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object is not an instance of declaring class on using java reflection

I am trying to load a class from a URL using java reflection and classLoader. The application is deployed on weblogic 12c server
File f = new File("/a/abc");
URL url = f.toURI().toURL();
ClassLoader classLoader = ClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(classLoader, url);
Class jobClass = classLoader.loadClass("com.test.abc");
However, I get the below error:
exception thrown:
java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
Please let me know what might be the issue here or if this is not the correct way to do it.

You are invoking the method 'addURL' on a class of type ClassLoader. The class ClassLoader does not contain the member method 'addURL'. That's why you get the error.
Instead you should call the Method's 'invoke' method with an object of type URLClassLoader.

Explicit loading - java.lang.ClassLoader

In first case, For explicit loading of test.ClassLoaderTest using below code,
public ClassLoaderTest{
public static void main(String[] args){
.....
Class.forName("test.ClassLoaderTest", true,
ClassLoaderTest.class.getClassLoader().getParent());
....
}
findClass() method of Launcher$ExtClassLoader instance gets invoked to load test.ClassLoaderTest with below error due to visibility principle,
java.lang.ClassNotFoundException: test.ClassLoaderTest
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at sun.misc.Launcher$ExtClassLoader.findClass(Launcher.java:229)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at test.ClassLoaderTest.main(ClassLoaderTest.java:29)
In second case, On explicit loading of test.ClassLoaderTest1, using
public ClassLoaderTest{
public static void main(String[] args){
.....
Class.forName("test.ClassLoaderTest1");
....
}
loadClass() method of Launcher$AppClassLoader instance is ultimately used to load test.ClassLoaderTest1 class,
where test.ClassLoaderTest1 is a wrong class file that lead to below error,
java.lang.ClassNotFoundException: test.ClassLoaderTest1
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at test.ClassLoaderTest1.main(ClassLoaderTest.java:16)
In both cases, class loading job is done by java.net.URLClassLoader.findClass()
Class.forName() internally invokes getClassLoader() to know the class loader that already loaded the class.
In second case, When Class gets a class loader instance(of type Launcher$AppClassLoader) by calling
ClassLoader cl = getClassLoader0(); to invoke the class loader instance again.
Is java.lang.ClassLoader mainly used for sub-classing custom class loader? that load classes not available in CLASSPATH but from network source etc...

From the Javadoc,
public **abstract** class ClassLoader
extends Object
There are different ClassLoader implementations that use different strategies for locating and reading the byte streams that compose a class.

Mockito + PowerMock LinkageError while mocking system class

I've got such a code snippet:
#RunWith(PowerMockRunner.class)
#PrepareForTest({Thread.class})
public class AllMeasuresDataTest {
#Before
public void setUp() throws Exception {
}
#Test
public void testGetMeasures() {
AllMeasuresData measure = new AllMeasuresData();
assertEquals(measure.getMeasures(), null);
HashMap<String, Measure> map = new HashMap<String, Measure>();
measure.setMeasures(map);
assertEquals(measure.getMeasures(), map);
measure.setMeasures(null);
assertEquals(measure.getMeasures(), null);
}
#Test
public void testAllMeasuresData() throws IOException {
ClassLoader loader = PowerMockito.mock(ClassLoader.class);
Thread threadMock = PowerMockito.mock(Thread.class);
Vector<URL> vec = new Vector<URL>();
Mockito.when(loader.getResources("measure")).thenReturn(vec.elements());
Mockito.when(threadMock.getContextClassLoader()).thenReturn(loader);
PowerMockito.mockStatic(Thread.class);
Mockito.when(Thread.currentThread()).thenReturn(threadMock);
...
}
}
While running this tests I got:
java.lang.LinkageError: loader constraint violation: loader (instance of org/powermock/core/classloader/MockClassLoader) previously initiated loading for a different type with name "javax/management/MBeanServer"
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:791)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at org.powermock.core.classloader.MockClassLoader.loadUnmockedClass(MockClassLoader.java:201)
at org.powermock.core.classloader.MockClassLoader.loadModifiedClass(MockClassLoader.java:149)
at org.powermock.core.classloader.DeferSupportingClassLoader.loadClass(DeferSupportingClassLoader.java:67)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.initializeMBean(ProtocolImpl.java:247)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.<init>(ProtocolImpl.java:237)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.getInstance(ProtocolImpl.java:185)
at measure.CodeCoverCoverageCounter$6ya5ud0ow79ijrr1dvjrp4nxx60qhxeua02ta2fzpmb1d.<clinit>(MeasureCalculatorsHolder.java:146)
at measure.MeasureCalculatorsHolder.<clinit>(MeasureCalculatorsHolder.java:17)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:188)
at javassist.runtime.Desc.getClassObject(Desc.java:43)
at javassist.runtime.Desc.getClassType(Desc.java:152)
at javassist.runtime.Desc.getType(Desc.java:122)
at javassist.runtime.Desc.getType(Desc.java:78)
at algorithm.AllMeasuresDataTest.testGetMeasures(AllMeasuresDataTest.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:66)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:312)
at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:86)
at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:94)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:296)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit49RunnerDelegateImpl$PowerMockJUnit49MethodRunner.executeTestInSuper(PowerMockJUnit49RunnerDelegateImpl.java:116)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit49RunnerDelegateImpl$PowerMockJUnit49MethodRunner.executeTest(PowerMockJUnit49RunnerDelegateImpl.java:77)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:284)
at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:84)
at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:49)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:209)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:148)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:122)
at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:34)
at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:44)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:120)
at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:101)
at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:53)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.codecover.juniteclipse.runner.EclipseTestRunner.main(EclipseTestRunner.java:40)
Do you know how can I prevent this? I maybe there is another way to mock such a piece of code:
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
...
Enumeration<URL> resources = classLoader.getResources(path);

Try adding this annotation to your Test class:
#PowerMockIgnore("javax.management.*")
Worked for me.

Similar to the accepted response here, I ended up having to exclude all of the SSL related classes:
#PowerMockIgnore({"javax.management.*", "org.apache.http.conn.ssl.*", "com.amazonaws.http.conn.ssl.*", "javax.net.ssl.*"})
Adding that to the top of my class resolved the error.

Classloader conflict, use this: #PowerMockIgnore("javax.management.*")
Let mock classloader do not load javax.*.
It works.

This may be a bit of an old topic, but I have also ran into this problem. Turns out that some of the java versions cannot handle powermockito when powermock finds out there are 2 classes with the same name in the same package (over different dependencies).
With any version higher than Java 7_25 it gives this error.

In PowerMock 1.7.0 a user-defined global configuration can be added to your project's classpath. PowerMockConfig
org/powermock/extensions/configuration.properties
Simply add a line in the properties file like:
powermock.global-ignore=javax.management.*
This will resolve the error for all the test classes in your project.

In order to mock system classes, prepare the class that is the target of the test, not Thread.class. There's no way PowerMock will be able to instrument Thread.class because it is required during JVM startup - well before PowerMock can instrument.
The way instrumentation works, once a class is loaded, it can no longer be intstrumented.
See the PowerMock wiki.

Depending on your individual setup, it may be necessary to add more methods to #PowerMockIgnore. I stumbled over this with slf4j, for using PowerMock and slf4j together, you'll need
#PowerMockIgnore({ "com.sun.org.apache.xerces.*", "javax.xml.*", "org.xml.*", "javax.management.*", "org.w3c.dom.*" })

What happens if import statements can not be resolved?

I am not clear on the following:
A class is loaded by JVM when needed, like lazy initialization, right?
Now if class A does an import of class B which class B actually is not in the file system (e.g. B.class was deleted or not delivered or any reason)
then does class A get loaded and runs if no method of class B is called?
Or class A is not able to run at all since the imports can not be resolved?
Or class A is loaded and run up to a certain point?

import statement is only important for the compiler. In bytecode all references to other classes are fully qualified. That's why superflous imports don't matter at runtime.
In your case JVM will try to load all classes that are required to load and verify A, so it will try to load B immediately, but dependant classes are loaded lazily only when they are needed. Check out the following example:
public class A {
public static void bar() {
new B().foo();
}
public static void main(String[] args) {
//bar();
}
}
Compile A.java and delete B.class. Without calling bar() method your program will run just fine. But once you uncomment piece of code actually using B class you'll get nasty:
Exception in thread "main" java.lang.NoClassDefFoundError: B
at A.bar(A.java:4)
at A.main(A.java:8)
Caused by: java.lang.ClassNotFoundException: B
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
... 2 more
If B is not available, you'll get NoClassDefFound or similar.

If A.class require B.class which is missing. A.class can not be loaded.
Loading class is a recursion operation.
When A.class require B.class, the JVM search B.class in PermGen. If B.class is loaded and store in PermGen, JVM will not reload the B.class but get it from PermGen directly, otherwrise the JVM will load B.class recursively.
When JVM can not find B.class, it throw NoClassDefFoundError.
See more about NoClassDefFoundError in [Java Specification] :page 319.

You will get NoClassDefFoundError when you call the method that uses the definition of B.class. This will cause the class loader to search for B.class and load it in memory.
AFAIK there is one exception to this rule, Annotations. When you have an annotation that cannot be found at run time it will be ignored in some cases, see: Why doesn't a missing annotation cause a ClassNotFoundException at runtime?

How to dynamically load a jar with common abstract Class?

I am trying to develop a plugin system, which provides a interface to load jar at runtime. Each jar contains a class that extends from a common abstract class. For example:
//BasicPlugin.java
package byv;
abstract class BasicPlugin {
abstract public int test(int a);
}
I implemented a subclass:
//PluginA.java
package byv;
import byv.BasicPlugin;
public class PluginA extends BasicPlugin {
#Override
public int test(int a) {
return a + a;
}
}
The subclass above was compiled and packaged into a jar file(PluginA.jar). This jar only contains PluginA.class. And then in the main project I load it using URLClassLoader:
private static void loadTest() throws Exception {
URL url = new File("PluginA.jar").toURI().toURL();
URLClassLoader ClassLoader = URLClassLoader.newInstance(new URL[] {url});
Class<?> clazz = Class.forName("byv.PluginA", true, ClassLoader);
BasicPlugin obj = (BasicPlugin) clazz.newInstance();
obj.test(2);
}
I have already added a reference to BasicPlugin in the main project. But errors still occured:
Exception in thread "main" java.lang.IllegalAccessError: class byv.PluginA cannot access its superclass byv.BasicPlugin
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at java.net.FactoryURLClassLoader.loadClass(URLClassLoader.java:615)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at byv.Main.loadTest1(Main.java:18)
at byv.Main.main(Main.java:11)
So how can I fix this problem?

Since it is an IllegalAccessError I suggest you make BasicPlugin public to solve the problem. As far as I know the class you defined is package protected. As such it is not accessible from a different class loader. Since that is what you require for your plugin, it makes not sense to make the class anything else than public.
One more thing, URLClassLoader has a second constructor in which you can specify the parent loader. This loader is then used to load the plugin class. In a more complex environment you may want to specify that loader. Currently your code uses more or less the system loader, which is ok in your example, but I don't know what you intend to do later on. BasicPlugin.class.getClassLoader() gives you the right loader for that class for sure.

Pass the classloader context to use when constructing your URLClassLoader instance:
URLClassLoader ClassLoader = URLClassLoader.newInstance(new URL[] {url},
this.getClassLoader());