Running Java Applications

Understanding the java command will save you from some frustrating errors.

While running Java program using command line/server, I always got confused about using -jar option and -cp option. We bundle the application in jar file using maven/gradle or any building tool. Now there may be some resource files which should be provided from outside of the application. This is very common scenario.

Now if you use

java -cp /path/to/resources -jar app.jar

Java won't read classpath resources and will look for classpath mentioned in app.jar manifest file.

So, in such case,

java -cp /path/to/resources:/path/to/jar package.main-classname

Now, Java will search for the jar file and resources in classpath (separated by ; in windows and : in unix).

if you are not providing any -cp option, java checks System's CLASSPATH and uses the directories.

If there is no envrionment variable CLASSPATH set, then it checks the current directory

java -cp . package.main-classname

Any arguments provided after the main class name will be considered as program arguments

java -cp /path/to/all/resources package.main-classname arg1 arg2 arg3

You can pass JVM arguments before the main class and after -cp option with -D as prefix

java -cp /path/to/resources -Dkey=value package.main-classname arg1 arg2

Have a nice day :)