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= Property Mapping
:description: Helidon config property mapping
:keywords: helidon, config
:feature-name: Config
:rootdir: {docdir}/../..
include::{rootdir}/includes/se.adoc[]
== Contents
- <>
- <>
- <>
- <>
- <>
- <>
== Overview
Although config values are originally text, you can use the config system's
built-in conversions or add your own to translate text
into Java primitive types and simple objects (such as `Double`), into `enum` values, and to
express parts of the config tree as complex types (`List`, `Map`, and
custom types specific to your application). This section introduces how to
use the built-in mappings and your own custom ones to convert to simple and
complex types.
== Converting Configuration to Simple Types
The link:{config-javadoc-base-url}/io/helidon/config/Config.html[`Config`] class itself provides many
conversions to Java types. See the JavaDoc for the complete list.
The methods which support Java primitive types and their related classes follow a
common pattern. The examples in the table below deal with conversion to a boolean
but the same pattern applies to many data types listed in the JavaDoc.
Assume a local variable has been assigned something like
[source,java]
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_1, indent=0]
----
<1> Shorthand method
<2> Generic method (for any type)
.Built-in Conversions to Simple Types (e.g., boolean)
|===
|Java type |Example usage ^1^
|`boolean` |`boolean b = value.get();` ^2^
`boolean defaultedB = value.orElse(true);` ^3^
|`Optional` | ConfigValue already has all methods of an Optional. If actual optional is needed:
`Optional b = value.asOptional();` ^4^
|`Supplier` |`Boolean b = value.supplier().get();`
`boolean defaultedB = value.supplier(true).get();`
|`Supplier>` |
`Boolean b = value.optionalSupplier().get().orElse(Boolean.TRUE);`
|===
[NOTE]
.Built-in Conversions to Simple Types
====
1. All conversions can throw `MissingValueException` (if no value exists at the
requested key and no default is provided) and
`ConfigMappingException` (if some error occurred while performing the data mapping).
2. The `Config.asXXX` methods internally use the Java-provided `XXX.parseXXX` methods, so here
a missing or un-parseable string gives `false` because that is how `Boolean.parseBoolean`
behaves.
3. User code defaults the value to `true`.
4. User code defaults the value to `Boolean.TRUE` if absent; otherwise parses
the value using `Boolean.parseBoolean`.
====
The numerous conversions defined on the `Config` class for other types (integers,
doubles, etc.) will satisfy many of your application's needs.
The link:{config-javadoc-base-url}/io/helidon/config/ConfigMappers.html[`ConfigMappers`] class
includes other related mappings from `String` (rather than from `Config`) to
Java types (described in the JavaDoc).
For additional type mapping, you can use these methods defined on `Config`:
[source,java]
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_2, indent=0]
----
which maps the current node to a type.
The next example, and later ones below showing complex type mapping, use the example
xref:introduction.adoc#built-in-formats[`application.properties`] configuration
from the config introduction. Part of that example includes this line:
[source,properties]
----
bl.initial-id = 10000000000
----
Your application can use `Config.as` to interpret the value as a `BigDecimal`:
[source,java]
BigDecimal initialId = config.get("bl.initial-id").as(BigDecimal.class);
== Converting Configuration to `enum` Values
Configuration can automatically map `Config` nodes to most `enum` types.
Your application code simply passes the enum class type to `config.as(Class type)`.
The built-in `enum` converter attempts to match the string value in the config node to the name of one of the values declared for that specific `enum` in the Java code.
=== Matching `enum` Names
The conversion applies the following algorithm to match config values to `enum` names, stopping as soon as it finds a match:
1. Select an exact match if one exists.
2. Treat hyphens (`-`) in config strings as underscores (`_`) and select an otherwise exact match if one exists.
3. Select a _case-insensitive_ match (with or without hyphen substitution) if there is _exactly one_ such match.
4. Finding no match or multiple case-insensitive matches, throw a `ConfigMappingException`.
=== Example
The following example illustrates how to use the built-in `enum` conversion feature. The example code builds a simple `Config` tree itself which contains simple test data; normally your application would load the config from a file or some other location.
[source,java]
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_3, indent=0]
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_4, indent=0]
----
<1> Retrieve the `Config` object corresponding to the key `house.tint`.
<2> Indicate that, when the value in that `Config` object is converted, Helidon should convert it to a `Color` `enum` value.
<3> Convert and retrieve the value.
+
The conversion triggered by invoking `get()` matches the string `blue-green`--expressed in lower case and with a hyphen -- to `Color.BLUE_GREEN` using the conversion rules described earlier.
<4> The config key `car.color` locates the mixed-case string `Red` which the converter matches to `Color.RED`.
<5> The config key `warning` locates `YELLOW` which the converter matches exactly to `Color.YELLOW`.
=== Why use heuristics in matching strings to `enum` values?
Short answer: ease-of-use.
Users composing config sources often adopt a style with hyphens within words to improve readability and lower-case keys and values.
With that style in mind, users typing an `enum` value into a config source might accidentally enter a hyphen instead of an underscore or use lower case instead of upper case. Users might even _prefer_ to make these changes so they can follow their preferred config style.
With the heuristics, Helidon allows users to adopt a common config style and prevents unnecessary runtime exceptions--and user frustration--from inconsequential typos.
Remember:
* Helidon always finds exact matches unambiguously, without relying on the heuristics.
In our `Color` example the text `BLUE_GREEN` always maps to `Color.BLUE_GREEN`.
* Because hyphens cannot appear in a valid Java `enum` value name, interpreting them as underscores during `enum` conversion introduces no ambiguity.
Only in the following unusual situation are the heuristics unable to unambiguously match a string to an `enum` value:
* The `enum` has values which differ _only_ in their case (such as `Red` and `RED`), _and_
* The string in the config source is not an exact match with an `enum` value name (such as `red`).
If your application must deal with such cases, write your own function which maps a `Config` node to the correct `enum` value, resolving the ambiguities however makes sense in your use case.
Your code tells config to use that function instead of the built-in `enum` conversion when it converts values. A xref:customConfigAs[later section] describes this technique which works for all types, not only `enum` types.
== Converting Configuration to Complex Types
The xref:hierarchical-features.adoc[hierarchical features] section describes
the tree structure used to represent config data. The config system can map subtrees
of a config tree to complex Java types.
=== Built-in Conversions to `List` and `Map`
The `Config` class exposes several methods for mapping a structured config node
to a Java `List` or `Map`. The link:{config-javadoc-base-url}/io/helidon/config/Config.html[JavaDoc]
contains complete details, but briefly your application can convert a structured `Config` node into:
* a `List` of a given type
* a `Map` in which each key is the fully-qualified key `String` for a
config entry and the value is its `String` value
=== Custom Conversions
Often your code will be simpler if you can treat parts of the configuration as
custom, application-specific Java objects, rather than as a group of `String` keys and
values. You will need customized conversions to do so.
The config system provides many ways to accomplish this, described in
the link:{config-javadoc-base-url}/io/helidon/config/package-summary.html#conversions[`io.helidon.config`
package JavaDoc].
Some of those approaches require that the target class -- the class to which
you want to convert the configuration data -- have certain characteristics
or that you add a method to the class to help do the mapping.
You might want to avoid changing the target class else you
might not even be able to if you do not control its source.
Here are two approaches that will always work without requiring changes
to the target class. For both approaches, you write your own conversion function.
The difference is in how your application triggers the use of that mapper.
[[customConfigAs]]
==== Use Custom Mapper Explicitly: `Config.as` method
Any time your application has a `Config` instance to map to the target class
it invokes `Config.as` passing an instance of the corresponding conversion function:
[source,java]
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_5, indent=0]
----
You do not necessarily need a new instance of the mapper every time you want to use
it.
In this approach, everywhere your application needs to perform this conversion it specifies the
mapper to use. If you decided to change which mapper to use you would need
to update each of those places in your application.
==== Register Custom Mapper Once, Use Implicitly: `Config.as` method
In this approach, your application:
1. Tells each `Config.Builder`
that needs to know about the custom mapper by either:
a. registering an instance of your mapper by invoking `Config.Builder.addMapper`, or
b. implementing
link:{config-javadoc-base-url}/io/helidon/config/spi/ConfigMapperProvider.html[`ConfigMapperProvider`]
so it returns an instance of your mapper (see the JavaDoc for complete information)
and creating or editing the file `io.helidon.config.spi.ConfigMapperProvider`
so it contains
a line with the fully-qualified class name of your `ConfigMapperProvider`. The
config system will use the Java service loader to find and invoke all
`ConfigMapperProvider` classes listed and add the mappers they provide to each
`Config.Builder` automatically.
2. Converts using the mapper by invoking the
`Config.as` method which accepts the target type to convert to, _not_ the
mapper itself that does the conversion.
If your application converts to the same
target type in several places in the code, this approach allows you to change which mapper it uses by
changing only the _registration_ of the mapper, not each use of it.
[[WebConfig]]
==== Continuing the `Web` Example
The following examples build on the example configuration from the
xref:introduction.adoc#built-in-formats[`application.properties`]
example file in the introduction.
[source,java]
.Java POJO to Hold `web` Properties Config
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_6, indent=0]
----
[source,java]
.Custom Mapper Class
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_7, indent=0]
----
[source,java]
.Explicitly Using the Mapper
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_8, indent=0]
----
[source,java]
.Registering and Implicitly Using the Mapper
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_9, indent=0]
----
Either of the two approaches just described will _always_ work without requiring you to change
the POJO class.
== Advanced Conversions using Explicit Mapping Logic
If the target Java class you want to use meets certain conditions -- or if you can change
it to meet one of those conditions -- you might not need to write a separate mapper
class. Instead, you add the mapping logic to the POJO itself in one of
several ways and the config system
uses Java reflection to search for those ways to perform the mapping.
Your application facilitates this implicit mapping either by adding to the
POJO class or by providing a builder class for it.
This feature is available in Object mapping module, and is added through Java `ServiceLoader`
mechanism. This is no longer part of core Config module, as it depends on reflection
and introduces a lot of magic (see the list of supported mapping methods below, also
uses reflection to invoke the methods and to map configuration values to fields/methods etc.).
[source,xml]
.Config object mapping Dependency in `pom.xml`
----
io.helidon.confighelidon-config-object-mapping
----
=== Adding the Mapping to the POJO
If you can change the target class you can add any one of the following methods or
constructors to the POJO class which the config system will find and use for mapping.
Continuing with the <> example introduced earlier:
.Methods Supporting Auto-mapping
|===
|`static WebConfig create(Config);`
|`static WebConfig from(Config);`
|`static WebConfig from(String);`
|`static WebConfig of(Config);`
|`static WebConfig of(String);`
|`static WebConfig valueOf(Config);`
|`static WebConfig valueOf(String);`
|`static WebConfig fromConfig(Config);`
|`static WebConfig fromString(String);`
|===
.Constructors Supporting Auto-mapping
|===
|`WebConfig(Config);`
|`WebConfig(String);`
|===
If the config system finds any of these methods or constructors when the
application invokes
[source,java]
WebConfig wc = config.as(WebConfig.class).get();
it will invoke the one it found to
map the config data to a new instance of the target class. You do not need to
write a separate class to do the mapping or register it with the `Config.Builder`
for the config instance.
=== Writing a Builder Method and Class for the POJO
You can limit the changes to the POJO class by adding a single
`builder` method to the POJO which returns a builder class for the POJO:
[source,java]
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_10, indent=0]
----
The builder class `WebConfigBuilder` is expected to be a Java Bean with
1. bean properties named for the config properties of interest, and
2. a method `WebConfig build()` which creates the mapped instance
from the builder's own bean properties.
When your application invokes `config.as(WebConfig.class)` the config system
1. finds and invokes the `WebConfig.builder()` method,
2. assigns the bean properties on the returned builder from the config subtree
rooted at `config`, and
3. invokes the builder's `build()` method yielding the resulting `WebConfig` instance.
== Conversions using JavaBean Deserialization
////
10. a factory method `from(...)` with parameters (loaded from config sub-nodes)
creates new instance of a bean.
11. a _factory_ constructor with parameters (loaded from config sub-nodes).
12. a no-parameter constructor to create new instance of type and apply
recursively same mapping behaviour
described above on each JavaBean property of such object,
a.k.a. <>.
////
The config system can also interpret your classes as JavaBeans and use
the normal bean naming conventions to map configuration data to your POJO classes,
using one of these patterns:
1. <> - The config system treats the target class itself as
a JavaBean, assigning values from the config to the bean properties of the POJO
class.
2. <> - The config system invokes the POJO's `builder()`
method to obtain a builder for that POJO type and treats the _builder_
class as a JavaBean, assigning values from the config to the builder's
bean properties and then invoking the builder's `build` method to create
an instance of the target POJO class.
3. <> - The
config system finds a `from` method or a constructor on
the POJO class itself which accepts annotated arguments, then invokes that method
or constructor
passing the specified arguments based on the config. The `from` method returns
an instance of the POJO class initialized with the values passed as arguments.
The following sections describe these patterns in more detail.
This feature is available in Object mapping module, and is added through Java `ServiceLoader`
mechanism. This is no longer part of core Config module, as it depends on reflection.
[source,xml]
.Config object mapping Dependency in `pom.xml`
----
io.helidon.confighelidon-config-object-mapping
----
[[pojoAsJavaBean]]
=== POJO as JavaBean
If your POJO target class is already a JavaBean -- or you can modify it
to become one -- you might be able to avoid writing any explicit
mapping code yourself.
The config system invokes the no-args constructor on the target class to create
a new instance. It treats each public setter method and each public non-final field
as a JavaBean property. The config system processes any non-primitive property
recursively as a JavaBean. In this way the config system builds up the target
object from the config data.
By default, the system matches potential JavaBean property names with
config keys in the configuration.
Use the link:{config-mapping-javadoc-base-url}/io/helidon/config/objectmapping/Value.html[`Value`] annotation to control some of
JavaBean processing for a given property.
.`Value` Annotation
|===
|Attribute |Usage
|`key` |Indicates which config key should match this JavaBean property
|`withDefault` |`String` used for the bean property default value if none is set in the config
|`withDefaultSupplier` |`Supplier` of the default bean property value if not is set in the config
|===
To exclude a bean property from the config system bean processing annotate it with
link:{config-mapping-javadoc-base-url}/io/helidon/config/objectmapping/Transient.html[`Transient`].
Here is an example using the `app` portion of the example configuration from the
introduction.
[source,java]
.Java bean to load `app` properties into via setters
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_11, indent=0]
----
<1> Public no-parameter constructor.
<2> Property `greeting` is not customized and will be set from the config node with
the key `greeting`, if present in the config.
<3> Property `pageSize` is matched to the config key `page-size`.
<4> If the `page-size` config node does not exist, the `pageSize` bean property defaults to `10`.
<5> Property `basicRange` is matched to the config key `basic-range`.
<6> If the `basic-range` config node does not exist, a `BasicRangeSupplier` instance will provide
the default value.
<7> The `timestamp` bean property is never set, even if the config contains a node
with the key `timestamp`.
<8> `BasicRangeSupplier` is used to supply the `List` default value.
Here is an example of code loading config and mapping part of it to the `AppConfig`
bean above.
[source,java]
.Map `app` config node into `AppConfig` class
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_12, indent=0]
----
<1> The config system finds no registered `ConfigMapper` for `AppConfig` and so
applies the JavaBean pattern to convert the config to an `AppConfig` instance.
<2> Because the bean property `timestamp` was marked as transient, the
config system did not set it.
[[builderAsJavaBean]]
=== Builder as JavaBean
If the target class includes the public static method `builder()` that returns any object,
then the config system will make sure that the return type has a method `build()`
which returns an instance of the target class. If so, the config system treats
the _builder_ as a JavaBean and
1. invokes the `builder()` method to instantiate the builder class,
2. treats the _builder_ as a JavaBean and maps the `Config` subtree to it,
3. invokes the builder's `build()` method to create the new instance of the target
class.
You can augment the target class with the public static `builder()` method:
[source,java]
.JavaBean for `app` properties, via a `Builder`
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_13, indent=0]
----
<1> The builder's property `greeting` is not customized and is set from config node with
`greeting` key, if one exists.
<2> The builder's property `pageSize` maps to the config key `page-size` and
defaults to `10` if absent.
<3> The builder's property `basicRange` maps to the config key `basic-range`
and uses a `BasicRangeSupplier` instance to get a default value if needed.
<4> Finally, the config system invokes the builder's public method `build()`,
creating the new instance of `AppConfig` for use by the application.
[[pojoWithFactoryMethodOrConstructor]]
=== Target Class with Annotated Factory Method or Constructor
Another option is to annotate the parameters to a _factory method_ or to a constructor
on the target class. You can add a _factory method_ to the target class, a `public static`
method `from` with parameters annotated to link them to the corresponding config
keys. Or you can add or modify a constructor with parameters, similarly annotated
to form the link from each parameter to the corresponding config key.
[WARNING]
=========
Be sure to annotate each parameter of the `from` method or constructor with `@Value`
and specify the key to use for the mapping. The parameter names in the Java code
are not always available at runtime to map to config keys. (They might be `arg0`,
`arg1`, etc.)
=========
[source,java]
.Target Class with Factory Method `from`
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_14, indent=0]
----
<1> The config system invokes the factory method `from(...)`, passing
arguments it has fetched from the correspondingly-named config subtrees.
The factory method returns the new initialized `AppConfig` instance.
Note the consistent use of `@Value(key = "...")` on each parameter.
<2> Because the property `greeting` does not specify a default value
the property is **mandatory** and must appear in the configuration source.
Otherwise, the config system throws a `ConfigMappingException`.
Alternatively, you can use an annotated constructor instead of a static factory
method. Revising the example above, make the constructor public, annotate its
parameters, and remove the now-unneeded `from` factory method.
[source,java]
.Target Class with Annotated Public Constructor
----
include::{sourcedir}/se/config/PropertyMappingSnippets.java[tag=snippet_15, indent=0]
----
<1> Constructor is `public`.
<2> Each parameter has the `ConfigValue` annotation to at least specify the
config key name.
When the application invokes `config.as(AppConfig.class)`, the config system locates
the public annotated constructor and invokes it, passing as arguments the data it fetches
from the configuration matching the annotation `key` names with the configuration
keys.
