Getting started with Java Jars and Classes

What is a Java Class?

A Java class is a branch of code which be referred to by the creation of new objects in Java. From a narrow point of view it can be called a sub, but a class is more flexible.

http://docs.oracle.com/javase/tutorial/java/concepts/class.html

What is a Java Jar

A jar is a collection of java code and metadata into an archive, normally a zip.

Working with classes and Jars

In this example a class for converting farenheit to celcius is created with java code that references it and is placed into a Jar:

Create TempratureConvertorBean.java

package tconvert;
public class TempratureConvertorBean {
  private double celsius = 0.0;
  private double fahrenheit = 32.0;
  public double getCelsius() {
    return celsius;
  }
  public void setCelsius(double c) {
    celsius = c;
    fahrenheit = 1.8*c + 32.0;
  }
  public double getFahrenheit() {
    return fahrenheit;
  }
  public void setFahrenheit(double f) {
    fahrenheit = f;
    celsius = (f-32.0)/1.8;
  }
  public String getInfo() {
    return new String("Temp Convert Bean");
  }
}

Create class subfolder
Compile the code into a java class into the 'class' subfolder: javac -d class TempratureConvertorBean.java
Create a jar from the class: jar cvf tconvert.jar -C class tconvert
View contents:

$ jar tf tconvert.jar
META-INF/
META-INF/MANIFEST.MF
tconvert/
tconvert/TempratureConvertorBean.class

Create F2C.java

import tconvert.TempratureConvertorBean;
public class F2C {
   public static void main(String[] arg) {
      TempratureConvertorBean b = new TempratureConvertorBean();
      double f = 0.0;
      if (arg.length>0) f = Double.parseDouble(arg[0]);
      b.setFahrenheit(f);
      double c = b.getCelsius();
      System.out.println("Fahrenheit = "+f);
      System.out.println("Celsius = "+c);
      System.out.println(b.getInfo());
   }
}

Compile the code into F2C.class (file name implied) whilst referencing the dependancy for the jar: javac -classpath tconvert.jar F2C.java

At this point the jar file cannot be executed by itself as the class has to be called for it to become an executable jar. If an attempt to execute is attempted now the following error occurs:

$ java -jar tconvert.jar 60.0
Failed to load Main-Class manifest attribute from tconvert.jar

An attribute must be added to the manifest pointing to our code.

Creating executable Jars

For the above example:

Create fs_f2c.txt

Main-Class: F2C

Add it to the jar

$ jar uvmf fs_f2c.txt tconvert.jar F2C.class
updated manifest
adding: F2C.class(in = 956) (out= 578)(deflated 39%)

The jar can now be executed standalone

$ java -jar tconvert.jar 60.0
Fahrenheit = 60.0
Celsius = 15.555555555555555
Temp Convert Bean

Category:Java Category:Scripting