Determine which JAR file a class is from

Yes. It works for all classes except classes loaded by bootstrap classloader. The other way to determine is:

Class klass = String.class;
URL location = klass.getResource('/' + klass.getName().replace('.', '/') + ".class");

As notnoop pointed out klass.getResource() method returns the location of the class file itself. For example:

jar:file:/jdk/jre/lib/rt.jar!/java/lang/String.class
file:/projects/classes/pkg/MyClass$1.class

The getProtectionDomain().getCodeSource().getLocation() method returns the location of the jar file or CLASSPATH

file:/Users/home/java/libs/ejb3-persistence-1.0.2.GA.jar
file:/projects/classes

Checkout the LiveInjector.findPathJar() from Lombok Patcher LiveInjector.java. Note that it special cases where the file doesn't actually live in a jar, and you might want to change that.

/**
 * If the provided class has been loaded from a jar file that is on the local file system, will find the absolute path to that jar file.
 * 
 * @param context The jar file that contained the class file that represents this class will be found. Specify {@code null} to let {@code LiveInjector}
 *                find its own jar.
 * @throws IllegalStateException If the specified class was loaded from a directory or in some other way (such as via HTTP, from a database, or some
 *                               other custom classloading device).
 */
public static String findPathJar(Class<?> context) throws IllegalStateException {
    if (context == null) context = LiveInjector.class;
    String rawName = context.getName();
    String classFileName;
    /* rawName is something like package.name.ContainingClass$ClassName. We need to turn this into ContainingClass$ClassName.class. */ {
        int idx = rawName.lastIndexOf('.');
        classFileName = (idx == -1 ? rawName : rawName.substring(idx+1)) + ".class";
    }

    String uri = context.getResource(classFileName).toString();
    if (uri.startsWith("file:")) throw new IllegalStateException("This class has been loaded from a directory and not from a jar file.");
    if (!uri.startsWith("jar:file:")) {
        int idx = uri.indexOf(':');
        String protocol = idx == -1 ? "(unknown)" : uri.substring(0, idx);
        throw new IllegalStateException("This class has been loaded remotely via the " + protocol +
                " protocol. Only loading from a jar on the local file system is supported.");
    }

    int idx = uri.indexOf('!');
    //As far as I know, the if statement below can't ever trigger, so it's more of a sanity check thing.
    if (idx == -1) throw new IllegalStateException("You appear to have loaded this class from a local jar file, but I can't make sense of the URL!");

    try {
        String fileName = URLDecoder.decode(uri.substring("jar:file:".length(), idx), Charset.defaultCharset().name());
        return new File(fileName).getAbsolutePath();
    } catch (UnsupportedEncodingException e) {
        throw new InternalError("default charset doesn't exist. Your VM is borked.");
    }
}

Use

String path = <Any of your class within the jar>.class.getProtectionDomain().getCodeSource().getLocation().getPath(); 

If this contains multiple entries then do some substring operation.

With Linux, I'm using a small script to help me find in which jar a class lies that can be used in a find -exec:

findclass.sh:

unzip -l "$1" 2>/dev/null | grep $2 >/dev/null 2>&1 && echo "$1"

Basically, as jars are zip, unzip -l will print the list of class resources, so you'll have to convert . to /. You could perform the replacement in the script with a tr, but it's not too much trouble to do it yourself when calling the script.

The, the idea is to use find on the root of your classpath to locate all jars, then runs findclass.sh on all found jars to look for a match.

It doesn't handle multi-directories, but if you carefully choose the root you can get it to work.

Now, find which jar contains class org.apache.commons.lang3.RandomUtils to you un-mavenize your project (...):

$ find ~/.m2/repository/ -type f -name '*.jar' -exec findclass.sh {} org/apache/commons/lang3/RandomUtils \;

.m2/repository/org/apache/commons/commons-lang3/3.7/commons-lang3-3.7.jar
.m2/repository/org/apache/commons/commons-lang3/3.6/commons-lang3-3.6.jar
.m2/repository/org/apache/commons/commons-lang3/3.6/commons-lang3-3.6-sources.jar
$