具有Maven内置jar的Classloader

Question

I am working on a project which contains a number of subprojects. The structure is something like Project 1, Project 2 and ProjectClassLoader. Using separate configuration files, I pass in the binary names of the classes from Projects 1 and 2 that need to be loaded each time as arguments to the ProjectClassLoader project.

The ProjectClassLoader gets a handle to the system classloader

ClassLoader loader = ClassLoader.getSystemClassLoader();

which in theory allows it to load any classes which are contained in the classpath.

I'm using Maven to build the projects and handle their associated dependences. Thus each project has it's own individual pom.xml file. The ProjectClassLoader defines a parent pom.xml over Projects 1 and 2 which inherit from this. The parent pom contains dependency entries for both Project 1 and 2.

My understanding was that any dependencies specified in the pom.xml files of these projects would get added to the classpath at runtime. However when trying to load classes using the system classloader, I'm getting class not found execptions.

I have tried using the mvn:exec plugin which I understand includes the dependencies in the classpath when executing jars on the command line but this has not worked.

I'd grately appreciate any help in furthering my understanding of why I can load the classes even though the dependencies are defined in the pom...Thanks in advance

Answer 1

Can you check if your pom matches this configuration a bit?

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    ...
    <properties>
        ...
        <exec.maven.plugin.version>1.2.1</exec.maven.plugin.version>
        ...
    </properties>

    <build>
        <plugins>
            ...
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
            </plugin>
            ...
        </plugins>

        <pluginManagement>
            <plugins>
                ...
                <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>exec-maven-plugin</artifactId>
                    <version>${exec.maven.plugin.version}</version>
                    <configuration>
                        <executable>java</executable>
                        <arguments>
                            <argument>-classpath</argument>
                            <!-- automatically creates the classpath using all project dependencies, also adding the project build directory -->
                            <classpath />
                            <argument>com.example.Main</argument><!-- your mainClass -->
                        </arguments>
                    </configuration>
                </plugin>
                ...
            </plugins>
        </pluginManagement>
    </build>

    <dependencies>
        ...
        <dependency>
            <groupId>groupId.for.project1</groupId>
            <artifactId>project1</artifactId>
        </dependency>
        <dependency>
            <groupId>groupId.for.project2</groupId>
            <artifactId>project2</artifactId>
        </dependency>
        ...
    </dependencies>

    <dependencyManagement>
        <dependencies>
            ...
            <dependency>
                <groupId>groupId.for.project1</groupId>
                <artifactId>project1</artifactId>
                <version>${project1.version}</version>
            </dependency>
            <dependency>
                <groupId>groupId.for.project2</groupId>
                <artifactId>project2</artifactId>
                <version>${project2.version}</version>
            </dependency>
            ...
        </dependencies>
    </dependencyManagement>
    ...
</project>

Where you fill them with the correct artifacts.

You should then be able to start it with:

mvn exec:exec

Can you post your configuration for your pom plz if it doesn't match, that way it's easier to understand what exactly you currently have in your pom.