I understand that there are many similar questions and answers, but none of the answers fit what I am trying to do. I have a file called gui.java and I am trying to turn it into a jar file, LifeGame.jar I keep the .java and the .class in a folder, and when I try to archive it, it works, but when I try to run the file, it gives me Could not find or load main class gui. I do give a manifest called "META-INF:MANIFEST.MF" and the folder is stored on my desktop.The manifest looks like:
Main-Class: gui
I would like some advice on what to do and how to fix this problem. (As I have already said, I understand this is a clone of many other questions, but the examples I've seen don't work for me in my situation)
EDIT:
Some details on my scenario:
I have multiple classes in gui.java but only one of them is public, the rest aren't private of public. It is on my desktop and when I try java gui it gives the same error as when I try archiving it into a Jar.
Name of your main class should be gui. check your .class file which is created to be gui.class or any other. It doesn't matter the name of .java file but the name of .class file.
Related
I want to use my "Final_Frame.java" in my "SampleFrameProject.java", sort of like interconnecting it by creating a "Final_Frame" object and then typing the "this.show()".
However, an error message appears, saying that "package com.toedter.components does not exist." That is the only error I've seen; I'm confident that my "Final Frame.java" is functioning well before I copy-paste it, hence I knew the issue was caused by my copy-pasting.
Btw all of the error is pointing to JCalender, and JComponents, I'm pretty sure I followed the instructions while installing them. They are working just fine before I move the "Final_Frame.java", but after I move it, it began to show errors. So is there any way for me to copy-paste it correctly? or are there any methods to call "Final_Frame.java" inside my "SampleFrameProject.java" without copy-pasting?
Thank you in advance!
package is a fundamental concept in Java. It is nothing but a namespace for the Java source file, reflecting the directory (folder) structure of your project after the src/main/java directory.
package is usually defined at the first line of the file. For example, given the directory structure from your root directory mavenproject2, the directory structure inside if it is src/main/java/com/foo/bar/FinalJava.java, then the corresponding package defined will be
package com.foo.bar;
// imports
public class FinalJava {
// the content of class
}
For your case, it would seem that you have copied com/toedter/components/FinalFrame.java from the Hotel_Management_System project (which already has defined a line package com.toedter.components in the file) to another project mavenproject2 which has a different directory structure (which is not clear in your question).
The solution would be to modify the package line according to your directory structure then it would solve the error.
Personally, I would suggest to learn some basics in Java, take some tutorials, or at least write some simple Java console applications before diving into GUI applications.
I currently don't have a working way to edit and run Java on my computer, so I'm trying to get Atom working with Java (I realize it's not a Java IDE and I'm not trying to make it one, I just want to be able to do some light Java work on my laptop). I've installed the script and instant-build packages for Atom and wrote the following test code in a file called "main.java' in my project folder:
class Main{
public static void main(String[] args) {
System.out.println("please");
}
}
When I try to run the code with cmd+i (I'm on a 2012 MacBook Pro) and get the following error message:Error: Could not find or load main class main.
I'd be happy to provide any further information; thanks for helping!
The huge problem of learning Java is that you need to launch the projects in a very strict way, and setting the classpath is always problematic. The solution lays in the following (pretty enigmatic) line of the "Script" documentation:
Project directory should be the source directory; subfolders imply packaging.
So, instead of opening a plain file, open the project (folder) with .java classes or define inside the file, the package to which your .java belong.
It's due to the fact that JDE needs to create a virtual target in form of .classess and single .java file definitely can't be launched as standalone file. I suppose that "Script" is not able to locate the source folder when you try to execute seperate .java file.
Before launching your .java files always "Add Project Folder..."
Please remember that it's not possible to have several folders opened if they don't belong to the same project. Such situation cause problems of locating the right classpath and in the end javac prompts the error.
You have to name your file with the first letter in upper case Main.java, since it must match the name of your class
I replicated the issue quite easily. If I created a new directory in Atom itself and then tried to run the code it didn't work as your error message came up for me as well. Error: Could not find or load main class main.
I used an existing directory and then created a file inside that folder in Atom and ran the same code it worked. I then copied and pasted that same file into the directory of my choice and it worked.
I am reading packaging java applications with jar tool. I noticed a manifest file is created under META-INF directory. For a simple application, it felt like, it's serves no purpose. I searched on stackoverflow to understand the usages of Manifest file. I came across UsesOfManifestFile. While reading the answer, i got confused on point 2 and 3 which are :-
What are download extensions? I am not able to understand the concept in the answer.
What does it mean to seal the jar? It has given an example below like this
Name: myCompany/MyPackage/ Sealed: true
What is the use of putting such information? Can someone elaborate on these points.
Thanks.
The manifest file is like the guidance instructions for the java program to run the jar. When the manifest is created it will hold crucial information, for example a reference to the main program. When the java program runs your jar it doesn't know where to start, so it look sin the manifest for a line telling it where the class with the main method is so it has a start point for the program. Another use is a classpath line, which will tell the program where to find any 3rd party library's in the jar are, otherwise again, the java program won't find them.
There is a range of data that can be stored in the manifest file, I would recommend checking out the Oracle information on them and seeing if that clears it up a bit more.
EDIT: From the Oracle site regarding your example:
Packages within JAR files can be optionally sealed, which means that all classes defined in that package must be archived in the same JAR file. You might want to seal a package, for example, to ensure version consistency among the classes in your software.
You seal a package in a JAR file by adding the Sealed header in the manifest, which has the general form:
Name: myCompany/myPackage/
Sealed: true
The value myCompany/myPackage/ is the name of the package to seal.
Note that the package name must end with a "/".
What this appears to mean is that any and all classes that you use in your program must be within the same jar file.
EDIT 2 (For comment response)
A manifest may contain the following line:
Main-Class: com.mkyong.awt.AwtExample
When the javaw.exe (I think) runs your runnable jar, it has no idea where to start from, all java programs run via a main method, but if you have say 50 class files in your jar, it has no idea which one has the main method to start from. It will look at the manifest file, reads your above line and thinks right, the main method is in the package com.mkyong.awt and the class is AwtExample, it then finds this class and runs the program from that main method. It's most basic function within the jar is to tell the java program running the jar where to find the stuff it needs to run your jar.
Note: that example manifest entry came from a little tutorial which shows how to create a simple runnable jar.
Good Luck!
I am writing a program that connects to a web host that has a directory containing a couple JAR files. Then, a GUI shows the JAR name and when you double click on the name, it will open the JAR file.
The problem that I am having is, not all of the JAR files have the main method in the same place, so I need some way of finding the main class.
I have tried doing an approach like this:
File file = new File("website/test.jar");
JarFile jar = new JarFile(file);
String mainClass = jar.getManifest().getMainAttributes().get("Main-Class").toString();
However, I get:
Exception in thread "main" java.util.zip.ZipException: error in opening zip file
On the line where a JarFile object is created. I have thought of two solutions: go through all the class files in the JAR and search for the one that contains the main method, or create a text file in the directory that tells the main file of each JAR.
I would prefer to use the first solution, because it does not actually require me to do extra steps every time I upload another JAR. However, I am concerned that it is kind of a "brute-force" alternative, and it feels inefficient.
Have any of you experienced a similar problems?
Thanks a lot!
See this :
http://docs.oracle.com/javase/tutorial/deployment/jar/jarclassloader.html
I'm trying to run theme studio for jscookmenu, by double-clicking the file js-cook-menu_2.30.jar. But when I do that, I get the following message:
"Failed to load Main-Class manifest attribute from C:\dev\tapestry-jsmenu_2.30\js-cook-menu_2.30.jar"
After searching on the internet, I found that it is necessary to specify a main class in the file manifest.mf of a jar file in order to run it. I thought that if I modified js-cook-menu_2.30.jar and I add a manifest.mf specifying a class with a main method, I could run the program.
I looked on the source code provided with the downloadable package, but no success.
Could someone help me with this?
Your error is exactly what it says it is. A main class and a main method of another class are very different things. Per your error message, the documentation and what you found from looking you need to add a main class, so that's what I would recommend doing.