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Need a Java system-only class loader

I would like to find or create a Java class loader that loads only system classes, excluding any classes on the application defined class path. My goal is to use that class loader to build a class loader that loads from a specific JAR file, resolving system classes using the system-only class loader, but not contaminating the classes from the JAR file with any classes defined by my application.
So far I have not found any way to create a system-only class loader that does not either use private APIs or make assumptions about the Java implementation. See code below that works in my current environment but makes undesirable assumptions.
If there is no way to create an implementation independent system-only class loader, is there a reason why not? Is what I am trying to do a bad idea?
private ClassLoader createJarClassLoader(File f)
{
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
if (systemClassLoader instanceof URLClassLoader) {
URLClassLoader cl = (URLClassLoader) systemClassLoader;
URL[] urls = cl.getURLs();
List<URL> wantedURLs = new ArrayList<>();
for (URL u : urls) {
if (isSystemURL(u)) {
wantedURLs.add(u);
}
}
try {
wantedURLs.add(f.toURI().toURL());
} catch (MalformedURLException ex) {
return null;
}
return new URLClassLoader(wantedURLs.toArray(new URL[0]), null);
}
return null;
}
private boolean isSystemURL(URL u)
{
return u.getPath().contains("/jre/");
}

You need to set the parent ClassLoader to be the bootstrap ClassLoader. Call getClassLoader() on a java.lang.String object to get the bootstrap ClassLoader and use that in your ClassLoader constructor.
public class MyClassLoader extends URLClassLoader {
protected MyClassLoader(URL[] urls, ClassLoader parent) {
super(urls, parent);
}
static MyClassLoader getInstance(URL[] urls) {
ClassLoader parent = "".getClass().getClassLoader();
return (new MyClassLoader(urls, parent));
}
}
The representation of the bootstrap classloader is documented as implementation dependent
Some implementations may use null to represent the bootstrap class loader. This method [getClassLoader] will return null in such implementations if this class was loaded by the bootstrap class loader.
but the same wording applies to parent ClassLoader in the ClassLoader constructor, which means that this solution is portable.

using URLClassLoader gets the same reference as the default ClassLoader

I'm trying to test that a class can be loaded to the jvm more then once but using different ClassLoader
so my code tries to load a class (class name "Tzvika") twice
first using the default ClassLoader
and in the second try using the URLClassLoader
the problem is that i get the same reference for the URLClassLoader and the default ClassLoader
what i'm doing wrong?
here code:
public static void main(String[] args) {
Tzvika t1 = new Tzvika();
System.out.println("t1 class loader: " + t1.getClass().getClassLoader());
Tzvika t2 = null;
try {
URLClassLoader clsLoader = URLClassLoader.newInstance(new URL[] {new URL("file:/C://data/workspace/ashrait/tests/SampleClassLoader/bin/com/tzvika/sample/")});
// same problem when i do this
//URLClassLoader clsLoader = new URLClassLoader(new URL[] {new URL("file:/C://data/workspace/ashrait/tests/SampleClassLoader/bin/com/tzvika/sample/")});
Class cls = clsLoader.loadClass("com.tzvika.sample.Tzvika");
t2 = (Tzvika)cls.newInstance();
System.out.println("t2 class loader: " + t2.getClass().getClassLoader());
} catch (Exception e) {
e.printStackTrace();
}
}
here my console output:
t1 class loader: sun.misc.Launcher$AppClassLoader#1a52fdf
t2 class loader: sun.misc.Launcher$AppClassLoader#1a52fdf

Create the URLClassLoader without a parent.
URLClassLoader clsLoader = URLClassLoader.newInstance(new URL[] {new URL("file:/C://data/workspace/ashrait/tests/SampleClassLoader/bin/com/tzvika/sample/")}, null);
Note also that you should specify the path to the root of the classpath, ie.
URLClassLoader clsLoader = URLClassLoader.newInstance(new URL[] {new URL("file:/C://data/workspace/ashrait/tests/SampleClassLoader/bin")});
To better clarify, the constructor of URLClassLoader states
The URLs will be searched in the order specified for classes and
resources after first searching in the specified parent class loader.
So when you try to load a class from this ClassLoader, it will first check the parent ClassLoader, which is the bootstrap classloader, for the existence of that class. Since that parent class loader has already loaded a com.tzvika.sample.Tzvika class, it will return it.
Now, since the classes loaded by the parent class loader and your clsLoader are different, the instances of these classes therefore belong to different classes. So although you have com.tzvika.sample.Tzvika from both ClassLoaders, they are very much different.
That's the whole point of different ClassLoaders.

findLoadedClass() returns null

According to the JVM spec, the class loader that initiates loading of a class is recorded as the initiating class loader by the JVM. Furthermore, according to the JavaDoc of ClassLoader#findLoadedClass() the method
Returns the class with the given binary name if this loader has been recorded by the Java virtual machine as an initiating loader of a class with that binary name.
(emphasis mine)
Consider a simple class loader
class SimpleClassLoader extends ClassLoader {
void foo() {
System.err.println(loadClass("foo.Bar"));
System.err.println(findLoadedClass("foo.Bar"));
}
}
Given that foo.Bar actually exists in the class path, new SimpleClassLoader().foo() prints
class foo.Bar
null
According to the reasons given above, SimpleClassLoader should be the initiating class loader and findLoadedClass("foo.Bar") should just return the successfully loaded class.
Now consider this second version:
class SimpleClassLoader2 extends ClassLoader {
SimpleClassLoader2() {
super(null); // disables delegation
}
protected Class<?> findClass(String name) {
try {
byte[] b = IOUtils.toByteArray(new FileInputStream("path/to/foo/Bar.class"));
return defineClass("foo.Bar", b, 0, b.length);
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
void foo() {
System.err.println(loadClass("foo.Bar"));
System.err.println(findLoadedClass("foo.Bar"));
}
}
This makes SimpleClassLoader2 both the initiating as well as the defining class loader of foo.Bar. Indeed, now new SimpleClassLoader2().foo() prints the desired
class foo.Bar
class foo.Bar
So either the documentation is wrong or I don't understand why SimpleClassLoader is not regarded as the initiating class loader of foo.Bar. Can someone please shed some light into this?

I did some more tests and I'm fairly sure the spec is correctly implemented. My mistake was thinking that reflectively loading a class is the same as having it load as part of the resolution step. It makes sense: Both the spec and the JavaDoc mention "recording" of a class loader as the initiating class loader. If I call loadClass() myself, the VM has no way of knowing what class loader should be the initiating class loader, so the defining class loader trivially becomes the initiating class loader as well.
This can be demonstrated by having the loaded class trigger loading of another class (foo.Baz) as part of dependency resolution but have another class loader do the actual loading.*
*I'm pretty sure this is not correct behavior of a valid class loader. I just do it to illustrate a point.
Consider the following classes (they are all in package foo):
public class Bar {
public Bar() {
new Baz();
}
}
and
public class Baz {
}
My custom class loader is now slightly modified:
public class SimpleClassLoader extends ClassLoader {
static final String PATH = "/path/to/classes";
public SimpleClassLoader() {
// disable parent delegation
super(null);
}
public void printLoadedClass(String name) throws Exception {
Class<?> cls = findLoadedClass(name);
System.err.println("findLoadedClass(" + name + ") = " + cls
+ ", has class loader " + cls.getClassLoader());
}
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
if (name.equals("foo.Baz")) {
// don't want to be defining class loader of foo.Baz
return getSystemClassLoader().loadClass(name);
}
// now we're loading foo.Bar
try {
byte[] b = IOUtils.toByteArray(new FileInputStream(PATH + "/foo/Bar.class"));
return defineClass(name, b, 0, b.length);
} catch (ClassFormatError | IOException e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
}
The test is straight forward:
public static void main(String[] args) throws Exception {
SimpleClassLoader cl = new SimpleClassLoader();
Class<?> cls = cl.loadClass("foo.Bar");
cls.newInstance(); // this triggers resolution
cl.printLoadedClass("foo.Bar");
cl.printLoadedClass("foo.Baz");
}
Output is
findLoadedClass(foo.Bar) = class foo.Bar, has class loader foo.SimpleClassLoader#3a65724d
findLoadedClass(foo.Baz) = class foo.Baz, has class loader sun.misc.Launcher$AppClassLoader#1a2b2cf8
As can be seen: SimpleClassLoader initiates loading of and also defines foo.Bar. Creating the instance triggers resolution of foo.Baz. This time, definition of the class is delegated to the
system class loader so it becomes the defining class loader. The output shows that SimpleClassLoader is initiating class loader for both classes but defines only the first class.

Java ClassLoader change

I have some class A:
public class A {
public A(String str) {
System.out.println("Create A instance: " + str);
}
public void methodA() {
System.out.println("#methodA1()");
}
}
And my class loader implementation:
public class MyClassLoader extends ClassLoader {
public MyClassLoader() {
super();
}
#Override
public synchronized Class<?> loadClass(String name)
throws ClassNotFoundException {
System.out.println("Load: " + name);
return super.loadClass(name);
}
}
And now I try to change default class loader in current thread:
import java.util.ArrayList;
import java.util.List;
public class ChangeLoaderTest {
public static void main(String[] args) {
// Save class loader so that we can restore later.
ClassLoader oldLoader = Thread.currentThread().getContextClassLoader();
MyClassLoader newLoader = new MyClassLoader();
try {
// Set new classloader.
Thread.currentThread().setContextClassLoader(newLoader);
// My class.
A a = new A("1");
a.methodA();
// Standard Java class.
List<Integer> list = new ArrayList<Integer>();
list.add(2);
list.add(3);
} finally {
// Restore.
Thread.currentThread().setContextClassLoader(oldLoader);
}
}
}
And ChangeLoaderTest output:
Create A instance: 1
#methodA1()
No one
Load: ...
Why? How I can change ClassLoader into some thread?

As Marko Topolnik points out the context classloader is for use by frameworks. To use the classloader yourself you have to call loadClass("somepackage.A") and then use the reflection API to create a new instance of A (Class.newInstance()).
You wont be able to use A or its methods in your source directly since the calling code does not know A - it uses a different classloader. An interface or baseclass of A that can be loaded by the normal classloader can be used to avoid reflection.
interface AIF{
void someMethod();
}
class A implements AIF{
public void someMethod(){}
}
public void test(){
MyLoader loader = new MyLoader();
Class cla = loader.loadClass("A");
AIF a = (AIF) cla.newInstance();
a.someMethod();
}

The contextClassLoader mechanisms is not used by the basic Java operations like new. It's only there so various frameworks can access the context class loader in charge and load resources, classes, etc. Java will always use the classloader that loaded the code that is executing. It's the one that you access via ChangeLoaderTest.class.getClassLoader() -- and there is nothing you can do about this one.

I think that what happens is that your application's class loader which is also your classloader's "parent" can locate A and load it. As a result your classloader will not be searched or used for loading A.
To be honest, I haven't much experience with classloaders but if you subclassed one that uses a URL for the path of the class (so that it can locate the class file) and the parent classloader can not load it (not part of classpath), your custom one will be used.

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.