I have been googling around, trying to understand what the Java Classpath and Path are. However, I am stil not quite sure if I have understood it or not. If feel that this topic is one of those grey areas.
Can someone explain me what those are? I mean, where do I find and set them (where is the actual text file on Mac/Windows)? Is there only one instance of each one? If so, how do I set the path for multiple classes?
As you might have notices, I am totally confused right now after reading so many different tutorials... So now I really would like to have a straight forward explanation.
Please help me, I just trying to learn :)
Thank you all
A path is just a folder location. The path is where your OS will look for programs by default. If java, javac, javap, etc, etc, are in your path then you can just type their names without the entire folder location.
Your classpath is similar. It is a set of folders that contain .class files describing classes(hence the name) and .jar files, which are basically files that contain .class files. All code that you're running is either out of the classpath, generated, or out of the java libaries(also part of the classpath, techncically).
With each run of a java program you can specify a classpath by parameters passed to the java executable. It also grabs classes out of "extension folders,", special folders Java keeps around to act as a system classpath, and finally, the "bootstrap classes", which are a set of important classes almost any Java program needs to run.
Simple mean of path is location of file system. if you want to access any file then you have to manually needs to go there location.
just example: d:\text1.txt then needs to go that d:\ location. same way java program have command like
javac -for compile
java - for run
.
.
.
etc.
that inside java-jdk\bin folder
so if you don't set into classpath. then you can execute java program like
run->cmd
c:\jdk1.6\bin> javac test.java
so without going explicit way you can set it into classpath, and direct execute java program from anywhere.
You can set java path as environment variable of computer.
The PATH is basically where your JDK is installed; this is essentially what your IDE will look for when trying to compile or create Javadoc or such; it's basically just the location of a folder on your hard drive, set as a Windows (or other OS) environment variable to make it easier to use.
The CLASSPATH is a property that tells the compiler where to look for classes. Basically if you download a library or such from somewhere, you need to add it to the CLASSPATH for the compiler to use it. Usually you can do this in your IDE, however, you should not need to directly access the CLASSPATH variable.
By the way, the Wikipedia article is pretty helpful.
1)java Path: it is location of binary executable files
example :javac , java
this file are used for compile and run
2)class Path: it is location of .class file(file create after compile your source code .java file)
Related
I am having issue running a Java program.
There's a line of code in a jar file that opens the FileInputStream to some file from "/apps/somefile" location,
however I do not have access to the root directory.
The issue is that I am unable to change the code since it is provided in a jar file, as such is there anyway to change to root directory that "/" points to when running the java program?
Here's the line for reference:
FileInputStream fileInputStream = new FileInputStream("/apps/somefile");
The line of code you present does not do anything directly. It is Java source code, and Java is ordinarily compiled to bytecode and presented in that form to the JVM. It sounds like your jar may contain source files along with the compiled class files, which is sometimes done. If that's the case then you can simply unpack the Java sources from the jar, modify them, recompile, and make a new jar from the results.
If you don't have source then you could try decompiling, as another answer suggests. That's a bit nasty, but it probably would work.
Alternatively, what you actually ask is
is there anyway to change to root directory that "/" points to when running the java program?
In fact, there may be, depending on the system on which the code is running. You could conceivably run your program under chroot, which has precisely that effect. That's a distinctly non-trivial exercise, however, and if you don't have access to the root directory (though I'm not sure how you can do anything in that case) then you very likely do not have sufficient access to set up or use a chroot environment, either.
you need to do a decompiler to that jar. You can use : http://jd.benow.ca/
Then you need to change that class that have the InputStream and then compile again and make the Jar file.
I currently wrote a simple GUI in Eclipse which runs as intended. I was hoping to export it so I can share it with my friend (who doesn't need to install eclipse and the java libraries). I tried all 3 library handling method Eclipse provides and none of them works. I read a little online and saw something about a manifest file, but wasn't quite sure what to do with it. Is it going to help?
This is where I placed the folder that comes with the .dll file.
This is the result. Am I doing something wrong?
As indicated by the error messages in the first screenshot, what you are missing here is the native library - the software library written and compiled to native code specific to the operating system. What you will need to do is provide the libraries specific to the operating system on which your software will run, eg. dlls for 32 or 64 bit Windows. The manifest does not provide the capability to include those libraries.
When the program is run on Windows, Java will look for native libraries in the following locations:
The current directory
The directories in the PATH environment variable
The directories in java.library.path (if it's specified)
It may be easiest to simply put all files in the one directory. If you do this, you should be able to run the program in the same way as you do now.
The java.library.path option is only needed if you want to put your native library files in a directory separate to the one in which you run your program and not on your PATH. It is only in this case that you will need to add java.library.path, eg. by adding -Djava.library.path=c:\path\to\your\lib after java. Also note that you may use a relative path, ie. a path that is relative to the directory you are in when you execute the command.
I also see from your later error messages that you have another dependency, but on a java library LeapJava.jar. As running a jar with -jar will only work if you have a single jar, but because you have more than one (your own program plus the dependency), you'll instead need to use the -classpath (or -cp for short) argument and add your main class. The classpath argument is a semicolon-separated list of classpath locations, while the main class is the one containing your public static void main method, eg. your.package.name.YourMainClass. So assuming your UI.jar is still in C:\Users\Ian\Desktop\Leap Data UI, you should be able to navigate to that directory and execute with:
java -cp UI.jar;UI_lib\LeapJava.jar -Djava.library.path="UI_lib\x64" your.package.name.YourMainClass
Pretty basic problem here. So I have a Java package that I have created that has three classes (one has the main method). I am trying to use a few Apache Jars, and have added these to my build path in Eclipse. However Eclipse wont let me build and run it properly, so I am trying the command line. I have added the env var CLASSPATH and pointed it to my lib directory which hold the Apache Jars. However, when I try to use javac I get a bunch of errors:
package org.apache.xmlrpc does not exist
import org.apache.xmlrpc.client.XmlRpcClient;
I was reading the man page for javac and it said that:
If neither CLASSPATH, -cp nor -classpath is specified, the user class path consists of the current directory.
So I tried copying the Jars to the same location as my three source files, but no change.
Can someone please tell me what I'm doing wrong?
Thanks.
Classpath variable (or command line option of javac) must contain all jars explicitly. It cannot go through jar files stored in specified directory.
You can compile this by specifying the option -cp on the command line:
javac -cp foo.jar:bar.jar foo/bar/Baz.java
You then run it with the same option:
java -cp foo.jar:bar.jar foo.bar.Baz
It sounds like you've just set the classpath to the directory containing the jar files. You need to set it to the individual jar files, or use java.ext.dirs to set an "extension" directory containing jar files. I'd recommend using the specific jar files. Something like:
// Assuming Windows...
CLASSPATH = c:\libs\foo.jar;c:\libs\bar.jar
I'd also personally recommend specifying the classpath on the command line instead of using an environment variable - the latter will work, but it ends up being a bit more fiddly if you want to compile different projects against different libraries.
However, I'd actually recommend getting Eclipse working first, rather than retreating to the command line. It should be fine - if you could give us more information about what's failing in Eclipse, we may be able to help you with that instead.
The jar files in the current directory are not automatically included; that only refers to .class files in normal package/directory hierarchy. Jar files must be added either explicitly, or via a wildcard like javac -cp ./* (Assuming JDK6+)
(Some OSes may require an escape of the * to avoid globbing; OSX does not.)
I agree with previous answers, but I would also recommend to use proper java build tool - like ant (perceived easier to use, but not necessary) or maven ( perceived more difficult to use, but really worth learning )
I am running a java program in command prompt. I have set the "C:\j2sdk1.4.2_09\bin" in PATH System variable.Then I am able to compile and run the program.But as I need to set the mysql-connector I have set this in "C:\mysql-connector-java-5.1.10\mysql-connector-java-5.1.5-bin.jar"
CLASSPATH variable but now I am able to compile the program as I run the program I get "Exception in thread main java.lang.NoClassFoundDefError".
How is this? Can anybody elaborate on this?
It's fairly unclear what you're asking here. Are you saying you changed your CLASSPATH and now the class can't be found?
One thing to keep in mind is that the CLASSPATH does not, by default, include the current directory. You have to add it (.) if you want it in the CLASSPATH, e.g.:
set CLASSPATH=.;C:\mysql-connector-java-5.1.10\mysql-connector-java-5.1.5-bin.jar
(Note I put a .; at the beginning of that.)
PATH and CLASSPATH are completely unrelated. PATH tells Windows where to find executable programs (and does implicitly include the current directory) when you run them without giving it an explicit path. CLASSPATH tells the Java runtime where it can load classes from.
Apologies if this is completely off-base; if so, could you clarify your question?
Agree with the answer above, because I've done this a million times myself. The exception you're getting is almost definitely related to your classpath not including the "." which tells Java to include the current directory in its death for classes.
It seems counter intuitive that Java wouldn't automatically include the current directory in its search, but anytime you manually override the classpath, you have to include this.
The reason? When you don't specify a classpath then a default one is used, which includes the ".", and which is transparent to you.
Before answering your question, just wann ans this simple question : Why we need PATH and CLASSPATH?
Answer:
1) PATH: You need to set PATH to compile Java source code, create JAVA CLASS FILES and Operating System to load classes at runtime.
2) CLASSPATH: Classpath in Java is path to directory or list of directory which is used by ClassLoaders to find and load class in Java program.This is used by JVM and not by OS.
Modify PATH variable (Windows System Environmental Variable) so that it points to bin dir which contains all exe for example: java,javac and etc. In my case it is like this : ;C:\Program Files\Java\jre7\bin.
Classpath can be specified using CLASSPATH environment variable which is case insensitive, -cp or -classpath command line option or Class-Path attribute in manifest.mf file inside JAR file in Java.
For CLASSPATH in details please refer this link "http://javarevisited.blogspot.in/2011/01/how-classpath-work-in-java.html"
I downloaded the Javax.mail package. I have jdk1.6.0_11.
Problem is...I cannot get javac or java to find those classes!
I can get apps to compile using JCreator LE ( by adding the mail jar
to its search list ) but, when I try to run the app in a command window,
it fails.
Can I add these new classes to the rt.jar without
hurting my jdk installation?
I know java has it wired up to look there for classes.
(And, the mail classes are inside a javax package - seems like
they could reasonably be added to the javax folder in rt.jar..
Thanks!
Phil D'
No you can't, nor should you.
Instead, figure out the problem with your classloader (probably paths?). You'll need that for the next library you need to access.
Messing with rt.jar means you can't run on any other JVM.
You should either specify the jar file in your classpath: preferably on the command line with the -cp option, but possibly with the CLASSPATH environment variable.
Alternatively, you can specify its directory in the java.ext.dirs system property. For more details, see the documentation for the extensions mechanism.
You shouldn't be messing around with rt.jar. That's very definitely not the way to make extra jar files available - it's akin to trying to add Microsoft Word to the Windows kernel ;)
Adding things to rt.jar seems like a bad idea, even though its possible and easy to accomplish.
Try compile your application from the command line like this:
javac -cp <path_to_3rd_libs>/jarfile.jar . MainClass.java
If the compiler still complains about the javax.mail package try to unpack/examine the jar file to see that javax.mail package (and its expected content) is there.
(On windows its easy to examine a jar file using 7zip.)
Most definitely no.
If you post the command you are running from the command line we will be able to point you on the right direction, but most likely you are just missing a classpath parameter.
java -classpath /path/to/mail.jar MyClass
You need to understand the CLASSPATH concept which allows you to add individual classes and jar files containing classes to the "universe" of defined classes available for the code you want to compile and/or run. It is similar in idea to the PATH variable in the Windows world.
For the Windows command line this is the documentation:
http://java.sun.com/javase/6/docs/technotes/tools/windows/classpath.html
The Java Tutorial surprised me by not being well-written for this particular concept:
http://java.sun.com/docs/books/tutorial/essential/environment/paths.html
You most likely need something along the lines:
C:> set CLASSPATH=c:\javamail\first.jar;c:\javamail\second.jar
after which both java and javac should know about these classes