How to run a JAR file

Question

I created a JAR file like this:

jar cf Predit.jar *.*

I ran this JAR file by double clicking on it (it didn't work). So I ran it from the DOS prompt like this:

java -jar Predit.jar

It raised "Failed to load main class" exceptions. So I extracted this JAR file:

jar -xf Predit.jar

and I ran the class file:

java Predit

It worked well. I do not know why the JAR file did not work. Please tell me the steps to run the JAR file

Answer 1

You need to specify a Main-Class in the jar file manifest.

Oracle's tutorial contains a complete demonstration, but here's another one from scratch. You need two files:

Test.java:

public class Test
{
    public static void main(String[] args)
    {
        System.out.println("Hello world");
    }
}

manifest.mf:

Manifest-version: 1.0
Main-Class: Test

Note that the text file must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.

Then run:

javac Test.java
jar cfm test.jar manifest.mf Test.class
java -jar test.jar

Output:

Hello world

Answer 2

java -classpath Predit.jar your.package.name.MainClass

Answer 3

Before run the jar check Main-Class: classname is available or not in MANIFEST.MF file. MANIFEST.MF is present in jar.

java -jar filename.jar

Answer 4

You have to add a manifest to the jar, which tells the java runtime what the main class is. Create a file 'Manifest.mf' with the following content:

Manifest-Version: 1.0
Main-Class: your.programs.MainClass

Change 'your.programs.MainClass' to your actual main class. Now put the file into the Jar-file, in a subfolder named 'META-INF'. You can use any ZIP-utility for that.

Answer 5

A very simple approach to create .class, .jar file.

Executing the jar file. No need to worry too much about manifest file. Make it simple and elgant.

Java sample Hello World Program

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

Compiling the class file

javac HelloWorld.java

Creating the jar file

jar cvfe HelloWorld.jar HelloWorld HelloWorld.class

or

jar cvfe HelloWorld.jar HelloWorld *.class

Running the jar file

 java -jar HelloWorld.jar

Or

java -cp HelloWorld.jar HelloWorld

Answer 6

If you don`t want to create a manifest just to run the jar file, you can reference the main-class directly from the command line when you run the jar file.

java -jar Predit.jar -classpath your.package.name.Test

This sets the which main-class to run in the jar file.

Answer 7

Java

class Hello{
   public static void main(String [] args){
    System.out.println("Hello Shahid");
   }
}

manifest.mf

Manifest-version: 1.0
Main-Class: Hello

On command Line:

$ jar cfm HelloMss.jar  manifest.mf Hello.class 
$ java -jar HelloMss.jar

Output:

Hello Shahid

Answer 8

Eclipse Runnable JAR File

Create a Java Project – RunnableJAR

• If any jar files are used then add them to project build path.
• Select the class having main() while creating Runnable Jar file.

Main Class

public class RunnableMainClass {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("Name : "+args[0]);
        System.out.println(" ID  : "+args[1]);
    }
}

Run Jar file using java program (cmd) by supplying arguments and get the output and display in eclipse console.

public class RunJar { 
    static StringBuilder sb = new StringBuilder();
    public static void main(String[] args) throws IOException {
        String jarfile = "D:\\JarLocation\\myRunnable.jar";
        String name = "Yash";
        String id = "777";

        try { // jarname arguments has to be saperated by spaces
            Process process = Runtime.getRuntime().exec("cmd.exe start /C java -jar "+jarfile+" "+name+" "+id);
                    //.exec("cmd.exe /C start dir java -jar "+jarfile+" "+name+" "+id+" dir");
            BufferedReader br = new BufferedReader(new InputStreamReader(process.getInputStream ()));
            String line = null;
            while ((line = br.readLine()) != null){
                sb.append(line).append("\n");
            }
            System.out.println("Console OUTPUT : \n"+sb.toString());
            process.destroy();
        }catch (Exception e){
            System.err.println(e.getMessage());
        }
   }
}

In Eclipse to find Short cuts:

Help ► Help Contents ► Java development user guide ► References ► Menus and Actions

Answer 9

I have this folder structure:

D:\JavaProjects\OlivePressApp\com\lynda\olivepress\Main.class D:\JavaProjects\OlivePressApp\com\lynda\olivepress\press\OlivePress.class D:\JavaProjects\OlivePressApp\com\lynda\olivepress\olives\Kalamata.class D:\JavaProjects\OlivePressApp\com\lynda\olivepress\olives\Ligurian.class D:\JavaProjects\OlivePressApp\com\lynda\olivepress\olives\Olive.class

Main.class is in package com.lynda.olivepress

There are two other packages:

com.lynda.olivepress.press

com.lynda.olivepress.olive

1) Create a file named "Manifest.txt" with Two Lines, First with Main-Class and a Second Empty Line.

Main-Class: com.lynda.olivepress.Main

D:\JavaProjects\OlivePressApp\ Manifest.txt

2) Create JAR with Manifest and Main-Class Entry Point

D:\JavaProjects\OlivePressApp>jar cfm OlivePressApp.jar Manifest.txt com/lynda/olivepress/Main.class com/lynda/olivepress/*

3) Run JAR

java -jar OlivePressApp.jar

Note: com/lynda/olivepress/* means including the other two packages mentioned above, before point 1)

Answer 10

If you don't want to deal with those details, you can also use the export jar assistants from Eclipse or NetBeans.

Answer 11

• Follow this answer, if you've got a jar file, and you need to run it
• See troubleshooting sections for hints to solve most common errors

Introduction

There are several ways to run java application:

1. java -jar myjar.jar - is the default option to run application
2. java -cp my-class-path my-main-class or java -classpath my-class-path my-main-class
3. java --module-path my-module-path --module my-module/my-main-class
4. Deployment to an enterprise server. It's when you have war or ear file. We'll omit the explanation for this case

In this answer I'll explain, how to run a jar if you have to run it manually, give hints to resolve common problems.

java -jar

Start with the most common option: run the jar file using the -jar. Example:

java -jar myjar.jar

If it fails:

• with no main manifest attribute, then the jar is not executable:
  • If you're trying to build a jar see Can't execute jar- file: "no main manifest attribute"
  • Otherwise, proceed with classpath or module-path solution below
• with other error, then see "Troubleshooting" section below

Classpath or module path

If -jar failed, then the jar should be run using classpath or module-path.

Module-path is used, when an application is modular itself. JPMS - Java Platform Module System - is a modern way to develop, distribute and run applications. For details:

1. Watch excellent Modular Development with JDK 9 by Alex Buckley
2. See awesome-java-module-system

To run a jar:

1. Determine if it's modular or not:
   1. Invoke:

      jar --describe-module --file=path-to-jar-file
      

2. Examine output:
   1. If you see No module descriptor found. in the first line, then proceed with classpath solution below
   2. If you see something similar to:

   org.diligentsnail.consoleconsumer@1.0-SNAPSHOT jar:file:///home/caco3/IdeaProjects/maven-multi-module-project-demo/jars/console-consumer.jar!/module-info.class
   requires java.base mandated
   requires org.diligensnail.hellolibrary
   

   continue with module-path solution below

See also: List modules in jar file

Classpath

Try the following:

java -cp my-jar.jar my-main-class

• -cp is the same as -classpath
• my-jar.jar is the jar to run
• my-main-class is name of the class with static void main(String[]) method

Example:

java -cp jars/console-consumer.jar org.diligentsnail.consoleconsumer.Main

Module-path

Try the following command:

java --module-path my-jar.jar --module my-module-name/my-main-class

• my-jar.jar is the jar to run
• my-module-name is name of the module where my-main-class belongs
  • Usually my-module-name is in the module-info.java file
• my-main-class - the class with the static void main(String[]) method

If it fails with FindException:

• Example of message:

  Error occurred during initialization of boot layer
  java.lang.module.FindException: Module javafx.fxml not found, required by org.diligentsnail.javafxconsumer
  

• Usually, this means the my-jar.jar has a dependency on the other jar. For example, the application uses a third party library. See "Supplying dependencies" below

Troubleshooting

UnsupportedClassVersionError

Update java. See List of Java class file format major version numbers?

NoClassDefFoundError, NoSuchMethodError, NoSuchFieldError

See:

1. "Supplying dependencies" section
2. Why am I getting a NoClassDefFoundError in Java?

Supplying dependencies

An Error or Exception is thrown when an application run with missing or out of date dependencies. Common exceptions and errors:

1. NoClassDefFoundError, NoSuchFieldError, NoSuchMethodError
2. ClassNotFoundException, NoSuchFieldException, NoSuchMethodException
3. FindException

To supply dependencies:

1. Determine the list of dependencies
   1. Usually it's a list of jar or can be a list of directories or both
2. Join the list with : if you're running Unix, ; - if you're on Windows
3. Invoke java with -classpath or --module-path

Example

• Project maven-multimodule-project-demo
• I'm trying to run console-consumer.jar:
  • Command:

    java -classpath jars/console-consumer.jar org.diligentsnail.consoleconsumer.Main
    

  • jars/console-consumer.jar is the jar I'm trying to run
  • org.diligentsnail.consoleconsumer.Main is the class with main method
• Error I get:

  Exception in thread "main" java.lang.NoClassDefFoundError: org/diligentsnail/hellolibrary/Hello
    at org.diligentsnail.consoleconsumer.Main.main(Main.java:11)
  Caused by: java.lang.ClassNotFoundException: org.diligentsnail.hellolibrary.Hello
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
    ... 1 more
  

• Missing dependency is jars/hello-library.jar
• Correct command:

  java -classpath jars/console-consumer.jar:jars/hello-library.jar org.diligentsnail.consoleconsumer.Main
  

Answer 12

To run jar, first u have to create

executable jar

then

java -jar xyz.jar

command will work