How to open class files

Syntax for Javap

Javap Command Example in Java

lets use the Java Decomplier (Javap tool) to decompile our Demo class.

Demo.java

Output :

upon executing javap command in cmd

javap com.javainterviewpoint.Demo

The below output will be produced

In the above code we have not used any options along with javap command in cmd, the most popular option is ‘-c’, this option prints instructions that comprise the Java bytecodes for each of the methods in the class.

How to use Javap command in Eclipse

Running javap command in eclipse can be done using External Tool Configuration only. Goto “Run –> External Tools –> External Tools Configurations” as shown in below image.

Enter the below information in the new Configuration window

We will get the below out displayed

you can also add different options in the Arguments block for different output. The complete list of the option which can be used along with Java Disassembler is present here.

CLASS File Extension

Java Class Format

What is CLASS file?

File with CLASS extension contains compiled source code written in Java programming language. CLASS file is generated by compiler from JAVA file that contains classes. CLASS file is thus the result of successful compilation process. Single CLASS file contains bytecode, that is, a binary code of given program that is executable when run by Java Virtual Machine (JVM).

Compilation process

CLASS files are generated automatically in the process of JAVA file compilation. JAVA files are the source files for any Java applications. CLASS file on the other hand contain bytecode that is platform independent. JAVA file can hold one or more classes, and each class gets its own CLASS file in the process of code compilation.

Additional information

Compilation of JAVA files into CLASS files is conducted using javac command included with JVM software package.

Most Java programming environments are capable of compiling CLASS files on the fly as programmers write the code.

Java Runtime Environment supports możliwe execution od CLASS files, but doesn’t allow its compilation since JRE doesn’t have javac command.

CAUTION

Because Java Virtual Machine is available in many versions, one must have in mind that not all CLASS files are compatible with all versions of the engine.

Programs which support CLASS file extension

Below you will find an index of software programs that can be used to open CLASS files divided into 4 categories according to system platform supported. CLASS files can be encountered on all system platforms, including mobile, yet there is no guarantee each will properly support such files.

Programs that support CLASS file

How to open file with CLASS extension?

Problems with accessing CLASS may be due to various reasons. On the bright side, the most encountered issues pertaining to Java Class Format files aren’t complex. In most cases they can be addressed swiftly and effectively without assistance from a specialist. The following is a list of guidelines that will help you identify and solve file-related problems.

Step 1. Install Java Runtime Environment software

The most common reason for such problems is the lack of proper applications that supports CLASS files installed on the system. This issue can be addressed by downloading and installing Java Runtime Environment or different compatible program, such as Eclipse, dirtyJOE, DJ Java Decompiler. Above you will find a complete listing of programs that support CLASS files, classified according to system platforms for which they are available. One of the most risk-free method of downloading software is using links provided by official distributors. Visit Java Runtime Environment website and download the installer.

Step 2. Check the version of Java Runtime Environment and update if needed

You still cannot access CLASS files although Java Runtime Environment is installed on your system? Make sure that the software is up to date. Software developers may implement support for more modern file formats in updated versions of their products. If you have an older version of Java Runtime Environment installed, it may not support CLASS format. All of the file formats that were handled just fine by the previous versions of given program should be also possible to open using Java Runtime Environment.

Step 3. Set the default application to open CLASS files to Java Runtime Environment

After installing Java Runtime Environment (the most recent version) make sure that it is set as the default application to open CLASS files. The method is quite simple and varies little across operating systems.

The procedure to change the default program in Windows

The procedure to change the default program in Mac OS

Step 4. Ensure that the CLASS file is complete and free of errors

If you followed the instructions form the previous steps yet the issue is still not solved, you should verify the CLASS file in question. Problems with opening the file may arise due to various reasons.

1. Verify that the CLASS in question is not infected with a computer virus

If the CLASS is indeed infected, it is possible that the malware is blocking it from opening. Immediately scan the file using an antivirus tool or scan the whole system to ensure the whole system is safe. If the CLASS file is indeed infected follow the instructions below.

2. Check whether the file is corrupted or damaged

If you obtained the problematic CLASS file from a third party, ask them to supply you with another copy. It is possible that the file has not been properly copied to a data storage and is incomplete and therefore cannot be opened. It could happen the the download process of file with CLASS extension was interrupted and the file data is defective. Download the file again from the same source.

3. Check if the user that you are logged as has administrative privileges.

Sometimes in order to access files user need to have administrative privileges. Log out of your current account and log in to an account with sufficient access privileges. Then open the Java Class Format file.

4. Verify that your device fulfills the requirements to be able to open Java Runtime Environment

The operating systems may note enough free resources to run the application that support CLASS files. Close all running programs and try opening the CLASS file.

5. Verify that your operating system and drivers are up to date

Regularly updated system, drivers, and programs keep your computer secure. This may also prevent problems with Java Class Format files. It is possible that one of the available system or driver updates may solve the problems with CLASS files affecting older versions of given software.

Do you want to help?

If you have additional information about the CLASS file, we will be grateful if you share it with our users. To do this, use the form here and send us your information on CLASS file.

How do I open a class file with notepad++?

I am opening a minecraft class folder with notepad++. But whenever I open it, it just has a bunch of boxes that say Nul, DC1, SO, SOH, FF, STX, ect. On guides I see that the class file opens just fine, what am I doing wrong?

3 Answers 3

Trending sort

Trending sort is based off of the default sorting method — by highest score — but it boosts votes that have happened recently, helping to surface more up-to-date answers.

It falls back to sorting by highest score if no posts are trending.

Switch to Trending sort

You are trying to edit a compiled Java class.

Check your guides again, you’ve likely strayed from the path on which they were intending to lead you.

you cannot view the source of the file if it is still compiled, you have to decompile the class file to a java file and then you can read the file with notepad/eclipse, there is a program that people have made to decompile the minecraft source so you can edit it, its called mcp

Just download this and continue as you’re meant to, then you can freely edit the code

If you want to open class file as is, use any binary editor. ghex is a good one. You can do:

javap is a built in tool to disassemble java class files. Here is an example:

This will provide you with lots of information about your class, its byte code and even explanation to what it is doing

I’m trying to figure out what a Java applet’s class file is doing under the hood. Opening it up with Notepad or Textpad just shows a bunch of gobbledy-gook.

Is there any way to wrangle it back into a somewhat-readable format so I can try to figure out what it’s doing?

17 Answers 17

Trending sort

Trending sort is based off of the default sorting method — by highest score — but it boosts votes that have happened recently, helping to surface more up-to-date answers.

It falls back to sorting by highest score if no posts are trending.

Switch to Trending sort

jd-gui is the best decompiler at the moment. it can handle newer features in Java, as compared to the getting-dusty JAD.

If you don’t mind reading bytecode, javap should work fine. It’s part of the standard JDK installation.

As pointed out by @MichaelMyers, use

to get the JVM assembly code. You may also redirect the output to a text file for better visibility.

You want a java decompiler, you can use the command line tool javap to do this. Also, Java Decompiler HOW-TO describes how you can decompile a class file.

what you are looking for is a java de-compiler. I recommend JAD http://www.kpdus.com/jad.html It’s free for non commercial use and gets the job done.

Note: this isn’t going to make the code exactly the same as what was written. i.e. you’re going to lose comments and possibly variable names, so it’s going to be a little bit harder than just reading normal source code. If the developer is really secretive they will have obfuscated their code as well, making it even harder to read.

cpuguru, if your applet has been compiled with javac 1.3 (or less), your best option is to use Jad.

Unfortunately, the last JDK supported by JAD 1.5.8 (Apr 14, 2001) is JDK 1.3.

If your applet has been compiled with a more recent compiler, you could try JD-GUI : this decompiler is under development, nevertheless, it generates correct Java sources, most of time, for classes compiled with the JDKs 1.4, 1.5 or 1.6.

you can also use the online java decompilers available. For e.g. http://www.javadecompilers.com

Using Jad to decompile it is probably your best option. Unless the code has been obfuscated, it will produce an okay result.

That’s compiled code, you’ll need to use a decompiler like JAD: http://www.kpdus.com/jad.html

You need to use a decompiler. Others have suggested JAD, there are other options, JAD is the best.

I’ll echo the comments that you may lose a bit compared to the original source code. It is going to look especially funny if the code used generics, due to erasure.

JAD and/or JADclipse Eclipse plugin, for sure.

If the class file you want to look into is open source, you should not decompile it, but instead attach the source files directly into your IDE. that way, you can just view the code of some library class as if it were your own

As suggested you can use JAD to decompile it and view the files. To make it easier to read you can use the JADclipse plugin for eclipse to integrate JAD directly to eclipse or use DJ Java Decompiler which is much easier to use than command line JAD

There is no need to decompile Applet.class. The public Java API classes sourcecode comes with the JDK (if you choose to install it), and is better readable than decompiled bytecode. You can find compressed in src.zip (located in your JDK installation folder).

Not the answer you’re looking for? Browse other questions tagged java windows class applet or ask your own question.

Linked

Related

Hot Network Questions

Subscribe to RSS

To subscribe to this RSS feed, copy and paste this URL into your RSS reader.

By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy.

How to open *.class files with decompiler by default in Eclipse?

When opening a *.class file in Eclipse, the default Class File Viewer editor is used showing a not coder-friendly view of the class.

Having several decompilers installed on my Eclipse instance, I can «manually» use any them to open my *.class file so it shows in the Class Decompiler Viewer editor.

What I would like though is to change the default editor from Class File Viewer to Class Decompiler Viewer so when I open a *.class file (e.g. by double-clicking it in the Package Explorer view or via the Open Type. dialog) it opens directly in the right view.

At the moment *.class files are consistently opening in the Class File Viewer editor regardless of my settings. I have to go back to the Package Explorer view, right-click the *.class file and select the desired decompiler explicitely in the Open Class With option to get it open in the Class Decompiler Viewer editor.

I have set the Class Decompiler Viewer editor as the default one for *.class files, applied, restarted Eclipse even, but still all the *.class files I open are coming through the Class File Viewer by default.

The decompiler preference also has an option to set himslef as the default editor of *.class files, but without success on my side:

Any idea of a solution to force the default *.class file editor to something else than the Class File Viewer one? Am I missing something obvious in my settings?