Question

Please, imagine a folder which has two files

1. MyApp.jar
2. B.class (real path is long)

Inside the jar there is some code to check whether B.class exists inside jar-file.

try {
    Class.forName("B");
    System.out.println("exists");
} catch (Exception ignored) {
    System.out.println("does not exist");
}

But even though B.class is not inside jar, the code above does not throw exception, because B.class exists outside of jar.

The jar is generated with Ant from Eclipse. So I thought class-path might be the reason

 <manifest>
     <attribute name="Main-Class" value="org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader"/>
     <attribute name="Rsrc-Main-Class" value="org.client.Client"/>
     <attribute name="Class-Path" value="."/>
     <attribute name="Rsrc-Class-Path" value="./ many_jar_here.jar"/>
  </manifest>

So I have changed only Class-Path like this

<attribute name="Class-Path" value=""/>

But now it gives an error like this:

 Exception in thread "main" java.lang.NoClassDefFoundError: B
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:264)
        at org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader.main(JarRs
der.java:56)
Caused by: java.lang.ClassNotFoundException: B
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        ... 3 more

Answer 1

In this scenario, where B is found by the class loader but it’s not in the jar, you could additionally check if

B.class.getProtectionDomain().getCodeSource() == ClassKnownToBeInJar.class.getProtectionDomain().getCodeSource()

or

B.class.getProtectionDomain().getCodeSource().getLocation().getPath().contains("MyApp.jar")

If either of those is true, B was loaded from MyApp.jar.

So, something like this:

try {
    Class bClass = Class.forName("B");
    System.out.println("exists");
    if (bClass.getProtectionDomain().getCodeSource().getLocation().getPath().contains("MyApp.jar")) {
        System.out.println("class loaded from MyApp.jar");
    } else {
        System.out.println("class not loaded from MyApp.jar");
    }
} catch (Exception ignored) {
    System.out.println("does not exist");
}