3.7. Building a Java Program

In Java, there is no deep distinction between a “library” and a “program” except that a JAR file that provides a program must have a main method. If a JAR file contains a main method, it can be executed, though it can also be used as a library. Here are the relevant files for the program example:


package com.example.basic;

import com.example.basic.BasicLibrary;

public class BasicProgram
    public static void main(String[] args)
        BasicLibrary l = new BasicLibrary(10);


name: java-program
platform-types: java
deps: java-library


parameters {
    java.jarName = 'java-program.jar'
    java.mainClass = 'com.example.basic.BasicProgram'
    java.wrapperName = 'java-program'
    abuild.rules = 'java'

A JAR file's manifest file may identify a class that contains a main method. Abuild adds the Main-Class attribute to the manifest file when the java.mainClass parameter is set in the Abuild.groovy. In addition, abuild will create a wrapper script if the java.wrapperName parameter is set. The wrapper script that abuild creates may be useful for casual execution of the Java program for testing purposes, but it is generally not a substitution for having your own deployment mechanism. In particular, the wrapper script references items from your classpath by their paths within the build structure, and additionally, abuild's wrapper scripts are not as portable as the Java code that they help to invoke. [9]

Here is the output of running abuild in this directory. As in the C++ program example, the output has been modified slightly: in addition to the --topdir-- substitution, we have also filtered out time stamps and other strings that could potentially differ between platforms:


abuild: build starting
abuild: java-library (abuild-java): all
    [mkdir] Created dir: --topdir--/basic/java-library/abuild-java/classes
    [javac] Compiling 1 source file to --topdir--/basic/java-library/abu\
    [mkdir] Created dir: --topdir--/basic/java-library/abuild-java/dist
      [jar] Building jar: --topdir--/basic/java-library/abuild-java/dist\
abuild: java-program (abuild-java): all
    [mkdir] Created dir: --topdir--/basic/java-program/abuild-java/classes
    [javac] Compiling 1 source file to --topdir--/basic/java-program/abu\
    [mkdir] Created dir: --topdir--/basic/java-program/abuild-java/dist
      [jar] Building jar: --topdir--/basic/java-program/abuild-java/dist\
abuild: build complete

[9] Specifically, abuild generates different wrapper scripts depending on whether you're running on Windows or not. Although it would work to build Java code on UNIX and run it on Windows, or vice versa, wrapper scripts generated on one platform are not portable to the other.