用类似java -cp的类路径描述MANIFEST.MF

Question

I have a command line Java application, which has dependencies to other libs/jars. Therefore I'm running the application with the -cp , eg java -cp libs/* <package.MainClass> <args> .

What I'm trying to do is to copy the dependency libs/jars in my Java application and describing correctly the MANIFEST.MF with the reason that I don't need the option -cp libs/* anymore.

My project is a maven project, therefore I'm using the maven-assembly-plugin to copy the libs in my JAR and describing the MANIFEST.MF. What I'm right now is the following:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-assembly-plugin</artifactId>
            <version>3.1.1</version>
            <configuration>
                <archive>
                    <manifest>
                        <mainClass>MainClass</mainClass>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                    </manifest>
                </archive>
            </configuration>
            <executions>
                <execution>
                    <id>dist</id>
                    <phase>package</phase>
                    <goals>
                        <goal>single</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

And my MANIFEST.MF looks like this atm:

Manifest-Version: 1.0
Built-By: user
Class-Path: lib/deps_A.jar lib/deps_B.jar
Created-By: Apache Maven 3.6.0
Build-Jdk: 1.8.0_202
Main-Class: MainClass

My Question is: How can describe the MANIFEST.MF file, so that I run my application with java -jar <my-jar-application.jar> <args> ?

Answer 1

The manifest entries are not listed among the URLs of the SystemClassLoader .

You have to manually get them from the manifest file.

            ClassLoader cl = ClassLoader.getSystemClassLoader();
            if (cl instanceof URLClassLoader) {
                URLClassLoader ucl = (URLClassLoader) cl;
                URL[] urLs = ucl.getURLs();
                for (URL url : urLs) {
                    File file = new File(URLDecoder.decode(url.getFile()));

                    try (JarFile jarFile = new JarFile(file)){
                        Manifest manifest = jarFile.getManifest();
                        String attrs = manifest.getMainAttributes().getValue(Attributes.Name.CLASS_PATH.toString());
                        if(attrs!=null) {
                            String[] splits = attrs.split(" ");
                            for (String split:  splits) {
                                // This is the name of the jar referenced in manifest.
                                System.out.println(split);
                            }
                        }

                        System.out.println(file);
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }

I took inspiration from guava lib, you can also check the sources of Classpath class.