Ron and Ella Wiki Page

You unit test Apache Camel by bootstrapping a CamelContext in JUnit 5, sending messages into real endpoints (direct:, seda:, REST), and asserting behaviour via responses or MockEndpoints, while keeping production RouteBuilders free of mocks and using Camel 4–friendly testing patterns.

Core building blocks

Modern Camel testing with JUnit 5 rests on three pillars: a managed CamelContext, controlled inputs, and observable outputs.

• CamelTestSupport manages the lifecycle of the CamelContext and exposes context, template (a ProducerTemplate), and getMockEndpoint.
• You inject messages with template.sendBody(...) or template.requestBody(...) into direct:, seda:, or HTTP endpoints
• You assert via:
  • MockEndpoint expectations (count, body, headers, order), or
  • Assertions on returned bodies.

Rationale: you want tests that execute the same routing logic as production, but in a fast, in‑JVM, repeatable way.

1. Testing direct component

A good practice is: no mock: in production routes; mocks are introduced only from tests. We start with a simple transformation route.

Route: only real direct: endpoints

package com.example;

import org.apache.camel.builder.RouteBuilder;

public class UppercaseRoute extends RouteBuilder {
    @Override
    public void configure() {
        from("direct:start")                // real entry endpoint
            .routeId("uppercase-route")
            .transform(simple("${body.toUpperCase()}"))
            .to("direct:result");           // real internal endpoint
    }
}

Rationale:

• direct:start is a synchronous, in‑JVM endpoint ideal as a “unit test entry point” and also usable in production wiring.
• direct:result is a real internal endpoint you can “tap” from tests using AdviceWith, keeping test concerns out of the RouteBuilder.

Test: apply AdviceWith in setup, then start context

In Camel 4, instead of overriding any flag, you apply AdviceWith in setup and then start the context explicitly.

package com.example;

import org.apache.camel.RoutesBuilder;
import org.apache.camel.builder.AdviceWith;
import org.apache.camel.component.mock.MockEndpoint;
import org.apache.camel.test.junit5.CamelTestSupport;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

class UppercaseRouteTest extends CamelTestSupport {

    @Override
    protected RoutesBuilder createRouteBuilder() {
        return new UppercaseRoute();
    }

    @BeforeEach
    void adviseRoute() throws Exception {
        // Apply advice *before* the context is fully started
        AdviceWith.adviceWith(context, "uppercase-route", route -> {
            route.weaveByToUri("direct:result")
                 .replace()
                 .to("mock:result");
        });

        // Ensure context is started after advice is applied
        if (!context.isStarted()) {
            context.start();
        }
    }

    @Test
    void shouldUppercaseBody() throws Exception {
        // 1. Expectations on the mock consumer
        MockEndpoint result = getMockEndpoint("mock:result");
        result.expectedMessageCount(1);
        result.expectedBodiesReceived("HELLO");

        // 2. Exercise the route via a real producer
        template.sendBody("direct:start", "hello");

        // 3. Verify expectations
        result.assertIsSatisfied();
    }
}

Rationale:

• The RouteBuilder is production-pure (direct: only); tests decide where to splice in mock: via AdviceWith.
• You apply advice in @BeforeEach while the context is created but before you use it, then explicitly start it, which aligns with modern Camel 4 test support guidance.

2. Testing seda component

For asynchronous flows, seda: is a common choice. You keep the route realistic and only intercept the tail for assertions.

Route: seda: producer and consumer

package com.example;

import org.apache.camel.builder.RouteBuilder;

public class UppercaseRouteSeda extends RouteBuilder {
    @Override
    public void configure() {
        from("seda:input")                // real async entry point
            .routeId("uppercase-route-seda")
            .transform(simple("${body.toUpperCase()}"))
            .to("seda:output");            // real async consumer endpoint
    }
}

Rationale:

• seda: simulates queue-like, asynchronous behaviour in‑JVM and is commonly used in real Camel topologies.

Test: intercept only the consumer side

package com.example;

import org.apache.camel.RoutesBuilder;
import org.apache.camel.builder.AdviceWith;
import org.apache.camel.component.mock.MockEndpoint;
import org.apache.camel.test.junit5.CamelTestSupport;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

class UppercaseRouteSedaTest extends CamelTestSupport {

    @Override
    protected RoutesBuilder createRouteBuilder() {
        return new UppercaseRouteSeda();
    }

    @BeforeEach
    void adviseRoute() throws Exception {
        AdviceWith.adviceWith(context, "uppercase-route-seda", route -> {
            route.weaveByToUri("seda:output")
                 .replace()
                 .to("mock:result");
        });

        if (!context.isStarted()) {
            context.start();
        }
    }

    @Test
    void shouldUppercaseBodyUsingSedaProducer() throws Exception {
        MockEndpoint result = getMockEndpoint("mock:result");
        result.expectedMessageCount(1);
        result.expectedBodiesReceived("HELLO");

        template.sendBody("seda:input", "hello");

        result.assertIsSatisfied();
    }
}

Rationale:

• The route used in production (seda:input → seda:output) is unchanged.
• The test uses AdviceWith to “cut off” the external consumer and replace it with mock:result, which is precisely where you want isolation.

3. REST DSL route with internal direct: logic

REST DSL adds a mapping layer (paths, verbs, binding) over internal routes that contain the business logic. Testing is easier when those are separated.

Route: REST DSL + internal direct: route

package com.example;

import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.model.RouteDefinition;
import org.apache.camel.model.rest.RestBindingMode;

public class RestRoute extends RouteBuilder {

    @Override
    public void configure() {
        // REST server configuration for tests / dev
        restConfiguration()
            .component("netty-http")
            .host("localhost")
            .port(8081)
            .bindingMode(RestBindingMode.off);

        // Internal route with business logic
        configureHelloRoute();

        // REST DSL: GET /api/hello/{name} - routes to the separate direct route
        rest("/api")
            .get("/hello/{name}")
                .routeId("rest-hello-route")
                .produces("application/json")
                .to("direct:hello");
    }

    /**
     * Configures the hello route with business logic.
     * This method is extracted to allow testing the route logic independently.
     */
    protected RouteDefinition configureHelloRoute() {
        return from("direct:hello")
            .routeId("direct-hello-route")
            .log("Processing direct:hello with headers: ${headers}")
            .setBody(simple("{\"message\": \"Hello, ${header.name}!\"}"))
            .setHeader("Content-Type", constant("application/json"));
    }
}

Rationale:

• configureHelloRoute() encapsulates the business logic in a reusable method that always creates from("direct:hello"). This gives you a stable seam for unit tests: any test that calls configureHelloRoute() will have a valid direct:hello consumer.
• The main configure() wires REST to that internal route, which is the transport layer. By keeping this wiring in configure() and the logic in configureHelloRoute(), you can selectively enable or bypass the REST layer in tests without duplicating code.

Note: using RestBindingMode.off is a pragmatic choice here, because the GET action does not carry a request body and you are constructing the JSON response yourself. This avoids any extra marshalling/unmarshalling machinery and keeps the example simple and predictable.

4. Unit test for REST internal route (direct:hello)

This test bypasses HTTP and focuses on the business logic behind the REST endpoint.

package com.example;

import org.apache.camel.RoutesBuilder;
import org.apache.camel.test.junit5.CamelTestSupport;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

/**
 * This test validates the business logic of RestRoute by testing the direct:hello route
 * using the actual RestRoute class, bypassing REST server configuration.
 */
class RestRouteDirectTest extends CamelTestSupport {

    @Override
    protected RoutesBuilder createRouteBuilder() {
        // Create a test-specific version of RestRoute that only configures the business logic
        return new RestRoute() {
            @Override
            public void configure() {
                // Only configure the hello route, skip REST server configuration
                configureHelloRoute();
            }
        };
    }

    @Test
    void shouldReturnGreetingForName() {
        String response = template.requestBodyAndHeader(
            "direct:hello",
            null,
            "name",
            "Alice",
            String.class
        );

        assertEquals("{\"message\": \"Hello, Alice!\"}", response);
    }

    @Test
    void shouldReturnGreetingForDifferentName() {
        String response = template.requestBodyAndHeader(
            "direct:hello",
            null,
            "name",
            "Bob",
            String.class
        );

        assertEquals("{\"message\": \"Hello, Bob!\"}", response);
    }
}

Rationale:

• Testing direct:hello directly gives you a fast, deterministic unit test with no HTTP stack involved.
• You reuse the exact same logic (configureHelloRoute()) that production uses, so there is no “test-only” copy of the route.
• By overriding configure() and calling only configureHelloRoute(), you intentionally skip restConfiguration() and rest("/api")..., which keeps this test focused solely on the business logic and avoids starting an HTTP server in this test.
• This is a very clean way to test the “core route” independent of any transport (REST, JMS, etc.), while still using the real production code path.
• Setting the name header matches how Rest DSL passes path parameters into the route, without needing a full HTTP roundtrip in this test.
• Assertions check only the JSON payload, which is exactly what the route is responsible for producing.

This is textbook “unit test the route behind the REST layer.”

5. Unit test for the full REST endpoint over HTTP

This test exercises the full REST mapping via HTTP using Camel’s netty-http client URI.

package com.example;

import org.apache.camel.RoutesBuilder;
import org.apache.camel.test.junit5.CamelTestSupport;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

class RestRouteHttpTest extends CamelTestSupport {

    @Override
    protected RoutesBuilder createRouteBuilder() {
        return new RestRoute();
    }

    @Test
    void shouldReturnGreetingOverHttp() {
        String response = template.requestBody(
            "netty-http:http://localhost:8081/api/hello/Bob",
            null,
            String.class
        );

        assertEquals("{\"message\": \"Hello, Bob!\"}", response);
    }
}

Rationale:

• Using netty-http:http://localhost:8081/... calls the REST endpoint as an HTTP client would, validating path, verb, port, and basic JSON response.
• This is integration-style, but still in‑JVM under CamelTestSupport, so it is relatively cheap to run.

6. Patterns to remember (Camel 4–friendly)

A quick pattern table to keep the approach straight:

The general rationale is:

• Design routes as you would for production, with real components only.
• Use AdviceWith in tests (configured before starting the context) to splice in mock: endpoints where you need observability or isolation.
• Layer tests: internal routes (direct:/seda:) for behaviour; REST/HTTP tests for contracts and configuration.

Apache Camel’s Java DSL lets you plug data formats directly into your routes using fluent marshal() and unmarshal() calls, including both built‑in and custom implementations.

Basics: marshal and unmarshal in Java DSL

In Java DSL, marshal() and unmarshal() are methods on RouteBuilder that apply a DataFormat to the message body.

• marshal() converts a Java object (or other in‑memory representation) into a binary or textual format for the “wire”.
• unmarshal() converts incoming bytes/text into a Java representation, often a POJO, Map, or List.

Example with a built‑in CSV data format:

public class CsvRoute extends RouteBuilder {
    @Override
    public void configure() {
        from("direct:format")
            .setBody(constant(Map.of("foo", "abc", "bar", 123)))
            .marshal().csv()        // Java Map -> CSV line
            .to("log:csv");
    }
}

The rationale is that your route expresses what transformation should happen (JSON, CSV, XML, etc.) while Camel’s data formats handle how to do it.

Three Java DSL styles for data formats

The Java DSL gives you several ways to use data formats.

1. Passing a DataFormat instance

You create and configure a DataFormat in Java and pass it to marshal()/unmarshal().

import org.apache.camel.dataformat.bindy.csv.BindyCsvDataFormat;

public class BindyRoute extends RouteBuilder {
    @Override
    public void configure() {
        DataFormat myCsv = new BindyCsvDataFormat(MyModel.class);

        from("file:data/in?noop=true")
            .unmarshal(myCsv)      // CSV -> MyModel instances
            .to("bean:processModel");
    }
}

Why use this: you get full type‑safe configuration in code and can reuse the same DataFormat instance across routes.

2. Short “dot” helpers: .marshal().json(), .unmarshal().csv()

For common formats, you can use the fluent helpers returned by marshal() and unmarshal().

public class JsonRoute extends RouteBuilder {
    @Override
    public void configure() {
        from("direct:toJson")
            .marshal().json()      // uses default JSON data format (e.g. Jackson)
            .to("log:json");
    }
}

This is concise but exposes only basic configuration options; you switch to the other styles if you need more control (like custom delimiters or modules).

3. Data Format DSL: marshal(dataFormat().csv().delimiter(","))

Camel 4 adds a dedicated Data Format DSL accessed via dataFormat(), which you can pass into marshal()/unmarshal().

public class CsvDslRoute extends RouteBuilder {
    @Override
    public void configure() {

        from("direct:format")
            .setBody(constant(Map.of("foo", "abc", "bar", 123)))
            .marshal(
                dataFormat()
                    .csv()          // choose CSV
                    .delimiter(",") // customize delimiter
                    .end()          // build DataFormat
            )
            .to("log:csv");
    }
}

Rationale: the Data Format DSL is still Java DSL, but it exposes the full configuration surface in a fluent, type‑safe way, without manually constructing the underlying DataFormat.

Custom DataFormat in Java DSL

You can plug any class implementing org.apache.camel.spi.DataFormat directly into the Java DSL. The following example uses the earlier “Hello‑line” format and wires it into routes.

Custom HelloLineDataFormat

import org.apache.camel.Exchange;
import org.apache.camel.spi.DataFormat;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.charset.StandardCharsets;

public class HelloLineDataFormat implements DataFormat {

    @Override
    public void marshal(Exchange exchange, Object graph, OutputStream stream) throws Exception {
        String body = exchange.getContext()
                .getTypeConverter()
                .mandatoryConvertTo(String.class, graph);

        String encoded = "HELLO|" + body + "\n";
        stream.write(encoded.getBytes(StandardCharsets.UTF_8));
    }

    @Override
    public Object unmarshal(Exchange exchange, InputStream stream) throws Exception {
        String text = toString(stream);
        if (!text.startsWith("HELLO|")) {
            throw new IllegalArgumentException("Invalid format, expected HELLO| prefix: " + text);
        }
        String withoutPrefix = text.substring("HELLO|".length());
        return withoutPrefix.trim();
    }

    private String toString(InputStream in) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        byte[] buf = new byte[1024];
        int len;
        while ((len = in.read(buf)) != -1) {
            out.write(buf, 0, len);
        }
        return out.toString(StandardCharsets.UTF_8);
    }

    @Override
    public void start() {
        // No special startup logic needed for this data format
    }

    @Override
    public void stop() {
        // No special shutdown logic needed for this data format
    }
}

Rationale: the custom format encapsulates both how to decorate the text and how to validate it, so routes never touch the "HELLO|" prefix logic directly.

Using the custom format with Java DSL

Here’s a minimal working example that wires the custom data format into Java DSL routes.

import org.apache.camel.CamelContext;
import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.impl.DefaultCamelContext;

public class HelloLineRouteBuilder extends RouteBuilder {

    private final HelloLineDataFormat helloFormat = new HelloLineDataFormat();

    @Override
    public void configure() {

        // Example: marshal plain text to custom wire format
        from("timer:hello?period=5000")
            .setBody(constant("World"))
            .marshal(helloFormat)           // "World" -> "HELLO|World\n"
            .to("stream:out")
            .to("direct:incomingHello");    // Send marshaled data to the direct route

        // Example: unmarshal from custom wire format back to String
        from("direct:incomingHello")
            .unmarshal(helloFormat)         // "HELLO|World\n" -> "World"
            .log("Decoded body = ${body}");
    }

    // Small bootstrapper
    void main() throws Exception {
        CamelContext context = new DefaultCamelContext();
        context.addRoutes(new HelloLineRouteBuilder());
        context.start();
        Thread.sleep(30_000);
        context.stop();
    }
}

Why this structure:

• The HelloLineDataFormat instance is a normal field, so you can reuse it in multiple routes in the same RouteBuilder.
• The Java DSL remains expressive: you can read the route as “take the timer body, marshal with helloFormat, send to stdout”, which keeps data‑format details out of the route’s core logic.
• The direct:incomingHello route applies .unmarshal(helloFormat) to turn "HELLO|World\n" back into "World", validating the round‑trip symmetry of your custom DataFormat in a realistic producer–consumer arrangement.

Choosing an approach in Java DSL

As a Java‑DSL user, you typically choose between:

• Helpers (.marshal().json(), .unmarshal().csv()): quickest for standard cases.
• Explicit instances (new JaxbDataFormat(...)): best when you need programmatic configuration or dependency‑injected collaborators.
• Data Format DSL (marshal(dataFormat().csv().delimiter(";"))): clean, fluent configuration for complex built‑in formats.
• Custom DataFormat classes (marshal(new MyCustomFormat())): when the wire format is non‑standard or proprietary.

Apache Camel’s REST DSL gives you a high-level way to describe REST endpoints, while the direct component and inline routes let you structure the implementation as clear, in‑process “function calls.” Together, they form a neat pattern for designing REST services that are both readable and operationally simple.

REST DSL in a nutshell

Camel’s REST DSL sits on top of the normal routing DSL and lets you define HTTP endpoints using verbs and paths instead of low‑level URIs.

Conceptually:

• The REST block defines the HTTP contract: paths, methods, parameters, and (optionally) types.
• Each REST method delegates to a Camel route, where you implement the business logic.

At runtime, Camel translates your REST definitions into regular consumer endpoints of an HTTP‑capable component (for example, netty-http, jetty, servlet, platform-http). The benefit is that your REST API is declared in one coherent place, rather than embedded inside a long list of from("netty-http:...") URIs.

The role of restConfiguration

Before you define REST endpoints, you configure the REST environment with restConfiguration():

• Choose the HTTP component (e.g., "netty-http", "jetty", "servlet", "platform-http").
• Set host and port (e.g., "localhost", 8080).
• Configure binding (how Camel marshals/unmarshals JSON/XML).
• Optionally enable inline routes.

Think of restConfiguration() as setting up the “web server” façade that Camel will use; the fluent REST DSL then hangs concrete endpoints off this façade.

Introducing the direct component as a REST “call target”

The direct component is a synchronous, in‑JVM entry point for routes. It behaves almost like a method call between routes:

• from("direct:x") defines a callable route.
• to("direct:x") invokes that route synchronously, on the same thread, without any transport overhead.

Why is this a good match for REST DSL?

• REST routes are just entry points; direct routes implement the logic behind them.
• The code reads like “for this HTTP operation, call this internal function.”
• You keep all HTTP concerns on the REST side and business concerns in direct routes.

This separation is particularly helpful in larger systems where you might later reuse direct routes from non‑HTTP entry points (e.g., timers, JMS, Kafka).

Inline routes: collapsing REST and direct into one route

By default, each REST method and each direct route exist as separate routes in Camel’s model:

• One route per REST method.
• One route per from("direct:...").

When you enable inline routes for REST, Camel can “inline” the direct route that a REST method calls, so that the REST and direct logic appear as a single route internally. The important characteristics are:

• The REST method is expected to call a unique direct endpoint (1:1 mapping).
• The inlined direct route is effectively fused into the REST route and no longer exists as an independent route in the model.
• This is primarily an operational convenience: fewer routes to manage, and a simpler topology when visualized or monitored.

As a design heuristic: use inline routes when each REST endpoint has its own dedicated direct route, used nowhere else. If a direct route is shared by multiple callers, you typically do not want it inlined.

What inline routes do not do: cascading direct: calls

A subtle but important detail: inline routes are only applied to the direct route that is directly linked from the REST definition.

Consider:

rest("/api").get("/a").to("direct:a");

from("direct:a")
    .to("direct:b");

from("direct:b")
    .to("log:result");

With inline routes enabled:

• The REST route and direct:a route are inlined into a single route.
• The direct:b route remains a separate, normal route.

Inline routes do not recursively flatten an entire graph of cascading direct: calls. They only merge the REST route with its immediate direct target under the 1:1 constraint. You can see this as “one level of inlining,” not a full program inliner.

From an architectural standpoint, this is reasonable: only the first hop (REST → direct) is known to be dedicated to that REST endpoint; deeper direct calls may be shared infrastructure or common logic.

Example

Below is a single‑class example that ties these concepts together:

• REST DSL defines GET /api/hello.
• REST routes to a direct: endpoint.
• inlineRoutes is enabled.

import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.main.Main;
import org.apache.camel.model.rest.RestBindingMode;

void main(String[] args) throws Exception {
    Main main = new Main();

    main.configure().addRoutesBuilder(new RouteBuilder() {
        @Override
        public void configure() throws Exception {

            // 1) Configure REST "server"
            restConfiguration()
                .component("netty-http")     // choose HTTP stack
                .host("localhost")
                .port(8082)
                .bindingMode(RestBindingMode.off)
                .inlineRoutes(true);         // inline REST → direct routes

            // 2) REST DSL: HTTP contract
            rest("/api")
                .get("/hello")
                    .to("direct:helloHandler");  // 1:1 mapping to direct

            // 3) Implementation route via direct
            from("direct:helloHandler")
                .setBody(constant("Hello from Camel REST DSL with inline direct route!"));
        }
    });

    // Add a startup listener to display info after Camel starts
    main.addMainListener(new org.apache.camel.main.MainListenerSupport() {
        @Override
        public void afterStart(org.apache.camel.main.BaseMainSupport main) {
            System.out.println("🚀 REST DSL Server started on http://localhost:8082");
            System.out.println("📡 Test the endpoint: curl http://localhost:8082/api/hello");
            System.out.println("⏹️  Press Ctrl+C to stop the server");
        }
    });

    // Run Camel (will run until Ctrl+C is pressed)
    main.run(args);
}

Rationale for each piece:

• restConfiguration() declares the HTTP environment and enables inline routes, so each REST endpoint plus its dedicated direct route is treated as one logical route.
• rest("/api").get("/hello") expresses the REST contract in domain terms (path + verb) without leaking HTTP component details all over the code.
• to("direct:helloHandler") clearly separates “transport” (REST) from “business logic” (the direct route). Think of it like calling a method in a service layer.
• from("direct:helloHandler") defines that service layer, and setBody(constant(...)) communicates that the response is fixed, not derived from input.

Design takeaways

A clean REST DSL + direct + inlineRoutes design gives you:

• A centralized, readable REST API declaration.
• A clear separation between HTTP entry points and internal processing.
• A simplified route graph when each REST method has its own dedicated direct route.

REST DSL defines the contract; direct routes implement it; inline routes keep the topology manageable when reuse is not required.

In Camel’s Java DSL, a variable is a key–value pair you associate with an Exchange, a route, or the CamelContext, and VariableReceive is a mode where incoming data is written into variables instead of the Message body and headers. Used properly, these give you a clean, explicit way to manage routing state without polluting payloads or headers.

1. Variables in Java DSL: the mental model

• A variable is a named value stored on the exchange, route, or CamelContext, accessed with setVariable / getVariable or via EIPs.
• Variables are separate from the message body, headers, and legacy exchange properties, and are meant as a scratchpad for state you need while routing.

Conceptually, you choose the scope based on who should see the value and how long it should live.

2. Local, global, and route‑scoped variables

2.1 Exchange‑local variable

Exchange‑local variables live only during processing of a single message.

from("direct:local-variable")
    .process(exchange -> {
        // store some state for this exchange only
        exchange.setVariable("customerId", 12345L);
    })
    .process(exchange -> {
        Long id = exchange.getVariable("customerId", Long.class);
        System.out.println("Customer ID = " + id);
    });

Rationale: use exchange‑local variables for state that is logically tied to a single message and must not leak to other messages or routes.

2.2 Global and route‑scoped variables

Global variables live in the CamelContext, and route‑scoped variables are global variables whose keys include a route identifier.

from("direct:global-and-route")
    .routeId("routeA")
    .process(exchange -> {
        CamelContext context = exchange.getContext();

        // global variable (shared across all routes)
        context.setVariable("globalFlag", true);

        // route‑scoped variable using naming convention
        exchange.setVariable("route:routeA:maxRetries", 3);
    })
    .process(exchange -> {
        CamelContext context = exchange.getContext();

        Boolean globalFlag =
            context.getVariable("globalFlag", Boolean.class);

        Integer maxRetries =
            context.getVariable("route:routeA:maxRetries", Integer.class);

        System.out.println("Global flag   = " + globalFlag);
        System.out.println("Max retries A = " + maxRetries);
    });

You can also read a global variable from an exchange with the global: prefix (if you prefer that naming scheme).

Rationale:

• Global variables are convenient for application‑level flags or configuration computed at runtime.
• Route‑scoped variables avoid collisions when different routes use similar semantic names like maxRetries or threshold.

3. SetVariable and SetVariables EIPs in Java DSL

While you can always call exchange.setVariable(...) directly, the Java DSL provides EIPs that make variable usage declarative in your routes.

3.1 Set a single variable

from("direct:set-variables")
    // single variable from constant
    .setVariable("status", constant("NEW"))

    // multiple variables in one go
    .setVariables(
        "randomNumber", simple("${random(1,100)}"),
        "copyOfBody",  body()
    )

    .process(exchange -> {
        String status  = exchange.getVariable("status", String.class);
        Integer random = exchange.getVariable("randomNumber", Integer.class);
        String bodyCopy = exchange.getVariable("copyOfBody", String.class);

        System.out.printf(
            "status=%s, random=%d, copy=%s%n",
            status, random, bodyCopy
        );
    });

Rationale: setVariable and setVariables keep the “I am defining variables here” logic inside the DSL layer rather than hiding it in processors, which makes routes easier to read and reason about.

4. VariableReceive on the consumer: fromV

VariableReceive changes how Camel receives data from endpoints: instead of populating the message body and headers, Camel stores them into variables.

Using it on the consumer side is done with fromV (the Java DSL variant of from):

fromV("direct:incoming", "originalBody")
    // body is empty here because the incoming payload is stored in variable "originalBody"
    .transform().simple("Processed: ${body}") // -> "Processed: "
    .process(exchange -> {
        String original =
            exchange.getVariable("originalBody", String.class);
        String processed =
            exchange.getMessage().getBody(String.class);

        System.out.println("Original  = " + original);
        System.out.println("Processed = " + processed);
    });

What actually happens:

• On entry, Camel stores the incoming body in variable originalBody.
• The Message body is empty (or at least not the original payload), so ${body} in the transform step produces "Processed: ".
• Later, you can inspect or restore the original payload by reading the originalBody variable.

Rationale: this is useful when you want to separate transport reception from business payload, or when you want to ensure that early processors do not accidentally work on the raw incoming data.

5. VariableReceive on the producer: toV

You can apply VariableReceive to producer calls with toV, which works like to but puts the response into a variable instead of overwriting the message body.

from("direct:call-service")
    // main message body is here
    .toV("http://localhost:8080/service", null, "serviceReply")
    .process(exchange -> {
        // original body is preserved
        String original =
            exchange.getMessage().getBody(String.class);

        // HTTP response body from the service
        String reply =
            exchange.getVariable("serviceReply", String.class);

        // response header X-Level as header variable
        String level =
            exchange.getVariable("header:serviceReply.X-Level", String.class);

        String combined = "Original=" + original
                        + ", Reply=" + reply
                        + ", Level=" + level;

        exchange.getMessage().setBody(combined);
        System.out.println(combined);
    });

Semantics:

• Request is sent to http://localhost:8080/service using the current body and headers.
• Response body is placed into variable serviceReply.
• Response headers are placed into variables named header:serviceReply.<HeaderName>.[camel.apache]
• The exchange’s message body and headers after toV remain what they were before the call.

Rationale: this is an enrichment‑style pattern where you treat external responses as additional data points rather than as replacements for your main message.

6. Comparing body, headers, properties, and variables

For Java DSL routes you’ll typically use all four concepts; the key is to pick the right tool for the job.

Rationale: using variables for scratchpad and enrichment state avoids overloading headers or properties with routing concerns, which improves maintainability and refactorability.

7. End‑to‑end Java program example (using only direct:)

This example shows how variables and toV/from can work together without any external HTTP endpoint. We simulate a “discount service” with another direct: route.

import org.apache.camel.CamelContext;
import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.impl.DefaultCamelContext;

void main() throws Exception {
    CamelContext context = new DefaultCamelContext();

    context.addRoutes(new RouteBuilder() {
        @Override
        public void configure() {

            // 1) "Discount service" route, purely in-memory
            from("direct:discountService")
                .process(exchange -> {
                    String originalOrder =
                        exchange.getMessage().getBody(String.class);

                    // pretend we call an external system and compute a discount
                    String discountReply =
                        "discount=10% for order " + originalOrder;

                    exchange.getMessage().setBody(discountReply);
                });

            // 2) Main route using variables + VariableReceive with toV
            from("direct:start")
                // keep the original order body in a variable
                .setVariable("originalOrder", body())

                // call the in-memory discountService; response -> variable "discountReply"
                .toV("direct:discountService", null, "discountReply")

                // build combined response
                .process(exchange -> {
                    String originalOrder =
                        exchange.getVariable("originalOrder", String.class);
                    String discountReply =
                        exchange.getVariable("discountReply", String.class);

                    String result = "Order=" + originalOrder
                                  + ", DiscountService=" + discountReply;

                    exchange.getMessage().setBody(result);
                    System.out.println(result);
                });
        }
    });

    context.start();

    // Send a test order into the main route
    context.createProducerTemplate()
           .sendBody("direct:start", "{\"id\":1,\"total\":100}");

    Thread.sleep(2000);
    context.stop();
}

• The direct:discountService route stands in for an external dependency but uses only the in‑memory direct component.
• toV("direct:discountService", null, "discountReply") still demonstrates VariableReceive on a producer: the reply body goes into the discountReply variable; the main message body is preserved.
• We then combine originalOrder and discountReply explicitly in the final processor, making the variable usage and flow of data very clear.