Can you tell on runtime if you're running java from within a jar?

Question

Well, you can tell whether or not a class has been loaded from a JAR file - use Foo.class.getResource("Foo.class") and see whether the returned URL begins with "jar:"

For example, take this program:

public class Foo {

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

Running it loading the file from the file system:

file:/C:/Users/Jon/Test/com/whatever/Foo.class

Running it from a jar file:

jar:file:/C:/Users/Jon/Test/foo.jar!/com/whatever/Foo.class

I have two better solutions. first:

URL url = this.getClass().getClassLoader().getResource("input.txt");
URLConnection urlConnection = url.openConnection();
if (urlConnection instanceof JarURLConnection) {
    // run in jar
} else {
    // run in ide
}

second:

String protocol = this.getClass().getResource("").getProtocol();
if(Objects.equals(protocol, "jar")){
    // run in jar
} else if(Objects.equals(protocol, "file")) {
    // run in ide
}

You could check the system class path property for the Equinox launcher:

if (System.getProperty("java.class.path").contains("org.eclipse.equinox.launcher")) {
    System.out.println("You're running inside Eclipse");
}

There are some other potential properties that you may check for, which you can find in Eclipse through Help -> About -> Configuration Details.

Jon's answer is good if you want to know whether you'r running from a JAR versus running from a bunch of class files. But if you use a JAR in both cases then it won't tell you what you need.

From How-To

package com.rgagnon;

public class HelloClass {
 public static void main(String[] args) {
    new HelloClass().say();
 }

 public void say() {
   String className = this.getClass().getName().replace('.', '/');
   String classJar =  
     this.getClass().getResource("/" + className + ".class").toString();
   if (classJar.startsWith("jar:")) {
     System.out.println("*** running from jar!");
   }
   System.out.println(classJar);
 }
}

Will give:

>jar cvfm Hello.jar manifest.mft com\rgagnon\HelloClass.class
added manifest
adding: com/rgagnon/HelloClass.class (in=1059) (out=601) (deflated 43%)

>java com.rgagnon.HelloClass
file:/C:/DEV/WORK/JAVA/com/rgagnon/HelloClass.class

>java -jar Hello.jar
*** running from jar!
jar:file:/C:/DEV/WORK/JAVA/Hello.jar!/com/rgagnon/HelloClass.class

---

As pointed out by Hosam Aly, this does not answer exactly the question.
I leave it there for general reference, as a wiki answer.

If you query the JAR file name it will work if running from a JAR file otherwise it will return something like classes so the following code can be used:

import java.io.File;

public class JarUtilities
{
    public static String getJarName()
    {
        return new File(JarUtilities.class.getProtectionDomain()
                                .getCodeSource()
                                .getLocation()
                                .getPath())
                       .getName();
    }

    public static boolean runningFromJar()
    {
        return getJarName().contains(".jar");
    }
}

EDIT:

If you need more accuracy and to be resistant against renaming of the file extension, checking whether the file contains the MANIFEST.MF should work:

public static boolean runningFromJAR()
{
    try
    {
        String jarFilePath = new File(JarUtilities.class.getProtectionDomain()
                .getCodeSource()
                .getLocation()
                .getPath()).
                toString();
        jarFilePath = URLDecoder.decode(jarFilePath, "UTF-8");

        try (ZipFile zipFile = new ZipFile(jarFilePath))
        {
            ZipEntry zipEntry = zipFile.getEntry("META-INF/MANIFEST.MF");

            return zipEntry != null;
        }
    } catch (Exception exception)
    {
        return false;
    }
}

Here is some code you can run from a normally or as a jar.

import java.applet.Applet;
import java.net.URL;
import java.security.CodeSource;

public class TestApplet extends Applet {

    public TestApplet(){    
        URL path = TestApplet.class.getResource("TestApplet.class");

        if(path.toString().startsWith("jar:"))
            System.out.println("I'm in a jar");
        else
            System.out.println("I'm not in a jar");
    }

    public static void main(String args[]) throws Exception{
        new TestApplet();
    }
}

To put it in a jar and run it use the following:

jar cf test.jar TestApplet.class
java -cp test.jar TestApplet