Rest DSL

Rest DSL

Available as of Camel 2.14

Apache Camel offers a REST styled DSL which can be used with Java or XML. The intention is to allow end users to define REST services using a REST style with verbs such as GET, POST, DELETE etc.

How it works

The Rest DSL is a facade that builds Rest endpoints as consumers for Camel routes. The actual REST transport is leveraged by using Camel REST components such as Restlet, Spark-rest, and others that has native REST integration.

Components supporting Rest DSL

The following Camel components supports the Rest DSL. See the bottom of this page for how to integrate a component with the Rest DSL.

• camel-coap
• camel-netty-http (also supports Swagger Java)
• camel-netty4-http (also supports Swagger Java)
• camel-jetty (also supports Swagger Java)
• camel-restlet (also supports Swagger Java)
• camel-servlet (also supports Swagger Java)
• camel-spark-rest (also supports Swagger Java from Camel 2.17)
• camel-undertow (also supports Swagger Java from Camel 2.17)

Rest DSL with Java

To use the Rest DSL in Java then just do as with regular Camel routes by extending the RouteBuilder and define the routes in the configure() method.

A simple REST service can be define as follows, where we use rest() to define the services as shown below:

    protected RouteBuilder createRouteBuilder() throws Exception {
        return new RouteBuilder() {
            @Override
            public void configure() throws Exception {
                rest("/say")
                    .get("/hello").to("direct:hello")
                    .get("/bye").consumes("application/json").to("direct:bye")
                    .post("/bye").to("mock:update");

                from("direct:hello")
                    .transform().constant("Hello World");
                from("direct:bye")
                    .transform().constant("Bye World");
            }
        };
    }

This defines a REST service with the following URL mappings:

Notice that in the REST service we route directly to a Camel endpoint using the to(). This is because the Rest DSL has a short-hand for routing directly to an endpoint using to(). An alternative is to embed a Camel route directly using route() - there is such an example further below.

Rest DSL with XML

The REST DSL supports the XML DSL also using either Spring or Blueprint. The example above can be define in XML as shown below:

  <camelContext xmlns="http://camel.apache.org/schema/spring">
    <rest path="/say">
      <get uri="/hello">
        <to uri="direct:hello"/>
      </get>
      <get uri="/bye" consumes="application/json">
        <to uri="direct:bye"/>
      </get>
      <post uri="/bye">
        <to uri="mock:update"/>
      </post>
    </rest>
    <route>
      <from uri="direct:hello"/>
      <transform>
        <constant>Hello World</constant>
      </transform>
    </route>
    <route>
      <from uri="direct:bye"/>
      <transform>
        <constant>Bye World</constant>
      </transform>
    </route>
  </camelContext>

Using Base Path

The REST DSL allows to define base path to make the DSL a bit more DRY. For example to define a customer path, we can set the base path in rest("/customer") and then provide the URI templates in the verbs, as shown below:

  rest("/customers/")
      .get("/{id}").to("direct:customerDetail")
      .get("/{id}/orders").to("direct:customerOrders")
      .post("/neworder").to("direct:customerNewOrder");

And using XML DSL it becomes:

    <rest path="/customers/">
      <get uri="/{id}">
        <to uri="direct:customerDetail"/>
      </get>
      <get uri="/{id}/orders">
        <to uri="direct:customerOrders"/>
      </get>
      <post uri="/neworder">
        <to uri="direct:customerNewOrder"/>
      </post>
    </rest>

It is not required to use both base path and URI templates. You can omit the bast path and define the base path and URI template in the verbs only. The example above can be defined as:

    <rest>
      <get uri="/customers/{id}">
        <to uri="direct:customerDetail"/>
      </get>
      <get uri="/customers/{id}/orders">
        <to uri="direct:customerOrders"/>
      </get>
      <post uri="/customers/neworder">
        <to uri="direct:customerNewOrder"/>
      </post>
    </rest>

Using Dynamic to()

Available as of Camel 2.16

The Rest DSL supports the new .toD or <toD> as dynamic to in the rest-dsl. For example to do a request/reply over JMS where the queue name is dynamic defined:

 public void configure() throws Exception {
   rest("/say")
     .get("/hello/{language}").toD("jms:queue:hello-${header.language}");
}

And in XML DSL

<rest uri="/say">
  <get uri="/hello//{language}">
    <toD uri="jms:queue:hello-${header.language}"/>
  </get>
<rest>

See more details at Message Endpoint about the dynamic to, and what syntax it supports. By default it uses the Simple language, but it has more power than so.

Embedding Camel Routes

Each of the rest service becomes a Camel route, so in the first example we have 2 x GET and 1 x POST REST service, which each become a Camel route. We also have two regular Camel routes. Therefore we have 3 + 2 = 5 routes in total. 

There are two route modes with the Rest DSL:

• mini using a singular to
• embedding a Camel route using route 

The first example is using the former with a singular to(). That's why we end up with 3 + 2 = 5 total routes.

The same example could use embedded Camel routes:

    protected RouteBuilder createRouteBuilder() throws Exception {
        return new RouteBuilder() {
            @Override
            public void configure() throws Exception {
                rest("/say/hello")
                    .get().route().transform().constant("Hello World");
                rest("/say/bye")
                    .get().consumes("application/json").route().transform().constant("Bye World").endRest()
                    .post().to("mock:update");
        };
    }

In the example above, we are embedding routes directly in the rest service using .route().

.get().route().routeId("myRestRoute").autoStartup(false).transform().constant("Hello World");

Managing Rest Services

Each of the rest service becomes a Camel route, so in the first example we have 2 x GET and 1 x POST REST service, which each become a Camel route. This makes it the same from Camel to manage and run these services - as they are just Camel routes. This means any tooling and API today that deals with Camel routes, also work with the REST services.

This means you can use JMX to stop/start routes, and also get the JMX metrics about the routes, such as number of message processed, and their performance statistics.

There is also a Rest Registry JMX MBean that contains a registry of all REST services which has been defined. 

Binding to POJOs Using

The Rest DSL supports automatic binding json/xml contents to/from POJOs using Camels Data Format. By default the binding mode is off, meaning there is no automatic binding happening for incoming and outgoing messages.

You may want to use binding if you develop POJOs that maps to your REST services request and response types. This allows you as a developer to work with the POJOs in Java code.

The binding modes are:

To use binding you must include the necessary data formats on the classpath, such as camel-jaxb and/or camel-jackson. And then enable the binding mode. You can configure the binding mode globally on the rest configuration, and then override per rest service as well.

To enable binding you configure this in Java DSL as shown below:

restConfiguration().component("restlet").host("localhost").port(portNum).bindingMode(RestBindingMode.auto);

And in XML DSL:

<restConfiguration bindingMode="auto" component="restlet" port="8080"/>

When binding is enabled Camel will bind the incoming and outgoing messages automatic, accordingly to the content type of the message. If the message is JSON, then JSON binding happens; and so if the message is XML then XML binding happens. The binding happens for incoming and reply messages. The table below summaries what binding occurs for incoming and reply messages. 

When using binding you must also configure what POJO type to map to. This is mandatory for incoming messages but optional for outgoing. 

For example, to map from xml/json to a POJO class UserPojo you do this in Java DSL as shown below:

// configure to use restlet on localhost with the given port
// and enable auto binding mode
restConfiguration().component("restlet").host("localhost").port(portNum).bindingMode(RestBindingMode.auto);

// use the rest DSL to define the rest services
rest("/users/")
    .post().type(UserPojo.class)
        .to("direct:newUser");

Notice we use type to define the incoming type. We can optionally define an outgoing type (which can be a good idea, to make it known from the DSL and also for tooling and JMX APIs to know both the incoming and outgoing types of the REST services.). To define the outgoing type we use outType as shown below:

// configure to use restlet on localhost with the given port
// and enable auto binding mode
restConfiguration().component("restlet").host("localhost").port(portNum).bindingMode(RestBindingMode.auto);

// use the rest DSL to define the rest services
rest("/users/")
    .post().type(UserPojo.class).outType(CountryPojo.class)
        .to("direct:newUser");

The UserPojo is just a plain POJO with getter/setter as shown:

public class UserPojo {
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

The UserPojo only supports JSON, as XML requires to use JAXB annotations, so we can add those annotations if we want to support XML also:

@XmlRootElement(name = "user")
@XmlAccessorType(XmlAccessType.FIELD)
public class UserPojo {
    @XmlAttribute
    private int id;
    @XmlAttribute
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

By having the JAXB annotations the POJO supports both JSON and XML bindings.

Configuring Rest DSL

The Rest DSL allows to configure the following options using a builder style

For example to configure to use the spark-rest component on port 9091, then we can do as follows:

restConfiguration().component("spark-rest").port(9091).componentProperty("foo", "123");

And with XML DSL

<restConfiguration component="spark-rest" port="9091"> 
  <componentProperty key="foo" value="123"/>
</restConfiguration>

You can configure properties on these levels. 

• component - Is used to set any options on the Component class. You can also configure these directly on the component.
• endpoint - Is used set any option on the endpoint level. Many of the Camel components has many options you can set on endpoint level.
• consumer - Is used to set any option on the consumer level. Some components has consumer options, which you can also configure from endpoint level by prefixing the option with "consumer." 
• data format - Is used to set any option on the data formats. For example to enable pretty print in the JSON data format.
• cors headers - If cors is enabled, then custom CORS headers can be set. See below for the default values which are in used. If a custom header is set then that value takes precedence over the default value.

You can set multiple options of the same level, so you can can for example configure 2 component options, and 3 endpoint options etc.

Enabling or Disabling Jackson JSON Features

Available as of Camel 2.15

When using JSON binding you may want to turn specific Jackson features on or off. For example to disable failing on unknown properties e.g., JSON input has a property which cannot be mapped to a POJO, then configure this using the dataFormatProperty as shown below:

restConfiguration().component("jetty").host("localhost").port(getPort()).bindingMode(RestBindingMode.json)
   .dataFormatProperty("json.in.disableFeatures", "FAIL_ON_UNKNOWN_PROPERTIES");

You can disable more features by separating the values using comma, such as:

   .dataFormatProperty("json.in.disableFeatures", "FAIL_ON_UNKNOWN_PROPERTIES,ADJUST_DATES_TO_CONTEXT_TIME_ZONE");

Likewise you can enable features using the enableFeatures such as:

restConfiguration().component("jetty").host("localhost").port(getPort()).bindingMode(RestBindingMode.json)
   .dataFormatProperty("json.in.disableFeatures", "FAIL_ON_UNKNOWN_PROPERTIES,ADJUST_DATES_TO_CONTEXT_TIME_ZONE")
   .dataFormatProperty("json.in.enableFeatures", "FAIL_ON_NUMBERS_FOR_ENUMS,USE_BIG_DECIMAL_FOR_FLOATS");

The values that can be used for enabling and disabling features on Jackson are the names of the enums from the following three Jackson classes

• com.fasterxml.jackson.databind.SerializationFeature
• com.fasterxml.jackson.databind.DeserializationFeature
• com.fasterxml.jackson.databind.MapperFeature

The rest configuration is of course also possible using XML DSL

<restConfiguration component="jetty" host="localhost" port="9090" bindingMode="json">
  <dataFormatProperty key="json.in.disableFeatures" value="FAIL_ON_UNKNOWN_PROPERTIES,ADJUST_DATES_TO_CONTEXT_TIME_ZONE"/>
  <dataFormatProperty key="json.in.enableFeatures" value="FAIL_ON_NUMBERS_FOR_ENUMS,USE_BIG_DECIMAL_FOR_FLOATS"/>
</restConfiguration>

Default CORS Headers

Available as of Camel 2.14.1

If CORS is enabled then the follow headers is in use by default. You can configure custom CORS headers which takes precedence over the default value.

Defining a Custom Error Message As-is

If you want to define custom error messages to be sent back to the client with a HTTP error code e.g., such as 400, 404 etc., then from Camel 2.14.1: you just set a header with the key Exchange.HTTP_RESPONSE_CODE to the error code (must be 300+) such as 404. And then the message body with any reply message, and optionally set the content-type header as well. There is a little example shown below:

restConfiguration().component("restlet").host("localhost").port(portNum).bindingMode(RestBindingMode.json);

// use the rest DSL to define the rest services
rest("/users/")
    .post("lives").type(UserPojo.class).outType(CountryPojo.class)
        .route()
            .choice()
                .when().simple("${body.id} < 100")
                    .bean(new UserErrorService(), "idToLowError")
                .otherwise()
                    .bean(new UserService(), "livesWhere");

In this example if the input id is a number that is below 100, we want to send back a custom error message, using the UserErrorService bean, which is implemented as shown:

public class UserErrorService {
    public void idToLowError(Exchange exchange) {
        exchange.getIn().setBody("id value is too low");
        exchange.getIn().setHeader(Exchange.CONTENT_TYPE, "text/plain");
        exchange.getIn().setHeader(Exchange.HTTP_RESPONSE_CODE, 400);
    }
}

In the UserErrorService bean we build our custom error message, and set the HTTP error code to 400. This is important, as that tells rest-dsl that this is a custom error message, and the message should not use the output POJO binding e.g., would otherwise bind to CountryPojo.

Returning a Custom Error Message for JsonParserException

From Camel 2.14.1: you return a custom message as-is (see previous section). So we can leverage this with Camel error handler to catch JsonParserException, handle that exception and build our custom response message. For example to return a HTTP error code 400 with a hard-coded message, we can do as shown below:

onException(JsonParseException.class)
    .handled(true)
    .setHeader(Exchange.HTTP_RESPONSE_CODE, constant(400))
    .setHeader(Exchange.CONTENT_TYPE, constant("text/plain"))
    .setBody().constant("Invalid json data");

Parameter Default Values

You can specify default values for parameters in the rest-dsl, such as the verbose parameter below:

  rest("/customers/")
      .get("/{id}").to("direct:customerDetail")
      .get("/{id}/orders")
        .param().name("verbose").type(RestParamType.query).defaultValue("false").description("Verbose order details").endParam()
          .to("direct:customerOrders")
      .post("/neworder").to("direct:customerNewOrder");

From Camel 2.17: the default value is automatically set as a header on the incoming Camel Message. So if the call the /customers/id/orders do not include a query parameter with key verbose then Camel will now include a header with key verbose=false because it was declared as the default value. This functionality is only applicable for query parameters.

Integrating a Camel Component with Rest DSL

Any Apache Camel component can integrate with the Rest DSL if they can be used as a REST service(e.g., as a REST consumer in Camel lingo. To integrate with the Rest DSL, then the component should implement the org.apache.camel.spi.RestConsumerFactory. The Rest DSL will then invoke the createConsumer method when it setup the Camel routes from the defined DSL. The component should then implement logic to create a Camel consumer that exposes the REST services based on the given parameters, such as path, verb, and other options. For example see the source code for camel-restlet, camel-spark-rest.

Swagger API

The Rest DSL supports Swagger Java by the camel-swagger-java module. See more details at  Swagger and the camel-swagger-java example from the Apache Camel distribution.

From Camel 2.16: you can define each parameter fine grained with details such as name, description, data type, parameter type and so on, using the <param>. For example to define the id path parameter you can do as shown below:

<!-- this is a rest GET to view an user by the given id -->
<get uri="/{id}" outType="org.apache.camel.example.rest.User">
  <description>Find user by id</description>
  <param name="id" type="path" description="The id of the user to get" dataType="int"/>
  <to uri="bean:userService?method=getUser(${header.id})"/>
</get>

And in Java DSL:

.get("/{id}").description("Find user by id").outType(User.class)
    .param().name("id").type(path).description("The id of the user to get").dataType("int").endParam()
    .to("bean:userService?method=getUser(${header.id})")

The body parameter type requires to use body as well for the name. For example a REST PUT operation to create/update an user could be done as:

<!-- this is a rest PUT to create/update an user -->
<put type="org.apache.camel.example.rest.User">
  <description>Updates or create a user</description>
  <param name="body" type="body" description="The user to update or create"/>
  <to uri="bean:userService?method=updateUser"/>
</put>

And in Java DSL:

.put().description("Updates or create a user").type(User.class)
    .param().name("body").type(body).description("The user to update or create").endParam()
    .to("bean:userService?method=updateUser")

For an example see the examples/camel-example-servlet-rest-tomcat of the Apache Camel distribution.

See Also

• DSL
• Rest
• Swagger Java
• Spark-rest
• How do I import rests from other XML files