Maven: specify the outputDirectory only for packaging a jar?

Thanks @Sean Patrick Floyd for the excellent explanation.

Instead of creating a profile and using mvn always by -P switch, I'd like to use another way that making a default value of property ${dir}.

Just define ${dir}'s default value as ${project.build.directory}

<properties>
    <dir>${project.build.directory}</dir>
</properties>

and same as @Sean Patrick Floyd, set outputDirectory.

<plugins>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>2.3.1</version>
        <configuration>
            <outputDirectory>${dir}</outputDirectory>
        </configuration>
    </plugin>
</plugins>

Now you can either do

# everything goes in someOtherDir instead of target
mvn clean install -Ddir=someOtherDir

or plain old

# everything goes in target
mvn clean install

on command line

-DoutputDirectory=<path>

and in pom.xml

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-jar-plugin</artifactId>
      <version>2.3.1</version>
      <configuration>
        <outputDirectory>/my/path</outputDirectory>
      </configuration>
    </plugin>
  </plugins>
</build>

Parameter Expressions

About command line usage:

The parameter documentation specifies that the parameter is initialized to the value of the property ${project.build.directory} (which is the property referring to the target folder)

Here's what this means:

For mojos that are intended to be executed directly from the CLI, their parameters usually provide a means to be configured via system properties instead of a <configuration/> section in the POM. The plugin documentation for those parameters will list an expression that denotes the system properties for the configuration. In the mojo above, the parameter url is associated with the expression ${query.url}, meaning its value can be specified by the system property query.url as shown below:

mvn myquery:query -Dquery.url=http://maven.apache.org

Reference:

• Guide to Configuring Plug-ins > Generic Configuration

Configuring ${project.build.directory}

However, ${project.build.directory} is not a system property, it's a property of the Project's Build object.

You can't set maven's internal properties directly on the command line, but you can get there with a little trick by adding placeholders in your pom.xml:

<build>
    <directory>${dir}</directory>
</build>

Now, the output directory is set via the property from the command line (using -Ddir=somedirectory). Downside: now you always have to use the -Ddir parameter on the command line.

Using Profiles

But there's help here, too. Just use a profile when you want to configure the directory:

<profiles>
    <profile>
        <id>conf</id>
        <build>
            <directory>${dir}</directory>
        </build>
    </profile>
</profiles>

Now you can either do

# everything goes in someOtherDir instead of target
mvn clean install -Pconf -Ddir=someOtherDir

or plain old

# everything goes in target
mvn clean install

Configuring the Jar Plugin

Now if you just want to change the jar outputDirectory from the command line without redirecting everything from target, we'll modify the profile to configure the plugin from a command line property:

<profiles>
    <profile>
        <id>conf</id>
        <build>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-jar-plugin</artifactId>
                    <version>2.3.1</version>
                    <configuration>
                        <outputDirectory>${dir}</outputDirectory>
                    </configuration>
                </plugin>
            </plugins>
        </build>
    </profile>
</profiles>

The usage is identical to above:

# everything goes in someOtherDir instead of target
mvn clean install -Pconf -Ddir=someOtherDir