So when i try to open a java class that's not in a package from the command prompt it all works fine, but when I try to open a class that's in a package it gives me NoClassDefFoundError.
And when I list the package when I try to open the class (java somepackage/someclass) it says that it can't load or find the main class.
Any help?
What I can infer is, you have two classes:
Test.java:
// no package defined here
class Test{
public static void main(String[] args){
System.out.println("Test");
}
}
so you can compile and run it using:
javac Test.java
java Test
Another class:
Test.java:
package test; // package defined here
class Test{
public static void main(String[] args){
System.out.println("Test");
}
}
And thus doing same thing gives you error.
For this you need to be in the parent directory of 'test' folder in your terminal or cmd and use:
java test.Test
No problem with compiler. You can compile as usual using javac Test.java from inside 'test' folder.
NoClassDefFoundError means that your JVM can't find your class at runtime. This could be because it's not visible (set to private or protected, or just no access modifier).
It could also be missing from your build path
package pkg1;
public class Dataguise{
public static void main(String [] args){
System.out.println("My name is Maninder Singh");
}
}
Suppose this is my code. My package name is pkg1.
1. First you need to create pkg1 dirrectory if not existed.
2. Run "javac Dataguise.java" command
3. It will generate the "Dataguise.class" file and move this file to "pkg1" folder
4. Now run "pkg1.Dataguise" command it will work.
I was having the same issue so sharing my experience.
What I assume is, you are creating a package and a class inside it, for example.
package com.vishwa.hello.commandLineArgs;
public class ComandLineArguments {
public static void main(String[] args) {
// TODO Auto-generated method stub
for (int i = 0; i < args.length; i++) {
System.out.println("Command line arg is "+args[i]);
}
}
}
When I try to compile and run the above code in command prompt (from the package folder) as
javac ComandLineArguments.java.
java ComandLineArguments 123 456.
You get the below error:
Error: Could not find or load main class ComandLineArguments
Caused by: java.lang.NoClassDefFoundError: com/vishwa/hello/commandLineArgs/ComandLineArguments (wrong name: ComandLineArguments)
There are 2 ways to solve this:
Go to the root path of your project and then run.
java com/vishwa/hello/commandLineArgs/ComandLineArguments 123 456
789.
Command line arg is 123.
Command line arg is 456.
Command line arg is 789.
If you want to run the program from the package directory, you need to specify the class path along with the complete package
reference.
java -cp
/Users/16399/Documents/workspace-spring-tool-suite-4-4.12.1.RELEASE/CoreJava/src/
com/vishwa/ hello/commandLineArgs/ComandLineArguments 123 456 789.
Command line arg is 123. Command line arg is 456. Command line arg is 789.
Related
For example I have Main.java and test.java. test has public static int bro = 5; so i try to print test.bro from Main but the class test is not found. An IDE like Eclipse takes care of this for me but how do I do this with an editor? Sorry noob question. I'm in cmd in the directory of deez files and i type javac Main.java, den java Main. Thanks.
file Main.java:
public class Main {
public static void main(String[] args) {
System.out.println(test.bro);
}
}
file test.java:
public class test {
public static int bro = 5;
}
So suppose you have two source files: Main.java and test.java then you need to compile them first.
You can do it via command javac Main.java test.java. That command will produce 2 files in your current directory: Main.class and test.class. Which contain compiled java code.
Now you need to run your main class with classpath which contains both of your classes. So you need to run command java -cp . Main. Where . represents directory with your compiled classes.
I have been trying to run Java with command line arguments, but for some reason the class can not be found. I am very certain the directory is correct. Is there any way to fix this?
CLDemo.java file
public class Demo {
public static void main(String[] args) {
System.out.print("It works!!!");
}
}
You need to do cd out\production before java CLDemo.
The default compile output of IntelliJ IDEA is under out\production folder, and Java needs to run at the corresponding package (folder) of your compile output.
When I run this code from eclips it will give the error print:
package Chapter08_JavaIOFundamentals;
import java.io.Console;
public class Echo {
public static void main(String []args) {
Console console = System.console();
if(console == null) {
System.err.println("Cannot retrive console object - are you running your application from an IDE? Exiting the application ... ");
ystem.exit(-1); }
console.printf(console.readLine());
}
}
But when i try to run it from a command line like this.
javac Echo.java //(this will gives no errors)
java Echo
it will give a error:
Error: Could not find or load main class Echo.
Other programma's like Hello World give no problem.
What is the problem here?
You have to call java -cp . Chapter08_JavaIOFundamentals.Echo from the parent directionry of Chapter08_JavaIOFundamentals
You need to check that you added the location of your .class file to your classpath. If it is in the current folder then you can a dot . in your classpath
Running into an interesting problem
When I run:
$cd /projects/MyProject/
$java -cp . S3Sample
This works fine. However if I run:
$java -cp /projects/MyProject /projects/MyProject/S3Sample
Error: Could not find or load main class .projects.MyProject.S3Sample
Why is that. Did a quick look and can't find the answer. Thanks!
I have this folder structure:
- home
- org
- test
+ Foo.java
+ Foo.class
And the code in Foo.java is a simple hello world application:
//Note the usage of the package statement here.
package org.test;
public class Foo {
public static void main(String[] args) {
System.out.println("Hello world");
}
}
Then, in command line, in order to execute Foo.class, I should provide the complete name of the class (I'm in "/home" folder in cmd):
$ java -cp "org/test;." Foo
Exception in thread "main" java.lang.NoClassDefFoundError: Foo (wrong name: org/test/Foo)
$ java -cp "org/test;." org.test.Foo
Hello world
Now, I edit the class above and remove the package sentence:
//no package specified
public class Foo {
public static void main(String[] args) {
System.out.println("Hello world");
}
}
After recompiling the class and executing the same command lines:
$ java -cp "org/test;." Foo
Hello world
$ java -cp "org/test;." org.test.Foo
Exception in thread "main" java.lang.NoClassDefFoundError: org/test/Foo (wrong name: Foo)
TL;DR
Make sure to always specify the full name of the class. Check if your class belongs to a package. Specifying the path of the class to execute is the same as writing the full name of the class, java program will replace / by ..
You should run
$ java -cp /projects/MyProject S3Sample
The path for class is already CLASSPATH-relative
With java, you specify the fully qualified name of a class containing the main method you want executed. (The launcher will replace / with .). This class needs to be in the classpath. The argument is not a path to a file.
I am trying to run a java based tool using a command line syntax as the following: java -cp archive.jar archiveFolder.theMainClassName.Although the class I am searching for, a main class, "theMainClassName" is in the archive.jar and in the archiveFolder given at input, I keep getting the error that my class is not seen. Does anybody have any ideas concerning this problem? Thank you in advance
Here's a concrete example of what does work, so you can compare your own situation.
Take this code and put it anywhere, in a file called MainClass.java. (I've assumed a directory called src later. Normally you'd arrange the source to match the package, of course.)
package archiveFolder;
public class MainClass
{
public static void main(String[] args)
{
System.out.println("I'm MainClass");
}
}
Then run each of these commands:
# Compile the source
javac -d . src/MainClass.java
# Build the jar file
jar cf archive.jar archiveFolder
# Remove the unpackaged binary, to prove it's not being used
rm -rf archiveFolder # Or rmdir /s /q archiveFolder on Windows
# Execute the class
java -cp archive.jar achiveFolder.MainClass
The result:
I'm MainClass
How are you building your jar file? Is the code in the appropriate package?
Does theMainClassName class have the following package line at the top:
package archiveFolder
You need the class file to be in the same directory structure as the declared package. So if you had something like:
org/jc/tests/TestClass.class
its source file would have to look like this:
package org.jc.tests;
public class TestClass {
public static void main(String[] args) {
System.out.printf("This is a test class!\n");
}
}
Then you could use the following to create the jar file and run it from the command line (assuming the current directory is at the top level, just above org):
$ jar -cf testJar.jar org/jc/tests/*.class
$ java -cp testJar.jar org.jc.tests.TestClass
Perhaps with java -jar archive.jar?
Of course, it supposes the manifest points to the right class...
You should give the exact message you got, it might shed more light.
EDIT: See Working with Manifest Files: The Basics for information on setting the application entry point (Main class) in your jar manifest file.
Usually this happens when a dependent class (static member) is not found - like this, using log4j:
public class MyClass {
private static Logger log = Logger.getLogger("com.example");
}
The reason is that the initialization of such a static member can be understood as part of the class loading - errors causing the class not to be available (loadable), resulting in the error you described.
Static constructors are another possible reason:
public class MyClass {
static {
// <b>any</b> error caused here will cause the class to
// not be loaded. Demonstrating with stupid typecast.
Object o = new String();
Integer i = (Integer) o;
}
}
I think others have covered some common stuff here. I'd jar tf the jar and make sure the class is listed. I'd also double-check that the class is public and the method is "public static void main(String[] arg)".