I'm trying to understand the inclusion of third party jar files in a java project using only the command line in Windows 10.
Specifically, I try to include the file json-20200518.jar in my "project" so that I can use the java object JSONObject in the project.
My java file:
package com.mypackage.example;
import org.json.JSONObject;
class Example {
public static void main(String[] args) {
// ... program logic
}
}
location of my java file (Examp.java):
./com/mypackage/example
location of jar file:
./jars
using cmd win10 I compile:
javac -cp "C:\Users\pfort\Desktop\java\jars\json-20200518.jar" "C:\Users\pfort\Desktop\java\com\mypackage\example\Examp.java"
compilation is successful.
Run:
java -cp "C:\Users\pfort\Desktop\java\jars\json-20200518.jar" com.mypackage.example.Examp
I get a report:
Error: Could not find or load main class com.mypackage.example.Pokus
Caused by: java.lang.ClassNotFoundException: com.mypackage.example.Pokus
Second attempt:
java -cp "C:\Users\pfort\Desktop\java\jars\json-20200518.jar" "C:\Users\pfort\Desktop\java\com\mypackage\example\Pokus"
But the same error message comes back to me.
Where am I going wrong? Is it the wrong structure? I don't get it, the compilation is successful but the run does not work.
The compiled Examp.class file isn't part of json-20200518.jar, so you'll need to add the directory containing it to the command line. Assuming it's the current directory (.):
java -cp "C:\Users\pfort\Desktop\java\jars\json-20200518.jar;." com.mypackage.example.Examp
I am trying to use the javax.jms library: https://docs.oracle.com/javaee/7/api/javax/jms/package-summary.html, for which I have downloaded the jar: http://www.java2s.com/Code/Jar/j/Downloadjavaxjms111sourcesjar.htm
Specifically, I am trying to use the MessageListener and Message classes, which I know to be in there based on the Jar decompilation. I put this jar file next to my java file, so that the file structure looks like this:
myDir
-|jms.jar
-|Main.java
Main.java:
import javax.jms.MessageListener;
import javax.jms.Message;
public class Main {
public static void main(String[] args) {
MessageListener listener = new MessageListener() {
#Override
public void onMessage(Message msg) {
}
};
System.out.println("Hello World");
}
}
I can compile this using javac -cp jms.jar Main.java from inside myDir. This creates Main.class. However, when I run java Main, I get:
Exception in thread "main" java.lang.NoClassDefFoundError: javax/jms/MessageListener
This would lead me to believe that MessageListener is not included in the jar, but it is and the file structure is javax/jms/MessageListener checks out. What dumb mistake am I making?
When I compile without -cp js, it fails, saying:
error: package javax.jms does not exist
thus, at least the compiler is looking in the jar.
I made 2 Mistakes
1: Credit to #NormR, .:jms.jar (or .; for Windows I surmise)
2: Link jar while executing. Therefore, the commands should've been:
javac -cp .:jms.jar Main.java
java -cp .:jms.jar Main
I was trying to connect to MySQL using JDBC API. I have downloaded the MySQL driver which is the "mysql-connector-java-5.1.28-bin jar" file. My OS is Windows 7 and I have set the Classpath of Java to following path:
"E:\Myclass"
I have copied the above jar file to this folder.
Then I have written the following code to test if I can load the driver.
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class LoadDriver {
public static void main(String[] args) {
try {
// The newInstance() call is a work around for some
// broken Java implementations
Class.forName("com.mysql.jdbc.Driver").newInstance();
} catch (Exception ex) {
// handle the error
System.out.println("Unable to load Driver Class");
return;
}
}
}
I expect everything should be working fine but I always get the "Unable to load Driver Class". Can anyone point out where was wrong? Thanks
Note: Thanks for all your answers. I have solved the problem. Since I am using Eclipse, I have add the JAR file to the classpath of the Eclipse.
You have to include the JAR in your classpath:
java -jar yourdriver.jar LoadDriver
JARs are filesystems. They should be added to your classpath the same way you add directories. Only classes will be loaded from the classpath you specified.
Use the below cmd to run it
java -cp E:\Myclass\mysql-connector-java-5.1.28-bin.jar; LoadDriver
As mentioned the mysql jar exist # E:\Myclass\mysql-connector-java-5.1.28-bin.jar , just set in the classpath and run it
This question already has answers here:
What does "Could not find or load main class" mean?
(61 answers)
Closed 7 years ago.
I am having trouble compiling and running my Java code, intended to allow me to interface Java with a shared object for Vensim, a simulation modeling package.
The following code compiles without error:
javac -d . -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel.java VensimHelper.java VensimException.java VensimContextRepository.java
However, when I try to run the following:
java -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel vars
I get the following error: "Error: Could not find or load main class SpatialModel
". My SpatialModel.java code does contain a 'main' method (below), so I'm not sure what the problem is - can anyone please help me out? Thanks.
import java.io.File;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.apache.log4j.Logger;
public class SpatialModel {
private VensimHelper vh;
public static final String DLL_LIBNAME_PARAM = "vensim_lib_nam";
public static final String MODEL_PATH_PARAM = "vensim_model_path";
private final static int VENSIM_CONTEXT_CREATION_MAX_FAILURE_COUNT = 10;
public SpatialModel() throws SpatialException {
String libName = System.getProperty(DLL_LIBNAME_PARAM);
String modelPath = System.getProperty(MODEL_PATH_PARAM);
if(libName == null || libName.trim().equals("")) {
log.error("Vensim library name has to be set with -D" + DLL_LIBNAME_PARAM);
throw new SpatialException("Vensim library name has to be set with -D" + DLL_LIBNAME_PARAM);
}
if(modelPath == null || modelPath.trim().equals("")) {
log.error("Model path has to set with -D" + MODEL_PATH_PARAM);
throw new SpatialException("Model path ahs to be set with -D" + MODEL_PATH_PARAM);
}
for (int i = 0; i < VENSIM_CONTEXT_CREATION_MAX_FAILURE_COUNT && vh == null; i++) {
try {
log.info("creating new vensim helper\n\tdll lib: " + libName + "\n\tmodel path: " + modelPath);
vh = new VensimHelper(libName, modelPath);
} catch (Throwable e) {
log.error("An exception was thrown when initializing Vensim, try: " + i, e);
}
}
if (vh == null) {
throw new SpatialException("Can't initialize Vensim");
}
}
public static void main(String[] args) throws VensimException {
long before = System.currentTimeMillis();
String libName = System.getProperty(DLL_LIBNAME_PARAM);
String modelPath = System.getProperty(MODEL_PATH_PARAM);
if (libName == null) {
libName = "libvensim";
}
if(modelPath == null) {
modelPath = "~/BassModel.vmf";
}
System.setProperty(DLL_LIBNAME_PARAM, libName);
System.setProperty(MODEL_PATH_PARAM, modelPath);
if (args.length > 0 && args[0].equals("info")) {
System.out.println(new VensimHelper(libName, modelPath).getVensimInfo());
} else if (args.length > 0 && args[0].equals("vars")) {
VensimHelper helper = new VensimHelper(libName, modelPath);
String[] vars = helper.getVariables();
for (String var : vars) {
System.out.println(helper.getVariableInfo(var));
}
} else {
File f = new File(".");
System.out.println(f.getAbsolutePath());
SpatialModel sm = new SpatialModel();
}
System.out.println("Execution time: " + (System.currentTimeMillis() - before));
}
}
You must ensure that you add the location of your .class file to your classpath. So, if its in the current folder, add . to your classpath.
Note that the Windows classpath separator is a semi-colon, i.e. a ;.
If the class is in a package
package thepackagename;
public class TheClassName {
public static final void main(String[] cmd_lineParams) {
System.out.println("Hello World!");
}
}
Then calling:
java -classpath . TheClassName
results in Error: Could not find or load main class TheClassName. This is because it must be called with its fully-qualified name:
java -classpath . thepackagename.TheClassName
And this thepackagename directory must exist in the classpath. In this example, ., meaning the current directory, is the entirety of classpath. Therefore this particular example must be called from the directory in which thepackagename exists.
To be clear, the name of this class is not TheClassName, It's thepackagename.TheClassName. Attempting to execute TheClassName does not work, because no class having that name exists. Not on the current classpath anyway.
Finally, note that the compiled (.class) version is executed, not the source code (.java) version. Hence “CLASSPATH.”
You can try these two when you are getting the error: 'could not find or load main class'
If your class file is saved in following directory with HelloWorld program name
d:\sample
java -cp d:\sample HelloWorld
java -cp . HelloWorld
I believe you need to add the current directory to the Java classpath
java -cp .:./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel vars
You have to include classpath to your javac and java commands
javac -cp . PackageName/*.java
java -cp . PackageName/ClassName_Having_main
suppose you have the following
Package Named: com.test
Class Name: Hello (Having main)
file is located inside "src/com/test/Hello.java"
from outside directory:
$ cd src
$ javac -cp . com/test/*.java
$ java -cp . com/test/Hello
In windows the same thing will be working too, I already tried
If you work in Eclipse, just make a cleanup (project\clean.. clean all projects) of the project.
You have to set the classpath if you get the error:
Could not find or load main class XYZ
For example:
E:\>set path="c:\programfiles\Java\jdk1.7.0_17\bin"
E:\>set classpath=%classpath%;.;
E:\>javac XYZ.java
E:\>java XYZ
I got this error because I was trying to run
javac HelloWorld.java && java HelloWorld.class
when I should have removed .class:
javac HelloWorld.java && java HelloWorld
Check your BuildPath, it could be that you are referencing a library that does not exist anymore.
If you're getting this error and you are using Maven to build your Jars, then there is a good chance that you simply do not have your Java classes in src/main/java/.
In my case I created my project in Eclipse which defaults to src (rather than src/main/java/.
So I ended up with something like mypackage.morepackage.myclass and a directory structure looking like src/mypackage/morepackage/myclass, which inherently has nothing wrong. But when you run mvn clean install it will look for src/main/java/mypackage/morepackage/myclass. It will not find the class but it won't error either. So it will successfully build and you when you run your outputted Jar the result is:
Error: Could not find or load main class mypackage.morepackage.myclass
Because it simply never included your class in the packaged Jar.
I know this question was tagged with linux, but on windows, you might need to separate your cp args with a ; instead of a :.
java -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar;./vensim.jar SpatialModel vars
http://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html
If you try to run a java application which needs JDK 1.6 and you are trying to run on JDK 1.4, you will come across this error. In general, trying to run a Java application on old JRE may fail. Try installing new JRE/JDK.
Problem is not about your main function. Check out for
javac -d . -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel.java VensimHelper.java VensimException.java VensimContextRepository.java
output and run it.
Project > Clean and then make sure BuildPath > Libraries has the correct Library.
java -verbose:class HelloWorld might help you understand which classes are being loaded.
Also, as mentioned before, remember to call the full qualified name (i.e. include package).
I was using Java 1.8, and this error suddenly occurred when I pressed "Build and clean" in NetBeans. I switched for a brief moment to 1.7 again, clicked OK, re-opened properties and switched back to 1.8, and everything worked perfectly.
I hope I can help someone out with this, as these errors can be quite time-consuming.
This problem occurred for me when I imported an existing project into eclipse. What happens is it copied all the files not in the package, but outside the package. Hence, when I tried run > run configurations, it couldn't find the main method because it was not in the package. All I did was copy the files into the package and Eclipse was then able to detect the main method. So ultimately make sure that Eclipse can find your main method, by making sure that your java files are in the right package.
If so simple than many people think, me included :)
cd to Project Folder/src/package there you should see yourClass.java then run javac yourClass.java which will create yourClass.class then cd out of the src folder and into the build folder there you can run java package.youClass
I am using the Terminal on Mac or you can accomplish the same task using Command Prompt on windows
If you are using Eclipse... I renamed my main class file and got that error. I went to "Run As" configurator and under the class path for that project, it had listed both files in the class path. I removed old class that I renamed and left the class that had the new name and it compiled and ran just fine.
This solved the issue for me today:
cd /path/to/project
cd build
rm -r classes
Then clean&build it and run the individual files you need.
I have a similar problem in Windows, it's related to the classpath. From the command line, navigate until the directory where it's located your Java file (*.java and *.class), then try again with your commands.
I use Anypoint Studio (an Eclipse based IDE). In my case everything worked well, until I found out that while running the java code, something totally different is executed. Then I have deleted the .class files. After this point I got the error message from this question's title. Cleaning the project didn't solve the problem.
After restarting the IDE everything worked well again.
I'm trying to understand how jars and packages work in Java. So to do this, I created a simple test JAR and am trying to use a class contained in that jar. Simple enough, but it is giving me errors like "class not found". Here's the setup:
1) I have a file called MyHelloWorld.java, which will be packaged in a JAR:
package com.mytest;
public class MyHelloWorld {
public String getHello() {
return "Hello";
}
public static void main(String[] args) {
System.out.println("Hello world!");
}
}
2) I have another file called 'HelloHello.java' which uses the function getHello() in com.mytest.MyHelloWorld
import com.mytest.*;
public class HelloHello {
public static void main (String[] args) {
MyHelloWorld hello = new MyHelloWorld();
System.out.println(hello.getHello());
}
}
3) To package the MyHelloWorld class inside a JAR, I created the folders com/mytest in the current directory, and moved MyHelloWorld.java to that folder
4) I compiled MyHelloWorld.java in that folder using javac MyHelloWorld.java
5) I ran jar -cf myhello.jar ./com/mytest/*.class from the root folder to create the JAR file (as described in http://www.javacoffeebreak.com/faq/faq0028.html)
6) I copied HelloHello.java and myhello.jar to a new folder with nothing else in it, to test this setup
7) javac -cp ./*.jar HelloHello.java [succeeds]
8) java -cp ./*.jar HelloHello [FAILS] (I also tried just `java HelloWorld', which failed too, with a different error message)
This last statement fails with the message:
$java -cp ./*.jar HelloHello
Exception in thread "main" java.lang.NoClassDefFoundError: HelloHello
Caused by: java.lang.ClassNotFoundException: HelloHello
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:315)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:330)
at java.lang.ClassLoader.loadClass(ClassLoader.java:250)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:398)
Any idea why it's failing? Any insights you can provide on why it works this way, and how package names are defined inside a JAR etc. would also be appreciated!
I believe it is looking in the jar for your HelloHello class. You probably need the current folder on the classpath too.
java -cp .:myhello.jar HelloHello
You should use:
java -cp .:./* HelloHello
java and javac treat -cp argument a bit differently. With java the * in cp will automatically load all the jars it finds in the given location.
Also, the colon : is the separator between different classpath elements.
Make sure if HelloHello.class is in appropriate directories structure (com/mytest) than change your 8th step:
8) java com.mytest.HelloHello //or java -cp .;*.jar com.mytest.HelloHello
well, java HelloHello works too