupId> <artifactId>commons-collections</artifactId> <version>3.2.1</version> </dependency> <dependency> <groupId>commons-beanutils</groupId> <artifactId>commons-beanutils</artifactId> <version>1.9.2</version> </dependency> </dependencies> ``` Why do I have to add the dependencies myself? I thought the whole point in using maven is to avoid having to do such things and in terms of javadocs and source files it does a pretty good job. By now I am convinced that the dependencies are not included in the hierarchy by design (is that so?), probably to avoid overhead. However is there a way to either simply get all dependencies at once or even better to get all dependencies I need? And why is it designed this way?

Original source

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