Kotlin API

Since Camel 4.4

This API is experimental support level and is not recommended being used for production

Kotlin API provides alternative approach to define routes.

Defining a route

To define route using Kotlin API you need to retrieve CamelContext and pass it to function camel:

import org.apache.camel.kotlin.camel

val ctx: CamelContext = ...

camel(ctx) { ... }

Then you can define route in route block:

camel(ctx) {
    route { (1)
        from { (2)
            component("direct")
            url("input")
        }
        steps { (3)
            to {
                component("mock")
                url("output")
            }
        }
    }
}
1 Definition of route
2 Definition of consuming endpoint
3 Definition of processing steps

You can find a number of handful methods in route block, for example, setting route id:

camel(ctx) {
    route {
        id("my-route")
    }
}

Defining endpoints

Here and further camel(ctx) block will be omitted to make code less annoying.

Raw endpoints

Raw endpoint constructs from three components: component, url and a number of property-es. They all connected with each other and form resulting uri by the following rule:

"$component:$url?${property1.key}=${property1.value}&..."

So to define consumer from netty-http you can write the following:

route {
    from {
        component("netty-http")
        url("http://localhost:8080")
        property("keepAlive", "false")
        property("reuseAddress", "false")
    }
}

Also you can omit component and write only url by the following schema:

route {
    from {
        url("netty-http:http://localhost:8080")
        property("keepAlive", "false")
        property("reuseAddress", "false")
    }
}

Moreover, you can omit property at all and write full uri in url function:

route {
    from {
        url("netty-http:http://localhost:8080?keepAlive=false&reuseAddress=false")
    }
}

This three definitions are equivalent.

property method accepts only String value type. This is because it builds raw uri, not Endpoint. That means that if you need to define property of complex type, you must define bean in registry. Also that behaviour may be very useful in the following situations:

  • usage of property placeholders

  • toD and enrich EIPs accepts simple language in uri, so it will be handful to use simple in property values

Endpoint DSL

Defining string-based uris may not be very handy. So there is Endpoint DSL for building uris. For each Camel component exists an extension function with the name of that component. An example:

import org.apache.camel.kotlin.components.`netty-http`
import org.apache.camel.kotlin.components.mock

route {
    from {
        `netty-http` {
            protocol("http")
            host("localhost")
            port(8080)

            keepAlive(false)
            reuseAddress(false)
        }
    }
    steps {
        to { mock { name("output") } }
    }
}

Rules remain the same: all non-primitive types must be defined as beans in registry and referenced in properties by #name.

Defining EIPs

In that section we will take a look at several important EIPs to demonstrate logic of their definition.

Marshal, Unmarshal and DataFormat DSL

Marshal and Unmarshal EIPs come with DataFormat DSL. This DSL is the only way to define both EIPs. Example:

import org.apache.camel.kotlin.dataformats.csv

route {
    from { direct { name("input") } }
    steps {
        unmarshal {
            csv {
                delimiter(";")
            }
        }
    }
}

LoadBalance and nested DSLs

Some of EIPs provide additional complex configuration for their fields. For example, Load Balance EIP: there we can define various variants of which algorithm to use. So all that options are wrapped into their own DSL. Example:

route {
    from { direct { name("input") } }
    steps {
        loadBalance {
            failover {
                maximumFailoverAttempts(1)
            }
        }
    }
}

Filter, Multicast, Pipeline and outputs

Some of EIPs defines their own subroutes, for example, Filter and Multicast. In that cases use outputs property of EIP’s block. Filter example:

route {
    from { direct { name("input") } }
    steps {
        filter(constant("true")) {
            outputs {
                log("only calls in filter block")
            }
        }
        log("calls after filter block executed")
    }
}

Multicast example:

route {
    from { direct { name("input") } }
    steps {
        multicast {
            outputs {
                to { direct { name("first") } }
                to { direct { name("second") } }
            }
        }
    }
}

That behaviour differs for Pipeline EIP, which has not any properties and so all nested steps defines in pipeline block:

route {
    from { direct { name("input") } }
    steps {
        pipeline {
            log("first pipeline")
        }
        pipeline {
            log("second pipeline")
        }
    }
}

Defining beans

Direct object binding

You can just provide instance of any type for Camel context:

val map = mapOf<String, String>()

camel(ctx) {
    bean("myMap", map)
}

Or use supplier-function:

camel(ctx) {
    bean("myMap") {
        mapOf<String, String>()
    }
}

Or construct bean using builder:

camel(ctx) {
    bean<MyClass>("myBean") {
        myField = "value"
    }
}

Runtime object binding

Use other way is to use declarative approach of defining beans, without referencing beans at compile-time:

class Example {
    lateinit var map: MutableMap<String, String>
}

camel(ctx) {
    bean("map", mutableMapOf(Pair("key", "value")))
    bean {
        name("test")
        type("org.apache.camel.kotlin.Example")
        property("map", "#map")
    }
}

When you cross-reference beans like in example (bean test reference map) be sure you declare beans in the dependency order: first dependencies, last dependents.

Using languages, expressions and predicates

There are a number of functions that provides Camel languages like constant or simple and a number of helper functions for building predicates/expressions like body or header. All of them are in the package org.apache.camel.kotlin.languages.

There are two useful extension functions:

  • Expression.toPredicate(): Predicate converts any expression to predicate type

  • Expression.which(): ValueBuilder converts any expression to ValueBuiler, makes possible to write expressions like:

    body().which().isInstanceOf(String::class)

Fallback to Java API

For camel block there is routeBuilder field which can help to define any Camel entity using Java API. For each other block there is def field present. It can be used if some functionality is missing. Example:

route {
    from { direct { name("input") } }
    steps {
        def.pipeline().log("Java API").end()
    }
}
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