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java class getResource can't find icons listed in jar file

I am really stumped. I'm just an old C X11/Motif programmer trying to write a little Java program. After a week of reading the Oracle Java Documentation, as well as the
Stack Overflow answers related to getResource, I still can not figure out how to retrieve the path to the icon files in my jar file.
My icons are contained within the jar file for my application. I wish to access them using the relative position within jar file. I am assuming the best way to do this is through the getResource method.
The core part of my code for my program called Fŭd (pronounced food - like the cat spells it in the comic strip "Get Fuzzy") is as follows:
package localhost.system1;
imports not shown for brevity.
public class Fud extends JPanel
implements FocusListener, ActionListener, ItemListener
{
private static final long serialVersionUID = 1L;
static Food data = null;
static int prev = 0;
static int next = 1;
static int plus = 2;
static int minus = 3;
public static void main(String args[]) throws Exception
{
LocalDate now = LocalDate.now();
int dateDifference = 0;
// load in the existing data
data = new Food(programName);
data.loadFood(programName);
// test to see if data is up to date. Add days if not
dateDifference = Math.abs((int)ChronoUnit.DAYS.between(now, data.day[0].date));
if ( dateDifference != 0)
{
data.adjustToToday(dateDifference, programName);
}
/////////////////////////////////////////////////
// create the GUI and switch running over to it.
/////////////////////////////////////////////////
Fud fud = new Fud();
Class<? extends Fud> fudClass = fud.getClass();
String className = fudClass.getName();
System.out.println("fudClass getname returns " + className);
URL testURL = fudClass.getResource("prev.png");
System.out.println("fudClass getResource returned " + testURL);
// Create GUI and turn the control over to it
javax.swing.SwingUtilities.invokeLater
(new Runnable()
{
public void run()
{
URL[] iconURL = new URL[4];
iconURL[prev] = Fud.class.getResource("prev.png");
iconURL[next] = Fud.class.getResource("next.png");
iconURL[plus] = Fud.class.getResource("plus.png");
iconURL[minus] = Fud.class.getResource("minus.png");
createAndShowGUI(fud, iconURL);
}
}
);
} // end of main
.
.
.
Rest of methods and subroutines needed
.
.
.
}
When run, the code returns the following results:
fudClass getname returns localhost.system1.Fud
fudClass getResource returned null
This has me quite frustrated. No matter what I try (and I have tried a number of things) the result remains the same. I keep getting NULL for a response from the getResource method. When I query the jar file with jar -tf Fud.jar I get the following:
jar tf Fud.jar
META-INF/MANIFEST.MF
localhost/
localhost/system1/
localhost/system1/Day.class
localhost/system1/Food.class
localhost/system1/Fud$1.class
localhost/system1/Fud$2.class
localhost/system1/Fud$3.class
localhost/system1/Fud$4.class
localhost/system1/Fud$5.class
localhost/system1/Fud$6.class
localhost/system1/Fud$7.class
localhost/system1/Fud.class
minus.png
next.png
plus.png
prev.png
So the icons are in the Jar file. Can anyone tell me what I am doing wrong? In Eclipse, my project explorer looks like:eclipse Project Explorer
I added the Image directory to my project Java build in eclipse as follows: Eclipse Java Build
I built the program using Eclipse Version: 2021-12 (4.22.0) Build id: 20211202-1639. Furthermore, I am using Java 17.0.1 2021-10-19 LTS on Windows 11 Pro build 22000.434.

You have to add a slash in front of the resource:
Fud.class.getResource("/prev.png");
otherwise java searching in the same folder as the class is located,
so it will search in localhost/system1

Java add Classpaths at runtime

There are many answers to this question in the stackoverflow?
But the most cast the ClassLoader.getSystemClassLoader() to URLClassLoader and this works anymore.
The classes must be found by the systemclassloader.
Is there an another solution?
- without restarting the jar
- without creating a own classloader (In this case I must replace the systemclassloader with my own)
The missing classes/jars must be added at the moment only on startup and I didn't want to add these in the manifest with "Classpath".
I found the Java Agent with the premain-Method. This can also work great, but in this case I want to start the premain method without calling "java -javaagent:... -jar ..."
Currently I restart my programm at the beginning with the missing classpaths:
public class LibLoader {
protected static List<File> files = new LinkedList<>();
public static void add(File file) {
files.add(file);
}
public static boolean containsLibraries() {
RuntimeMXBean runtimeMxBean = ManagementFactory.getRuntimeMXBean();
String[] classpaths = runtimeMxBean.getClassPath().split(System.getProperty("path.separator"));
List<File> classpathfiles = new LinkedList<>();
for(String string : classpaths) classpathfiles.add(new File(string));
for(File file : files) {
if(!classpathfiles.contains(file)) return false;
}
return true;
}
public static String getNewClassPaths() {
StringBuilder builder = new StringBuilder();
RuntimeMXBean runtimeMxBean = ManagementFactory.getRuntimeMXBean();
builder.append(runtimeMxBean.getClassPath());
for(File file : files) {
if(builder.length() > 0) builder.append(System.getProperty("path.separator"));
builder.append(file.getAbsolutePath());
}
return builder.toString();
}
public static boolean restartWithLibrary(Class<?> main, String[] args) throws IOException {
if(containsLibraries()) return false;
List<String> runc = new LinkedList<>();
runc.add(System.getProperty("java.home") + "\\bin\\javaw.exe");
RuntimeMXBean runtimeMxBean = ManagementFactory.getRuntimeMXBean();
List<String> arguments = runtimeMxBean.getInputArguments();
runc.addAll(arguments);
File me = new File(LibLoader.class.getProtectionDomain().getCodeSource().getLocation().getPath());
String classpaths = getNewClassPaths();
if(!classpaths.isEmpty()) {
runc.add("-cp");
runc.add(classpaths);
}
if(me.isFile()) {
runc.add("-jar");
runc.add(me.getAbsolutePath().replace("%20", " "));
} else {
runc.add(main.getName());
}
for(String arg : args) runc.add(arg);
ProcessBuilder processBuilder = new ProcessBuilder(runc);
processBuilder.directory(new File("."));
processBuilder.redirectOutput(Redirect.INHERIT);
processBuilder.redirectError(Redirect.INHERIT);
processBuilder.redirectInput(Redirect.INHERIT);
Process process = processBuilder.start();
try {
process.waitFor();
} catch (InterruptedException e) {
e.printStackTrace();
}
return true;
}
}
Hope someone has a better solution.

Problem is, the classes must be found my the system ClassLoader not by a new ClassLoader.
It sound like your current solution of relaunching the JVM is the only clean way to do it.
The system ClassLoader cannot be changed, and you cannot add extra JARs to it at runtime.
(If you tried to use reflection to mess with the system classloader's data structures, at best it will be non-portable and version dependent. At worst it will be either error prone ... or blocked by the JVM's runtime security mechanisms.)
The solution suggested by Johannes Kuhn in a comment won't work. The java.system.class.loader property is consulted during JVM bootstrap. By the time your application is running, making changes to it will have no effect. I am not convinced that the approach in his Answer would work either.
Here is one possible alternative way to handle this ... if you can work out what the missing JARs are early enough.
Write yourself a Launcher class that does the following:
Save the command line arguments
Find the application JAR file
Extract the Main-Class and Class-Path attributes from the MANIFEST.MF.
Work out what the real classpath should be based on the above ... and other application specific logic.
Create a new URLClassLoader with the correct classpath, and the system classloader as its parent.
Use it to load the main class.
Use reflection to find the main classes main method.
Call it passing the save command line arguments.
This is essentially the approach that Spring Bootstrap and OneJar (and other things) take to handle the "jars in a jar" problem and so on. It avoids launching 2 VMs.

How to run groovy in Java

Hi everyone tried different ways to run a groovy in java with no luck, had read some documentation but things aren't that clear at the moment.
Anyone may know how to run this groovy?
package com.test.dev.search;
public class SearchQueryBase implements SearchQuery {
public QueryString getMatterQuery( SearchFilter filter ) {
String[] terms = filter.getSearchTerm().toLowerCase().split( " " );
...
...
...
}
}
This is a .groovy file (the one from above), I've tried the follow to run it without luck.
Down here is the Java class in which I want to run the above Groovy and execute getMatterQuery() to see the output from java main.
public static void main(String args[]) throws CGException {
String TEMPLATE_PACKAGE_PREFIX = "<path_to_groovy_file.";
String templateFileName = TEMPLATE_PACKAGE_PREFIX + "SearchQueryBase";
SearchFilter test = null;
Binding binding = new Binding();
binding.setVariable("filter", test);
GroovyShell shell = new GroovyShell(binding);
shell.evaluate(templateFileName);
System.out.println("Finish");
}
EDIT #1
This is the error I'm getting when I run it;
Exception in thread "main" groovy.lang.MissingPropertyException: No such property: Common for class: Script1
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:50)
at org.codehaus.groovy.runtime.callsite.PogoGetPropertySite.getProperty(PogoGetPropertySite.java:49)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callGroovyObjectGetProperty(AbstractCallSite.java:231)
at Script1.run(Script1.groovy:1)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:580)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:618)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:589)

1.
the GroovyShell.evaluate(java.lang.String scriptText) accepts string as a groovy text (content), and you try to call it with filename instead. use shell.evaluate( new File(templateFileName) )
2.
you can continue using shell.evaluate( new File(...) ) but keep in your groovy file only content of the method getMatterQuery():
String[] terms = filter.getSearchTerm().toLowerCase().split( " " );
...
...
...
so you'll have groovy script, and your code should work
3.
if you want to keep groovy as a class and call the method getMatterQuery() from this class with parameter, then your java code should be like this:
import groovy.lang.*;
...
public static void main(String[]s)throws Exception{
GroovyClassLoader cl=new GroovyClassLoader();
//path to base folder where groovy classes located
cl.addClasspath(path_to_groovy_root);
//the groovy file with SearchQueryBase.groovy
//must be located in "com/test/dev/search" subfolder under path_to_groovy_root
Class c = cl.loadClass("com.test.dev.search.SearchQueryBase");
SearchQuery o = (SearchQuery) c.newInstance();
System.out.println( o.getMatterQuery(test) );
}

Listing all of App resources within an Android Library

My aim is to write an android Library that can list all the resources (layouts, drawables. ids, etc.) of the caller application. The caller application need to pass to the library, nothing more than App Name or the package namespace.
E.g:
The MyLibrary function:
public void listDrawables(String namespace) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
String resourceNameSpace = namespace + ".R";
String drawableNamespace = resourceNameSpace + ".drawable";
final Class drawableClass = Class.forName(drawableNamespace);
Object drawableInstance = drawableClass.newInstance();
final Field[] fields = drawableClass.getDeclaredFields();
for (int i = 0, max = fields.length; i < max; i++) {
final int resourceId;
try {
resourceId = fields[i].getInt(drawableInstance);
// Use resourceId further
} catch (Exception e) {
continue;
}
}
The caller code (From the App Activity, say com.example.sample.MainActivity.java)
mylibrary.listDrawables("com.example.sample");
I have setup the MyLibrary as a dependent android library for the Sample app.
When this is run, I get this exception inside library at the Class.forName statement:
java.lang.ClassNotFoundException: com.example.sample.R.drawable
I am not able to understand fully, why library can't refer to the class. May be I missed a basic lesson in Java classpath and how build works, but isn't the class already loaded when library runs?
I would also like to know id there is any alternative way to list resources in an external library.

Since Drawable is a inner class, you need to access it by using $.
String resourceNameSpace = namespace + ".R";
String drawableNamespace = resourceNameSpace + "$drawable";

Apache commons Fileutils

I downloaded apache commons FileUtils to perform a copy directory and added them under libraries in eclipse as well. When I say Fileutils.copyDirectory(s,d) as give below eclipse says " Multiple markers at this line -Syntax error on token "(", delete this token
-Syntax error on token ")", delete this token". Can someone help
import org.apache.commons.io.FileUtils;
Public class b {
File s = new file("C://Tom//eso");
File d = new file("C://Tom//pos");
FileUtils.copyDirectory(s,d);
}

Try this:
import org.apache.commons.io.FileUtils;
public class B {
public static void main(String[] args) throws Exception {
File s = new File("C:/Tom/eso");
File d = new File("C:/Tom/pos");
FileUtils.copyDirectory(s,d);
}
}
There are several errors in your code:
Classes start with an uppercase char - it's File, not file. And it's class B, not class b (remember to also rename the file to B.java)
You must not use double / chars, just one
The code must reside inside a method, not at the class level
It's public, not Public
You're not handling exceptions, either throw them or catch them

File s = new file("C://Tom//eso");
File d = new file("C://Tom//pos");
file should be capitalized. It should be new File(....
Side note: Usually for windows the path looks like C:\\Tom\\eso, you have forward-slashes instead of backward.

You're trying to call a method outside of the body of a method...try something more along the lines of;
public class b {
public static void main(String args[]) {
File s = new File("C:/Tom/eso");
File d = new File("C:/Tom/pos");
try {
FileUtils.copyDirectory(s,d);
} catch (IOException exp) {
exp.printStackTrace();
}
}
}
Just to highlight...
Public should be public
file should be File
// should be either / or \\ (most people prefer /)
Execution code must be executed from the context of a method or static init section
I'd also recommend that you take the time to learn the Java naming conventions as well as have a read through the tutorials under the Trails Covering the Basics section

Two errors.
First
File s = new file("C://Tom//eso");
File d = new file("C://Tom//pos");
should be
File s = new File("C://Tom//eso");
File d = new File("C://Tom//pos");
Second
FileUtils.copyDirectory(s,d);
should in main method.