JVM Explorer

JVM Explorer is a Java desktop application for browsing loaded class files inside locally running Java Virtual Machines.

Features

• Browse local JVMs
• View all classes inside each JVM
• Search to find specific classes (supports regex)
• View classloader hierarchy
• View decompiled bytecode
• View disassembled bytecode
• Re-assemble bytecode
• Re-compile class file
• Modify method implementations (insert, replace method body with java code)
• Execute code in local JVMs
• Export loaded classes to a JAR file
• Browse the state of all static variables in a class (and their nested fields)
• Modify the state of static variables (and their nested fields)
• Replace class file definitions
• View system properties of running JVMs
• Run JAR files with a patched ProcessBuilder to remove any -XX:+DisableAttachMechanism argument

Demos

Browse Classes And Fields

Notes:

• Ctrl + Click on a class name in the class file viewer to open it
• Search and press enter to open a class with the specified name
• Edit a field value (under Fields tab) through Right Click -> Edit - only works for primitives and Strings

Execute Code In Remote JVM

Notes:

• You can call methods in the remote classes - the loaded classes are used for the compiler classpath

Show Class Loader Hierarchy

Launch Jar With Patched ProcessBuilder

Export Classes In Package

Modify Method

Notes:

• You can insert code to the beginning of a method or replace the entire method body
• The compiler currently uses the loaded classes as the classpath, so if a parameter type or return type isn't loaded, it may fail (you can replace the type with Object to make it compile if you don't need that type)
• To access fields in the class, add a field with the same name and type in the class to compile - the reference will automatically be replaced

Modify Class

Notes:

• Ctrl+S to re-recompile and patch class (or Right-Click -> Save Changes)
• Often, decompiled code is not valid Java code, so it may not compile in some cases - the modify method feature is the workaround for this
• The compiler currently uses the loaded classes as the classpath, so if some required type isn't loaded, it may fail

Modify Bytecode

Notes:

• Ctrl+S to re-assemble and patch class (or Right-Click -> Save Changes)
• The disassembler/assembler isn't perfect - it works most of the time but struggles with some cases (it uses OpenJDK AsmTools)

Getting Started

There are three ways to run the application. Execute a provided platform-specific installer, a provided JAR file, or build and run it yourself. This application is intended to run on Java 11+ and can attach to JVMs running Java 7+.

Run Installer:

The platform-specific installers currently support Windows, Linux, and Mac with Intel 64-bit arch

1. Download a platform-specific installer (look at file extension) from the latest release
2. Run the installer
3. Launch the installed program

Run JAR:

A platform-specific JAR is provided for each OS/arch combinations

1. Download a jvm-explorer.jar file for your OS/arch (look at file name) from the latest release
2. Run the JAR (Java 11+)

Build And Run:

This approach will work on all platforms and architectures.

1. Clone with Git

git clone https://github.com/Naton1/jvm-explorer.git

2. Change into the project directory

cd jvm-explorer

3. Run with Gradle (Java 11+)

./gradlew run

Troubleshooting

Two logs files application.log and agent.log are created at [User Home]/jvm-explorer/logs

• Must run the application with a Java version of at least Java 11
• Must attach to a JVM running a Java version of at least Java 7
• Must attach to a JVM running the same architecture - a 32-bit JVM must attach to a 32-bit JVM
• May have to attach to a JVM that the same user started
• Must attach to a JVM that supports the Java Attach API
• If you run an old version of this application and attach it to a JVM, then update to a newer version and try to re-attach, the agent will be outdated. If there are changes in the client-server protocol, the agent will fail. Restart the target JVM to fix this.

Note: this uses the Java Attach API so any limitations will apply here.

Project Principles

• Maintain a simple and easy-to-use interface
• Take advantage of attaching to running JVMs - this is not a traditional GUI decompiler

Possible Enhancements

• Plugins/scripting
• Tabs/open multiple classes
• Search code/improved search mechanism
• Option to load all classes
• Internationalization
• Observe changes in fields

License

MIT License