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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.

Java class is present in classpath but startup fails with Error: Could not find or load main class

I have a jar file foobar.jar containing the following two classes:
public class Foo {
public static void main(String[] args) {
System.out.println("Foo");
}
}
The other class looks like this:
import javax.batch.api.chunk.ItemProcessor;
public class Bar implements ItemProcessor {
public static void main(String[] args) {
System.out.println("Bar");
}
#Override
public Object processItem(Object item) throws Exception {
return item;
}
}
If I execute the program using the following command, the program behaves as expected and prints Foo:
$ java -cp foobar.jar Foo
Foo
$
But if I try to start the program using the main method in the class Bar, the JVM prints a startup error and exits:
$ java -cp foobar.jar Bar
Error: Could not find or load main class Bar
$
This is the same error as if I would try to start the program using a class which is not in the jar, e.g.
$ java -cp foobar.jar BarNotThere
Error: Could not find or load main class BarNotThere
$
Why do I get this error? The fact that the Foo.main method can be started and I'm able to decompile the class Bar from the jar proves, that the class should be available on the classpath. I realize that this could have something to do with the interface ItemProcessor not being on the classpath. But shouldn't I get a java.lang.ClassNotFoundException in that case?

The problem is indeed that the interface ItemProcessor is not on the classpath. Notice that the error states "find or load main class". In the case of BarNotThere the JVM is really not able to find the main class. But in the Bar case, it is not able to load the main class.
In order to completely load a class, the JVM also need instances of each superclass objects. During this process for Bar, the JVM tries to load the class object for ItemProcessor. But since this interface is not on the classpath, loading of the main class Bar fails and the startup terminates with the Error: Could not find or load main class Bar.
If you struggle with finding the problematic class in question (because the is no message saying so), you can use the jdeps tool to inspect the classpath. Just use the same classpath, but run jdeps instead of java:
$ jdeps -cp foobar.jar Bar
foobar.jar -> java.base
foobar.jar -> not found
<unnamed> (foobar.jar)
-> java.io
-> java.lang
-> javax.batch.api.chunk not found
(This was created using openjdk-9, actual output may vary heavily depending on the Java version)
This should give you enough hints as where to look for the missing class.
Further explanation
Notice the difference between loading and initializing a class. If classloading fails during initialization (which means the class was successfully found and loaded), you will get your expected ClassNotFoundException. See the following example:
import javax.batch.api.chunk.ItemProcessor;
public class FooBar {
private static ItemProcessor i = new ItemProcessor() {
#Override
public Object processItem(Object item) throws Exception {
return item;
}
};
public static void main(String[] args) {
System.out.println("Foo");
}
}
In this case, the class FooBar can be loaded during startup. But it can not be initialized, since the static field i needs the ItemProcessor class, which is not on the classpath. Initialization is a precondition if a static method on a class is executed, which is the case, when the JVM tries to invoke the main method.
$ java -cp foobar.jar FooBar
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NoClassDefFoundError: javax/batch/api/chunk/ItemProcessor
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
at java.lang.Class.privateGetMethodRecursive(Class.java:3048)
at java.lang.Class.getMethod0(Class.java:3018)
at java.lang.Class.getMethod(Class.java:1784)
at sun.launcher.LauncherHelper.validateMainClass(LauncherHelper.java:544)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:526)
Caused by: java.lang.ClassNotFoundException: javax.batch.api.chunk.ItemProcessor
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 7 more
$

How to invoke Akka helloworld?

Below is the code taken from http://doc.akka.io/docs/akka/2.2.3/AkkaScala.pdf
import akka.actor.Actor
object Greeter {
case object Greet
case object Done
}
class Greeter extends Actor {
def receive = {
case Greeter.Greet =>
println("Hello World!")
sender ! Greeter.Done
}
}
In it, it says :
How can I run this as a standalone within Eclipse ?
I've tried creating a new Run configuration, setting com.example.HelloWorld as the main class with the program argument being akka.Main but I receive "main class not found" exception.
Update :
Based on answer by TheTerribleSwiftTomato I have
input akka.Main into the Main class field, and
added com.example.HelloWorld as the sole argument in the Arguments tab.
but I receive below error :
Exception in thread "main" java.lang.ClassNotFoundException: com.example.HelloWorld
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at akka.actor.ReflectiveDynamicAccess$$anonfun$getClassFor$1.apply(DynamicAccess.scala:67)
at akka.actor.ReflectiveDynamicAccess$$anonfun$getClassFor$1.apply(DynamicAccess.scala:66)
at scala.util.Try$.apply(Try.scala:161)
at akka.actor.ReflectiveDynamicAccess.getClassFor(DynamicAccess.scala:66)
at akka.Main$.main(Main.scala:32)
at akka.Main.main(Main.scala)
I have added the akka Maven dependency, is there something else I'm missing ?

akka.Main is not an argument, it is the launcher class (i.e. the one that contains the main method). In this case, as described in the documentation, it will set up the ActorSystem instance and other necessary infrastructure.
So, in Eclipse, you would:
input akka.Main into the Main class field, and
add com.example.HelloWorld as the sole argument in the Arguments tab.
Re edit: I see two problems:
in the snippet you posted in the question, you don't actually appear to have the HelloWorld class from the example. Did you remember to include in your project?
even if you did remember to include it, there's a slight error in the HelloWorld class. It's missing the package declaration:
package com.example
Correct that (move it to the right package) and you should be on your way to writing Actor systems.

NoClassDefFoundError when running a class [closed]

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Closed 11 years ago.
I am trying to run a class, but I get the following error:
java.lang.NoClassDefFoundError: MyClass
Caused by: java.lang.ClassNotFoundException: MyClass
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Exception in thread "main"
It happens, although the main method is in MyClass and I run directly this class.
Why is this class not found, although I launch the program from it?
Here is some code:
public class MyClass extends A implements B{
public MyClass() throws Exception {
//make some initializations
}
public static void main(final String[] args) throws Exception {
MyClass myClass = new MyClass();
//do stuff with myClass
}
}
PS: I am using Eclipse Indigo.
EDIT
I have run the class in the Command Line twice:
D:\Eclipse JEE\Workspace2\Example\target\classes\com\example\main>java com.example.main.MyClass
Exception in thread "main" java.lang.NoClassDefFoundError: com/example/main/MyClass
Caused by: java.lang.ClassNotFoundException: com.example.main.MyClass
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: com.example.main.MyClass. Program will exit.
D:\Eclipse JEE\Workspace2\Example\target\classes\com\example\main>java GeoDAOImpl
Exception in thread "main" java.lang.NoClassDefFoundError: MyClass(wrong name: com/example/main/MyClass)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: MyClass. Program will exit.

You haven't really given us enough information to say for sure, but my guess is that either:
You're not giving the right fully-qualified class name
Your class isn't on the classpath
For an example of the first case, if your code looks like this:
package foo;
public class MyClass {
public static void main(String[] args) {
}
}
Then you should be running:
java foo.MyClass
EDIT: With the extra information provided, I suspect I know what's happened. I suspect the class was created not in a package, and a Run Configuration was created which just runs MyClass. It's now been moved into com.example.main, but without the Run Configuration being updated.
Go into the Run Configuration editor (click on the dropdown next to the "run" button, and select Run Configurations...) and find "MyClass", then check which class is going to be run, and edit it to put the right package name in.

You first need to compile your source file if you haven't already done so. Assuming that your class MyClass is not in a package, and you are in the directory that contains the source file MyClass.java, compile it with:
javac MyClass.java
You should now have a file named MyClass.class.
Then, to run it, you need to make sure that the directory that contains the MyClass.class file is in the classpath. If you do not have the CLASSPATH environment variable set, Java will by default look in the current directory, so you should be able to run it with:
java MyClass
If this doesn't work, you can explicitly put the current directory . in the classpath using the -cp option:
java -cp . MyClass

NoClassDefFoundError: wrong name

I wrote a java program to test RESTful web services by using Netbeans7.0.1 and it works fine there. Now I wrote the build.xml file to compile the code and when I try to run the generated .class file I always got this exception:
Exception in thread "main" java.lang.NoClassDefFoundError: ClientREST (wrong name: clientrest/ClientREST)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: ClientREST. Program will exit.
The name and path are correct, so any thoughts why I'm getting this exception?

Exception in thread "main" java.lang.NoClassDefFoundError: ClientREST
So, you ran it as java ClientREST. It's expecting a ClientREST.class without any package.
(wrong name: clientrest/ClientREST)
Hey, the class is trying to tell you that it has a package clientrest;. You need to run it from the package root on. Go one folder up so that you're in the folder which in turn contains the clientrest folder representing the package and then execute java clientrest.ClientREST.
You should not go inside the clientrest package folder and execute java ClientREST.

I encountered this error using command line java:
java -cp stuff/src/mypackage Test
where Test.java resides in the package mypackage.
Instead, you need to set the classpath -cp to the base folder, in this case, src, then prepend the package to the file name.
So it will end up looking like this:
java -cp stuff/src mypackage.Test

To further note on Garry's reply: The class path is the base directory where the class itself resides. So if the class file is here -
/home/person/javastuff/classes/package1/subpackage/javaThing.class
You would need to reference the class path as follows:
/home/person/javastuff/classes
So to run from the command line, the full command would be -
java -cp /home/person/javastuff/classes package1/subpackage/javaThing
i.e. the template for the above is
java_executable -cp classpath the_class_itself_within_the_class_path
That's how I finally got mine to work without having the class path in the environment

Probably the location you are generating your classes in doesnt exists on the class path. While running use the jvm arg -verbose while running and check the log whether the class is being loaded or not.
The output will also give you clue as to where the clasess are being loaded from, make sure that your class files are present in that location.

Try the below syntax:
Suppose java File resides here: fm/src/com/gsd/FileName.java
So you can run using the below syntax:
(Make current directory to 'fm')
java src.com.gsd.FileName

Suppose you have class A
and a class B
public class A{
public static void main(String[] args){
....
.....
//creating classB object
new classB();
}
}
class B{
}
this issue can be resolved by moving class B inside of class A and using static keyword
public class A{
public static void main(String[] args){
....
.....
//creating class B
new classB();
static class B{
}
}

Here is my class structure
package org.handson.basics;
public class WithoutMain {
public static void main() {
System.out.println("With main()...");
}
}
To compile this program, I had to use absolute path. So from src/main/java I ran:
javac org/handson/basics/WithoutMain.java
Initially I tried with the below command from basics folder and it didn't work
basics % java WithoutMain
Error: Could not find or load main class WithoutMain
Caused by: java.lang.NoClassDefFoundError: org/handson/basics/WithoutMain (wrong name: WithoutMain)
Later I went back to src\main\java folder and ran the class with relevant package structure, which worked as expected.
java % java org.handson.basics.WithoutMain
With main()...

I also have encountered this error on Windows when using Class.forName() where the class name I use is correct except for case.
My guess is that Java is able to find the file at the path (because Windows paths are case-insensitive) but the parsed class's name does not match the name given to Class.forName().
Fixing the case in the class name argument fixed the error.