I am coming back to Java after quite some time. Having a little trouble with classpath. Would really appreciate it if someone could please point me in the right direction!
My folder structure is as follow:
├── lib
│ └── algs4.jar
└── src
└── HelloWorld.java
HelloWorld.java
import edu.princeton.cs.algs4.StdOut;
public class HelloWorld {
public static void main(String[] args) {
StdOut.println("Hello World");
}
}
I compiled my program with the following cmd
cd src
javac -cp ../lib/* HelloWorld.java
However, when I run my program using java HelloWorld, I get the following error.
Exception in thread "main" java.lang.NoClassDefFoundError: edu/princeton/cs/algs4/StdOut
at HelloWorld.main(HelloWorld.java:5)
Caused by: java.lang.ClassNotFoundException: edu.princeton.cs.algs4.StdOut
at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
I dont understand how it compiles fine, but then its unable to find classes at runtime. Could someone please shed some light on this ? Thank you in advance!
It was painful, but I finally fixed it. Turns out when running the program, you not only have to specify the location of the JAR files, but you also have to specify the location of your own file. (Seems obvious now >.<)
For example, in my case, running the following is not good enough.
java -cp ../lib/* HelloWorld
We included the the JARs but didn't include the folder that actually contains HelloWorld.class.
To fix this, I had to run the following. We are including the location of the JAR and the location of the file being executed.
java -cp ../lib/*:. HelloWorld
(Some of you might have to escape * depending on your OS / shell settings)
To execute your program you also need to pass the classpath to the java command like this.
java -cp ../lib/* HelloWorld
When you've compiled your code you told the compiler to use the ../lib/* directory(and files) to look up for class definitions, of course, the compiler found them, and write the byte code in the HelloWorld.class file but this bytecode only contains your code (the lines you wrote in HelloWorld.java) for any external library you use there the compiler will only store a sort of reference with the full package name and the method name. No bytecode from the jar will be stored in the HelloWorld.class file.
So to execute HelloWorld you need to tell the JVM to load first all the external classes that you in the code passing them with the -cp parameter.
Then, the JVM will execute your code, look for the reference to the jar code via package/method name and execute them.
If you don't provide -cp ../lib/* the JVM will load only the standard library (All the classes in JDK) and your HelloWorld.class thus it won't find the external jar in memory.
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)
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?
////////////////////////////////////////////////////////////////////
UPDATE
////////////////////////////////////////////////////////////////////
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.
Apologies in advance - I know there are many, many, many very similar questions but I wanted to ask something specific to my situation.
I have a bunch of java and jar files in the same directory. I am able to compile fine and thus end up with a number of class files in the same dir. But when I go to execute the program it gives a NoClassDefFoundError saying it can't find the specified class:
C:\Users\DB\Desktop\nextreports-integration-demo\src\ro\nextreports\integration>
java -cp ".;*.jar" SimpleDemo
Exception in thread "main" java.lang.NoClassDefFoundError: SimpleDemo (wrong nam
e: ro/nextreports/integration/SimpleDemo)
I tried the same thing from a higher-level dir but it made no difference:
C:\Users\DB\Desktop\nextreports-integration-demo\src>java -cp ".\ro\nextreports\
integration\*.jar;.\ro\nextreports\integration" SimpleDemo
Exception in thread "main" java.lang.NoClassDefFoundError: SimpleDemo (wrong nam
e: ro/nextreports/integration/SimpleDemo)
The package statement in the source file is:
package ro.nextreports.integration;
I have a feeling I'm overlooking something very elementary. Thanks in advance.
Edit: Thanks very much. It works with the following:
java -cp ".\ro\nextreports\integration\nextreports-engine-6.3.jar;.\ro\nextreports\integration\commons-jexl-2.1.1.jar;.\ro\nextreports\integration\commons-logging-1.1.1.jar;.\ro\nextreports\integration\derby-10.10.1.1.jar;.\ro\nextreports\integration\itext-2.1.7.jar;.\ro\nextreports\integration\itext-rtf-2.1.7.jar;.\ro\nextreports\integration\itextpdf-5.0.6.jar;.\ro\nextreports\integration\jcalendar-1.3.2.jar;.\ro\nextreports\integration\jcommon-1.0.15.jar;.\ro\nextreports\integration\jfreechart-1.0.12.jar;.\ro\nextreports\integration\jofc2-1.0.1.jar;.\ro\nextreports\integration\mysql-connector-java-5.1.23-bin.jar;.\ro\nextreports\integration\mysql-connector-java-5.1.23-bin.jar;.\ro\nextreports\integration\poi-3.7.jar;.\ro\nextreports\integration\winstone-lite-0.9.10.jar;.\ro\nextreports\integration\xstream-1.3.1.jar;" ro.nextreports.integration.SimpleDemo
But why can I not use wildcards for the *.jar files? For instance, the following leads to a NoClassDefFoundError for a class in any jar file I don't make explicit:
java -cp ".;.\ro\nextreports\integration\*.jar" ro.nextreports.integration.
SimpleDemo
Suppose this is your directory tree:
src/
ro/
nextreports/
integration/
SimpleDemo.class
From your package declaration, your compiled class should be in the integration subdiretory. Check that there really is a SimpleDemo.class in that directory. If that is correct, its classpath includes the contents of the src directory.
That means that, if you didn't have any JAR dependencies, you could run your application from the src directory like this:
java -cp . ro.nextreports.integration.SimpleDemo
You need to use the fully qualified class name.
Since you do have jars, you have to include them in the classpath as well. Suppose you have one JAR in the directory above src, and another in the current directory, you could use:
java -cp ../one.jar;another.jar;. ro.nextreports.integration.SimpleDemo
If you run it from another directory, it will still work if you review the classpath's relative directories or use absolute directories to describe your classpath. I am not sure but usually the current directory is always included in the classpath by the java executable.
I am trying to run a file from command line. The file is a .class file and is apart of a larger project that I compiled in Netbeans. I navigated to the .class file and ran
java MyFile
And I got:
Exception in thread "main" java.lang.NoClassDefFoundError: PersonTest/class
Caused by: java.lang.ClassNotFoundException: PersonTest.class
at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: PersonTest.class. Program will exit
Whats up with that? (I should mention that i'm running ubuntu)
You need to check this useful link java - the Java application launcher:
By default, the first non-option
argument is the name of the class to
be invoked. A fully-qualified class
name should be used
So, you have to write the full qualified name of the class (this includes the package name).
So, the right way to execute your command is this (from the root dir where your class files are stored):
> java my.package.MyFile
Also, make sure to include all the needed dependencies at the classpath (-cp) argument (check the referenced link).
UPDATE: to include a classpath setting example:
java -classpath C:\MyProject\classes;C:\MyProject\lib\utility.jar my.package.MyFile
With this, the java runtime will search for the classes at the C:\MyProject\classes directory, and at the C:\MyProject\lib\utility.jar JAR file. You'll need not only your class direct dependencies, but the dependencies needed by the referenced files (the whole tree).
The answer appears to be in this line:
Exception in thread "main" java.lang.NoClassDefFoundError: PersonTest/class
It means you didn't type:
java MyFile
as you said in your original post, you typed
java PersonTest.class
you should have typed
java PersonTest
Yes you can, they are compiled by a java compiler. If you have the right version of the jvm (often other versions work aswell) than it can be run. The information about your error is not enough to tell what went wrong.
Your probably in the wrong folder, mistyped the classname, used a class in your code that couldn't be found, etc.
Unless your class is entirely standalone (i.e. only references java.lang classes like String), you'll need to add other classes/JARs to the classpath when you invoke Java.
The NoClassDefFoundError (which usually states the name of the class by the way, and always includes a stacktrace) indicates that an external class that was available when your class was compiled, is not available on the classpath at runtime.
EDIT based on update:
You're invoking your process incorrectly. You don't need to append the .class suffix of the file - doing so makes Java look for a file class class in a subpackage.
(P.S. you said you ran java MyFile. That's a lie, you actually ran java PersonTest.class. If you'd noted that to start with, it would have made it much easier for people to answer the question!)
Just consider this example
say I already have a folder src and I wrote in my notepad
package test.oye;
class testclass {
static public void main (String [] args)
{
int a=3;
System.out.println(a);
}
}
then what go to src folder and you ,yourself create a folder named test and inside it oye . Then put your .java file in it . Then cd src/test/oye only(in Command prompt or terminal).From there itself
javac testclass.java
cd src
java test.oye.testclass
This will work for sure.
If you don’t want to put .java file there … then just compile your .java file and get the .class file .
Now create the test folder and then oye inside it ….and put .class file inside it ….
Now go back to src …and then type
java test.oye.testclass;
according to terminal ide, android requires classes in DEX format when running them.
Try:
dx --dex --output=practice.jar practice.class
Then run using this:
java -jar practice.jar practice
I'm new to working with Java from the command line and I don't get it. I read previous CLASSPATH questions but still didn't get my problem to work.
I have the following class in C:\Temp\a\b\c
package a.b.c;
public class Hello
{
public static void main(String args[])
{
System.out.println("Hello World!");
}
}
The package name is intentional.
I compiled it fine and I put the Hello.class file inside C:\Temp\a\target
Now in the command line I go to C:\Temp\ and execute the following:
java -cp .\a\target a.b.c.Hello
It complains that it cannot find the class a.b.c.Hello
Thanks in advance!!
and I put the Hello.class file inside C:\Temp\a\target
This is wrong. It should be placed in the same folder as the .java file. The source code itself is declared to be in the package a.b.c; so, the .class file should really be kept in \a\b\c folder.
Then, to execute it just do:
C:\Temp>java -cp . a.b.c.Hello
Avoid "putting" the classfiles anywhere. The following should work:
javac -d c:\temp c:\temp\a\b\c\Hello.java
# creates Hello.class in c:\temp\a\b\c
java -cp c:\temp a.b.c.Hello
To expand on BalusC's point: the classpath defines a "root". When java is looking for your classes, it will start at each root (or jar) in your class path and drill down through the directories to match the package strucutre. You still need to have you class in a directory structure that matches its package name. In your case, to execute
java -cp .\a\target a.b.c.Hello
you would move the file to
.\a\target\a\b\c\Hello.class
Years ago, I too found this baffling.
Java will try to search for a directory structure a\b\c from starting in target and as you notice, it wont work.
Move the whole directory into target and you'll be fine, it should look like:
C:\Temp\a\target\a\b\c\Hello.class
You may compile it with the -d option which tall the compiler where to put the class file.
Many project structures are like this.
C:\whatever\projectname\src
C:\whatever\projectname\classes
C:\whatever\projectname\bin
C:\whatever\projectname\lib
C:\whatever\projectname\doc
That way you can always step on your project directory and type:
javac -d classes src\*.java
Which will compile all the sources in the src directory and will place them in the classes directory.
Then execute your program:
java -cp classes a.b.c.Hello
You may optionally place required jars in lib
This works pretty fine for small programs ( < 10 src files and 2 - 3 jar libraries ) If it grows beyond that, you could probably use an IDE or ant
The good thing about following this project structure is that some IDES ( as IntellJ idea ) just pick them very easily when you create a new project. You select "Create project from existing sources" and then you can continue from there.
I like compiling and editing at the command line a lot!!