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How to Download and Use JAR Files in Java




If you are a Java developer or a user of Java applications, you may have encountered JAR files. JAR stands for Java ARchive, and it is a file format that can store multiple files in a compressed form. In this article, you will learn what JAR files are, how to create them, how to view their contents, how to extract them, how to update them, and how to run them.


What is a JAR File?




A JAR file is a package file format that can contain Java class files, metadata, resources, and manifest files. A JAR file can be used to distribute application software or libraries on the Java platform. A JAR file can also be executable if it has a manifest file that specifies the main class.




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Benefits of JAR Files




Some of the benefits of using JAR files are:


  • They reduce the size of the files by compressing them.



  • They simplify the distribution of software by bundling multiple files into one.



  • They improve the performance of Java applications by reducing the loading time of classes.



  • They enhance the security of Java applications by allowing digital signatures and encryption.



Structure of JAR Files




A JAR file has a ZIP file structure, which means it consists of entries that can be files or directories. Each entry has a name, a size, a date and time, and optional attributes. A JAR file can also have a manifest file, which is a special entry that contains information about the other entries and the JAR file itself. The manifest file has the name META-INF/MANIFEST.MF and it is located at the root of the JAR file. The manifest file can specify attributes such as:


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  • The main class of an executable JAR file.



  • The classpath of dependencies for a JAR file.



  • The version and vendor of a JAR file.



  • The digital signature and certificate of a JAR file.



How to Create a JAR File




There are two main ways to create a JAR file: using the Jar tool or using an integrated development environment (IDE).


Using the Jar Tool




The Jar tool is a command-line utility that is part of the Java Development Kit (JDK). You can use it to create, view, extract, and update JAR files. To use the Jar tool, you need to have JDK installed on your system and set up your environment variables accordingly. The basic syntax for creating a JAR file using the Jar tool is:


jar cf jar-file input-file(s)


This command creates a JAR file named jar-file from the input files or directories specified. For example, if you have a package named pack that contains two classes named class1 and class2 in your current directory, you can create a JAR file named pack.jar by typing:


jar cf pack.jar pack


This will create pack.jar in your current directory with the following structure:


META-INF/ META-INF/MANIFEST.MF pack/ pack/class1.class pack/class2.class


You can also specify individual files or use wildcards to include multiple files in a JAR file. For example, if you want to include all the class files in your current directory, you can type:


jar cf classes.jar *.class


If you want to create an executable JAR file, you need to specify the main class in the manifest file. You can do this by using the -e option or by creating a custom manifest file. For example, if you have a class named Main that has a main method in your current directory, you can create an executable JAR file named main.jar by typing:


jar cfe main.jar Main Main.class


This will create main.jar with the following manifest file:


Manifest-Version: 1.0 Created-By: 1.8.0_292 (Oracle Corporation) Main-Class: Main


You can also create a custom manifest file and use the -m option to include it in the JAR file. For example, if you have a manifest file named mymanifest.txt that contains the following content:


Manifest-Version: 1.0 Created-By: 1.8.0_292 (Oracle Corporation) Main-Class: Main Class-Path: lib.jar


You can create an executable JAR file named main.jar by typing:


jar cfm main.jar mymanifest.txt Main.class


Using an IDE




An IDE is an integrated development environment that provides tools and features for developing, testing, debugging, and deploying Java applications. Most IDEs support creating JAR files from your projects or modules. For example, if you are using Eclipse, you can follow these steps to create a JAR file:


  • Select the project or module that you want to export as a JAR file.



  • Right-click on it and choose Export.



  • Select Java > JAR file and click Next.



  • Choose the destination folder and name for the JAR file.



  • Select the resources and classes that you want to include in the JAR file.



  • If you want to create an executable JAR file, check the option Launch configuration and select the main class.



  • Click Finish.



How to View the Contents of a JAR File




There are two main ways to view the contents of a JAR file: using the Jar tool or using a file explorer.


Using the Jar Tool




The Jar tool can also be used to list the entries of a JAR file. The basic syntax for viewing the contents of a JAR file using the Jar tool is:


jar tf jar-file


This command displays the names of the entries in the JAR file. For example, if you want to view the contents of pack.jar that you created earlier, you can type:


jar tf pack.jar


This will display the following output:


META-INF/ META-INF/MANIFEST.MF pack/ pack/class1.class pack/class2.class


If you want to view more details about the entries, such as their sizes and dates, you can use the -v option. For example, if you type:


jar tvf pack.jar


This will display the following output:


0 Wed Jun 21 02:36:46 GMT 2023 META-INF/ 69 Wed Jun 21 02:36:46 GMT 2023 META-INF/MANIFEST.MF 0 Wed Jun 21 02:36:46 GMT 2023 pack/ 412 Wed Jun 21 02:36:46 GMT 2023 pack/class1.class 412 Wed Jun 21 02:36:46 GMT 2023 pack/class2.class Using a File Explorer




A file explorer is a graphical user interface that allows you to browse and manage files and folders on your system. Most file explorers can also open and view the contents of a JAR file as if it were a ZIP file. For example, if you are using Windows, you can follow these steps to view the contents of a JAR file:


  • Locate the JAR file that you want to view.



  • Double-click on it or right-click on it and choose Open with > Windows Explorer.



  • You will see the entries of the JAR file in a new window.



  • You can double-click on any entry to open it or extract it to another location.



How to Extract the Contents of a JAR File




There are two main ways to extract the contents of a JAR file: using the Jar tool or using a file explorer.


Using the Jar Tool




The Jar tool can also be used to extract the entries of a JAR file. The basic syntax for extracting the contents of a JAR file using the Jar tool is:


jar xf jar-file [input-file(s)]


This command extracts all or some of the entries of the JAR file to the current directory. For example, if you want to extract all the entries of pack.jar that you created earlier, you can type:


jar xf pack.jar


This will create a directory named pack with two class files in your current directory. If you want to extract only some of the entries, you can specify their names after the JAR file name. For example, if you want to extract only class1.class from pack.jar, you can type:


jar xf pack.jar pack/class1.class


Using a File Explorer




A file explorer can also be used to extract the contents of a JAR file as if it were a ZIP file. For example, if you are using Windows, you can f


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