JShell Web

A fully functional JShell (Java's built-in REPL) running entirely in the browser.

Live Demo

What it does

Type Java code, press Enter, see results. Variables, methods, and classes persist between evaluations, exactly like the real jshell command.

jshell> int x = 10
x ==> 10
jshell> x * 2
$2 ==> 20
jshell> String greet(String name) { return "Hello, " + name + "!"; }
|  created method greet(String)
jshell> greet("World")
$4 ==> "Hello, World!"

Output format matches real jshell line-for-line, including $N scratch variables, | created/replaced/modified messages, and | Exception java.lang.X: msg for thrown errors.

Features

• All standard Java syntax: variables, methods, classes, records, enums, interfaces, abstract classes, generics, lambdas, method references, pattern matching, streams.
• Real $N scratch variables: 1 then $1 returns 1, just like real jshell. Reusable in later expressions and method calls.
• Auto-imports for java.io.*, java.math.*, java.net.*, java.nio.file.*, java.util.*, java.util.concurrent.*, java.util.function.*, java.util.prefs.*, java.util.regex.*, java.util.stream.* — same set as real jshell's default startup.
• Multiline input via Shift+Enter; auto-semicolon insertion for incomplete statements.
• Forward references: methods can reference vars/methods defined later, with the same cannot be invoked until X is declared warning.

Commands

Command	Description
/help	Show available commands
/clear	Clear the console
/reset	Reset session (clear all user state)
/dev	Enable developer mode (test suites + diagnostics)

In developer mode, additional commands are available:

Command	Description
/test	Run all 5 test suites (218 tests)
/test-language	Java language features
/test-format	Exact value display per type
/test-scratch	$N scratch-variable semantics
/test-errors	Compile errors, runtime exceptions, single-execution
/test-advanced	Multiline + integration programs
/probe	Diagnostic snapshot (varValue, event status, reset budget)
/diag <code>	Per-snippet inspector — what JShell sees for any input

Keyboard shortcuts

• Enter — run code
• Shift+Enter — new line (multiline input)
• Up/Down — command history

How it works

• CheerpJ: Full JVM running in WebAssembly (loaded from CDN).
• jdk.compiler_17.jar: Java 17 compiler (javac), extracted from Temurin JDK 17.
• jdk.jshell.jar: JShell engine + dependencies, with TaskFactory patched to use ServiceLoader (CheerpJ's ToolProvider.getSystemJavaCompiler() returns null otherwise).
• MhExecutionControl: a custom LocalExecutionControl subclass (patched into the jdk.jshell.execution package) that invokes the snippet's do_it$ via MethodHandle.invokeWithArguments instead of Method.invoke. CheerpJ silently drops exceptions through Method.invoke; the MethodHandle path propagates them correctly, so SnippetEvent.value() and .exception() work natively.
• JShellBridge.java: bridge between JavaScript and the JShell API. Handles $N referenceability via top-level var wrapping, prints exceptions + jshell-style at (#N:L) stack frames, and runs a hybrid soft/hard reset that bypasses CheerpJ's 15-cycle close+build limit.

Total download: ~5.6 MB of JARs + CheerpJ runtime from CDN.

Building from source

See BUILD.md for instructions on rebuilding the JARs from a JDK 17 distribution. To rebuild the bridge:

bash src/build.sh

To run a local dev server:

serve.bat        # Windows
python serve.py  # any platform with Python 3

The dev server adds HTTP Range support (required by CheerpJ to load JARs) and no-cache headers (so edits land without hard-reload).

Known limitations

These are CheerpJ-specific deviations from real Java semantics. None affect normal user code; they're documented for transparency.

• 1 % 0 silently returns 0 — CheerpJ's WASM doesn't trap mod-by-zero. Real Java throws ArithmeticException. (Tested as a "canary" in /test-errors.)
• new int[-1] throws ArithmeticException instead of NegativeArraySizeException. (Also a canary.)
• JVM-thrown exception messages are often null — e.g. real ArithmeticException from 1/0 carries : / by zero; CheerpJ's WASM raises the exception without setting the message field. User-thrown exceptions (throw new RuntimeException("boom")) keep their message intact.
• shell.close() + new JShell.builder().build() is hardcoded to ~15 cycles per page in CheerpJ; after that, javac's Names.Table corrupts and any compile fails. We work around it by soft-resetting (drop snippets, keep JShell alive) and only doing a hard reset every 20 evals — budget capped at 14, with a warning before the cliff.
• cheerpjRunLibrary can only be called once per page. The only true "deep reset" is reloading the page.

Credits

• CheerpJ by Leaning Technologies — JVM in the browser.
• SnapCode by Jeff Martin — inspired the jdk.compiler extraction approach.
• Eclipse Temurin JDK 17 — source for jdk.compiler_17.jar and jdk.jshell.jar.

License

CheerpJ Community License (free for non-commercial use from the CDN). JDK components are under the GPL v2 with Classpath Exception.