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:
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:
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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 follow these steps to extract the contents of a JAR file:
Locate the JAR file that you want to extract.
Right-click on it and choose Extract All.
Choose the destination folder and click Extract.
You will see the extracted entries in the destination folder.
How to Update a JAR File
There are two main ways to update a JAR file: using the Jar tool or using an IDE.
Using the Jar Tool
The Jar tool can also be used to update the entries of a JAR file. The basic syntax for updating a JAR file using the Jar tool is:
jar uf jar-file input-file(s)
This command updates or adds the input files or directories to the existing JAR file. For example, if you have updated class1.class in your pack directory and you want to update it in pack.jar, you can type:
jar uf pack.jar pack/class1.class
This will replace the old class1.class with the new one in pack.jar. If you want to add new files or directories to the JAR file, you can specify them after the JAR file name. For example, if you want to add a new class named class3.class to pack.jar, you can type:
jar uf pack.jar pack/class3.class
Using an IDE
An IDE can also be used to update a JAR file from your projects or modules. Most IDEs support exporting your changes to an existing JAR file. For example, if you are using Eclipse, you can follow these steps to update a JAR file:
Select the project or module that contains the changes that you want to export.
Right-click on it and choose Export.
Select Java > JAR file and click Next.
Browse and select the existing JAR file that you want to update.
Select the resources and classes that you want to update in the JAR file.
If you want to update an executable JAR file, check the option Launch configuration and select the main class.
Click Finish.
How to Run a JAR File
There are two main ways to run a JAR file: using the Java command or using a file explorer.
Using the Java Command
The Java command is a command-line utility that is part of the Java Runtime Environment (JRE). You can use it to run executable JAR files that have a manifest file that specifies the main class. To use the Java command, you need to have JRE installed on your system and set up your environment variables accordingly. The basic syntax for running a JAR file using the Java command is:
java -jar jar-file
This command runs the JAR file by invoking the main class specified in the manifest file. For example, if you want to run main.jar that you created earlier, you can type:
java -jar main.jar
This will execute the main method of the Main class in main.jar. You can also pass arguments to the main method by appending them after the JAR file name. For example, if you want to pass two arguments "Hello" and "World" to the main method, you can type:
java -jar main.jar Hello World
Using a File Explorer
A file explorer can also be used to run executable JAR files that have a manifest file that specifies the main class. For example, if you are using Windows, you can follow these steps to run a JAR file:
Locate the JAR file that you want to run.
Double-click on it or right-click on it and choose Open with > Java(TM) Platform SE binary.
The JAR file will run and display the output in a console window.
You can close the console window when the execution is finished.
Conclusion
JAR files are a convenient and efficient way to package and distribute Java applications and libraries. They can reduce the size, simplify the deployment, improve the performance, and enhance the security of Java software. You can create, view, extract, update, and run JAR files using various tools and methods. In this article, you learned how to use the Jar tool, an IDE, and a file explorer to perform these tasks. You also learned about the structure and benefits of JAR files and how to specify attributes in the manifest file.
FAQs
Q: How do I download a JAR file?
A: You can download a JAR file from any website or source that provides it. You just need to click on the link or button that initiates the download and save the JAR file to your desired location. For example, if you want to download joda-time-2.0.jar, which is a library for handling date and time in Java, you can go to this website and click on Download joda-time-2.0.jar (1.5 MB). Then, you can choose where to save the JAR file on your system.
Q: How do I add a JAR file to my classpath?
A: The classpath is a list of locations where Java looks for classes and resources. You can add a JAR file to your classpath by using one of these methods:
Using the -cp or -classpath option when running the Java command. For example, if you want to add joda-time-2.0.jar to your classpath, you can type:
java -cp joda-time-2.0.jar Main
Using the CLASSPATH environment variable. You can set or append the path of the JAR file to this variable in your system settings or in your script or batch file. For example, if you want to add joda-time-2.0.jar to your CLASSPATH variable, you can type:
set CLASSPATH=%CLASSPATH%;joda-time-2.0.jar
Using the Class-Path attribute in the manifest file of another JAR file. You can specify the relative or absolute path of the JAR file that you want to include in your classpath in this attribute. For example, if you want to add joda-time-2.0.jar to your classpath in main.jar, you can write this in your manifest file:
Class-Path: joda-time-2.0.jar
Q: How do I create a custom manifest file?
A: A custom manifest file is a text file that contains information about your JAR file and its entries. You can create a custom manifest file by using any text editor and following these rules:
The file name must be META-INF/MANIFEST.MF and it must be located at the root of the JAR file.
The file must have a blank line at the end.
The file must have one or more sections, each starting with a name followed by a colon and ending with a new line.
Each section can have one or more attributes, each starting with a name followed by a colon and a space and ending with a new line.
If an attribute value is longer than 70 characters, it can be continued on the next line by starting with a space.
The first section is the main section and it contains information about the JAR file itself. It can have attributes such as Manifest-Version, Created-By, Main-Class, Class-Path, etc.
The subsequent sections are individual sections and they contain information about specific entries in the JAR file. They can have attributes such as Name, Content-Type, Digest-Algorithms, etc.
For example, a custom manifest file that specifies the main class, the classpath, and the digital signature of a JAR file can look like this:
Manifest-Version: 1.0 Created-By: 1.8.0_292 (Oracle Corporation) Main-Class: Main Class-Path: lib.jar Name: Main.class Content-Type: application/java-byte-code Digest-Algorithms: SHA-256 SHA-256-Digest: 7a1f001d51b4e6f36d0beae9566f5b59c55c5b4b3c4cd888e7e5f8b0e9a8a371
Q: How do I sign a JAR file?
A: Signing a JAR file is a process of adding a digital signature to it that verifies its authenticity and integrity. You can sign a JAR file by using the Jarsigner tool or using an IDE.
Using the Jarsigner Tool
The Jarsigner tool is a command-line utility that is part of the JDK. You can use it to sign and verify JAR files. To use the Jarsigner tool, you need to have JDK installed on your system and set up your environment variables accordingly. You also need to have a keystore that contains your private key and certificate. A keystore is a file that stores cryptographic keys and certificates in a secure way. You can create a keystore by using the Keytool utility or using an IDE. The basic syntax for signing a JAR file using the Jarsigner tool is:
jarsigner -keystore keystore-file jar-file alias
This command signs the JAR file with the private key and certificate associated with the alias in the keystore file. You will be prompted to enter the password for the keystore and the alias. For example, if you have a keystore named mykeystore.jks that contains your private key and certificate with the alias myalias, you can sign pack.jar by typing:
jarsigner -keystore mykeystore.jks pack.jar myalias
This will add a digital signature to pack.jar that can be verified by anyone who has your public key or certificate.
Using an IDE
An IDE can also be used to sign a JAR file from your projects or modules. Most IDEs support exporting your projects or modules as signed JAR files. For example, if you are using Eclipse, you can follow these steps to sign a JAR file:
Select the project or module that you want to export as a signed JAR file.
Right-click on it and choose Export.
Select Java > Runnable JAR file and click Next.
Choose the launch configuration and the destination folder and name for the JAR file.
Check the option Package required libraries into generated JAR.
Click Next.
Select Use an existing keystore and browse and select your keystore file.
Select your alias from the list and enter your password.
Click Finish.
Q: How do I download joda-time-2.0.jar?
A: joda-time-2.0.jar is a library for handling date and time in Java. You can download it from this website: You just need to click on Download joda-time-2.0.jar (1.5 MB) and save the JAR file to your desired location. You can also use the Maven dependency or the Gradle dependency to include the library in your project. For example, if you are using Maven, you can add this to your pom.xml file:
<dependency> <groupId>joda-time</groupId> <artifactId>joda-time</artifactId> <version>2.0</version> </dependency>
If you are using Gradle, you can add this to your build.gradle file:
dependencies implementation 'joda-time:joda-time:2.0'
After downloading or including the library, you can use it in your Java code by importing the classes from the org.joda.time package. For example, if you want to create a DateTime object and print it, you can write:
import org.joda.time.DateTime; public class Main public static void main(String[] args) DateTime dt = new DateTime(); System.out.println(dt);
This will print something like this:
2023-06-21T02:36:46.000Z 44f88ac181
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