Develop Reference

Unable to run downloaded jar from other jar

I currently have the following problem:
I have created a updater jar from which a client jar is downloaded and placed in some directory (just somewhere on the disk, not associated with the directory of the updater jar). I use the following code the run the client jar from the updater:
private void startApplication() {
String url = getFilePath()+"client.jar";
URL parsedURL = null;
try {
parsedURL = new File(url).toURI().toURL();
} catch (MalformedURLException e) {
e.printStackTrace();
}
ClassLoader loader = URLClassLoader.newInstance(new URL[]{parsedURL}, getClass().getClassLoader());
Class<?> cl = null;
try {
cl = Class.forName("org.myApp.client.mainPackage.Main", true, loader);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
finally {
loader = null;
}
Class<? extends Application> runClass = cl.asSubclass(Application.class);
// Avoid Class.newInstance, for it is evil.
Constructor<? extends Application> ctor = null;
try {
ctor = runClass.getConstructor();
} catch (NoSuchMethodException | SecurityException e) {
e.printStackTrace();
}
Application doRun = null;
try {
doRun = ctor.newInstance();
} catch (InstantiationException | IllegalAccessException
| IllegalArgumentException | InvocationTargetException e) {
e.printStackTrace();
}
try {
doRun.start(primaryStage);
} catch (Exception e) {
e.printStackTrace();
}
}
This code seems to work, because the Main of the client.jar gets runned. However, after its Main is started, I get an exception from the client jar. The Main from the client jar tries to load a FXML file in the upper pane. This is the exception:
ClassNotFoundException: org.myApp.client.lockscreen.LockscreenController when loading a FXML file
I do not know what triggers this error. The client jar just runs as should be, when I run it standalone.
Do I need to load all classes from the client jar from the updater jar?
Any help is greatly appreciated!

Everybody thanks for your help. I was able to fix it like this (thanks Jool, you will get all the credits):
I downloaded and runned the client jar, assuming it would have its own references. However, as Jool said, I had to add the director to the class path. What I did wrong, was that I added the directory, and not the Jar file. You have to add the JAR file too ! I did that with this code:
public void addPath(String s) throws Exception {
File f = new File(s);
URI u = f.toURI();
URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
Class<URLClassLoader> urlClass = URLClassLoader.class;
Method method = urlClass.getDeclaredMethod("addURL", new Class[]{URL.class});
method.setAccessible(true);
method.invoke(urlClassLoader, new Object[]{u.toURL()});
}
And then I just called addPath(url) before running the client jar.

It is saying that the class cannot be found because it is not on your classpath.
This depends on how you build your application (Ant, Maven etc), since this determines how the location of the .jar file is known, and where the .jar file is.
If you are using an IDE, there would usually be some sort of Libraries placeholder in which you define .jars that you are dependent upon.

How to get resource files when loading a jar using an URLClassLoader?

I have written an application which manages several plugins which are provided as jars. I load the plugin classes using an URLClassLoader which works as supposed.
But now I am writing a plugin which loads some resources which are stored inside the jar. If I start this plugin as a standalone application everything works, but if I start it from inside my application I get a NullPointerException when I try to open the resources InputStream.
I open the stream like this:
this.getClass().getResourceAsStream("/templates/template.html");
My Eclipse project structure looks like:
src
|
+ My source files
resources
|
+ templates
|
+ template.html
The following loads my plugins:
private List<Class<?>> loadClasses(final File[] jars) {
List<Class<?>> classes = new ArrayList<Class<?>>();
URL[] urls = getJarURLs(jars);
URLClassLoader loader = new URLClassLoader(urls);
for (File jar : jars) {
JarFile jarFile = null;
try {
jarFile = new JarFile(jar);
} catch (IOException e) {
// Skip this jar if it can not be opened
continue;
}
Enumeration<JarEntry> entries = jarFile.entries();
while (entries.hasMoreElements()) {
JarEntry entry = entries.nextElement();
if (isClassFile(entry.getName())) {
String className = entry.getName().replace("/", ".").replace(".class", "");
Class<?> cls = null;
try {
cls = loader.loadClass(className);
} catch (ClassNotFoundException e) {
// Skip this jar if a class inside it can not be loaded
continue;
}
classes.add(cls);
}
}
try {
jarFile.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
try {
loader.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return classes;
}
/**
* Checks if a path points to a class file or not.
*
* #param path the filename to check
* #return {#code true} if the path points to a class file or {#code false}
* if not
*/
private boolean isClassFile(final String path) {
return path.toLowerCase().endsWith(".class") && !path.toLowerCase().contains("package-info");
}
Then I make instances from this classes using newInstance().
I think that the root path of the plugins jar is not the same as the root path of the application or that not all contents of the jar files are loaded or both...
Can someone help me?

First note, that using getClass().getResource(...) also delegates to a ClassLoader, which is also responsible for loading resources. Which class loader is used? It is the same class loader, with which the class was loaded. Point.
In your code, you build up an URLClassLoader for loading some classes. So the same URLClassLoader will then be used for loading the resources, if the above mentioned call comes from a class inside your plugin.
This all seems to be ok ... but you did a little mistake. At the end of loading you also closed the loader. This will prevent subsequent calls to loadClass or getResource from returning anything meaningful. In fact, it could null, as now the loader cannot load the resource anymore.
Conclusion: Do not close the URLClassLoader, if you still need it ofr loading purposes. Instead keep the reference to this class loader and close it at the end of your program runtime.

Get java.lang.Class object of a class by reading its source(.java) file

I want to get the java.lang.Class object of a class by reading its source file using FileReader.
Actually I want to get all methods, constructors, parent class, overridden methods and imported packages of the class by selecting its source file using JFileChooser. So, I think I got these all things by using its class Class object methods like getConstructors() etc.
I have tried this, but it gives java.lang.ClassNotFoundException...
public static void main(String[] args) {
File file = new File(
"F:\\study\\projects\\saralbhakti\\src\\signup\\SignupServlet.java");
try {
// Convert File to a URL
URL url = file.toURL(); // file:/c:/myclasses/
URL[] urls = new URL[] { url };
// Create a new class loader with the directory
ClassLoader cl = new URLClassLoader(urls);
// Load in the class; MyClass.class should be located in
// the directory file:/c:/myclasses/com/mycompany
Class cls = cl.loadClass("signup.SignupServlet");
System.out.println("Class Name : " + cls.getName());
Method[] declaredMethods = cls.getDeclaredMethods();
System.out.println("All Methods : " + declaredMethods.length);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}

Classes are loaded from .class files, not .java files. You have two options:
1) Use a different API, like AST parsing, which is designed to read and understand .java files (but not execute the code in them)
2) Programmatically compile the .java file, then read the .class file. This is ugly and wonky and horrible and full of caveats and probably not what you want to do.

Add folder, which contains java sources, to classpath at runtime

Is it possible to add a folder which contains java source code as a classpath element. I have tried a few things and it seems that the classloadr is not picking up java soruce files? One of my attempts is shown below....
File uncompressedSrc = new File("uncompressed" + File.separator + "src" + File.separator);
URL uncompressedSrcURL = null;
try {
uncompressedSrcURL = new URL("file://"
+ uncompressedSrc.getAbsolutePath());
} catch (MalformedURLException e) {
e.printStackTrace();
}
URL elements[] = { uncompressedSrcURL };
new URLClassLoader(elements, ClassLoader.getSystemClassLoader());

I have found a solution to my problem... I used the following dirty "hack" to add a folder to class path...
public static void addUrl(URL u) {
URLClassLoader sysloader = (URLClassLoader) ClassLoader
.getSystemClassLoader();
Class<URLClassLoader> sysclass = URLClassLoader.class;
try {
Method method = sysclass.getDeclaredMethod("addURL", parameters);
method.setAccessible(true);
method.invoke(sysloader, new Object[] { u });
} catch (Throwable t) {
t.printStackTrace();
try {
throw new IOException(
"Error, could not add URL to system classloader");
} catch (IOException e) {
logger.log(Level.SEVERE, e.getMessage());
}
}
}

Java source code is nothing the JVM can handle by itself. Only compiled class files may be loaded by the classloader. So you may only add the content of JAR files or CLASS files to the classpath.

Sorry. I skimmed through your question way to fast.
As #Daniel says, the JVM can not read .java files, only .class files.
The java-files can be compiled into class-files and loaded in the JVM programatically as described here: Programmatically Compile and Execute with Java
The key ingredient is the following
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager filemanager = compiler.getStandardFileManager( null, null, null );
try {
Iterable compilationUnits = filemanager.getJavaFileObjects( file );
compiler.getTask( null, filemanager, null, null, null, compilationUnits ).call();
filemanager.close();
} catch ( IOException e ) {
e.printStackTrace();
}
Hope that helps!

ClassNotFound exception when extracting a class from a jar

I've been unsuccessfully trying to get a class from a .jar file. The .jar is located at C:\CTF.jar and contains a .class file CaptureTheFlagRules in a folder named CTF. The following code does not work:
try {
File jarFile = new File("C:\\CTF.jar");
URLClassLoader urlClassLoader = URLClassLoader.newInstance(new URL[]
{ jarFile.toURI().toURL() }, getClass.getClassLoader());
Class<?> unknownClass = Class.forName("CaptureTheFlagRules",
true, urlClassLoader);
....
} catch (MalformedURLException e) {
} catch (ClassNotFoundException e) {
}
The code throughs a ClassNotFoundException at the forName() call. What do I need to do to get my class?

Try:
Class<?> unknownClass = Class.forName("CTF.CaptureTheFlagRules",
true, urlClassLoader);
If it's in the CTF folder, it's probably in the CTF package.