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

User · Accepted Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt String  Class lt   gt  gt  ENTRY POINTS           new HashMap lt String  Class lt   gt  gt         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  lt  1                  throw exception  not enough args                   final Class lt   gt  entryPoint   ENTRY POINTS get args 0            if entryPoint  null                  throw exception  entry point doesn t exist                   final String   argsCopy               args length  gt  1                   Arrays copyOfRange args  1  args length                    new String 0           entryPoint getMethod  main   String   class  invoke null               Object  argsCopy

User · Answer

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

User · Answer

This answer is for Spring-boot users   If your JAR was from a Spring-boot project and created using the command mvn package spring-boot repackage  the above  -cp  method won t work  You will get      Error  Could not find or load main class your alternative class path   even if you can see the class in the JAR by jar tvf yours jar   In this case  run your alternative class by the following command   java -cp yours jar -Dloader main your alternative class path org springframework boot loader PropertiesLauncher   As I understood  the Spring-boot s org springframework boot loader PropertiesLauncher class serves as a dispatching entrance class  and the -Dloader main parameter tells it what to run   Reference  https   github com spring-projects spring-boot issues 20404

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

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