We Keep Coding

How to use URLClassloader in an auto-update jar launcher?

I've come across many posts about these two topics: Auto-Updating and URLClassloaders. I'll start with the auto updating goal. I found this post here that talks about a 2 jar system. One jar that launches the main app jar: From Stephen C:
The launcher could be a Java application that creates a classloader for the new JAR, loads an entrypoint class and calls some method on it. If you do it this way, you have to watch for classloader storage leaks, but that's not difficult. (You just need to make sure that no objects with classes loaded from the JAR are reachable after you relaunch.)
This is the approach I'm taking, but I'm open to other ideas if they prove easier and/or more reliable. The Coordinator has posted some pretty cool launcher code to which I plan on incorporating some of this reload type code in my launcher, but first I need to get it to work.
My issue is that my main app jar has many other dependencies, and I cannot get some of those classes to load despite the fact that all the jars have been added to the URL's array. This brings up the second topic URLClassloader.
Side Note for future readers: When passing a URL to the URLClassloader that is a directory, a helpful note that would have saved me (an embarrassingly large) amount of time is that the contents of the directory must be .class files! I was originally pointing to my dependent jar directory, no good.
Context for the code below, my launcher jar resides in the same directory as my app jar, which is why I'm using user.dir. I will probably change this, but for now the code works and gets far enough into my app's code to request a connection to a sqlite database before failing.
Launcher:
public class Launcher {
public static void main(String[] args) {
try {
String userdir = System.getProperty("user.dir");
File parentDir = new File(userdir);
ArrayList<URL> urls = getJarURLs(parentDir);
URL[] jarURLs = new URL[urls.size()];
int index = 0;
for (URL u : urls) {
System.out.println(u.toString());
jarURLs[index] = u;
index ++;
}
URLClassLoader urlCL = new URLClassLoader(jarURLs);
Class<?> c = urlCL.loadClass("main.AppStart");
Object [] args2 = new Object[] {new String[] {}};
c.getMethod("main", String[].class).invoke(null, args2);
urlCL.close();
} catch (Exception e1) {
e1.printStackTrace();
}
}
public static ArrayList<URL> getJarURLs(File parentDir) throws MalformedURLException {
ArrayList<URL> list = new ArrayList<>();
for (File f : parentDir.listFiles()) {
if (f.isDirectory()) {
list.addAll(getJarURLs(f));
} else {
String name = f.getName();
if (name.endsWith(".jar")) {
list.add(f.toURI().toURL());
}
}
}
return list;
}
}
Here's an example of the URL output added to the array:
file:/C:/my/path/to/dependent/jars/sqlite-jdbc-3.32.3.2.jar
file:/C:/my/path/to/main/app.jar
file: ... [10 more]
The URLClassloader seems to work well enough to load my main method in app.jar. The main executes a some startup type stuff, before attempting to load a login screen. When the request is made to get the user info database, my message screen loads and displays (<-this is important for later)
the stacktrace containing:
java.sql.SQLException: No suitable driver found for jdbc:sqlite:C:\...\users.db
I understand that this is because that jar is not on the class path, but it's loaded via the class loader, so why can't it find the classes from the jar? From this post JamesB suggested adding Class.forName("org.sqlite.JDBC"); before the connection request. I rebuilt the app jar with this line of code and it worked!
The weird thing that happened next, is that my message screen class can no longer be found even though earlier it loaded and displayed correctly. The message screen is a class inside my main app.jar and not in a dependent jar, which is why I'm baffled. Am I going to have to add Class.forName before every instance of any of my classes? That seems rude..
So what could I be doing wrong with the class loader? Why does it load some classes and not others despite that fact that all the jars have been added to the URL array?
Some other relative info: My app works perfectly as intended when launched from windows command line when the classpath is specified: java -cp "main-app.jar;my/dependent/jar/directory/*" main.AppStart. It's only when I try launching the app via this classloader that I have these issues.
By the way, is this java command universal? Will it work on all operating systems with java installed? If so, could I not just scrap this launcher, and use a process builder to execute the above command? Bonus points for someone who can tell me how to execute the command from a jre packaged with my app, as that's what I plan on doing so the user does not have to download Java.

EDIT
I figured out one of the answers to one of the questions below. Turns out, I didn't need to do any of the code below. My main method loads a login screen but after it's loaded it returns back to the AppLauncher code, thus closing the URLClassLoader! Of course, at that point any requested class will not be found as the loader has been closed! What an oof! Hopefully I will save someone a headache in the future...
Original
Well, after more time, effort, research, and effective use of Eclipse's debugging tool, I was able to figure out what I needed to do to resolve my issues.
So the first issue was my JDBC driver was never registered when passing the jars to the URLClassloader. This is the part I sorta don't understand, so advisement would be welcomed, but there is a static block in the JDBC class that registers the driver so it can be used by DriverManager see code below. Loading the class is what executes that static block, hence why calling Class.forName works.
static {
try {
DriverManager.registerDriver(new JDBC());
} catch (SQLException e) {
e.printStackTrace();
}
}
What I don't understand, is how class loading works if jars are specified via the class path. The URLClassLoader doesn't load any of those classes until they are called, and I never directly work with the JDBC class, thus no suitable driver exception, but are all the classes specified via the classpath loaded initially? Seems that way for static blocks to execute.
Anyhow, to resolve my other issue with some of my app's classes not being found I had to implement my own classloader. I get what I did and how it works well, but still don't understand why I had to do it. All of my jars were loaded to the original URLClassloader so if I could find them and the files within, why couldn't it do it?
Basically, I had to override the findClass and findResource methods to return jarEntry information that I had to store. I hope this code helps someone!
public class SBURLClassLoader extends URLClassLoader {
private HashMap<String, Storage> map;
public SBURLClassLoader(URL[] urls) {
super(urls);
map = new HashMap<>();
try {
storeClasses(urls);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
private void storeClasses(URL[] urls) throws ClassNotFoundException {
for (URL u : urls) {
try {
JarFile jarFile = new JarFile(new File(u.getFile()));
Enumeration<JarEntry> e = jarFile.entries();
while (e.hasMoreElements()) {
JarEntry jar = e.nextElement();
String entryName = jar.getName();
if (jar.isDirectory()) continue;
if (!entryName.endsWith(".class")) {
//still need to store these non-class files as resources
//let code continue to store entry un-altered
} else {
entryName = entryName.replace(".class", "");
entryName = entryName.replace("/", ".");
}
map.put(entryName, new Storage(jarFile, jar));
System.out.println(entryName);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
Class<?> c = null;
try {
c = super.findClass(name);
} catch (ClassNotFoundException e) {
Storage s = map.get(name);
try {
InputStream in = s.jf.getInputStream(s.je);
int len = in.available();
c = defineClass(name, in.readAllBytes(), 0, len);
resolveClass(c);
} catch (IOException e1) {
e1.printStackTrace();
}
if (c == null) throw e;
}
return c;
}
#Override
public URL findResource(String name) {
URL url = super.findResource(name);
if (url == null) {
Storage s = map.get(name);
if (s != null) {
try {
url = new URL("jar:"+s.base.toString() + "!/" + name);
} catch (Exception e) {
e.printStackTrace();
}
}
}
return url;
}
private class Storage {
public JarFile jf;
public JarEntry je;
public URL base;
public Storage(JarFile jf, JarEntry je) {
this.jf = jf;
this.je = je;
try {
base = Path.of(jf.getName()).toUri().toURL();
} catch (MalformedURLException e) {
e.printStackTrace();
}
}
}
}

Java Properties Class

using java 8, tomcat 8
Hi, i am loading a file using properties, but i have a check before loading which returns the same properties object if its already been loaded (not null). which is a normal case scenario but i want to know if there is any way that if any change occur in target file, and some trigger should be called and refreshes all the properties objects. here is my code.
public static String loadConnectionFile(String keyname) {
String message = "";
getMessageFromConnectionFile();
if (propertiesForConnection.containsKey(keyname))
message = propertiesForConnection.getProperty(keyname);
return message;
}
public static synchronized void getMessageFromConnectionFile() {
if (propertiesForConnection == null) {
FileInputStream fileInput = null;
try {
File file = new File(Constants.GET_CONNECTION_FILE_PATH);
fileInput = new FileInputStream(file);
Reader reader = new InputStreamReader(fileInput, "UTF-8");
propertiesForConnection = new Properties();
propertiesForConnection.load(reader);
} catch (Exception e) {
Utilities.printErrorLog(Utilities.convertStackTraceToString(e), logger);
} finally {
try {
fileInput.close();
} catch (Exception e) {
Utilities.printErrorLog(Utilities.convertStackTraceToString(e), logger);
}
}
}
}
the loadConnectionFile method executes first and calls getMessageFromConnectionFile which has check implemented for "null", now if we remove that check it will definitely load updated file every time but it will slower the performance. i want an alternate way.
hope i explained my question.
thanks in advance.

Java has a file watcher service. It is an API. You can "listen" for changes in files and directories. So you can listen for changes to your properties file, or the directory in which your properties file is located. The Java Tutorials on Oracle's OTN Web site has a section on the watcher service.
Good Luck,
Avi.

Java Android FTP Upload Issue & "Async"

I am attempting to upload via ftp a file, named "advancedsettings.xml" located in path "/storage/emulated/0/advancedsettings.xml" from my Android device. It doesn't seem to be working; the file does not upload and the following exception is thrown:
01-06 17:56:17.498 28084-28084/com.name.example.appname E/SmsReceiver﹕ android.os.NetworkOnMainThreadException
I discovered that basically, an application cannot attempt to perform a networking operation "on its main thread".
I am new at Java but I understand, following from this, I must implement "ASync"; I haven't understood how to implement it. Could somebody help describe this to me and how I might implement it in respect of the below code?
My code is as follows:
public class FtpUpload {
// use this method to upload the file using file path global var and ftp code,
//then return the link string.
//TO DO: UID file name to prevent file already exists overwrite on server?
public void total() {
FTPClient con = null;
String dest_fname = "advancedsettings.xml"; // Added to create a destination file with a dynamically created name (same as the file name in /sdcard/ftp/)
try
{
con = new FTPClient();
con.connect("ftp.domain.co.uk");
// Check your USERNAME e.g myuser#mywebspace.com and check your PASSWORD to ensure they are OK.
if (con.login("username", "password"))
{
con.enterLocalPassiveMode(); // important!
con.setFileType(FTP.BINARY_FILE_TYPE);
String data = "/storage/emulated/0/advancedsettings.xml";
FileInputStream in = new FileInputStream(data);
boolean result = con.storeFile(dest_fname, in);
in.close();
if (result) Log.v("upload result", "succeeded");
con.logout();
con.disconnect();
} else { // This Error Log was created
// Create error log as a file
File log_file = new File("/storage/emulated/0/error.txt");
try {
FileWriter lfw = new FileWriter(log_file);
BufferedWriter lout = new BufferedWriter(lfw);
// Continue
lout.write("Upload Connection Failed!");
lout.close();
} catch (IOException e1) {
// TODO Auto-generated catch block
Log.e("SmsReceiver", e1.toString());
}
}
}
catch (Exception e)
{
Log.e("SmsReceiver", e.toString());
}
}
Thank you in advance.
K

Shamefully I didn't initially come across this documentation on Async Tasks, which has proven invaluable.
With a bit of improv though, I got it working. I just modified my class as such:
private class FtpUpload extends AsyncTask<Void, Void, Void> {
protected Void doInBackground(Void... params) {
//code here
}
And used the following to call the above Async method:
new FtpUpload().execute();
Of course, you won't get very far with FTP networking without declaring the following user permissions in your manifest file (outside of the "application" tags):
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

JavaScript to Java Applet using DeployJava.js to run commandline

I am pretty new to Java. I want to create a Java Applet that will allow my JavaScript to pass a commandline to the Java Applet. This will only ever be run on my development machine - no need to remind me what a security issue that is. The use-case is that I have an introspector for my ExtJS app that allows me to display the classes. I want to be able to click a class, pass the relevant pathname to the Applet and have that file open in Eclipse for editing.
I am using Win7x64, jre 1.7
So, to get Eclipse to open the file from the commandline the command is:
D:\Eclipse\eclipse.exe --launcher.openFile C:\mytestfile.js
This works.
I have written the Applet, self signed it and tested the say() method using the code shown below. That works. However when I run the executecmd() method, I don't get any output. If I comment out the whole try/catch block so that I am simply returning the cmd string passed in, the method works. Therefore, I suspect that I have the try catch incorrectly setup and since my Java skills and knowledge of the exceptions are primitive I am lost.
Can anyone help me please? At least to get some output returned, if not how to actually run the command line passed in?
And, I am passing the whole command line because when I have this working I would like to share it (since the Ext introspector is really useful). Other developers will be using different editors so this way they can use it by passing their specific commandline.
Thanks!
Murray
My HTML test page:
<html>
<head>
<meta charset="UTF-8">
<title>Test Run</title>
<script src="http://www.java.com/js/deployJava.js"></script>
<script>
var attributes = { id:'testapp', code:'systemcmd.Runcmd', archive:'runcmd.jar', width:300, height:50} ;
var parameters = {} ;
deployJava.runApplet(attributes, parameters, '1.6');
</script>
</head>
<body>
<p>Hello</p>
<script type="text/javascript">
//alert(testapp.say("Hello test")); // This works
var command = "D:\Eclipse\eclipse.exe --launcher.openFile C:\mytestfile.js";
alert(testapp.executecmd(command)); // Nothing returned at all.
</script>
</body>
</html>
My class:
package systemcmd;
import java.applet.Applet;
import java.io.IOException;
import java.security.AccessController;
//import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
public class Runcmd extends Applet {
private static final long serialVersionUID = -4370650602318597069L;
/**
* #param args
*/
public static void main(String[] args) {
}
public String say(String arg)
{
String msg[] = {null};
msg[0] = "In Say. You said: " + arg;
String output ="";
for(String str: msg)
output=output+str;
return output;
}
public String executecmd(final String cmd) throws IOException
{
final String msg[] = {null};
String output ="";
msg[0] = "In executecmd, cmd="+cmd;
try {
try {
AccessController.doPrivileged(
new PrivilegedExceptionAction() {
public Object run() throws IOException { //RuntimeException,
msg[1] = " Pre exec()";
Runtime.getRuntime().exec(cmd);
msg[2] = " Post exec()";
return null;
}
}
);
} catch (PrivilegedActionException e) {
msg[3] = " Caught PrivilegedActionException:"+ e.toString();
throw (IOException) e.getException();
}
}
catch (Exception e) {
msg[4] = " Command:" + cmd + ". Exception:" + e.toString();
}
msg[5] = " End of executecmd.";
for(String str: msg)
output=output+str;
return output;
}
}

Set Eclipse as the default consumer for .java files and use Desktop.open(File) which..
Launches the associated application to open the file.

Ok, #Andrew. Some progress, thank you!
I set the default program for *.js files to Eclipse and if I double click a file it opens in Eclipse. All good.
I then had success running the following using RunAs Java Application - the test file opened in Eclipse. Getting closer!
public class Runcmd extends Applet {
File file;
private static Desktop desktop;
private static final long serialVersionUID = -4370650602318597069L;
/**
* #param args
* #throws IOException
*/
public static void main(String[] args) throws IOException {
System.out.println("hello");
if (Desktop.isDesktopSupported()) {
desktop = Desktop.getDesktop();
}
File file = new File("C:\\sites\\test.js");
// This works if I execute it from the Eclipse RunsAs Java Application.
// ie the file is opened in Eclipse for editing.
// And, if I specify a non-existant file, it correctly throws and prints the error
try {
desktop.open(file);
} catch (Exception ioe) {
ioe.printStackTrace();
System.out.println("Error: " + ioe.toString());
}
}}
However, when I added the following method and ran it via the DeployJava.js (as per my original post above), I get the following output returned with the error appearing whether or not the jar is self signed.
Started: , Desktop is supported , Error:
java.security.AccessControlException: access denied
("java.awt.AWTPermission" "showWindowWithoutWarningBanner")
public static String openfile(String arg) {
String output = "Started: ";
File file = new File("C:\\sites\\test.js");
if (Desktop.isDesktopSupported()) {
desktop = Desktop.getDesktop();
output = output + ", Desktop is supported ";
}
try {
desktop.open(file);
} catch (Exception ioe) {
output = output + ", Error: " + ioe.toString();
}
return output + arg;
}
So, what do I need to add to get around the apparent security issue? I have read the docs and the tutorials and I am going around in circles! There seems to be a lot of conflicting advice. :-(
Thanks again,
Murray

Serializing objects in an applet

Ok, simply put I am making a quiz game in a java applet, and I want to serialize an object which stores the high scores. When I do this it works perfectly in eclipse but not in a browser.
Here is the code of my applet where it reads the file:
and yes I have all of the appropriate imports
package histApplet;
public class QuizApplet extends Applet
{
private static final String TRACKERLOC = "histApplet/track.ser";
private StatsTracker tracker;
private int difflevel = 1;
//other instance variables
public void init()
{
//other code
if(new File(TRACKERLOC).exists())
{
tracker = null;
FileInputStream fis = null;
ObjectInputStream in = null;
try
{
fis = new FileInputStream(TRACKERLOC);
in = new ObjectInputStream(fis);
tracker = (StatsTracker)in.readObject();
in.close();
}
catch(IOException ex)
{
ex.printStackTrace();
}
catch(ClassNotFoundException ex)
{
ex.printStackTrace();
}
}
else
{
tracker = new StatsTracker(difflevel);
}
//other code
}
And here is my html code
<html>
<head><title>QuizApplet</title></head>
<body>
<center><applet code="histApplet/QuizApplet.class" height=550 width=700>
</applet></center>
</body>
</html>
If I comment out this code it works in a browser but otherwise doesn't. I'm not sure why this doesn't work, and any help would be greatly appreciated.

See my answer on how to write into a text file in Java.

Java Applets execute in a sandbox within the browser, so have limited access to resources in the client machine running the applet (into the browser). File system can't be accessed by an Applet, as explained in several sites SecuringJava, Oracle.
You need to sign your Applet (trusted code) in order to get access to the file system, Oracle.

As written by David, applets can't access the local file system.
They can send data to the host they came from (and receive answers from there), so you could store the highscores on the server, if you have some server-side program which accepts these highscores there.
An alternative would be using a JNLP-deployed applet, then your applet could access an applet-specific local storage with a PersistenceService.