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How to instantiate .class file which i get at run time

I get a class file at runtime and I am saving it in a particular location. How can I create an object of this class:
Class.forName(MyDynamicClass);
This does not work. The class does not have any package, so how to instantiate this class ?
I just have the MyDynamicClass.class file which is in my home folder
I tried "Amir Afghani" answer which throws the ClassNotFound Exception.

Steps that you can try to debug:
Check your class path
When you perform Class.forName("MyDynamicClass");
It checks in your classpath for the file MyDynamicClass.class and loads it
When you run your program, run with -verbose argument
This will help you identify how this class is searched and is loaded once you fix the class path issue
If you do not want to add this to your classpath, then you will most definitely have to write your own class loader (See java.net.URLClassLoader which provides an easier way to do this). This will allow you to put custom paths to be loaded

I have tested with and without package and both cases working fine.
Below are the classes
Default Package:
public class MyDynamicClass {
}
Under package com.test
package com.test;
public class MyDynamicClass {
}
package com.test;
public class MyDynamicClassTest {
public static void main(String[] args) {
try {
Class clazz = Class.forName("com.test.MyDynamicClass");
Class clazz2 = Class.forName("MyDynamicClass");
System.out.println("clazz :: "+ clazz);
System.out.println("clazz :: "+ clazz2);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
Output:
clazz :: class com.test.MyDynamicClass
clazz :: class MyDynamicClass
You can call newInstance() to create object once you have class object.
How are you compiling your class? Can you make sure there is .class file exist?

ClassLoader myClassLoader = ClassLoader.getSystemClassLoader();
// Step 2: Define a class to be loaded.
String classNameToBeLoaded = "MyDynamicClass";
// Step 3: Load the class
Class myClass = myClassLoader.loadClass(classNameToBeLoaded);
// Step 4: create a new instance of that class
Object whatInstance = myClass.newInstance();

Extending class with only private constructors

The problem is: I have a class with only private constructor available (and I cannot modify it's source code), and I need to extend it.
Since reflections allow us to create instances of such classes whenever we want (with getting constructors and calling for newInstance()), is there any way to create an instance of an extended version of such class (I mean, really any way, even if it is against OOP)?
I know, it is a bad practice, but looks like I have no choice: I need to intercept some calls to one class (it is a singleton, and it's not an interface realization, so dynamic proxies do not work here).
Minimal example (as requested):
public class Singleton {
static private Singleton instance;
private Singleton() {
}
public static Singleton getFactory() {
if (instance == null)
instance = new Singleton();
return instance;
}
public void doWork(String arg) {
System.out.println(arg);
}}
all I want to do is to construct my own wrapper (like this one)
class Extension extends Singleton {
#Override
public void doWork(String arg) {
super.doWork("Processed: " + arg);
}}
and the inject it into Factory using reflection:
Singleton.class.getField("instance").set(null, new Extension());
But I do not see any way to construct such object cause its superclass's constructor is private. The question is "is that possible at all".

It is possible (but a bad hack) if
you have the source code of the class with the private constructors or you can reconstitute it from bytecode
the class is loaded by the application class loader
you can modify the jvm's classpath
You can than create a patch that is binary compatible with the original class.
I will call the class you want to extend PrivateConstructorClass in the following section.
Take the source code of PrivateConstructorClass and copy it to a source file. The package and class name must not be changed.
Change the constructors of the PrivateConstructorClass from private to protected.
Re-compile the modified source file of PrivateConstructorClass.
Package the compiled class file into a jar archive. E.g. called "patch.jar"
Create a class that extends the first one and compile it against the class in the patch.jar
Change the jvm's classpath so that the patch.jar is the first entry in the classpath.
Now some example code that let you examine how it works:
Expect the following folder structure
+-- workspace
+- private
+- patch
+- client
Create the PrivateConstructor class in the private folder
public class PrivateConstructor {
private String test;
private PrivateConstructor(String test){
this.test = test;
}
#Override
public String toString() {
return test;
}
}
Open a command prompt in the private folder, compile and package it.
$ javac PrivateConstructor.java
$ jar cvf private.jar PrivateConstructor.class
Now create the patch file in the patch folder:
public class PrivateConstructor {
private String test;
protected PrivateConstructor(String test){
this.test = test;
}
#Override
public String toString() {
return test;
}
}
Compile and package it
$ javac PrivateConstructor.java
$ jar cvf patch.jar PrivateConstructor.class
Now comes the interresting part.
Create a class that extends the PrivateConstructor in the client folder.
public class ExtendedPrivateConstructor extends PrivateConstructor {
public ExtendedPrivateConstructor(String test){
super(test);
}
}
and a main class to test it
public class Main {
public static void main(String str[]) {
PrivateConstructor privateConstructor = new ExtendedPrivateConstructor("Gotcha");
System.out.println(privateConstructor);
}
}
Now compile the client folder's source files against the patch.jar
$ javac -cp ..\patch\patch.jar ExtendedPrivateConstructor.java Main.java
and now run it with both jars on the classpath and see what happens.
If the patch.jar comes before the private.jar than the PrivateConstructor class is loaded from the patch.jar, because the application class loader is a URLClassLoader.
$ java -cp .;..\patch\patch.jar;..\private\private.jar Main // This works
$ java -cp .;..\private\private.jar;..\patch\patch.jar Main // This will fail

The solution by #René Link was good enough, but not in my case: I wrote I'm hacking an Eclipse IDE plugin, and this means we're working under OSGi, and this means we cannot control the classpath resolving order (it will load our "hacked" class in our bundle, and vanilla victim class in another bundle, and it will do this with different classloaders, and then we would have problems with casting such objects one to another). Possibly OSGi has some tools to solve this problems, but I don't know it well enough, and also I found no info on this.
So we invented another solution. It is worse than previous one, but at least it works in our case (and so it's more flexible).
The solution is simple: javaagent. It's a standard tool, which allows to manipulate bytecode at the time it is loaded. So the task was solved by using it and java ASM library: the victim's bytecode was modified to make it's constructor public, the remaining was easy.
public class MyAgent {
public static void premain(String agentArguments, Instrumentation instrumentation) {
instrumentation.addTransformer(new ClassFileTransformer() {
#Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer)
throws IllegalClassFormatException {
if (className.equals("org/victim/PrivateClass")) { //name of class you want to modify
try {
ClassReader cr = new ClassReader(classfileBuffer);
ClassNode cn = new ClassNode();
cr.accept(cn, 0);
for (Object methodInst : cn.methods) {
MethodNode method = (MethodNode) methodInst;
if (method.name.equals("<init>") && method.desc.equals("()V")) { //we get constructor with no arguments, you can filter whatever you want
method.access &= ~Opcodes.ACC_PRIVATE;
method.access |= Opcodes.ACC_PUBLIC; //removed "private" flag, set "public" flag
}
}
ClassWriter result = new ClassWriter(0);
cn.accept(result);
return result.toByteArray();
} catch (Throwable e) {
return null; //or you can somehow log failure here
}
}
return null;
}
});
}
}
Next this javaagent must be activated with JVM flag, and then everything just works: now you can have subclasses which can call super() constructor without any problem. Or this can blow your whole leg off.

EDIT: This clearly doesn't work with the newly posted code examples edited into the question above, but I will keep the answer here for future posterity should it help someone else.
One method available to you which may or may not work depending on your situation is to use the Delegation pattern. For example:
public class PrivateClass {
private PrivateClass instance = new PrivateClass();
private PrivateClass() {/*You can't subclass me!*/
public static PrivateClass getInstance() { return instance; }
public void doSomething() {}
}
public class WrapperClass {
private PrivateClass privateInstance = PrivateClass.getInstance();
public void doSomething() {
//your additional logic here
privateInstance.doSomething();
}
}
You now have a class, WrapperClass, which has the same API as PrivateClass but delegates all the functionality to PrivateClass (after doing some pre or post work itself). Obviously, WrapperClass is not associated with the type heirarchy of PrivateClass but can be setup to do everything PrivateClass can.

Extending a class that may not be in classpath

I have a class that extends from a package class that may or may not be in the classpath when the program is ran, it isn't called unless the dependency is met,
however it seems to annoy the java verifier that throws a NoClassDefFoundError on attempting to load the program,
Any way around this?

Any way around this?
In practice, no. The superclass has to be available on the classpath for the loading, linking and verification of the subclass to succeed. That has to happen before the class can be initialized, and instances of it created.
If you can't be sure that the superclass is going to be available, you need to remove all direct and indirect static dependencies on the subclass(es), and then load the subclasses dynamically using Class.forName(). That will fail if the superclass is "missing", but you will get a different exception (not an Error) and there is the possibility that your application can continue, if it is designed to cope with the missing classes.

Frameworks such as Spring which have "optionally used" code depending on other libraries, use a "Strategy pattern" to put that dependency-specific code into an "inner class" or into a separate class.
The outer class can be loaded & run fine; it's only when you try & instantiate the inner class that the NoClassDefFoundError will be thrown.
So the outer class typically tries (try-catch) instantiating one strategy to use, and then if that fails instantiates a fallback strategy.
public class MyService {
protected MyStrategy strategy;
// constructor;
// -- choose our strategy.
public MyService() {
try {
this.strategy = new ExternalLib_Strategy();
} catch (NoClassDefFoundError x) {
// external library not available.
this.strategy = new Standard_Strategy ();
}
}
// --------------------------------------------------------
protected interface MyStrategy {
public void doSomething();
}
protected static class ExternalLib_Strategy implements MyStrategy {
ExternalLib lib = org.thirdparty.ExternalLib.getInstance(); // linkage may
public void doSomething() {
// ... use the library for additional functionality.
}
}
protected static class Standard_Strategy {
public void doSomething() {
// ... basic/ fallback functionality.
}
}
}

As a work around to this problem, If your class (subclass) is available in the classpath, you can check whether the parent class is available in the classpath by loading the class file as a resource using the method ClassLoader.getResource(). This method will never throw a class not found exception. But this will return null if the class is not found. You can aviod using your class if the resource is null.
See this sample code below:
public class Test {
/**
* #param args
*/
public static void main(String[] args) {
Object instance = Test.class.getClassLoader().getResource("com/test/package/Base.class");
Derived derived = null;
if(instance !=null) {
derived = new Derived();
System.out.println(derived.getString()); // call the getString method in base class
}
else {
// The class is not available. But no Exception
System.out.println("No Hope");
}
}
}

How to access java classes in a subfolder

I'm trying to make a program that can load an unknown set of plugins from a sub-folder, "Plugins". All of these plugins implement the same interface. What I need to know is how do I find all of the classes in this folder so that I can instantiate and use them?

MyInterface.java
A stub interface.
package test;
public interface MyInterface {
public void printSomething();
}
TestClass.java
A test class to be loaded, implementing your interface.
import test.MyInterface;
public class TestClass implements MyInterface {
public void printSomething() {
System.out.println("Hello World, from TestClass");
}
}
(Compiled class file placed in "subfolder/".)
Test.java
A complete test program that loads all class files from "subfolder/" and instantiates and runs the interface method on it.
package test;
import java.io.File;
public class Test {
public static void main(String[] args) {
try {
ClassLoader cl = ClassLoader.getSystemClassLoader();
File subfolder = new File("subfolder");
for (File f : subfolder.listFiles()) {
String s = f.getName();
System.out.println("Loading " + s);
Class cls = cl.loadClass(s.substring(0, s.lastIndexOf('.')));
MyInterface o = (MyInterface) cls.newInstance();
o.printSomething();
}
} catch (ClassNotFoundException e) {
} catch (InstantiationException e) {
} catch (IllegalAccessException e) {
}
}
}
Output from Test program above:
Loading TestClass.class
Hello World, from TestClass

Check java.util.ServiceLoader
A service is a well-known set of interfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interfaces and subclass the classes defined in the service itself. Service providers can be installed in an implementation of the Java platform in the form of extensions, that is, jar files placed into any of the usual extension directories. Providers can also be made available by adding them to the application's class path or by some other platform-specific means.
This article explains the details.

Look through the folder with File.listFiles() and use a JarClassLoader instance to load the classes in there.
Or, add a description.xml in each of those jars if they are on the classpath, and use getClass().getClassLoader().findResources("description.xml") to load all descriptions, and then you have all the plugin classes to load.

Annotate your implementation classes with a custom annotation and use scannotation it does byte code scanning of the class files, and is orders of magnitudes faster than anything else, you can use it to search the entirety of a very large classpath instantly.

how Java classloaders work for "regular" circumstances (nonexplicit use of classloaders)

I'm looking into dynamic modification of classpath. I found one solution that works nicely but it does so using an explicit call to addURL(). (presumably at startup)
However, I would like to intercept the class-loading process at runtime to locate classes if the default classloader can't seem to find them. I tried to subclass ClassLoader so it just delegates findClass() and loadClass() to the default, and print out a debug line telling me these methods have been called, but they never seem to get called when my class uses dependent classes via implicit classloading, e.g.
// regular object instantiation with 'new'
BrowserLauncher launcher;
launcher = new BrowserLauncher();
// static methods
Foobar.doSomethingOrOther();
// Class.forName()
Class cl = Class.forName("foo.bar.baz");
// reflection on a Class object obtained statically
Class<Foobar> cl = Foobar.class;
// do something with cl, like call static methods or newInstance()
How does classloading work under these circumstances? (vs. the simpler case where Classloader.loadClass() is called explicitly)
Here's my attempt at a custom classloader, below. If I use DynClassLoader0.main() with an arguments list of {"some.package.SomeClass", "foo", "bar", "baz"}, and some.package.SomeClass references other classes found in external .jar files, using one of the methods listed above, why doesn't my DynClassLoader0's findClass() and loadClass() get called? The only time loadClass gets called is the explicit call to loadClass in the main() function below.
package com.example.test.classloader;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class DynClassLoader0 extends ClassLoader {
public DynClassLoader0()
{
super();
}
public DynClassLoader0(ClassLoader parent)
{
super(parent);
}
public void runMain(String classname, String[] args) throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException
{
// [***] here we explicitly use our classloader.
Class<?> cl = loadClass(classname);
Method main = cl.getMethod("main", String[].class);
main.invoke(null, new Object[] {args});
}
#Override protected Class<?> findClass(String name) throws ClassNotFoundException
{
System.out.println("findClass("+name+")");
return super.findClass(name);
}
#Override public Class<?> loadClass(String name) throws ClassNotFoundException
{
System.out.println("loadClass("+name+")");
return super.loadClass(name);
}
static public void main(String[] args)
{
// classname, then args
if (args.length >= 1)
{
String[] classArgs = new String[args.length-1];
System.arraycopy(args, 1, classArgs, 0, args.length-1);
ClassLoader currentThreadClassLoader
= Thread.currentThread().getContextClassLoader();
DynClassLoader0 classLoader = new DynClassLoader0(currentThreadClassLoader);
// Replace the thread classloader - assumes
// you have permissions to do so
Thread.currentThread().setContextClassLoader(classLoader);
try {
classLoader.runMain(args[0], classArgs);
}
catch (Exception e) {
e.printStackTrace();
}
}
else
{
System.out.println("usage: DynClassLoader {classname} [arg0] [arg1] ...");
}
}
}
edit: I have looked through these questions already:
How do you change the CLASSPATH within Java?
Is it possible to “add” to classpath dynamically in java?
Adding files to java classpath at runtime.
edit: I thought what kdgregory is saying below is correct, that once I use my classloader explicitly (see line in code with [***] as a comment), all the code that executes from that class will cause implicit classloading from the same classloader. Yet my DynClassLoader0.loadClass() never gets called except during the outermost explicit call.

To quote from the ClassLoader JavaDoc:
The methods and constructors of
objects created by a class loader may
reference other classes. To determine
the class(es) referred to, the Java
virtual machine invokes the loadClass
method of the class loader that
originally created the class.
In other words, once you load a class, that class tries to load other classes through the classloader that loaded it. In a normal Java application, that is the system classloader, which represents the classpath passed to the JVM, or the boot classloader, used to load the JVM runtime.
Depending on your needs, there's a variant of Class.forName() that takes a classloader as an argument. If you use this to load a particular class, then references within that class should use the specified classloader.
Edit: I started tracing through your example, but decided it would just be easier to give my own. If you're going to write your own classloader, I suggest starting with the existing URLClassLoader, because it handles a lot of the behind-the-scenes stuff.
So, MyClassLoader takes a single JARfile/directory and loads classes for that directory alone. I've overridden the three methods called to load a class, and simply log their invocation (using System.err because it doesn't buffer output, unlike System.out).
My example uses a library that I'm currently working on; it was convenient, but you can pick any library you want as long as it's not already in your classpath.
The main() method loads a class via MyLoader. Then I invoke a method on that class, in a way that I know will throw an exception that's also part of the library. Note that I invoke the method by reflection: since the library is not on my Eclipse classpath, I couldn't compile it with an explicit reference.
When I run this program (under Sun JDK 1.5 for Linux), I see a lot of calls to loadClass(), both for classes in my library and for those on the classpath. This is expected: the ParseUtil class references a lot of other classes, and will use MyLoader (ie, its classloader) to load them. For those classes that MyLoader can't find locally, it delegates up the loader tree.
The exception is thrown, and when I print out its classloader I see that it's the same as the MyLoader instance I created. I also print out the loader for Exception.class, and it's null -- which the JavaDoc for Class.getClassLoader() says indicates the boot classloader.
import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
public class ClassLoaderExample
{
private static class MyClassLoader
extends URLClassLoader
{
public MyClassLoader(String path)
throws Exception
{
super(new URL[] { new File(path).toURL() });
}
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException
{
System.err.println("findClass(" + name + ")");
return super.findClass(name);
}
#Override
protected synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
System.err.println("loadClass(" + name + "," + resolve + ")");
return super.loadClass(name, resolve);
}
#Override
public Class<?> loadClass(String name) throws ClassNotFoundException
{
System.err.println("loadClass(" + name + ")");
return super.loadClass(name);
}
}
public static void main(String[] argv)
throws Exception
{
ClassLoader myLoader = new MyClassLoader("/home/kgregory/Workspace/PracticalXml-1.1/target/classes/");
System.out.println("myLoader = " + myLoader);
Class<?> parseUtilKlass = myLoader.loadClass("net.sf.practicalxml.ParseUtil");
Method parseMethod = parseUtilKlass.getDeclaredMethod("parse", String.class);
try
{
parseMethod.invoke(null, "not at all valid XML");
}
catch (InvocationTargetException e)
{
Throwable ee = e.getCause();
System.out.println("exception:" + ee);
System.out.println("exception loader = " + ee.getClass().getClassLoader());
System.out.println("Exception.class loader = " + Exception.class.getClassLoader());
}
}
}
Edit #2, based on today's comments.
A classloader is expected to delegate requests to its parent before it attempts to fulfill the request itself (this is in the ClassLoader JavaDoc). There are a couple of benefits to this practice, foremost being that you won't unintentionally load incompatible instances of the same class.
J2EE classloaders amend this model: the classloader used to load a WAR will attempt to resolve classes before the loader for a containing EAR, which in turn attempts to resolve classes before the container's classloader. The goal here is isolation: if both the WAR and its EAR contain the same library, it's probably because the two need differing versions (that, or they have a sloppy build process). Even in the J2EE case, I believe that the container classloader delegates in the standard way.

In your code the call to super.loadClass() delegates the loading of the class to the parent classloader (just look at the implementation of java.lang.ClassLoader#loadClass). So it is not your instance of DynClassLoader0 that loads the class, but the currentThreadClassLoader (which you took from Thread.currentThread().getContextClassLoader()) that you passed as a constructor parameter to DynClassLoader0. And when the loaded class refers to other classes, they are then also loaded by that classloader and not your DynClassLoader0.