Basically you want to implement extensibility or plugin design pattern. There are multiple ways to implement this scenario.
Which ever the component you want to allow someone else to reload the module, define an interface and implement your own implementation as a default one. For example, Here I am trying to provide a HelloInterface which each country can implement and load anytime,
public interface HelloInterface {
public String sayHello(String input);
..
}
public class HelloImplDefault implements HelloInterface {
public String sayHello(String input) {
return "Hello World";
}
}
Now allow user to add a plugin (custom implementation) files to a pre-configured path. You can either user FileSystemWatcher or a background thread to scan this path and try to compile and load the file.
To compile java file,
private void compile(List<File> files) throws IOException{
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);
Iterable<? extends JavaFileObject> compilationUnits = fileManager
.getJavaFileObjectsFromFiles(files);
JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager, diagnostics, null,
null, compilationUnits);
boolean success = task.call();
fileManager.close();
}
To load class file,
private void load(List<File> files) throws MalformedURLException, InstantiationException, IllegalAccessException, ClassNotFoundException{
ClassLoader cl = Thread.currentThread().getContextClassLoader();
for(File f: files){
if(f.getName().endsWith(".class") && !loadedClass.contains(f.getName())){
URL url = f.toURL();
URL[] urls = new URL[]{url};
Object obj = cl.loadClass(f.getName().replace(".class", "")).newInstance();
if(obj instanceof HelloInterface){
HelloProviders.addProvider((HelloInterface)obj);
System.out.println("Loaded "+ ((HelloInterface)obj).getProviderName());
}else{
//Add more classes if you want
}
loadedClass.add(f.getName());
}
}
}
At this point you can read custom implementation and loaded in system class loader. Now you are ready to go. There are security implications to this approach which you need learn from internet.
I implemented one sample code and posted in github, please take a look. Happy coding!
