clear the class loaded through class.forName in java

Question

I have created program which loads a java(JPanel) file which user chooses.User basically chooses a Java file which gets compiled by JavaCompiler and next generated class file is loaded. But problem is coming when any changes are done in the java file(JPanel) through some text editor ,since any new changes are not reflected in the class file even after closing the program and re-running the project.

I think same class file is loaded again and again from memory.

is there any way to clear the loaded class from memory?

Compilation:

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
if (compiler != null) {
        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
        StandardJavaFileManager stdFileManager = compiler.getStandardFileManager(diagnostics, null, null);
        Iterable<? extends JavaFileObject> fileObjects = stdFileManager.getJavaFileObjectsFromFiles(filesToCompile);
        List<String> optionList = new ArrayList<String>();
        // set compiler's classpath to be same as the runtime's
        rootDir=Utility.createRootDir();
        optionList.addAll(Arrays.asList("-d", rootDir.getAbsolutePath(), "-classpath", System.getProperty("java.class.path")));
        // optionList.add(()
        try {
            stdFileManager.flush();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        CompilationTask task = compiler.getTask(null, stdFileManager,null, optionList, null, fileObjects);
        Boolean result = task.call();
        try {
            stdFileManager.flush();
            stdFileManager.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
}

Loading:

loader = new URLClassLoader(new URL[] { rootDir.toURI().toURL() });
cls = Class.forName(Utility.extractFQDN(sourceFile)+"."+Utility.extractClassName(sourceFile),true, loader); 

panel= (JPanel) cls.newInstance();

I have checked the compiled class file with decompiler it has updated code but I don't know why previous class file is being loaded from memory by class loader.

Answer 1

Edit:

Here's an SSCCE compiling strings repeatedly to the same class name and demonstrating new behavior. To avoid the whole mess with files it does everything in memory. I think this should be easily adaptable to your application.

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.lang.reflect.Method;
import java.net.URI;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import javax.tools.FileObject;
import javax.tools.ForwardingJavaFileManager;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.JavaFileObject.Kind;
import javax.tools.SimpleJavaFileObject;
import javax.tools.ToolProvider;


public class Compile {
    static class OutFile extends SimpleJavaFileObject {
        private final ByteArrayOutputStream out = new ByteArrayOutputStream();

        OutFile(String name) {
            super(URI.create("memory:///" + name.replace('.','/') + Kind.CLASS.extension), Kind.CLASS);
        }

        @Override
        public OutputStream openOutputStream() throws IOException {
            return out;
        }
    }

    static class InFile extends SimpleJavaFileObject {
        final String code;

        InFile(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;
        }
    }

    static class Loader extends ClassLoader {
        private final Map<String, OutFile> files = new HashMap<String, OutFile>();

        public OutFile add(String className) {
            OutFile file = new OutFile(className);
            files.put(className, file);
            return file;
        }

        @Override
        protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
            Class<?> c = findLoadedClass(name);
            if(c == null) {
                OutFile file = files.get(name);
                if(file == null) {
                    return super.loadClass(name, resolve);
                }

                c = defineClass(name, file.out.toByteArray(), 0, file.out.size());
            }

            if(resolve) {
                resolveClass(c);
            }

            return c;
        }
    }

    static class FileManager extends ForwardingJavaFileManager<JavaFileManager> {
        private final Loader loader = new Loader();

        protected FileManager(JavaFileManager fileManager) {
            super(fileManager);
        }

        public Class<?> getClass(String name) throws ClassNotFoundException {
            return loader.loadClass(name);
        }

        @Override
        public JavaFileObject getJavaFileForOutput(Location location, String className, Kind kind, FileObject sibling) throws IOException {
            return loader.add(className);
        }
    }

    public static void compileAndRun(String source) throws Exception {
        InFile in = new InFile("Main", "class Main {\npublic static void main(String[] args) {\n" + source + "\n}\n}");

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        FileManager manager = new FileManager(compiler.getStandardFileManager(null, null, null));

        compiler.getTask(null, manager, null, null, null, Collections.singletonList(in)).call();

        Method method = manager.getClass("Main").getMethod("main", String[].class);
        method.setAccessible(true);
        method.invoke(null, (Object)new String[0]);
    }

    public static void main(String[] args) throws Exception {
        compileAndRun("System.out.println(\"Hello\");");
        compileAndRun("System.out.println(\"World\");");
    }
}

Original:

ClassLoader (and subclasses like URLClassLoader) will always ask the parent class loader to load the class, if there is a parent. If you don't explicitly set a parent when constructing it, the parent is set to the system class loader. So, all the new class loaders you create are deferring back to the system class loader, which already has the class defined.

To get the behavior that you want, set the parent to null:

loader = new URLClassLoader(new URL[] { rootDir.toURI().toURL() }, null);

Edit: Note that is only a problem because you are compiling your classes to the root directory, which is also on the classpath of the system class loader. If you compiled to some temp directory, the system class loader would not be able to find the class, and the URL loader would load the class itself.