Maven: add a folder or jar file into current classpath - maven-2

Question

I am using maven-compile plugin to compile classes. Now I would like to add one jar file into the current classpath. That file stays in another location (let's say c:/jars/abc.jar . I prefer to leave this file here). How can I do that?
If I use classpath in the argument:
<configuration>
<compilerArguments>
<classpath>c:/jars/abc.jar</classpath>
</compilerArguments>
</configuration>
it will not work because it will override the current classpath (that includes all the dependencies)

Answer 1

This might have been asked before. See Can I add jars to maven 2 build classpath without installing them?
In a nutshell: include your jar as dependency with system scope. This requires specifying the absolute path to the jar.
See also http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html

Answer 2

The classpath setting of the compiler plugin are two args. Changed it like this and it worked for me:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.6.1</version>
<configuration>
<compilerArgs>
<arg>-cp</arg>
<arg>${cp}:${basedir}/lib/bad.jar</arg>
</compilerArgs>
</configuration>
</plugin>
I used the gmavenplus-plugin to read the path and create the property 'cp':
<plugin>
<!--
Use Groovy to read classpath and store into
file named value of property <cpfile>
In second step use Groovy to read the contents of
the file into a new property named <cp>
In the compiler plugin this is used to create a
valid classpath
-->
<groupId>org.codehaus.gmavenplus</groupId>
<artifactId>gmavenplus-plugin</artifactId>
<version>1.12.0</version>
<dependencies>
<dependency>
<groupId>org.codehaus.groovy</groupId>
<artifactId>groovy-all</artifactId>
<!-- any version of Groovy \>= 1.5.0 should work here -->
<version>3.0.6</version>
<type>pom</type>
<scope>runtime</scope>
</dependency>
</dependencies>
<executions>
<execution>
<id>read-classpath</id>
<phase>validate</phase>
<goals>
<goal>execute</goal>
</goals>
</execution>
</executions>
<configuration>
<scripts>
<script><![CDATA[
def file = new File(project.properties.cpfile)
/* create a new property named 'cp'*/
project.properties.cp = file.getText()
println '<<< Retrieving classpath into new property named <cp> >>>'
println 'cp = ' + project.properties.cp
]]></script>
</scripts>
</configuration>
</plugin>

Answer 3

From docs and example it is not clear that classpath manipulation is not allowed.
<configuration>
<compilerArgs>
<arg>classpath=${basedir}/lib/bad.jar</arg>
</compilerArgs>
</configuration>
But see Java docs (also https://www.cis.upenn.edu/~bcpierce/courses/629/jdkdocs/tooldocs/solaris/javac.html)
-classpath path Specifies the path javac uses to look up classes needed to run javac or being referenced by other classes you are
compiling. Overrides the default or the CLASSPATH environment variable
if it is set.
Maybe it is possible to get current classpath and extend it,
see in maven, how output the classpath being used?
<properties>
<cpfile>cp.txt</cpfile>
</properties>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>2.9</version>
<executions>
<execution>
<id>build-classpath</id>
<phase>generate-sources</phase>
<goals>
<goal>build-classpath</goal>
</goals>
<configuration>
<outputFile>${cpfile}</outputFile>
</configuration>
</execution>
</executions>
</plugin>
Read file (Read a file into a Maven property)
<plugin>
<groupId>org.codehaus.gmaven</groupId>
<artifactId>gmaven-plugin</artifactId>
<version>1.4</version>
<executions>
<execution>
<phase>generate-resources</phase>
<goals>
<goal>execute</goal>
</goals>
<configuration>
<source>
def file = new File(project.properties.cpfile)
project.properties.cp = file.getText()
</source>
</configuration>
</execution>
</executions>
</plugin>
and finally
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.6.1</version>
<configuration>
<compilerArgs>
<arg>classpath=${cp}:${basedir}/lib/bad.jar</arg>
</compilerArgs>
</configuration>
</plugin>

Answer 4

We mixed two of the answers found here to solve a similar problem. Our project needs a JAR only in compile stage, but add a local dependency, using system scope, it is unuseful because Maven refuse the artifact publication with an error related to a missing dependency.
The snippets used are the following:
<properties>
<classpathfile>${basedir}/classpathfile.classpath</classpathfile>
</properties>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>2.9</version>
<executions>
<execution>
<id>build-classpath</id>
<phase>generate-sources</phase>
<goals>
<goal>build-classpath</goal>
</goals>
<configuration>
<outputFile>${classpathfile}</outputFile>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.codehaus.gmaven</groupId>
<artifactId>gmaven-plugin</artifactId>
<version>1.4</version>
<executions>
<execution>
<phase>generate-resources</phase>
<goals>
<goal>execute</goal>
</goals>
<configuration>
<source>
def file = new File(project.properties.classpathfile)
project.properties.originalClassPath = file.getText()
</source>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<compilerArgs>
<arg>-cp</arg>
<arg>${originalClassPath}${path.separator}${basedir}/../../../bin/POM_RUNTIME_PLACEHOLDER/ExtraJar.jar</arg>
</compilerArgs>
</configuration>
</plugin>
Maven is able to compile and successfully deploy the artifacts.
If anyone is interested the full POM is available in GitHub under project NuReflector, defaultPOM.template under src/NuReflector.