docs/components/modules/ROOT/pages/properties-component.adoc

[[properties-component]]
= Properties Component
//THIS FILE IS COPIED: EDIT THE SOURCE FILE:
:page-source: core/camel-base/src/main/docs/properties-component.adoc
//Written by hand, not generated.
:docTitle: Properties
:artifactId: camel-base
:description: The properties component is used for property placeholders in your Camel application, such as endpoint URIs.
:since: 2.3
:supportLevel: Stable

*Since Camel {since}*

The properties component is used for property placeholders in your Camel application, such as endpoint URIs.
It is *not* a regular Camel component with producer and consumer for routing messages. However for historical
reasons it was named `PropertiesComponent` and this name is commonly known and therfore we keep using it.

== Spring Boot Auto-Configuration

The component supports 10 options, which are listed below.



[width="100%",cols="2,5,^1,2",options="header"]
|===
| Name | Description | Default | Type
| *camel.component.properties.auto-discover-properties-sources* | Whether to automatically discovery instances of PropertiesSource from registry and service factory. | true | Boolean
| *camel.component.properties.default-fallback-enabled* | If false, the component does not attempt to find a default for the key by looking after the colon separator. | true | Boolean
| *camel.component.properties.encoding* | Encoding to use when loading properties file from the file system or classpath. If no encoding has been set, then the properties files is loaded using ISO-8859-1 encoding (latin-1) as documented by java.util.Properties#load(java.io.InputStream) |  | String
| *camel.component.properties.environment-variable-mode* | Sets the OS environment variables mode (0 = never, 1 = fallback, 2 = override). The default mode (override) is to use OS environment variables if present, and override any existing properties. OS environment variable mode is checked before JVM system property mode | 2 | Integer
| *camel.component.properties.ignore-missing-location* | Whether to silently ignore if a location cannot be located, such as a properties file not found. | false | Boolean
| *camel.component.properties.initial-properties* | Sets initial properties which will be used before any locations are resolved. The option is a java.util.Properties type. |  | String
| *camel.component.properties.location* | A list of locations to load properties. You can use comma to separate multiple locations. This option will override any default locations and only use the locations from this option. |  | String
| *camel.component.properties.override-properties* | Sets a special list of override properties that take precedence and will use first, if a property exist. The option is a java.util.Properties type. |  | String
| *camel.component.properties.properties-parser* | To use a custom PropertiesParser. The option is a org.apache.camel.component.properties.PropertiesParser type. |  | String
| *camel.component.properties.system-properties-mode* | Sets the JVM system property mode (0 = never, 1 = fallback, 2 = override). The default mode (override) is to use system properties if present, and override any existing properties. OS environment variable mode is checked before JVM system property mode | 2 | Integer
|===

[TIP]
**Resolving property from Java code** +
You can use the method `resolvePropertyPlaceholders` on the
`CamelContext` to resolve a property from any Java code.

== Using PropertyPlaceholder

Camel now provides a new `PropertiesComponent` in *camel-core* which
allows you to use property placeholders when defining Camel
Endpoint URIs.

This works much like you would do if using Spring's
`<property-placeholder>` tag. However Spring have a limitation which
prevents 3rd party frameworks to leverage Spring property placeholders
to the fullest. See more at
xref:manual:faq:how-do-i-use-spring-property-placeholder-with-camel-xml.adoc[How do
I use Spring Property Placeholder with Camel XML].

[TIP]
**Bridging Spring and Camel property placeholders** +
You can bridge the Spring property placeholder
with Camel, see further below for more details.

The property placeholder is generally in use when doing:

* lookup or creating endpoints
* lookup of beans in the Registry
* additional supported in Spring XML (see below in examples)
* using Blueprint PropertyPlaceholder with Camel
xref:properties-component.adoc[Properties] component
* using `@PropertyInject` to inject a property in a POJO
* Using default value if a property does not exists
* Include out of the box functions, to lookup property
values from OS environment variables, JVM system properties, or the
service idiom.
* Using custom functions, which can be plugged into the
property component.

== Syntax

The syntax to use Camel's property placeholder is to use `{\{key}}` for
example `{{file.uri}}` where `file.uri` is the property key.

You can use property placeholders in parts of the endpoint URI's which
for example you can use placeholders for parameters in the URIs.

You can specify a default value to use if
a property with the key does not exists, eg `file.url:/some/path` where
the default value is the text after the colon (eg /some/path).

[NOTE]
====
Do not use colon in the property key. The colon is used as a separator
token when you are providing a default value.
====

== Defining location

The properties component needs to know a location(s) where to resolve the
properties. You can define 1 to many locations. If you define the
location in a single String property you can separate multiple locations
with comma such as:

[source,java]
----
pc.setLocation("com/mycompany/myprop.properties,com/mycompany/other.properties");
----

You can set which location can be discarded if missing by by setting the ``optional`` attribute, which is false by default, i.e:

[source,java]
----
pc.setLocations(
    "com/mycompany/override.properties;optional=true"
    "com/mycompany/defaults.properties");
----

== Using system and environment variables in locations

The location now supports using placeholders for JVM system properties
and OS environments variables.

For example:

[source]
----
location=file:${karaf.home}/etc/foo.properties
----

In the location above we defined a location using the file scheme using
the JVM system property with key `karaf.home`.

To use an OS environment variable instead you would have to prefix with
env:

[source]
----
location=file:${env:APP_HOME}/etc/foo.properties
----

Where `APP_HOME` is an OS environment.

[NOTE]
====
Some OS'es (such as Linux) do not support dashes in environment variable names,
so here we are using `APP_HOME`. But if you specify `APP-HOME` then Camel 3 will automatic lookup
the value as `APP_HOME` (with underscore) as fallback.
====

You can have multiple placeholders in the same location, such as:

[source]
----
location=file:${env:APP_HOME}/etc/${prop.name}.properties
----

== Configuring in Java DSL

You have to create and register the `PropertiesComponent` under the name
`properties` such as:

[source,java]
----
PropertiesComponent pc = camelContext.getPropertiesComponent();
pc.setLocation("classpath:com/mycompany/myprop.properties");
----

== Configuring in Spring XML

Spring XML offers two variations to configure. You can define a spring
bean as a `PropertiesComponent` which resembles the way done in Java
DSL. Or you can use the `<propertyPlaceholder>` tag.

[source,xml]
----
<bean id="properties" class="org.apache.camel.component.properties.PropertiesComponent">
    <property name="location" value="classpath:com/mycompany/myprop.properties"/>
</bean>
----

Using the `<propertyPlaceholder>` tag makes the configuration a bit more
fresh such as:

[source,xml]
----
<camelContext ...>
   <propertyPlaceholder id="properties" location="com/mycompany/myprop.properties"/>
</camelContext>
----

Setting the properties location through the location tag works just fine but sometime you have a number of resources to take into account and starting from *Camel 2.19.0* you can set the properties location with a dedicated propertiesLocation:

[source,xml]
----
<camelContext ...>
  <propertyPlaceholder id="myPropertyPlaceholder">
    <propertiesLocation
      resolver = "classpath"
      path     = "com/my/company/something/my-properties-1.properties"
      optional = "false"/>
    <propertiesLocation
      resolver = "classpath"
      path     = "com/my/company/something/my-properties-2.properties"
      optional = "false"/>
    <propertiesLocation
      resolver = "file"
      path     = "${karaf.home}/etc/my-override.properties"
      optional = "true"/>
   </propertyPlaceholder>
</camelContext>
----

[TIP]
**Specifying the cache option inside XML** +
Camel supports specifying a value for the cache option both
inside the Spring as well as the Blueprint XML.

== Using a Properties from the Registry

For example in OSGi you may want to expose a service which returns the
properties as a `java.util.Properties` object.

Then you could setup the xref:properties-component.adoc[Properties] component as
follows:

[source,xml]
----
 <propertyPlaceholder id="properties" location="ref:myProperties"/>
----

Where `myProperties` is the id to use for lookup in the OSGi registry.
Notice we use the `ref:` prefix to tell Camel that it should lookup the
properties for the Registry.

== Examples using properties component

When using property placeholders in the endpoint URIs you can either use
the `properties:` component or define the placeholders directly in the
URI. We will show example of both cases, starting with the former.

[source,java]
----
// properties
cool.end=mock:result

// route
from("direct:start").to("{{cool.end}}");
----

You can also use placeholders as a part of the endpoint uri:

[source,java]
----
// properties
cool.foo=result

// route
from("direct:start").to("mock:{{cool.foo}}");
----

In the example above the to endpoint will be resolved to `mock:result`.

You can also have properties with refer to each other such as:

[source,java]
----
// properties
cool.foo=result
cool.concat=mock:{{cool.foo}}

// route
from("direct:start").to("mock:{{cool.concat}}");
----

Notice how `cool.concat` refer to another property.

And you can use placeholders several times:

[source,java]
----
// properties
cool.start=direct:start
cool.showid=true
cool.result=result

// route
from("{{cool.start}}")
    .to("log:{{cool.start}}?showBodyType=false&showExchangeId={{cool.showid}}")
    .to("mock:{{cool.result}}");
----

You can also your property placeholders when using
ProducerTemplate for example:

[source,java]
----
template.sendBody("{{cool.start}}", "Hello World");
----

== Example with xref:languages:simple-language.adoc[Simple] language

The xref:languages:simple-language.adoc[Simple] language now also support using property
placeholders, for example in the route below:

[source,java]
----
// properties
cheese.quote=Camel rocks

// route
from("direct:start")
    .transform().simple("Hi ${body} do you think ${properties:cheese.quote}?");
----

== Additional property placeholder supported in Spring XML

The property placeholders is also supported in many of the Camel Spring
XML tags such as
`<package>, <packageScan>, <contextScan>, <jmxAgent>, <endpoint>, <routeBuilder>, <proxy>`
and the others.

The example below has property placeholder in the `<jmxAgent>` tag:

[source,xml]
----
<camelContext xmlns="http://camel.apache.org/schema/spring">
    <propertyPlaceholder id="properties" location="org/apache/camel/spring/jmx.properties"/>
    <!-- we can use property placeholders when we define the JMX agent -->
    <jmxAgent id="agent" disabled="{{myjmx.disabled}}"
              usePlatformMBeanServer="{{myjmx.usePlatform}}"
              statisticsLevel="RoutesOnly" useHostIPAddress="true"/>
    <route id="foo" autoStartup="false">
        <from uri="seda:start"/>
        <to uri="mock:result"/>
    </route>
</camelContext>
----

You can also define property placeholders in the various attributes on
the `<camelContext>` tag such as `trace` as shown here:

[source,xml]
----
<camelContext trace="{{foo.trace}}" xmlns="http://camel.apache.org/schema/spring">
    <propertyPlaceholder id="properties" location="org/apache/camel/spring/processor/myprop.properties"/>
    <template id="camelTemplate" defaultEndpoint="{{foo.cool}}"/>
    <route>
        <from uri="direct:start"/>
        <setHeader name="{{foo.header}}">
            <simple>${in.body} World!</simple>
        </setHeader>
        <to uri="mock:result"/>
    </route>
</camelContext>
----

== Using JVM system properties or Environment variables as override or fallback values

The properties components supports using JVM system properties and also OS environment variables
as values which can either be used as override or fallback values.

The default mode is that both of them are in override mode, and they are check in the following order:

1. OS environment variable (override mode)
2. JVM system property  (override mode)
3. Property files and other locations
4. OS environment variable (fallback mode)
5. JVM system property  (fallback mode)

The check stops at first found property value for the key.

You can control these modes using the `systemPropertiesMode` and `environmentVariableMode`
options on the properties component.

== Using property placeholders for any kind of attribute in the XML DSL

In the example below we use the `prop` prefix for the namespace
camel.apache.org/schema/placeholder by which we can use the
`prop` prefix in the attributes in the XML DSLs. Notice how we use that
in the Multicast to indicate that the option
`stopOnException` should be the value of the placeholder with the key
"stop".

In our properties file we have the value defined as

[source]
----
stop=true
----

== Using Blueprint property placeholder with Camel routes

Camel supports Blueprint
which also offers a property placeholder service. Camel supports
convention over configuration, so all you have to do is to define the
OSGi Blueprint property placeholder in the XML file as shown below:

[source,xml]
----
<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:cm="http://aries.apache.org/blueprint/xmlns/blueprint-cm/v1.0.0"
           xsi:schemaLocation="
           http://www.osgi.org/xmlns/blueprint/v1.0.0 https://www.osgi.org/xmlns/blueprint/v1.0.0/blueprint.xsd">

    <!-- OSGI blueprint property placeholder -->
    <cm:property-placeholder id="myblueprint.placeholder" persistent-id="camel.blueprint">
        <!-- list some properties as needed -->
        <cm:default-properties>
            <cm:property name="result" value="mock:result"/>
        </cm:default-properties>
    </cm:property-placeholder>

    <camelContext xmlns="http://camel.apache.org/schema/blueprint">
        <!-- in the route we can use {{ }} placeholders which will lookup in blueprint
             as Camel will auto detect the OSGi blueprint property placeholder and use it -->
        <route>
            <from uri="direct:start"/>
            <to uri="mock:foo"/>
            <to uri="{{result}}"/>
        </route>
    </camelContext>
</blueprint>
----

=== Using OSGi blueprint property placeholders in Camel routes

By default Camel detects and uses OSGi blueprint property placeholder
service. You can disable this by setting the attribute
`useBlueprintPropertyResolver` to false on the `<camelContext>`
definition.

=== About placeholder syntax

Notice how we can use the Camel syntax for placeholders `{{` and `}}` in the
Camel route, which will lookup the value from OSGi blueprint.

The blueprint syntax for placeholders is `${ }`. So outside the
`<camelContext>` you must use the `${ }` syntax. Where as inside
`<camelContext>` you must use `{{` and `}}` syntax.

OSGi blueprint allows you to configure the syntax, so you can actually
align those if you want.

You can also explicit refer to a specific OSGi blueprint property
placeholder by its id. For that you need to use the Camel's
`<propertyPlaceholder>` as shown in the example below:

[source,xml]
----
<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:cm="http://aries.apache.org/blueprint/xmlns/blueprint-cm/v1.0.0"
           xsi:schemaLocation="
           http://www.osgi.org/xmlns/blueprint/v1.0.0 https://www.osgi.org/xmlns/blueprint/v1.0.0/blueprint.xsd">

    <!-- OSGI blueprint property placeholder -->
    <cm:property-placeholder id="myblueprint.placeholder" persistent-id="camel.blueprint">
        <!-- list some properties as needed -->
        <cm:default-properties>
            <cm:property name="prefix.result" value="mock:result"/>
        </cm:default-properties>
    </cm:property-placeholder>

    <camelContext xmlns="http://camel.apache.org/schema/blueprint">
        <!-- using Camel properties component and refer to the blueprint property placeholder by its id -->
        <propertyPlaceholder id="properties" location="blueprint:myblueprint.placeholder"/>

        <!-- in the route we can use {{ }} placeholders which will lookup in blueprint -->
        <route>
            <from uri="direct:start"/>
            <to uri="mock:foo"/>
            <to uri="{{prefix.result}}"/>
        </route>
    </camelContext>
</blueprint>
----


== Explicit referring to a OSGi blueprint placeholder in Camel

Notice how we use the `blueprint` scheme to refer to the OSGi blueprint
placeholder by its id. This allows you to mix and match, for example you
can also have additional schemes in the location. For example to load a
file from the classpath you can do:

[source]
----
location="blueprint:myblueprint.placeholder,classpath:myproperties.properties"
----

Each location is separated by comma.

== Overriding Blueprint property placeholders outside CamelContext

When using Blueprint property placeholder in the Blueprint XML file, you
can declare the properties directly in the XML file as shown below:

[source,xml]
----
<!-- blueprint property placeholders -->
<cm:property-placeholder persistent-id="my-placeholders" update-strategy="reload">
  <cm:default-properties>
    <cm:property name="greeting" value="Hello"/>
    <cm:property name="destination" value="mock:result"/>
  </cm:default-properties>
</cm:property-placeholder>

<!-- a bean that uses a blueprint property placeholder -->
<bean id="myCoolBean" class="org.apache.camel.test.blueprint.MyCoolBean">
  <property name="say" value="${greeting}"/>
</bean>

<camelContext xmlns="http://camel.apache.org/schema/blueprint">

  <route>
    <from uri="direct:start"/>
    <bean ref="myCoolBean" method="saySomething"/>
    <to uri="{{destination}}"/>
  </route>

</camelContext>
----

Notice that we have a `<bean>` which refers to one of the properties. And
in the Camel route we refer to the other using the `{{` and `}}` notation.

Now if you want to override these Blueprint properties from an unit
test, you can do this as shown below:

[source,java]
----
protected String useOverridePropertiesWithConfigAdmin(Dictionary props) {
    // add the properties we want to override
    props.put("greeting", "Bye");

    // return the PID of the config-admin we are using in the blueprint xml file
    return "my-placeholders";
}
----

To do this we override and implement the
`useOverridePropertiesWithConfigAdmin` method. We can then put the
properties we want to override on the given props parameter. And the
return value *must* be the `persistence-id` of the
`<cm:property-placeholder>` tag, which you define in the blueprint XML
file.

== Using .cfg or .properties file for Blueprint property placeholders

When using Blueprint property placeholder in the Blueprint XML file, you
can declare the properties in a `.properties` or `.cfg` file. If you use
Apache ServieMix / Karaf then this container has a convention that it
loads the properties from a file in the etc directory with the naming
`etc/pid.cfg`, where `pid` is the `persistence-id`.

For example in the blueprint XML file we have the
`persistence-id="stuff"`, which mean it will load the configuration file
as `etc/stuff.cfg`.

Now if you want to unit test this blueprint XML file, then you can
override the `loadConfigAdminConfigurationFile` and tell Camel which
file to load as shown below:

[source,java]
----
@Override
protected String[] loadConfigAdminConfigurationFile() {
    // String[0] = tell Camel the path of the .cfg file to use for OSGi ConfigAdmin in the blueprint XML file
    // String[1] = tell Camel the persistence-id of the cm:property-placeholder in the blueprint XML file
    return new String[]{"src/test/resources/etc/stuff.cfg", "stuff"};
}
----

Notice that this method requires to return a `String[]` with 2 values. The
1st value is the path for the configuration file to load.
The 2nd value is the `persistence-id` of the `<cm:property-placeholder>`
tag.

The `stuff.cfg` file is just a plain properties file with the property
placeholders such as:

[source]
----
== this is a comment
greeting=Bye
----

== Using .cfg file and overriding properties for Blueprint property placeholders

You can do both as well. Here is a complete example. First we have the
Blueprint XML file:

And in the unit test class we do as follows:

And the `etc/stuff.cfg` configuration file contains

[source]
----
greeting=Bye
echo=Yay
destination=mock:result
----

== Bridging Spring and Camel property placeholders

The Spring Framework does not allow 3rd party frameworks such as Apache
Camel to seamless hook into the Spring property placeholder mechanism.
However you can easily bridge Spring and Camel by declaring a Spring
bean with the type
`org.apache.camel.spring.spi.BridgePropertyPlaceholderConfigurer`, which
is a Spring
`org.springframework.beans.factory.config.PropertyPlaceholderConfigurer`
type.

To bridge Spring and Camel you must define a single bean as shown below:

[source,xml]
----
include::{examplesdir}/components/camel-spring/src/test/resources/org/apache/camel/components/properties/CamelSpringPropertyPlaceholderConfigurerTest.xml[tags=e1]
----

You *must not* use the spring `<context:property-placeholder>` namespace
at the same time; this is not possible.

After declaring this bean, you can define property placeholders using
both the Spring style, and the Camel style within the `<camelContext>` tag
as shown below:

[source,xml]
----
include::{examplesdir}/components/camel-spring/src/test/resources/org/apache/camel/components/properties/CamelSpringPropertyPlaceholderConfigurerTest.xml[tags=e2]
----

Notice how the hello bean is using pure Spring property placeholders
using the `${ }` notation. And in the Camel routes we use the Camel
placeholder notation with `{{` and `}}`.

== Clashing Spring property placeholders with Camels Simple language

Take notice when using Spring bridging placeholder then the spring `${ }`
syntax clashes with the xref:languages:simple-language.adoc[Simple] in Camel, and therefore
take care. For example:

[source,xml]
----
<setHeader name="Exchange.FILE_NAME">
  <simple>{{file.rootdir}}/${in.header.CamelFileName}</simple>
</setHeader>
----

clashes with Spring property placeholders, and you should use `$simple{ }`
to indicate using the xref:languages:simple-language.adoc[Simple] language in Camel.

[source,xml]
----
<setHeader name="Exchange.FILE_NAME">
  <simple>{{file.rootdir}}/$simple{in.header.CamelFileName}</simple>
</setHeader>
----

An alternative is to configure the `PropertyPlaceholderConfigurer` with
`ignoreUnresolvablePlaceholders` option to `true`.

== Overriding properties from Camel test kit

When Testing with Camel and using the
xref:properties-component.adoc[Properties] component, you may want to be able to
provide the properties to be used from directly within the unit test
source code. +
Camel test kits, eg `CamelTestSupport` class offers the following methods

* `useOverridePropertiesWithPropertiesComponent`
* `ignoreMissingLocationWithPropertiesComponent`

So for example in your unit test classes, you can override the
`useOverridePropertiesWithPropertiesComponent` method and return a
`java.util.Properties` that contains the properties which should be
preferred to be used.

=== Providing properties from within unit test source

This can be done from any of the Camel Test kits, such as camel-test,
camel-test-spring, and camel-test-blueprint.

The `ignoreMissingLocationWithPropertiesComponent` can be used to
instruct Camel to ignore any locations which was not discoverable, for
example if you run the unit test, in an environment that does not have
access to the location of the properties.

== Using @PropertyInject

Camel allows to inject property placeholders in POJOs using the
`@PropertyInject` annotation which can be set on fields and setter
methods.

For example you can use that with `RouteBuilder` classes, such as shown
below:

[source,java]
----
public class MyRouteBuilder extends RouteBuilder {

    @PropertyInject("hello")
    private String greeting;

    @Override
    public void configure() throws Exception {
        from("direct:start")
            .transform().constant(greeting)
            .to("{{result}}");
    }

}
----

Notice we have annotated the greeting field with `@PropertyInject` and
define it to use the key `"hello"`. Camel will then lookup the property
with this key and inject its value, converted to a String type.

You can also use multiple placeholders and text in the key, for example
we can do:

[source,java]
----
@PropertyInject("Hello {{name}} how are you?")
private String greeting;
----

This will lookup the placeholder with they key `"name"`.

You can also add a default value if the key does not exists, such as:

[source,java]
----
@PropertyInject(value = "myTimeout", defaultValue = "5000")
private int timeout;
----

== Using out of the box functions

The xref:properties-component.adoc[Properties] component includes the following
functions out of the box

* `env` - A function to lookup the property from OS environment variables
* `sys` - A function to lookup the property from Java JVM system
properties
* `service` - A function to lookup the property from OS environment
variables using the service naming idiom
* `service.name` - A function to lookup the
property from OS environment variables using the service naming idiom
returning the hostname part only
* `service.port` - A function to lookup the
property from OS environment variables using the service naming idiom
returning the port part only

As you can see these functions is intended to make it easy to lookup
values from the environment. As they are provided out of the box, they
can easily be used as shown below:

[source,xml]
----
  <camelContext xmlns="http://camel.apache.org/schema/blueprint">
    <route>
      <from uri="direct:start"/>
      <to uri="{`{env:SOMENAME}`}"/>
      <to uri="{`{sys:MyJvmPropertyName}`}"/>
    </route>
  </camelContext>
----

You can use default values as well, so if the property does not exists,
you can define a default value as shown below, where the default value
is a `log:foo` and `log:bar` value.

[source,xml]
----
  <camelContext xmlns="http://camel.apache.org/schema/blueprint">
    <route>
      <from uri="direct:start"/>
      <to uri="{`{env:SOMENAME:log:foo}`}"/>
      <to uri="{`{sys:MyJvmPropertyName:log:bar}`}"/>
    </route>
  </camelContext>
----

The service function is for looking up a service which is defined using
OS environment variables using the service naming idiom, to refer to a
service location using `hostname : port`

* __NAME__**_SERVICE_HOST**
* __NAME__**_SERVICE_PORT**

in other words the service uses `_SERVICE_HOST` and `_SERVICE_PORT` as
prefix. So if the service is named FOO, then the OS environment
variables should be set as

[source]
----
export $FOO_SERVICE_HOST=myserver
export $FOO_SERVICE_PORT=8888
----

For example if the FOO service a remote HTTP service, then we can refer
to the service in the Camel endpoint uri, and use
the HTTP component to make the HTTP call:

[source,xml]
----
<camelContext xmlns="http://camel.apache.org/schema/blueprint">
  <route>
    <from uri="direct:start"/>
    <to uri="http://{`{service:FOO}`}/myapp"/>
  </route>
</camelContext>
----

And we can use default values if the service has not been defined, for
example to call a service on localhost, maybe for unit testing etc

[source,xml]
----
<camelContext xmlns="http://camel.apache.org/schema/blueprint">
  <route>
    <from uri="direct:start"/>
    <to uri="http://{`{service:FOO:localhost:8080}`}/myapp"/>
  </route>
</camelContext>
----

== Using custom functions (advanced)

The xref:properties-component.adoc[Properties] component allow to plugin 3rd party
functions which can be used during parsing of the property placeholders.
These functions are then able to do custom logic to resolve the
placeholders, such as looking up in databases, do custom computations,
or whatnot. The name of the function becomes the prefix used in the
placeholder. This is best illustrated in the example code below

[source,xml]
----
<bean id="beerFunction" class="MyBeerFunction"/>

<camelContext xmlns="http://camel.apache.org/schema/blueprint">
  <propertyPlaceholder id="properties">
    <propertiesFunction ref="beerFunction"/>
  </propertyPlaceholder>

  <route>
    <from uri="direct:start"/>
    <to uri="{`{beer:FOO}`}"/>
    <to uri="{`{beer:BAR}`}"/>
  </route>
</camelContext>
----

[NOTE]
====
The location attribute (on propertyPlaceholder tag) is not mandatory
====

Here we have a Camel XML route where we have defined the
`<propertyPlaceholder>` to use a custom function, which we refer to be the
bean id - eg the `beerFunction`. As the beer function uses `"beer"` as its
name, then the placeholder syntax can trigger the beer function by
starting with `beer:value`.

The implementation of the function is only two methods as shown below:

[source,java]
----
public static final class MyBeerFunction implements PropertiesFunction {

    @Override
    public String getName() {
        return "beer";
    }

    @Override
    public String apply(String remainder) {
        return "mock:" + remainder.toLowerCase();
    }
}
----

The function must implement
the `org.apache.camel.component.properties.PropertiesFunction`
interface. The method `getName` is  the name of the function, eg beer.
And the `apply` method is where we implement the custom logic to do. As
the sample code is from an unit test, it just returns a value to refer
to a mock endpoint.

To register a custom function from Java code is as shown below:

[source,java]
----
PropertiesComponent pc = (org.apache.camel.componennt.properties.PropertiesComponent) context.getPropertiesComponent();
pc.addFunction(new MyBeerFunction());
----


== Using 3rd-party properties sources

The properties component allows to plugin 3rd party sources to load and lookup properties via the `PropertySource`
API from camel-api. For example the `camel-microprofile-config` component is implemented using this.
The 3rd-party `PropertySource` can automatic be discoverd from classpath when Camel is starting up.
This is done by include the file `META-INF/services/org/apache/camel/property-source-factory` file
which refers to the fully qualified class name of the `PropertySource` implementation.
See the `camel-microprofile-config` for an example.

You can also register 3rd-part property sources via Java API

[source,java]
----
PropertiesComponent pc = ...
pc.addPropertySource(myPropertySource);
----

=== LoadablePropertySource

A `PropertySource` can define that it supports loading all its properties from the source at once,
for example from file system. This allows Camel properties component to load these properties at once
during startup.

=== PropertySource

The regular `PropertySource` will lookup the property on-demand, for example to lookup
values from a backend source such as a database or HashiCorp Vault etc.