Friday, May 24, 2013

Compile .java file with .jar in class path



How to Compile and Run Java Code with jar from a Command Line


Fast Remedy

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javac -cp fileName.jar className.java 
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Task

Lets say we have a fairly standard java project that consists of three top level folders:
/bin - empty folder that will contain compiled .class files
/lib - contains third party .jar files
/src - contains .java source files
Our task would be to compile and launch the project from its root folder. We will use Windows OS as example (on Unix systems the only difference would be path separation symbol - ":" instead of ";").

Compiling Java Code

The first step is compiling plain text .java sources into Java Virtual Machine byte code (.classfiles). This is done with javac utility that comes with JDK.
Assuming we are at the application root folder trying to compile Application.java file fromcom.example package that uses lib1.jar and lib2.jar libraries from lib folder to a destination binfolder, compilation command should have the following format:
javac -d bin -sourcepath src -cp lib/lib1.jar;lib/lib2.jar src/com/example/Application.java
As a result bin/com/example/Application.class file should be created. If Application.javauses other classes from the project, they all should be automatically compiled and put into corresponding folders.

Running Java Code

To launch a .class file we just compiled, another JDK utility called java would be needed.
Assuming we are at the application root folder trying to launch Application.class file fromcom.example package that uses lib1.jar and lib2.jar libraries from lib folder, the launch command should have the following format
java -cp bin;lib/lib1.jar;lib/lib2.jar com.example.Application
Note that we don't provide a filename here, only an actual class name that java would attempt to find based on provided classpath.

Some Notes About Classpath

Lets say during Application.java compilation a compiler stumbles upon somecom.example.Util class. How to find it in the file system? According to Java file naming rules, Utilclass has to be located somewhere in Util.java file under /com/example/ folder, but where to start searching for this path? Here is where classpath comes into play which sets the starting folder for searching for classes. Classpath can be set in 3 different ways:
  • If no --classpath parameter is passed, CLASSPATH environment variable is used
  • If CLASSPATH environment variable is not found, current folder (".") is used by default
  • If --classpath is explicitly set as a command line parameter, it overrides all other values
The fact that classpath when set overrides default value (current folder) can cause some unexpected results.
For example if we don't use any third party libraries, only our own com.example.Util class, and try to compile Application.java from the src folder:
javac com/example/Application.java
this would work, but then if we decide to add a third party libarary to the classpath:
javac -cp lib/lib1.jar com/example/Application.java
it would cause an error:
package com.example.Util does not exist
This happens because when we set -cp lib/lib1.jar we override default value for the classpath - current folder. Now a compiler will be looking for all classes only inside that jar file. To fix this we need to explicitly add the current folder to the classpath:
javac -cp .;lib/lib1.jar com/example/Application.java

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