Book Image

Java 9 Programming By Example

By : Peter Verhas
Book Image

Java 9 Programming By Example

By: Peter Verhas

Overview of this book

This book gets you started with essential software development easily and quickly, guiding you through Java’s different facets. By adopting this approach, you can bridge the gap between learning and doing immediately. You will learn the new features of Java 9 quickly and experience a simple and powerful approach to software development. You will be able to use the Java runtime tools, understand the Java environment, and create Java programs. We then cover more simple examples to build your foundation before diving to some complex data structure problems that will solidify your Java 9 skills. With a special focus on modularity and HTTP 2.0, this book will guide you to get employed as a top notch Java developer. By the end of the book, you will have a firm foundation to continue your journey towards becoming a professional Java developer.
Table of Contents (17 chapters)
Title Page
Credits
About the Author
About the Reviewer
www.PacktPub.com
Customer Feedback
Preface

Packaging classes into a JAR file


When you deliver a Java application, usually the code is packaged into JAR, WAR, EAR, or some other packaged format. We learn something again that seems to be obscure at first sight, but in reality, this is not that complex. They are all ZIP files. You can open any of these files using WinZip or some other zip manager that you have a license for. The extra requirement is that, for example, in the case of a JAR file, the archive should contain a directory named META-INF and inside it a file named MANIFEST.MF. This file is a text file and contains meta information in the format, which is as follows:

Manifest-Version: 1.0 
Created-By: 9-ea (Oracle Corporation)

There can be a lot of other information in the file, but this is the minimum that the Java provided tool jar puts there if we package our class file into a jar issuing the following command:

         jar -cf hello.jar HelloWorld.class

The -c option tells the JAR archiver to create a new JAR file and the option f is used to specify the name of the new archive. The one we specified here is hello.jar and the file added to it is the class file.

The packaged JAR file can also be used to start the Java application. Java can read directly from JAR archives and load classes from there. The only requirement is that they are on the classpath.

Note

Note that you cannot put individual classes on the classpath, only directories. As JAR files are archives with an internal directory structure in them, they behave like a directory.

Check that the JAR file was created using ls hello.jar and remove the rm HelloWorld.class class file just to ensure that when we issue the command line, the code is executed from the JAR file and not the class.

$ java -cp hello.jar HelloWorld
Hello World

To see the content of the JAR file, however, it is recommended that you use the JAR tool and not WinZip even though that may be cozier. Real professionals use the Java tools to handle Java files.

$ jar -tf hello.jar 
META-INF/ 
META-INF/MANIFEST.MF 
HelloWorld.class