REPL-Driven Development

One of Enkan’s most distinctive features is its built-in REPL (Read-Eval-Print Loop).
Unlike typical Java development cycles that require a full JVM restart for every change,
Enkan’s REPL lets you start, stop, and reload the application while the JVM stays running —
and gives you a live interactive shell into the running system.

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What the REPL Is

The Enkan REPL is a JShell-based interactive environment that runs inside the same JVM as
your application. It is not just a control console — it is a full Java expression evaluator.
You can:

• Start and stop the HTTP server with a command
• Reload application code in ~1 second without restarting the JVM
• Inspect the live middleware stack and routing table
• Execute arbitrary Java expressions against live components (database, cache, etc.)
• Test domain objects interactively without writing test classes

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Starting the REPL

Bootstrap the REPL from your main method:

import enkan.system.repl.JShellRepl;
import enkan.system.repl.ReplBoot;
import kotowari.system.KotowariCommandRegister;

public class DevMain {
    public static void main(String[] args) {
        JShellRepl repl = new JShellRepl(MySystemFactory.class.getName());

        new ReplBoot(repl)
                .register(new KotowariCommandRegister())
                .onReady("/start")
                .start();
    }
}

Run it:

% mvn -e compile exec:java -Dexec.mainClass=com.example.DevMain

The REPL starts, executes /start automatically, and presents a prompt:

enkan>

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The Basic Development Loop

enkan> /start                   # start the HTTP server
System started.

# ... edit Java source files in your IDE ...
# ... IDE compiles to target/classes/ ...

enkan> /reset                   # reload changed classes, restart app layer
System reset in 743ms.          # DB pool, caches — still alive

enkan> /stop                    # graceful shutdown
System stopped.

/reset discards and recreates the application ClassLoader, picking up any recompiled
.class files from target/classes/. Long-lived components (connection pools, caches)
are not restarted unless they depend on the application layer.

Automatic reset on file change

Register the /autoreset command to watch for compiled class changes:

new ReplBoot(repl)
        .register(new KotowariCommandRegister())
        .register(r -> r.registerLocalCommand("autoreset", new AutoResetCommand(repl)))
        .start();

enkan> /autoreset
Start to watch modification of an application.

# Now just save + compile in your IDE — the app resets automatically

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Inspecting the Live System

Middleware stack

enkan> /middleware app list
ANY   defaultCharset   (enkan.middleware.DefaultCharsetMiddleware@4929dbc3)
ANY   trace            (enkan.middleware.TraceMiddleware@1c985ffd)
ANY   session          (enkan.middleware.SessionMiddleware@32424a32)
ANY   routing          (kotowari.middleware.RoutingMiddleware@226c7147)
ANY   controllerInvoker (kotowari.middleware.ControllerInvokerMiddleware@5f4da5c3)

Routing table

enkan> /routes app
GET    /                       HomeController#index
GET    /customers               CustomerController#index
GET    /customers/:id           CustomerController#show
POST   /customers               CustomerController#create

Metrics

enkan> /metrics
requests.count = 142
requests.mean_rate = 3.21/s
requests.p99 = 12.4ms

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Live Java Evaluation

The REPL is a full JShell session. Every component in the system is accessible via the
system variable, which is bound to your running EnkanSystem instance.

Query live data through your domain layer

enkan> var ds = system.getComponent("datasource", HikariCPComponent.class)
enkan> var dao = new CustomerDao(ds.getDataSource())
enkan> dao.findById(42L)
$3 ==> Customer{id=42, name="Alice", email="alice@example.com"}

Test business logic without HTTP

enkan> var order = new Order("customer-42", List.of(new OrderItem("SKU-001", 3)))
enkan> order.totalAmount()
$5 ==> 4800

enkan> order.applyDiscount(DiscountPolicy.LOYALTY)
enkan> order.totalAmount()
$7 ==> 4320

Toggle maintenance mode live

enkan> /middleware app predicate serviceUnavailable ANY
# All requests now return 503 — no redeploy needed

enkan> /middleware app predicate serviceUnavailable NONE
# Back to normal

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Connecting to a Remote Process

The REPL server listens on a ZeroMQ port (random by default; set repl.port to fix it).
A separate client process can attach to a running application — useful for production
inspection without a restart.

Server side (your application)

JShellRepl repl = new JShellRepl(MySystemFactory.class.getName());
new ReplBoot(repl).start();
// Logs: "Listen 64815"

Or fix the port:

java -Drepl.port=64815 -jar myapp.jar

Client side

# From the enkan-repl-client JAR
java -jar enkan-repl-client.jar 64815
Connected to server (port = 64815)
enkan> /metrics

Or from within another REPL session:

enkan> /connect 64815
Connected to server (port = 64815)

Remote connections can run any registered command. Arbitrary JShell expressions are
restricted to local connections for security.

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Available Commands

See the REPL Commands reference for the full list of built-in,
Kotowari, Devel, and Metrics commands.