Java reflection, how to reload a modified and recompiled class at runtime?

Question

I am working on a project similar to the IDE, where user-modified code is recompiled by JavaCompiler at runtime and needs a reboot to execute the modified code. I use reflection for this, but the problem is that the class once loaded by ClassLoader never gets changed when the code below is repeated, it remains static, but when I exit the full application and restart it, I see the changes in the recompiled code. Below is my code that I use:

Class<?> clazz = Class.forName("Projects.Demo."+classname); Constructor<?> ctor = clazz.getConstructor(App.class, Ctrl.class); Object object = ctor.newInstance(new Object[] { app , ctrl}); 

One of the solutions I found is on java2s.com, which is called "Dynamic reloading of a modified class":

  import java.io.File; import java.net.URL; import java.net.URLClassLoader; class MyClass{ public String myMethod() { return "a message"; } } public class Main { public static void main(String[] argv) throws Exception { URL[] urls = null; File dir = new File(System.getProperty("user.dir") + File.separator + "dir" + File.separator); URL url = dir.toURI().toURL(); urls = new URL[] { url }; ClassLoader cl = new URLClassLoader(urls); Class cls = cl.loadClass("MyClass"); MyClass myObj = (MyClass) cls.newInstance(); } 

but it does not work for me, since the changed class is never reloaded with this code.

Please help me or suggest me if there is another option for this.

Answer 1

Well, this is what I have to work on.

Be careful if Myclass is in the standard class path, this will not work.

 package nz.test.loader; public interface Executer { public void execute(); } package nz.test.loader; import javax.tools.*; import java.io.File; import java.io.IOException; import java.io.PrintWriter; import java.io.StringWriter; import java.net.URI; import java.net.URL; import java.net.URLClassLoader; import java.util.*; public class LoadClass { public static void main(String[] argv) throws Exception { URL[] urls = null; File dir = new File(System.getProperty("user.dir") + File.separator + "out/dir" + File.separator); File classFile = new File(dir,"nz/co.test/loader/MyClass.class"); long lastModified = classFile.lastModified(); URL url = dir.toURI().toURL(); urls = new URL[] { url }; ClassLoader cl = new URLClassLoader(urls); compileClass("first class", dir.getAbsolutePath()); Class cls = cl.loadClass("nz.test.loader.MyClass"); Executer myObj = (Executer) cls.newInstance(); myObj.execute(); compileClass("another class", dir.getAbsolutePath()); cl = new URLClassLoader(urls); cls = cl.loadClass("nz.test.loader.MyClass"); myObj = (Executer) cls.newInstance(); myObj.execute(); } public static void compileClass(String message, String destination) throws IOException { JavaCompiler compiler = ToolProvider.getSystemJavaCompiler(); DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>(); StringWriter writer = new StringWriter(); PrintWriter out = new PrintWriter(writer); out.println("package nz.test.loader;"); out.println("public class MyClass implements Executer{"); out.println(" public void execute() {"); out.println(" System.out.println(\""+message+"\");"); out.println(" }"); out.println("}"); out.close(); JavaFileObject file = new JavaSourceFromString("nz.test.loader.MyClass", writer.toString()); Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(file); List<String> optionList = new ArrayList<String>(); JavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null,null); List<String> params = new ArrayList(); params.add(destination); fileManager.handleOption("-d",params.iterator()); JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager, diagnostics, optionList, null, compilationUnits); boolean success = task.call(); for (Diagnostic diagnostic : diagnostics.getDiagnostics()) { System.out.println(diagnostic.getCode()); System.out.println(diagnostic.getKind()); System.out.println(diagnostic.getPosition()); System.out.println(diagnostic.getStartPosition()); System.out.println(diagnostic.getEndPosition()); System.out.println(diagnostic.getSource()); System.out.println(diagnostic.getMessage(null)); } System.out.println("Success: " + success); } } class JavaSourceFromString extends SimpleJavaFileObject { final String code; JavaSourceFromString(String name, String code) { super(URI.create("string:///" + name.replace('.', '/') + Kind.SOURCE.extension),Kind.SOURCE); this.code = code; } @Override public CharSequence getCharContent(boolean ignoreEncodingErrors) { return code; } }