You don't need to extract the jar file to make this work. Just open the jar with one of those apps, go to de directory where is the class file to be replaced, drag-and-drop the new file to replace the old one and save.
One option is to call File. listFiles() on the File that denotes the folder, then iterate the resulting array. To traverse trees of nested folders, use recursion. Scanning the files of a JAR file can be done using the JarFile API ... and you don't need to recurse to traverse nested "folders".
Reloading existing classes with existing data is likely to break things.
You can load new code into new class loaders relatively easily:
ClassLoader loader = URLClassLoader.newInstance(
new URL[] { yourURL },
getClass().getClassLoader()
);
Class<?> clazz = Class.forName("mypackage.MyClass", true, loader);
Class<? extends Runnable> runClass = clazz.asSubclass(Runnable.class);
// Avoid Class.newInstance, for it is evil.
Constructor<? extends Runnable> ctor = runClass.getConstructor();
Runnable doRun = ctor.newInstance();
doRun.run();
Class loaders no longer used can be garbage collected (unless there is a memory leak, as is often the case with using ThreadLocal, JDBC drivers, java.beans
, etc).
If you want to keep the object data, then I suggest a persistence mechanism such as Serialisation, or whatever you are used to.
Of course debugging systems can do fancier things, but are more hacky and less reliable.
It is possible to add new classes into a class loader. For instance, using URLClassLoader.addURL
. However, if a class fails to load (because, say, you haven't added it), then it will never load in that class loader instance.
This works for me:
File file = new File("c:\\myjar.jar");
URL url = file.toURL();
URL[] urls = new URL[]{url};
ClassLoader cl = new URLClassLoader(urls);
Class cls = cl.loadClass("com.mypackage.myclass");
I was asked to build a java system that will have the ability to load new code while running
You might want to base your system on OSGi (or at least take a lot at it), which was made for exactly this situation.
Messing with classloaders is really tricky business, mostly because of how class visibility works, and you do not want to run into hard-to-debug problems later on. For example, Class.forName(), which is widely used in many libraries does not work too well on a fragmented classloader space.
I googled a bit, and found this code here:
File file = getJarFileToLoadFrom();
String lcStr = getNameOfClassToLoad();
URL jarfile = new URL("jar", "","file:" + file.getAbsolutePath()+"!/");
URLClassLoader cl = URLClassLoader.newInstance(new URL[] {jarfile });
Class loadedClass = cl.loadClass(lcStr);
Can anyone share opinions/comments/answers regarding this approach?
Use org.openide.util.Lookup and ClassLoader to dynamically load the Jar plugin, as shown here.
public LoadEngine() {
Lookup ocrengineLookup;
Collection<OCREngine> ocrengines;
Template ocrengineTemplate;
Result ocrengineResults;
try {
//ocrengineLookup = Lookup.getDefault(); this only load OCREngine in classpath of application
ocrengineLookup = Lookups.metaInfServices(getClassLoaderForExtraModule());//this load the OCREngine in the extra module as well
ocrengineTemplate = new Template(OCREngine.class);
ocrengineResults = ocrengineLookup.lookup(ocrengineTemplate);
ocrengines = ocrengineResults.allInstances();//all OCREngines must implement the defined interface in OCREngine. Reference to guideline of implement org.openide.util.Lookup for more information
} catch (Exception ex) {
}
}
public ClassLoader getClassLoaderForExtraModule() throws IOException {
List<URL> urls = new ArrayList<URL>(5);
//foreach( filepath: external file *.JAR) with each external file *.JAR, do as follows
File jar = new File(filepath);
JarFile jf = new JarFile(jar);
urls.add(jar.toURI().toURL());
Manifest mf = jf.getManifest(); // If the jar has a class-path in it's manifest add it's entries
if (mf
!= null) {
String cp =
mf.getMainAttributes().getValue("class-path");
if (cp
!= null) {
for (String cpe : cp.split("\\s+")) {
File lib =
new File(jar.getParentFile(), cpe);
urls.add(lib.toURI().toURL());
}
}
}
ClassLoader cl = ClassLoader.getSystemClassLoader();
if (urls.size() > 0) {
cl = new URLClassLoader(urls.toArray(new URL[urls.size()]), ClassLoader.getSystemClassLoader());
}
return cl;
}
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