Tukukkk

Do you want to do it automatically or manually? If manually, a JAR file is really just a ZIP file, so you should be able to open it with any ZIP reader. (You may need to change the extension first.) If you want to update the JAR file automatically via Eclipse, you may want to look into Ant support in Eclipse and look at the zip task.

Simply drag and drop your new class file to the JAR using 7-Zip or Winzip. You can even modify a JAR file that is included in a WAR file using the parent folder icon, and click Ok when 7zip detects that the inside file has been modified

Jar is an archive, you can replace a file in it by yourself in your favourite file manager (Total Commander for example).

1. Use jar -xvf to extract the files to a directory.


2. Make your changes and replace the classes.


3. Use jar -cvf to create a new jar file.

A JAR file is just a .zip in disguise. The zipped folder contains .class files.

If you're on macOS:

1. Rename the file to possess the '.zip' extension. e.g. myJar.jar -> myJar.zip.


2. Decompress the '.zip' (double click on it). A new folder called 'myJar' will appear


3. Find and replace the .class file with your new .class file.


4. Select all the contents of the folder 'myJar' and choose 'Compress x items'. DO NOT ZIP THE FOLDER ITSELF, ONLY ITS CONTENTS
   

Miscellaneous - Compiling a single .class file, with reference to a original jar, on macOS

1. Make a file myClass.java, containing your code.


2. Open terminal from Spotlight.


3. 
   javac -classpath originalJar.jar myClass.java This will create your compiled class called myClass.class.

From here, follow the steps above. You can also use Eclipse to compile it, simply reference the original jar by right clicking on the project, 'Build Path' -> 'Add External Archives'. From here you should be able to compile it as a jar, and use the zip technique above to retrieve the class from the jar.

1) you can extract the file into a folder called

jarname.jar

and then replace the file in the folder, handy if you are updating the class a lot while debugging

2) you can extract the jar replace the file then the jar it up again

3) Open the jar with 7 zip and drag and drop your new class in to copy over the old one

Editing properties/my_app.properties file inside jar:

"zip -u /var/opt/my-jar-with-dependencies.jar properties/my_app.properties". Basically "zip -u <source> <dest>", where dest is relative to the jar extract folder.

You can find source code of any .jar file online, import the same project in your IDE with basic setups. Make necessary changes in .java file and compile it for .class files.

Once compilation is done You need to extract the jar file, replace the old .class file with new one.

And use below command for reconstruct .jar file

Jar cf test.jar *

Note : I have done so many time this changes in our project, hope you will find it useful.

High-level steps:

Setup the environment

Use JD-GUI to peek into the JAR file

Unpack the JAR file

Modify the .class file with a Java Bytecode Editor

Update the modified classes into existing JAR file

Verify it with JD-GUI

Refer below link for detailed steps and methods to do it,

https://www.talksinfo.com/how-to-edit-class-file-from-a-jar/

An alternative is not to replace the .class file in the jar file. Instead put it into a new jar file and ensure that it appears earlier on your classpath than the original jar file.

Not sure I would recommend this for production software but for development it is quick and easy.