Lesson 2	JAR files
Objective	Find out how JAR files are used to package and bundle multiple files

Archive Jar Files

Java 1.1 introduced a new feature known as Java archives. Java archives provide a means of bundling multiple files into a single, compressed archive file. Java archive files, also called JAR files, are very similar to ZIP or TAR files that are popular on Windows and Unix platforms, respectively. The primary reason for JavaSoft developing the JAR technology was to allow Java class files and resources to be bundled together for easier and more efficient distribution.

JAR files and JavaBeans

JAR files play a very important role in JavaBeans because they provide the standard distribution method for Beans.
Application builder tools expect Beans to be packaged in JAR files, as opposed to individual class files and resources. All Beans must be packaged into JAR files before they can be distributed and reused.

Java Archive (JAR) Files in JavaBeans: Technical Overview

In the realm of Oracle's Java technology stack, the Java Archive (JAR) file plays a crucial role in packaging, bundling, and distributing Java-based software components. It provides a standardized mechanism for aggregating multiple Java class files, metadata, resources, and auxiliary configurations into a single distributable unit.

Structure and Composition of JAR Files: JAR files follow the ZIP file format, which allows for efficient compression and consolidation of multiple files. Inside a JAR file, you might find:
1. Java Class Files: These are compiled `.class` files that represent the bytecode of your Java application or library .
2. Metadata & Manifest: The `META-INF/MANIFEST.MF` file, a manifest file, contains meta-information about the JAR, such as its version, the main class for executable JARs, and other attribute specifications.
3. Resource Files: This can include images, property files, configuration files, etc.
4. Auxiliary Libraries: Other dependent JARs or library files that the primary software component relies on.
Creation of JAR Files: The `jar` tool, bundled with the Java Development Kit (JDK), is the de facto utility for creating, updating, and extracting JAR files. The basic syntax for creating a JAR file is:
```
jar cf jar-file input-file(s)
```
For instance, to package multiple JavaBeans classes into a JAR named `MyBeans.jar`, one would execute:
```
jar cf MyBeans.jar Bean1.class Bean2.class Bean3.class
```
Deployment and Distribution: Once bundled as a JAR, JavaBeans components or any Java-based software can be distributed and deployed seamlessly. This JAR can be added to the classpath of applications, ensuring that the packaged JavaBeans components are discoverable and executable by the Java runtime environment.
Advantages of JAR Packaging:
1. Portability: JAR files encapsulate all necessary components, making distribution and installation straightforward.
2. Versioning: Through manifest attributes, developers can specify version information, ensuring clarity on component dependencies.
3. Security: Digital signatures can be applied to JAR files, ensuring authenticity and data integrity.
4. Compression: Being based on the ZIP format, JAR files are inherently compressed, making them efficient for storage and transfer.
Recommendations:
For optimal organization and maintainability:
1. Always include a descriptive manifest file in your JARs.
2. Structure your JAR content with package conventions, ensuring organized directories that mirror your Java package structures.
3. Consider using build tools like Apache Maven or Gradle for automated JAR packaging and dependency management.

In conclusion, JAR files stand as an integral part of the Java ecosystem, providing an efficient, standardized, and compact means of bundling, distributing, and deploying Java applications and components, including JavaBeans. Leveraging JARs effectively is paramount for scalable and maintainable Java software development.

Command-line options for the jar Program

Only one of the required command-line options (c, t, or x) can be selected at a time. The rest of the options can be combined or omitted as needed. All of the command-line options for the jar program are described below.

Option	Description
c	Create a new archive file.
t	List the table of contents for the archive file.
x	Extract the named files from the archive. If no filenames are specified, all files are extracted.
v	Generate verbose output.
f	Specify the archive filename following the options. If this option is not used, standard in and standard out are used.
m	Include manifest information from the specified manifest file.
0	Do not use compression.
M	Do not create a manifest file for the archive entries.

In the next lesson, a special piece of information in a JAR file that is critical to JavaBeans will be discussed: the manifest file.