How to run a class from Jar which is not the Main-Class in its Manifest file

Question

I have a JAR with 4 classes, each one has Main method. I want to be able to run each one of those as per the need. I am trying to run it from command-line on Linux box.

E.g. The name of my JAR is MyJar.jar

It has directory structure for the main classes as follows:

com/mycomp/myproj/dir1/MainClass1.class
com/mycomp/myproj/dir2/MainClass2.class
com/mycomp/myproj/dir3/MainClass3.class
com/mycomp/myproj/dir4/MainClass4.class

I know that I can specify one class as main in my Manifest file. But is there any way by which I can specify some argument on command line to run whichever class I wish to run?

I tried this:

jar cfe MyJar.jar com.mycomp.myproj.dir2.MainClass2 com/mycomp/myproj/dir2/MainClass2.class /home/myhome/datasource.properties /home/myhome/input.txt

And I got this error:

com/mycomp/myproj/dir2/MainClass2.class : no such file or directory

(In the above command, '/home/myhome/datasource.properties' and '/home/myhome/input.txt' are the command line arguments).

Answer 1

You can create your jar without Main-Class in its Manifest file. Then :

java -cp MyJar.jar com.mycomp.myproj.dir2.MainClass2 /home/myhome/datasource.properties /home/myhome/input.txt

Answer 2

You can execute any class which has a public final static main method from a JAR file, even if the jar file has a Main-Class defined.

Execute Main-Class:

java -jar MyJar.jar  // will execute the Main-Class

Execute another class with a public static void main method:

java -cp MyJar.jar com.mycomp.myproj.AnotherClassWithMainMethod

Note: the first uses -jar, the second uses -cp.

Answer 3

Apart from calling java -jar myjar.jar com.mycompany.Myclass, you can also make the main class in your Manifest a Dispatcher class.

Example:

public class Dispatcher{

    private static final Map<String, Class<?>> ENTRY_POINTS =
        new HashMap<String, Class<?>>();
    static{
        ENTRY_POINTS.put("foo", Foo.class);
        ENTRY_POINTS.put("bar", Bar.class);
        ENTRY_POINTS.put("baz", Baz.class);
    }

    public static void main(final String[] args) throws Exception{

        if(args.length < 1){
            // throw exception, not enough args
        }
        final Class<?> entryPoint = ENTRY_POINTS.get(args[0]);
        if(entryPoint==null){
            // throw exception, entry point doesn't exist
        }
        final String[] argsCopy =
            args.length > 1
                ? Arrays.copyOfRange(args, 1, args.length)
                : new String[0];
        entryPoint.getMethod("main", String[].class).invoke(null,
            (Object) argsCopy);

    }
}

Answer 4

First of all jar creates a jar, and does not run it. Try java -jar instead.

Second, why do you pass the class twice, as FQCN (com.mycomp.myproj.dir2.MainClass2) and as file (com/mycomp/myproj/dir2/MainClass2.class)?

Edit:

It seems as if java -jar requires a main class to be specified. You could try java -cp your.jar com.mycomp.myproj.dir2.MainClass2 ... instead. -cp sets the jar on the classpath and enables java to look up the main class there.

Answer 5

Another similar option that I think Nick briefly alluded to in the comments is to create multiple wrapper jars. I haven't tried it, but I think they could be completely empty other than the manifest file, which should specify the main class to load as well as the inclusion of the MyJar.jar to the classpath.

MyJar1.jar\META-INF\MANIFEST.MF

Manifest-Version: 1.0
Main-Class: com.mycomp.myproj.dir1.MainClass1
Class-Path: MyJar.jar

MyJar2.jar\META-INF\MANIFEST.MF

Manifest-Version: 1.0
Main-Class: com.mycomp.myproj.dir2.MainClass2
Class-Path: MyJar.jar

etc. Then just run it with java -jar MyJar2.jar

Answer 6

Add below plugin in your pom.xml file and specify the Main Class with fully qualified name in element.

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>1.2.5.RELEASE</version>
    <configuration>
        <mainClass>**main Class name with full qualified name**</mainClass>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>