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Guide to Selecting Alternative Wagon Providers

By default, Maven uses the java.net.URLConnection (HttpURLConnection) classes provided with the JDK to access repositories that use the HTTP/HTTPs protocols. Unfortunately, since this implementation contains certain bugs, Maven users may find themselves unable to connect to servers that demand some configurations. A couple examples of weird behavior include line-wrapping the Authorization header's Base64 value when passwords are long, and using cached values in preemptive authentication for successive connections to the same server.

Maven 2.2.0 attempted to amend this problem by switching over to a Wagon implementation that's based on Apache HttpClient. Unfortunately, it soon became apparent that HttpClient doesn't support NTLM (at least, version 2), which effectively means users behind NTLMv2 proxies cannot use Maven 2.2.0.

To hopefully resolve this once and for all, Maven 2.2.1 will contain support for specifying which Wagon provider you want to use for a given protocol during the build. The provider name will then be appended to the protocol using the format <protocol>-<provider> to form the component role-hint for the Wagon.

As of Maven 2.2.1, there are two ways to specify which Wagon provider should be used: via the command line, or in the <server> configuration section of the settings.xml.

Command-Line Configuration

To specify the Wagon provider from the command line, simply use the -Dmaven.wagon.provider.<protocol>=<provider-name> command-line option, like the following:

mvn -Dmaven.wagon.provider.http=httpclient clean install

This instructs Maven to use the HttpClient-based Wagon implementation for connections to HTTP repositories.

settings.xml Configuration

To specify which Wagon provider to use for a particular server, modify your settings.xml file to add the <wagonProvider> configuration to your <server> entry, like the following:

<settings>
  [...]
  <servers>
    <server>
      <id>central</id>
      <configuration>
        <wagonProvider>httpclient</wagonProvider>
        [...]
      </configuration>
    </server>

Available Wagon Providers

Maven 2.2.1 provides two providers for HTTP/HTTPS Wagons: lightweight and httpclient. If you add a new HTTP Wagon implementation via build extension, you'll need to make sure the extension binds its Wagon components to role-hints of the form <protocol>-<provider> in order to allow users to specify your alternative Wagon provider. For instance, the HttpClient HTTP Wagon component definition looks like this:

<component>
  <role>org.apache.maven.wagon.Wagon</role>
  <role-hint>http-httpclient</role-hint>
  <implementation>org.apache.maven.wagon.providers.http.HttpWagon</implementation>
  <instantiation-strategy>per-lookup</instantiation-strategy>
</component>

NOTE: The default provider for HTTP/HTTPS Wagons is lightweight.