Develop Reference

Load a class from a jar having dependency on another jar

My project structure is the following (very simplified of course):
So under lib-ext i download on a daily basis from a Jenkins server 2 jar files 'jar1 and jar2' to be checked by my program, i need one file from 'jar1' lets call it: "Class2Bloaded".
The issue is that this file implements an interface that is to be found in 'jar2', lets call this 'Dependency'
What i would like to do is, from my class under src "ClassThatLoads.java", load 'Class2Bloaded.class' and tell the class loader to look into 'jar2' to search for the implementing interface "Dependency.class"
My code so far (omitting exceptions handling):
//Create the URL pointing to Jar1
private URL getJarUrl(JarFile jarFile)
{
return new File(jarFile.getName()).toURI().toURL();
}
URL jar1Url = getJarUrl(jar1);
ClassLoader jar1classLoader = new URLClassLoader(new URL[] { jar1Url });
Class<?> Class2Bloaded = Class.forName(fullClassName, false, jar1classLoader );
So the problem happens within the Class.forName invocation, because the class i want to load implements an interface that is in jar 2.
Exception in thread "main" java.lang.NoClassDefFoundError: com/packagewithinJar2/Dependency
So eventually i have prepared another class loader that points to 'jar2', and i have even got the actual Interface i need:
URL jar2Url = getJarUrl(jar2);
ClassLoader jar2classLoader = new URLClassLoader(new URL[] { jar2Url });
Class<?> Interface2Bloaded = Class.forName(fullClassName, false, jar2classLoader );
Where 'fullClassName' in the second case is the fully qualified name of the interface from which 'Class2Bloaded' depends on.
Is just that i cant find anything in the javadocs of ClassLoader that allows me to 'inject' an additional class loader for the dependencies.
I hope my explanation is clear.

The first thing to do would be to add jar2 to the list of jars your URLClassLoader reads:
ClassLoader jarclassLoader = new URLClassLoader(new URL[] { jar1Url, jar2Url });
BUT the normal thing to do would be to add jar1 and jar2 on your classpath from the beginning.
To do so you would use the -cp parameter of the java executable.
for example, if you compile your classes into the bin directory:
java -cp libext/jar1.jar:libext/jar2.jar:bin ClassThatLoads
That way, you could use the classes seamless in your own java source and get rid of the cumbersome loading part :
public class ClassThatLoads {
public static void main(String[] args) {
Class2Bloaded stuff = new Class2Bloaded();
//use stuff from here...
}
}

Unable to invoke method by java reflection: NoSuchMethodException

We are working on a dynamic class loader project and trying to invoke a method in a dynamically loaded class via URLClassLoader. It works pretty well when it's run in Eclipse, so the invoking and dynamically loaded classes are bundled in to two different jars then deployed into Server because it’s part of requirement to split into two jars.
NoSuchMethodException exception thrown when calling mapClass.getDeclaredMethod("run", oneParam) using reflection api.
We have verified Jar and it confirms methods being invoked is present in class.
Here is sample code..
import com.altova.io.Input;
import com.altova.io.FileInput;
Input sourceInput = new FileInput(inputFileFullPath);
Class oneParam[] = { Input.class };
Object mapObj = mapClass.newInstance();
Method method = mapClass.getDeclaredMethod(RUN, oneParam);
li = (List) method.invoke(mapObj, sourceInput);
Exception is
java.lang.NoSuchMethodException:
com.sample.test.TrackingService.run(com.altova.io.Input)
Only difference I doubt is Input(in method signature) and FileInput (in the parameter).
It works well in Eclipse, but not in server. It's tested with JDK 1.6 & 1.7, but server runs on jdk 1.7
Second option.. tried as shown below but this time it throws
java.lang.IllegalArgumentException: argument type mismatch
Code
for (Method method : mapClass.getDeclaredMethods()) {
method.setAccessible(true);
if (method.getName().equals("run")) {
Class<?>[] params = method.getParameterTypes();
if (params.length == 2) {
if (params[0].isInstance(sourceInput) && params[1].isInstance(outputStream)) {
li = (List<ByteArrayOutputStream>) method.invoke(mapObj, sourceInput, outputStream);
System.out.println(" Parsing is complete:");
}
}
}}

It seems that you are trying to invoke non-public method. All you need is to make it accessible:
Method method = mapClass.getDeclaredMethod(RUN, oneParam);
method.setAccessible(true);

It worked when the class loader changed as shown below.
URLClassLoader loader = new URLClassLoader(new URL[] { new URL("file:/opt/jars/Tracking.jar") }, this.getClass().getClassLoader());
Rest everything same... as I said, with-out adding the current class loader also worked when run in eclipse.

Instantiating classes from a jar that implement a common interface, and then assigning the instance to the interface causes ClassCastException

This is a classloader issue that I am struggling with. I understand the root cause of the issue (different classloaders), but I'm not sure about the best way to fix it.
I have project with some common interfaces; let's call it api. I have two other projects called runner and module that both use api as a dependency.
The job of runner is to dynamically load a module artifact (from a jar; it's a fat one that includes its dependencies) and then execute it. runner expects module to provide certain concrete implementations from api. To make sure that classes from different versions of module.jar don't clobber each other, I create a new classloader with a URL to module.jar, and set the parent classloader to the classloader of the class that loads and processes module.jar. This works without any issues.
The problem arose when I used runner as a dependency inside a webapp (a spring boot app to be specific), and quickly found that I couldn't load some classes from module.jar because they conflict with classes that already exist in the current classpath (from other dependencies in the webapp).
Since module.jar really only needs the classes from api, I thought that I could create a new URLClassLoader (without a parent) that only has classes from api.jar, and then use that as the parent classloader when I load up the module. This is where I started running into trouble:
CommonInterface commonInterface = null;
Class<CommonInterface> commonInterfaceClass = null;
ClassLoader myClassLoader = URLClassLoader.newInstance(moduleJarURL, apiClassesClassLoader);
//...
//...
//clazz is a concrete implementation from module.jar
if(myClassLoader.loadClass(CommonInterface.class.getName()).isAssignableFrom(clazz)) {
commonInterfaceClass = clazz;
}
commonInterface = commonInterfaceClass.newInstance(); //ClassCastException
I understand that my original problem is due to the fact that the classloader first checks to see if the class has already been loaded before attempting to load it, which meant that when it was resolved using the name from module.jar, it was linking against an incompatible version of the class.
What's a good way to deal with this issue? Instead of creating a URL classloader that only has classes from api, does it make sense to create my own implementation that delegates to the parent only if the requested class is one from api?

You have loaded CommonInterface from two different class loaders. Classes with the same name but different class loaders are different classes to the JVM. (Even if the classes are 100% identical in the .class file - the problem is not incompatibility but the fact that they're from different class loaders)
If you do a
System.out.println(CommonInterface.class == myClassLoader.loadClass(CommonInterface.class.getName()));
You'll find that this prints false.
The way your create your classloader:
ClassLoader myClassLoader = URLClassLoader.newInstance(moduleJarURL, apiClassesClassLoader);
.. would only work if apiClassesClassLoader is also a parent class loader of the class that contains this code.
You could try:
ClassLoader myClassLoader = URLClassLoader.newInstance(moduleJarURL,
getClass().getClassLoader());
But from your description (it's a "fat" jar that contains its own dependencies) and the intricacies of the web classloader (child first) this may not solve your problem.
In that case, the only solution is to make your module jar "lean" to ensure that you only load each class once with one class loader only.

I forgot to update this question with my solution. I was able to solve this issue by creating a custom class-loader that extends URLClassLoader. This classloader does not have a parent.
I then overrode loadClass to control how classes were being loaded. I first check to see if the class exists in module.jar. If so, I load it from there. Otherwise, I load it using the current classloader. Since my custom classloader doesn't have a parent, it can load classes from module.jar even if they were already loaded by the main classloader, because they do not exist in my custom classloader's hierarchy.
The basic approach was like this:
public class MyClassLoader extends URLClassLoader {
private final ClassLoader mainClassLoader = MyClassLoader.class.getClassLoader();
private final Set<String> moduleClasses;
private MyClassLoader(URL url) {
super(new URL[]{ url });
try {
JarURLConnection connection = (JarURLConnection) url.openConnection();
this.moduleClasses = connection.getJarFile().stream()
.map(JarEntry::getName)
.filter(name -> name.endsWith(".class"))
.map(name -> name.replace(".class", "").replaceAll("/", "."))
.collect(Collectors.toSet());
} catch(IOException e) {
throw new IllegalArgumentException(String.format("Unexpected error while reading module jar: %s", e.getMessage()));
}
}
public static MyClassLoader newInstance(JarFile libraryJar) {
try {
return new MyClassLoader(new URL(String.format("jar:file:%s!/", libraryJar.getName())));
} catch(MalformedURLException e) {
throw new IllegalArgumentException(String.format("Path to module jar could not be converted into proper URL: %s", e.getMessage()));
}
}
#Override
public Class<?> loadClass(String name) throws ClassNotFoundException {
if(moduleClasses.contains(name)) {
Class<?> clazz = findLoadedClass(name);
if(clazz != null) {
return clazz;
} else {
return findClass(name);
}
} else {
return mainClassLoader.loadClass(name);
}
}
}

How to run Java source code within a Java program

I have wrote some code to compile a Java source code. It then produces the .class file. The problem is how do I run it?
For example, I am ok with the name of the program and class being set, I've used prog p = new prog(), in this case, however, the class file does not yet exist until I compile it. Not really sure what to do. Can someone give me an advice?
btw, the class looks like this:
public void compile{
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
int compilationResult = compiler.run(null, null, null, fileToCompile);
}
public void run(){
Prog prog = new Prog();
prog.run();
}

If you just want to run it, you could launch a java process using
Runtime.exec or ProcessBuilder. These will create a seperate java process to run your java program. This is more likely what you want. You can essentially do the equivelant of:
>java someClass
from within your application. This link may help.
If you want to actually load the classfile and use it in your current application, I think something along the lines of this, or dynamically loading Java Classes ought to help. Basically (directly from the link, slightly modified):
public class MainClass {
public static void main(String[] args){
ClassLoader classLoader = MainClass.class.getClassLoader();
try {
Class aClass = classLoader.loadClass("MyClass");
System.out.println("aClass.getName() = " + aClass.getName());
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
Once you loaded the class, you have a Class object, and you can create an instance of the class represented by aClass by calling aClass.newInstance(), which is like
MyClass newObj = new MyClass()
Or you can use any of the other methods the Class object exposes.
As pointed out by davmac, the code sample above presumes that the code you're loading is on your applications classpath. If the class files you want to run are not in your classpath, you might want to look into URLClassLoader

Load it by URLClassLoader.
File root = new File("/java"); // The package root.
URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { root.toURI().toURL() });
Class<?> cls = Class.forName("test.Test", true, classLoader); // Assuming package test and class Test.
Object instance = cls.newInstance();
// ...
See also:
How do I instantiate a class dynamically in Java?

You need to create a classloader (a URLClassLoader will probably be fine) which will load the just-compiled class file. (So for a URLClassLoader, the compilation output path should be one of the URLs).
Then, load the compiled class using the classloader, and execute it using reflection.
Class c = cl.loadClass("ClassName");
... etc.

How do I use JDK6 ToolProvider and JavaCompiler with the context classloader?

My usage case is compiling generated source files from a java program using the ToolProvider and JavaCompiler classes provided in JDK 6. The source files contain references to classes in the context classloader (it runs in a J2EE container), but not in the system classloader. My understanding is that by default the ToolProvider will create the JavaCompiler instance with the system classloader.
Is there a way to specify a classloader for JavaCompiler to use?
I tried this approach, modified from something on IBM DeveloperWorks:
FileManagerImpl fm =
new FileManagerImpl(compiler.getStandardFileManager(null, null, null););
with FileManagerImpl defined as:
static final class FileManagerImpl
extends ForwardingJavaFileManager<JavaFileManager> {
public FileManagerImpl(JavaFileManager fileManager) {
super(fileManager);
}
#Override
public ClassLoader getClassLoader(JavaFileManager.Location location) {
new Exception().printStackTrace();
return Thread.currentThread().getContextClassLoader();
}
}
The stacktrace indicates it's only called once during annotation processing. I verified the class referenced in the source file to be compiled is not on the system classpath but is available from the context classloader.

If you know the classpath to the files that are known to the contextclassloader you can pass them to the compiler:
StandardJavaFileManager fileManager = compiler.getStandardFileManager(this /* diagnosticlistener */, null, null);
// get compilationunits from somewhere, for instance via fileManager.getJavaFileObjectsFromFiles(List<file> files)
List<String> options = new ArrayList<String>();
options.add("-classpath");
StringBuilder sb = new StringBuilder();
URLClassLoader urlClassLoader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
for (URL url : urlClassLoader.getURLs())
sb.append(url.getFile()).append(File.pathSeparator);
options.add(sb.toString());
CompilationTask task = compiler.getTask(null, fileManager, this /* diagnosticlistener */, options, null, compilationUnits);
task.call();
This example assumes you're using a URLClassloader (which allows you to retrieve the classpath) but you could insert your own classpath if you wanted to.

Another option is to use Commons JCI.

You're asking two separate questions here.
One is how to compile classes not found in the system classpath. This is easily solved by passing the "-classpath" command-line argument to the compiler (as first mentioned by Leihca).
The second is how to instantiate ToolProvider and JavaCompiler on the thread context classloader. At the time of this writing, this is an unsolved question: Using javax.tools.ToolProvider from a custom classloader?