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What's a JAR File and How Do You Open One?

What's a JAR File and How Do You Open One? Image
  • Posted on 20th Jul, 2022 15:51 PM
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A file with the JAR file extension is a Java Archive file. Learn how to open or convert one to ZIP, EXE, or some other file format.

What to Know

  • A JAR file is a Java Archive file.
  • Open one with a browser (Java must be installed).
  • Convert to EXE with Eclipse.

This article explains what a JAR file is, the different ways you can open one depending on how you want to use it, and how to convert one to EXE or ZIP.

What Is a JAR File?

A file with the .JAR file extension is a Java Archive file used for storing Java programs and games in a single file. Some contain files that make them work as standalone apps, and others hold program libraries for other programs to use.

JAR files are ZIP compressed and often store things like CLASS files, a manifest file, and application resources like images, sound clips, and security certificates. Since they can hold hundreds or even thousands of files in a compressed format, it's easy to share and move them.

Java-capable mobile devices might use this format for game files, and some web browsers hold themes and add-ons in the JAR format.

How to Open JAR Files

The Java Runtime Environment (JRE) must be installed in order to open executable JAR files, but note that not all JAR files are executables. Once installed, you can just double-click the file to open it.

Some mobile devices have JRE built-in. Once installed, Java applications can be opened in a web browser, too, like Firefox, Safari, Edge, etc. (but not Chrome).

Since JAR files are compressed with ZIP, any file decompressor can open one to see the contents that are inside. This includes programs like 7-Zip, PeaZip and jZip

Another way to open the file is to use the following command in Command Prompt, replacing yourfile.jar with the name of your own JAR file:

 java -jar yourfile.jar 

Since you might need different programs to open different JAR files, see How to Change the Default Program for a Specific File Extension in Windows if it's automatically opening in a program that you don't want to use it with.

Errors Opening JAR Files

Because of security settings in the Windows operating system and within some web browsers, it's not at all uncommon to see errors when attempting to access Java applications.

For example, "Java Application Blocked" might be seen when trying to load a Java applet. "Your security settings have blocked an untrusted application from running." can be fixed by setting the security level within the Java Control Panel applet.

If you can't open Java applets even after installing JRE, first make sure Java is enabled in your browser and that Control Panel is properly set up to use Java. Then, completely restart your browser by closing down all open windows and then reopening the whole program.

Also, check that you're running the latest version of Java. If you're not, return to that JRE link above and install the latest version. Or, learn how to update your current Java installation.

Although unlikely, you might be receiving errors because you don't really have a JAR file, and so the program you're using to open it doesn't support the format. This can happen if you've misread the file extension. There's more on this below.

How to Convert a JAR File

You can decompile a JAR file's CLASS files to Java files with the help of JavaDecompilers.com. Upload your file there and choose which decompiler to use.

Converting a Java application so that it can be used on the Android platform would require a JAR to APK file conversion. One option might be to run the JAR file in an Android emulator so that the program creates an APK file automatically. However, it seems that the easiest way to get a Java program on Android is to simply compile the APK from the original source code.

You can make executable JAR files in programming applications like Eclipse.

WAR files are Java Web Archive files, but you can't convert a JAR file directly to a WAR file, since the WAR format has a particular structure that JARs do not. Instead, you can build a WAR and then add the JAR file into the lib directory so that the classes inside the JAR file are available for use. WizToWar might help you do this.

To make a ZIP file from a JAR file is as easy as renaming the file extension from .JAR to .ZIP. This doesn't actually perform a file conversion, but it does let programs that use ZIP files, like 7-Zip or PeaZip, more easily open the JAR file.

Still Can't Open It?

Some files share a few of the same file extension letters, which can cause confusion if the file isn't opening like you think it should. What's happening is that you're most likely misreading the file extension.

For example, JARVIS files open with a text editor, and JARC and ARJ files are archives.

More Information on the JAR Format

If you need help packing programs into JAR files, follow that link for instructions on the Oracle website.

Only one manifest file can be included in a JAR archive, and it has to be at the META-INF/MANIFEST.MF location. It should follow the syntax of the name and value separated by a colon, like Manifest-Version: 1.0. This MF file can specify the classes the application should load.

Java developers can digitally sign their applications, but it doesn't sign the JAR file itself. Instead, the files inside the archive are listed with their signed checksums.

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