JAR files in Java

In today's rapidly digitalizing world, effective data compression is crucial, especially when it needs to be lossless to preserve data integrity. This is where the concept of a JAR file in Java becomes significant. JAR (Java ARchive) files offer a solution for aggregating all components of an application into a single entity, enabling them to be downloaded via a single HTTP call. This not only streamlines the transfer and archiving of files but also enhances efficiency, particularly in high-latency network environments. JAR files are versatile, platform-independent, and particularly useful in constrained computing environments like mobile devices, where they efficiently store application files in a compressed format.

Working with JAR Files

1. Creating a JAR File in Java

Now that we're aware of JAR files, let's create a JAR file in java.

Here's the command for creating a JAR file in java.

Let's have a detailed look at the above command. c denotes that we are trying to create a jar file, v stands for verbose (provides more information in the logs than the typical logs), it gives some details of the command onto the console, and f specifies to store the output of the command into the file specified in the command prompt. jar-filename is the name of the jar file in which the output of the command is saved. And the file-input is the path/filename of the file that has to be compressed into the JAR file.

Example:

The above code will compress the SampleJavaClass.class file into SampleJarFile.jar. We can directly specify the file name if the command prompt is opened in the current directory, else an absolute path has to be given. In our case we are considering the command prompt opened in the current directory.

2. Viewing a JAR File

Just like how a file system shows the contents of a folder, Java also provides a way to view the contents of a JAR file.

Executing the following command displays the contents of a JAR file.

Let's have a granular look over the command, t specifies to display the contents of the jar file in tabular format, and f specifies that the path/filename for the jar file is provided in the command prompt itself. And the jar-filename in the command expects the filename of the jar file of which the contents are to be displayed.

Example:

The above code willshow the contents of the SampleJarFIle.jar in a tabular manner.

3. Extracting a JAR File

To extract contents from a JAR file while ensuring data integrity, use the jar xf command, where x signifies extraction and f indicates the JAR file's name. For specific file extraction, like SampleJavaClass.class from SampleJarFile.jar, the command is:

To extract all contents of SampleJarFile.jar, simply omit the specific file name:

This command unpacks every file within the JAR, maintaining the original structure and contents, including the manifest file which is generated by default during the JAR's creation.

4. Updating a JAR File

UUpdating a JAR file can involve adding new files or modifying its metadata. Use the jar uf command to update, where u signifies update and f specifies the JAR file name. For example, to add AddonJavaClass.class to SampleJarFile.jar, the command is:

This adds AddonJavaClass.class directly to the JAR. To add it to a specific folder within the JAR, such as an "Addons" folder, the command modifies slightly:

Note: This requires the "Addons" folder to exist within the JAR file.

To update the JAR's manifest file, use:

This replaces the manifest content of SampleJarFile.jar with that from NewManifest.txt.

5. Running a JAR File

Running a JAR file in Java involves specifying the main class that contains the main method. This is done using the java -cp command, where -cp stands for classpath. For example, to execute a main class named SampleJavaClass from a JAR file named SampleJarFile.jar, the command would be:

This command tells Java to execute the SampleJavaClass class from the specified JAR. Without the -cp option, Java might not find the main class, leading to a "no main manifest attribute" error.