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

Question

I am working on an IDE like project where code changed by user gets recompiled by JavaCompiler at runtime and needs to be reloaded to execute the changed code, I am using reflection to do that, but the problem is the class once loaded by ClassLoader never gets changed on re executing the code below it remains static, but when I exit the complete application and restart it then i can see the changes in recompiled code. Below is my code which I am using:

    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 solution I found is on java2s.com which is titled as "Dynamically Reloading 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 its not working for me as the changed class never gets reloaded by this code.

Please help me or suggest me if any other option is available to do this.

Answer 1

All right this is something that i got to work.

Be careful if Myclass is in the standard class path it 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;
    }
}