Java Compiler Sandbox - Free Online Java Compiler & REST API

Compile and run Java code from your browser or from any language with a single HTTP request. Free, no sign-up, no API key. Built with Spring Boot 3 and Java 21.

🌐 Live app: java-sandbox.sajitkhadka.com 📖 API docs: java-sandbox.sajitkhadka.com/docs

🚀 Features

• Online Java editor with syntax highlighting (CodeMirror)
• Free REST API: POST /api/compile compiles and runs a snippet
• Standard-input support (whitespace-separated values fed to Scanner(System.in))
• Line-accurate compiler errors
• Per-IP rate limiting with Bucket4j
• Container-ready with Jib

🆓 Using the Free API

No authentication or API key is required.

Base URL: https://java-sandbox.sajitkhadka.com Endpoint: POST /api/compile Content-Type: application/json

Request

Field	Type	Required	Description
code	string	yes	Full Java source containing a class with public static void main.
input	string	no	Whitespace-separated values sent to stdin, one per line (max 100 values).

Response

Successful run:

{
    "status": "success",
    "message": "Hello, World!"
}

Compilation or runtime failure:

{
    "status": "error",
    "errors": [{ "line": 3, "message": "';' expected" }]
}

Compile errors, runtime errors and timeouts are returned with HTTP 200 and "status": "error"; check the status field. Malformed requests use real HTTP codes with the same body shape:

HTTP	When
400	code is missing/blank, or contains no class declaration
413	code is longer than 64 KB (65,536 characters)
429	rate limit exceeded (see below)
503	the server is already running its maximum parallel programs; retry after the Retry-After seconds

Examples

curl

curl -X POST https://java-sandbox.sajitkhadka.com/api/compile \
  -H "Content-Type: application/json" \
  -d '{"code": "public class Main { public static void main(String[] args) { System.out.println(\"Hello, World!\"); } }"}'

With input

curl -X POST https://java-sandbox.sajitkhadka.com/api/compile \
  -H "Content-Type: application/json" \
  -d '{"code": "import java.util.Scanner; public class Main { public static void main(String[] a) { Scanner s = new Scanner(System.in); System.out.println(s.nextInt() + s.nextInt()); } }", "input": "2 3"}'

JavaScript

const res = await fetch("https://java-sandbox.sajitkhadka.com/api/compile", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ code: "public class Main { public static void main(String[] a){ System.out.println(1+1); } }" }),
});
console.log(await res.json());

Python

import requests

r = requests.post(
    "https://java-sandbox.sajitkhadka.com/api/compile",
    json={"code": "public class Main { public static void main(String[] a){ System.out.println(1+1); } }"},
)
print(r.json())

⏱️ Rate limits and quotas

Limit	Value
Request rate	30 requests per 30 seconds per client IP
Execution timeout	5 seconds per run
Input values	at most 100 per request
Code size	64 KB
Output size	first 64 KB of output is returned, the rest is truncated
Program memory	128 MB heap
Parallel runs	4 programs at once, server-wide
Cost	Free

When you exceed the request rate the API returns HTTP 429 Too Many Requests:

{ "error": "Too many requests. Please try again later." }

Every response carries X-RateLimit-Limit and X-RateLimit-Remaining; a 429 also carries Retry-After (seconds). Tokens refill continuously, so wait that long and retry. For batch workloads, space requests out (about one per second sustained) or run your own copy using the instructions below. The service is provided as-is with no uptime guarantee; don't rely on it for production traffic.

Note: CORS is restricted to sajitkhadka.com origins and localhost:3000. Calls from other websites' browser code will be blocked; call the API from a server, script or CLI instead.

🛠️ Building and Running Locally

Requires JDK 21 (a full JDK, not a JRE, since the app uses the system Java compiler).

git clone https://github.com/sajitkhadka/java-compiler-sandbox.git
cd java-compiler-sandbox
./mvnw spring-boot:run

The app is then available at http://localhost:8080 (UI at /, docs at /docs).

Build a jar with ./mvnw clean package.

🐳 Docker

# Build the image with Jib
./mvnw compile jib:dockerBuild

# Run it
docker run -p 8080:8080 online-java-compiler:latest

🤝 Contributions

Contributions, issues and feature requests are welcome - open a pull request or an issue.

📄 License

MIT