Demo: 2. Modify the Class file

July 2025

1. Unzip a WAR file

2. Decompile class files using Procyon

3. Edit Java source directly and deploy (with expected errors)

4. Modify class files using a Java class editor (without recompiling) and deploy successfully

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🧩 1. Unzip the WAR File

To inspect or modify the contents of a WAR file:

jar -xvf ../demo.war

Or use any archive tool like 7-Zip or WinRAR to extract it.

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🔍 2. Decompile Class Files Using Procyon

Procyon is a powerful Java decompiler. To use it:

1. Download the Procyon CLI jar from GitHub.

2. Run the decompiler:

java -jar procyon-decompiler.jar Demo2App.class > Demo2App.java

The decomplied coding is different from the orginal version

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⚠️ 3. Directly Edit Class File and Deploy (With Errors)

If you edit the Java .class file directly and try to repackage and deploy without recompiling:

1. Modify the .java file (e.g., change the message in Demo2App.getMessage()).

2. Repackage the WAR (without compiling):

jar -cvf demo.war *

1. Deploy to Tomcat by copying the WAR to the webapps/ folder.

🛑 This will fail because Tomcat runs .class files, and your .java edits are ignored unless recompiled.

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🛠️ 4. Edit Using a Java Class Editor (Without Recompiling)

To modify behaviour without source code or recompilation:

1. Use a bytecode editor like:

   • CFR + Bytecode Viewer

   • Java Class File Editor

   • JBE GUI

2. Open Demo2App.class

3. Locate the incorrect URL

1. Modify the return string in the bytecode.

2. Save the .class file and repackage the WAR:

jar -cvf Demo.war *

1. Deploy to Tomcat again — this time it works without compilation errors.

2. The problem is fixed

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