Open .CLASS files online
A .class file contains Java bytecode — the compiled, machine-independent code the Java Virtual Machine executes. Every .class starts with the magic bytes CA FE BA BE, followed by a format version, a constant pool, and the class's fields and methods as JVM instructions. You cannot read it as text; to understand it, you decompile it back toward source.
ZipTool decompiles .class files right in your browser using CFR, a well-known open-source decompiler, running on a WebAssembly JVM. The first decompile downloads the engine once (about 10–20 MB, cached afterwards). Your class file itself is never uploaded — the decompiled source you see was reconstructed on your own device. Most online decompilers take the opposite approach and send your code to their servers.
What is a `.class` file?
When javac compiles a .java source file, it emits a .class file: a strictly defined binary format holding a constant pool (strings, class and method references), field and method descriptors, and bytecode instructions for a stack-based virtual machine. The format is what makes Java portable — the same .class runs on any JVM.
Each .class represents one class or interface; the first four bytes are always CA FE BA BE (a recognizable 'magic number'), and bytes 4–7 encode the minor and major version, which maps to the Java language level it was compiled for (e.g. major 52 = Java 8, 61 = Java 17). Class files are usually packaged together into .jar archives.
- Compiled application code inside
.jar/.war/.earpackages - Libraries and frameworks distributed as bytecode
- Android apps (compiled to DEX from class files)
- Inspecting third-party or legacy code when source is unavailable
How to open a `.class` online
Open ZipTool in a new tab, then drop your .class onto the page — or click to browse for it, or paste a URL. The file opens in the Java decompiler, which reconstructs readable .java source from the bytecode, read entirely on your device.
- Drag your
.classonto the page — or click to browse, or paste a URL. - It opens immediately in the Java decompiler, which reconstructs readable
.javasource from the bytecode. No install, no sign-up. - Read, search, or copy it. Your file never leaves your browser.
Open `.class` without uploading
Most "open class online" sites work by uploading your file to a server and processing it there. ZipTool does the opposite: it loads its parser once, then reads your .class locally in the browser tab. The contents stay on your device.
You can prove that in seconds. Open the page, press F12 for DevTools, switch to the Network tab, and open your file — no request carrying the file's contents is sent. You can also turn off Wi-Fi after the page loads and the file still opens. See security and privacy for the full picture.
Frequently asked questions
How do I open a .class file online?
Drag the .class file onto the page (or click to browse, or paste a URL). ZipTool decompiles it locally and shows the reconstructed Java source in a code editor with syntax highlighting. The first run fetches the decompilation engine once (≈10–20 MB, then browser-cached); later classes decompile in about a second.
Can I recover the original .java source from a .class file?
Not exactly — decompilation reconstructs *equivalent* source, not the original. Comments, formatting, and some variable names are lost at compile time and cannot be restored. CFR produces clean, readable Java that behaves identically, which is usually enough to understand, audit, or port the code. For the packaged form, see decompile a JAR online.
Is it safe to decompile a class file without uploading it?
Decompiling never *executes* the bytecode, so the class cannot run while you inspect it. ZipTool does everything in your browser: the file is read, decompiled (by CFR on a WebAssembly JVM), and displayed locally — no upload, ever. The one network fetch is the engine itself, which downloads code from its CDN, not your file's contents. You can verify this in DevTools (F12 → Network) while opening a class.
Why does my .class file show as garbage in a text editor?
Class files are binary bytecode, not text — a text editor shows mojibake. The intended way to read one is a decompiler. If a file you believe is a .class does not start with the bytes CA FE BA BE, it is probably not a real class file, and ZipTool will tell you so instead of decompiling nonsense.