Property binding in Camel

Camel supports binding property values (key=value) in many places such as configuration of Camel components, endpoints, EIPs, routes, and Camel bootstrap configuration.

Together with property placeholders, property placeholder functions, then there is plenty of power, but also something that takes a little learning to master.

Property binding features

The core of Camels property binding is implemented in PropertyBindingSupport.java which is used internally in Camel, and as well can be used by Camel component developers.

The PropertyBindingSupport class supports binding String valued properties to an instance which uses a set of conventions:

  • property placeholders - Keys and values using Camels property placeholder will be resolved.

  • nested - Properties can be nested using the dot syntax (OGNL and builder pattern using with as prefix), eg foo.bar=123.

  • map - Properties can lookup in Map’s using map syntax, eg foo[bar] where foo is the name of the property that is a Map instance, and bar is the name of the key.

  • list - Properties can refer or add to in List’s using list syntax, eg foo[0] where foo is the name of the property that is a List instance, and 0 is the index. To refer to the last element, then use last as key.

  • reference by property placeholder id - Values can refer to a property placeholder key with #property:myKey

  • reference by bean id - Values can refer to other beans in the registry by prefixing with # or #bean: eg #myBean or #bean:myBean. It is recommended to favour using #bean: syntax to make it obvious it’s a bean reference.

  • reference by type - Values can refer to singleton beans by their type in the registry by prefixing with #type: syntax, eg #type:com.foo.MyClassType.

  • autowire by type - Values can refer to singleton beans by auto wiring by setting the value to #autowired.

  • reference new class - Values can refer to creating new beans by their class name by prefixing with #class, eg #class:com.foo.MyClassType. The class is created using a default no-arg constructor, however if you need to create the instance via a factory method then you specify the method as shown: #class:com.foo.MyClassType#myFactoryMethod. And if the factory method requires parameters they can be specified as follows: #class:com.foo.MyClassType#myFactoryMethod('Hello World', 5, true). Or if you need to create the instance via constructor parameters then you can specify the parameters as shown: #class:com.foo.MyClass('Hello World', 5, true).

  • ignore case - Whether to ignore case for property keys (will ignore by default)

Property binding basics

Do not get overwhelmed by the set of features and what they really do.

At the basics the property binding are used for setting values on Java objects from string values (key=value).

For example to set brokers on the Kafka component you can do:

camel.component.kafka.brokers = mykafka1,mykafka2

This will essentially be equivavlent to configuring Kafka component in regular Java code via setters:

KafkaComponent kafka = ...
kafka.setBrokers("mykafka1,mykafka2");
For configuring Camel components in Java code, there is also Component DSL.

The configuration of Camel components, endpoints, routes etc can often require more flexibility and therefore the property binding has many features to bind by looking up existing objects by id, or anonymously by their type, and as well to walk down an object graph to bind nested parameters.

Using PropertyBindingSupport in Java

Although PropertyBindingSupport is not primary intended for end users to use, but nevertheless its possible to use, and also you may get a better understanding of this feature by seeing how this class is used with pure Java.

Suppose we have the following two POJOs Foo.java and Bar.java:

public class Foo {
    private String name;
    private Bar bar = new Bar();

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Bar getBar() {
        return bar;
    }

    public void setBar(Bar bar) {
        this.bar = bar;
    }
}

public class Bar {
    private int age;
    private boolean rider;

    public int getAge() {
        return age;
    }

    public boolean isRider() {
        return rider;
    }

    // this has no setter but only builders and mix the builders with both styles

    public Bar withAge(int age) {
        this.age = age;
        return this;
    }

    public Bar withRider(boolean rider) {
        this.rider = rider;
        return this;
    }
}

Then we can use PropertyBindingSupport to bind properties to these POJOs:

Foo foo = new Foo();

Map<String, Object> prop = new HashMap<>();
prop.put("name", "James");
prop.put("bar.age", "33");
prop.put("bar.rider", "true");

PropertyBindingSupport.bindProperties(context, foo, prop);

This will then set the POJOs to have the following values:

Foo.name = James
Foo.Bar.age = 33
Foo.Bar.rider = true

Instead of providing a map with all the parameters then a single parameter can also be set using builder style as shown:

Foo foo = new Foo();

PropertyBindingSupport.build().bind(context, foo, "name", "James");
PropertyBindingSupport.build().bind(context, foo, "bar.age", "33");
PropertyBindingSupport.build().bind(context, foo, "bar.rider", "true");

Which is more common to do as follows:

Foo foo = new Foo();

PropertyBindingSupport.build().withCamelContext(context).withTarget(foo)
    .withProperty("name", "James");
    .withProperty("bar.age", "33");
    .withProperty("bar.rider", "true")
    .bind();

In the example above then we are setting nested values on foo via bar.age and bar.rider. This is possible because Foo class has a getBar method that returns the Bar instance to use:

    private Bar bar = new Bar();

    public Bar getBar() {
        return bar;
    }

It’s a common practice for POJO classes to not create nested instances, but instead on demand. So suppose private Bar bar = new Bar(); was not present in the Foo class. In this situation then Camel will automatic create a new instance of Bar using its default no-arg constructor. For more advanced use-cases then you can specify how the Bar instance should be created, such as via a factory method, or pass in constructor parameters.

For example suppose Bar has a constructor parameter that accepts a boolean, we can pass that information via #class: as shown:

PropertyBindingSupport.build().withCamelContext(context).withTarget(foo)
    .withProperty("name", "James");
    .withProperty("bar", "#class:com.mycompany.Bar(true)")
    .withProperty("bar.age", "33");
    .withProperty("bar.rider", "true")
    .bind();

More details

Property binding is noteably used when running Camel in standalone mode with Camel Main, or using Camel Spring Boot, Camel K, Camel Kafka Connector, or Camel Quarkus. All these runtimes have a similar way of configuring via property bindings such as from application.properties files.

See more at Camel Main

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