I want to run a java project in windows. I first compiled the .class file in linux. Copy back to windows. Now under the path H:\deletefiles has delete.class, delete.java, a.jar, b.jar. The package for class delete is deleteFiles.
My java class path is C:\program Files\Java\jre7\bin, Where I have no access to write.
I run in command prompt C:\program Files\Java\jre7\bin>
java -cp H:\deleteFiles\deleteFiles.delete
always has the problem could not find or load main class, what's the problem? thanks
You are missing the actual class to be run. The -cp H:\deleteFiles\deleteFiles.delete only defines the classpath to be used, but not which class you want to run (and you limit the classpath to a single class as well).
What you want is:
java -cp H:\deleteFiles\deleteFiles delete
Note the blank (space) between H:\deleteFiles\deleteFiles which means you are passing two parameters to the java command:
-cp H:\deleteFiles\deleteFiles - the classpath to use
delete - the class to run
If you need the classes that are part of the jar files, you need to add them to the classpath as well:
java -cp H:\deleteFiles\deleteFiles;H:\deleteFiles\deleteFiles\a.jar;H:\deleteFiles\deleteFiles\b.jar delete
You need to set the classpath to the location which contains the package hierarchy. If your package is named deleteFiles the location needs to contain a directory named deleteFiles which contains the class file.
In your example you would run it with
java -cp H:\ deleteFiles.delete
you should call delete.class in your java command line, like this:
java -cp H:\deleteFiles\delete
To execute a Java program you have two options. Using a class file or using a jar file.
If your program only contains a single source file, executing the class file would be fine. But if you have multiple sources, you would have to copy all of them. Then a jar would be more practicable.
For class:
java -cp <class path> <class name>
For jar (if the main class is set):
java -jar <jar file>
Related
I need to compile and run simple code using the gson library, but I can't use Maven, Gradle or the IDE.
The directory contains Main.java and gson-2.9.0.jar
javac -cp gson-2.9.0.jar Main.java works correctly and creates Main.class
But when I run java -cp ./*: Main, I get
Error: Could not find or load main class Main.
Caused by: java.lang.ClassNotFoundException: Main
I also tried the following commands:
java -cp gson-2.9.0.jar Main
java -cp gson-2.9.0.jar: Main
java -cp ./gson-2.9.0.jar:./* Main
But all these commands give the same result. I've never had to run code from the command line without Maven or IDEA before, so I think it's the classpath specification that's the problem. What am i doing wrong here?
If your main class is in a package (has a package ... declaration), you need to include the package name in the java call, e.g. java -cp ... mypackage.Main.
Additionally, the java documentation says about -cp / --class-path:
As a special convenience, a class path element that contains a base name of an asterisk (*) is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR.
Therefore, using * for .class files does not work; instead you have to specify the directory name. Based on your question it looks like you are using Linux, and that the .class file and the JAR are in the same directory, so the following should work in your case:
java -cp gson-2.9.0.jar:. Main
(note the . after the :, indicating to include the current directory for the classpath)
I wonder if it's possible to run a program without packaging it into a jar.
For instance we have this:
-AppRoot
Main.class
-Misc
Math.class
OtherTools.class
-YetAnotherFolder
UsefulFunctions.class
Is this possible? The main method should be executed from command line or similar.
You can run this way from the AppRoot directory
javac -cp Misc/*:YetAnotherFolder/* Main.java //To compile
java -cp Misc/*:YetAnotherFolder/* Main // To run
Below is some documentation
-classpath classpath
-cp classpath
Specifies a list of directories, JAR archives, and ZIP archives to search for class files. Class
path entries are separated by colons (:). Specifying -classpath or -cp overrides any setting of the
CLASSPATH environment variable.
If -classpath and -cp are not used and CLASSPATH is not set, the user class path consists of the cur-
rent directory (.).
Add all (sub-)directories containing class files to classpath and use the class with the main method as argument of the java executable.
The directory structure is your package structure.
java -cp ./:./AppRoot:./AppRoot/Misc:./AppRoot/YetAnotherFolder AppRoot.Main
This should work if all dependencies are resolved and on the classpath.
I am trying to learn more about javac and how to use developer tools for Java using the command line.
As far as I understood, the option -classpath is needed to specify the path where javac searches for our classes and resource files, if we are not in the current directory, because usually the class path is set to our current working directory.
This is my current working directory:
/Users/user1/Desktop
And I am trying to compile a .java file which is in:
/Users/user1/Desktop/PF/
and the file is called MainClass.java.
I am trying to compile it using the following command:
javac -classpath /PF MainClass.java
But it does not seem to work, in fact I keep receiving the following:
javac: file not found: MainClass.java
Usage: javac <options> <source files>
use -help for a list of possible options
What am I doing wrong?
Classpath is for .class files, not for .java files.
javac command needs correct path to the .java file to compile it. So
javac ./PF/MainClass.java
Will create the class file in current directory.
If your MainClass.java depends on any class files to compile correctly, then you put those class/jar files in classpath.
That isn't how the classpath works. You use the classpath to point to classes that your Java file needs in order to compile. You don't use the classpath to point to the Java file itself.
Either go into the PF directory and do this:
javac MainClass.java
That will create the MainClass.class file inside the PF directory. If instead you want to create the MainClass.class file on your desktop, then from your desktop, do this:
javac PF/MainClass.java
-classpath
Specifies the path javac uses to look up classes needed to run javac
or being referenced by other classes you are compiling. Overrides the
default or the CLASSPATH environment variable if it is set.
Directories are separated by colons. It is often useful for the
directory containing the source files to be on the class path. You
should always include the system classes at the end of the path.
class path is used to specify the compiled sources that need to be used in your class. For example in this code if you are accessing another class then you should specify the location of the compiled sources of the that class.
In your case if don't have any class dependency then simply remove classpath option and compile using[navigate inside folder]
javac Mainclass.java
Remove the -classpath. And if you are in the place where the java file is required (which currently you arent) you can remove that PF/ too.
So I'm still a noob in Java and I'm experimenting around with a few things.
I recently created a .jar file for my class using jar cvf <name>.jar <source files> and then used that jar to compile my driver class (javac -cp <name>.jar Driver.java) though how do I now run that class using the jar?
I've tried the following 2 commands:
java Driver and,
java -cp <name>.jar Driver.
The first gives me a NoClassDefFoundError for the class used, whereas the latter just gave me a single line error.
Error: Could not find or load man class Driver
What am I doing wrong? Is it possible I'm confusing this for something else?
I'm trying to do as much as I can without the use of any IDE.
You should put jar file and compiler output into classpath and specify main class:
java -classpath "<name.jar>;classes" Driver
EDIT (thanks to Kayaman):
If you are running command from linux/unix you have to use ":" as separator (in Windows works ";"). "classes" is a path to folder containing compiler output.
When creating an executable jar ( jar which contain a class with the main method) you should tell the jar which is the mainClass to be executed and for that you should create a file called 'Manifest.mf'.
The file should contain this:
Main-Class: MyPackage.MyClass
And when creating the jar you should use this to include your manifest:
jar cfm MyJar.jar Manifest.mf MyPackage/*.class
And for launching your jar :
java -jar MyJar.jar
Java and Gradle beginner's question.
I made a project directory for java and gradle test:
The directory hierarchy :
HelloWorld.java:
package foo.bar;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}
build.gradle:
apply plugin:'java'
Then,gradle build this project and generated what i need.
As you see above, my problem is why doesn't this execute correctly? Even through I cd to .class path.
======================================================================
While, if I remove package foo.bar; in HelloWorld.java, and repeat gradle commands and execute at he.bak directory then the error remained the same.
But when I cd to the directory where HelloWorld.java placed. everything goes OK！Why? something related with CLASSPATH environment variables or other causes?
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UPDATE
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Thought you guys' warm replies, I know that I should combine the CLASSPATH and the period-separated executable .class file to figure out what's going on when executing java class file.
I experiment my thought resulting in 2 point to this question:
The -cp option path parameter A/B plus the executable file c.d.e.class finally form the A/B/c.d.e.class full path where the class is actually located.
If I specify the package in source code file with package d,I must split the full path in the form of java -cp A/B/c/d e.class. split in other ways all will result in errors.
something I am not sure here is :
When I specify my package path in my source code file, It determined the only classpath when executing corresponding executable, right?
If it is the truth, How does a project with lots of package and sources files work?
What's the root principle?
When in build/classes/main try java foo.bar.HelloWorld instead of java HelloWorld
The reason you need to specify foo.bar.HelloWorld is because you specified package foo.bar;. This tells java that the class should be in foo/bar/HelloWorld and the fully qualified name for HelloWorld is foo.bar.HelloWorld. If you want to execute the class from a different working directory however, you can specify the classpath explicitly using the -cp option, e.g., java -cp c:\myproject\build\classes\main foo.bar.HelloWorld.
By the way, the classpath default is the current working directory (i.e., .) but java -cp c:\myproject\build\classes\main foo.bar.HelloWorld will NOT have the classpath set to the current working directory if it is explicitly set using the -cp option. If you want to include the current working directory but explicitly set it, or even add more directories, you can chain them using semicolons like this: java -cp .;c:\myproject\build\classes\main foo.bar.HelloWorld. So this will include both the current working directory and the directory I specified.