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apache-commons-config

dependency for Apache Commons Configuration2 declared as optional

I try to simple usage of simple Apache Commons Configuration2 loading configuration from a properties file. Here is my dependency:

    <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-configuration2</artifactId>
        <version>2.1</version>
    </dependency>

I try to start my web application and get this:

java.lang.ClassNotFoundException: org.apache.commons.beanutils.DynaBean
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1285)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1119)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at com.sun.proxy.$Proxy21.<clinit>(Unknown Source)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:739)
    at org.apache.commons.configuration2.builder.fluent.Parameters.createParametersProxy(Parameters.java:294)
    at org.apache.commons.configuration2.builder.fluent.Parameters.properties(Parameters.java:245)

Lovely. Since when can't we use Maven to get dependencies automatically? I look at the commons-configuration2 POM on Maven Central and see that commons-beanutil is declared as optional.

<dependency>
  <groupId>commons-beanutils</groupId>
  <artifactId>commons-beanutils</artifactId>
  <version>1.9.2</version>
  <optional>true</optional>
</dependency>

Why is it declared as "optional" when obviously I need it?

Solution

  • This dependency is marked as optional because it is only required for some non central features of the library, which means you can still use most of the library without having this dependency installed.

    As per the documentation: Runtime dependencies for Commons Configuration 2.0

    Commons Configuration 2.0 requires Java 6 or later.

    A lot of dependencies are declared in the Maven POM. These are all needed during compile time. On runtime however you only need to add the dependencies to your classpath that are required by the parts of the Commons Configuration package you are using. The following table helps you to determine which dependencies you have to include based on the components you intend to use:

    Considering tables are not easily included in a SO answer, I'll just list the features from commons-configuration-2 which requires you to include beanutils:

    • Configuration builders
    • ConfigurationDynaBean

    In your case, based on the stack trace you provided, you are using a configuration builder and thus need to manually included the beanutils dependency in your pom.