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Effective Use of Java Command Line Tools

  • January 8, 2016
  • By Manoj Debnath
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Java command line tools are a sophisticated way to give a finer touch in the application development process. Java is bundled with several such tools. They can be found in the bin folder of the JDK installation directory. When working with our favourite IDE, we often do not notice the real power behind the tools because the IDE takes care of them in the background, giving the programmer a headache-free productive experience. This is quite appreciable, yet in cases blinds the programmer of the real essence of command line tools. Oracle provides extensive documentation on Java tools and their utilities. In this article, we'll explore some of the tools that come in quite handy when programming in Java.

And, for the time being, forget about your IDE. We'll go command line from here and use a very basic text editor, something like Notepad or gedit for coding.

Compilation and Execution

Once the program code is complete, for compilation we need javac (a Java compiler that transforms source code into byte code or a .java file into a .class file) and to run the command, we need java.

The java command initiates the runtime environment (JRE), loads specific classes, and searches for the start-up and other application specific classes (in the location: bootstrap class path, installed extensions and user class path). The start-up class contains a program initiation method such as main, which is finally launched within the periphery of Java Virtual Machine.

Sample Program

class testapp{
   public static void main(String[] args){
      System.out.println("Testing command line tools");
   }
}

To compile the preceding code, we would write the following:

javac testapp.java

This will create the testapp.class, which is a byte code file in the current directory. To execute the program, we simply can write the following command:

java testapp

Now, suppose, Java source files are distributed across a complex hierarchy of packages in the current directory. How would we compile and then run the program?

Let's create the scenario.

Create three folders, one inside another, as /org/mano/example in the current directory. Put the following Java source files, such as p1.java, inside /org, p2.java inside /org/mano, p3.java, and testapp1.java inside /org/mano/example.

p1.java

package org;
/** This is my javadoc comment about the class*/
public class p1 {
   String str="from org package";
   /**
    * the method return void and prints String constant
    */
   public void printStr(){
      System.out.println(str);
   }
}

p2.java

package org.mano;
/** This is my javadoc comment about the class*/
public class p2 {
   String str="from org.mano package";
   /**
    * the method return void and prints String constant
    */
   public void printStr(){
      System.out.println(str);
   }
}

p3.java

package org.mano.example;
/** This is my javadoc comment about the class*/
public class p3 {
   String str="from org.mano.example package";
   /**
    * the method return void and prints String constant
    */
   public void printStr(){
      System.out.println(str);
   }
}

testapp1.java

package org.mano.example;
import org.p1;
import org.mano.p2;
/** This is my avadoc comment about the class*/
public class testapp1 {

   /**
    * this is main method
    */
   public static void main(String[] args){
      p1 a=new p1();
      a.printStr();
      p2 b=new p2();
      b.printStr();
      p3 c=new p3();
      c.printStr();
   }
}

To compile, use javac as follows:

javac -cp . org/*.java org/mano/*.java org/
   mano/example/*.java

Note that the "-cp ." option denotes the class path to the current directory. The "*" denotes all Java source files in the particular directory. It, however, can be replaced by a specific file name, but "*" seems more convenient for our purpose at the moment. The command thus would compile and create .class files in the respective directory.

Now, to execute testapp1 (the testapp1 class contains the main function) use the following command:

java org.mano.example.testapp1

Tool1
Figure 1: Output in a command line terminal

There are several other optional parameters we can use both with the javac and java commands to tweak compilation and program launch process, such as putting .class files in a specific directory, enabling/disabling assertions, loading a splash image before application launch, and so forth. Refer to documentation for more information about the optional parameters.

Packaging and Executing a JAR File

Archiving and packaging with the jar utility is another important command line tool to combine multiple files into a single package, called a JAR file. A JAR contains a manifest file, called the content describer, and, optionally, a signature file for security.

Let's create a JAR archive for the preceding example. First, create a Manifest.txt file in the current directory (in my case, it is java_pro, refer to Figure 1) with the content as follows:

Manifest-Version: 1.0
Created-By: 1.8.0_65 (Oracle Corporation)
Main-Class: org.mano.example.testapp1

Now, use the following command to create the archive:

jar cmf Manifest.txt testapp1.jar org

This will create a JAR file named testapp1.jar in the current directory. Now, to display the content of the jar file, use the following command:

jar tf testapp1.jar

This command will display the contents as (shown in Figure 2). Now, to launch the JAR file, use the java command as follows:

java -jar testapp1.jar

That's it. The package is now ready to port and launch in a different machine (JVM).

Tool2
Figure 2: Packaging with the jar utility and launching the application

Creating Documentation

The javadoc utility can be used to create documentation from the source code. The comments we have written in the source code in the preceding example will be used for method and class description. Now, to create documentation of the source code, we may create a docs folder in the current directory (otherwise, all HTML files will be created in the current directory) and use the following command:

javadoc -d ./docs/ org org.mano org.mano.example
   p1.java p2.java p3.java testapp1.java

Tool3
Figure 3: Java documentation files

Creating a usable and good documentation file is very important. There are standard procedures for creating one in Java. The purpose here is to show the usage of the javadoc tool, so it is better not to pay attention to the useless comments written in the source code but to the essence of javadoc command line utility. :)

Conclusion

There are numerous other command line utilities but these four are the basics. Here, I have tried to cover the fourfold utility of the command line tools such as compilation, execution, packaging, and documentation, with very specific implementation examples. If you are an IDE buff, I suggest that you stoop down to the command line sometime and feel the fun of using utility tools. You'll thank me later.


Tags: Java, command line, JAR, IDE, JVM, JDK, utility, documentation, MANIFEST FILES, command line interface, JRE




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