content/extensions/mod_collections.adoc

:jbake-type: page
:jbake-status: published

= Apache Tamaya -- Extension: Collection Support

toc::[]

[[Optional]]
== Tamaya Collection Support (Extension Module)
=== Overview

All configuration in Tamaya is expressed as simple key, value pairs. Nevertheless this concept allows similarly
the modelling of collection typed values such as lists, sets, maps or simple collections of things. The Tamaya
Collections extension adds this functionality to the Tamaya eco-system.

=== Compatibility

The module is based on Java 7, so it will not run on Java 7 and beyond.


=== Installation

To use Tamaya collection support you only must add the corresponding dependency to your module:

[source, xml]
-----------------------------------------------
<dependency>
  <groupId>org.apache.tamaya.ext</groupId>
  <artifactId>tamaya-collections</artifactId>
  <version>{tamaya_version}</version>
</dependency>
-----------------------------------------------


=== Overview

Tamaya Collections adds +PropertyConverter+ implementations that are able to access configuration data
as lists, maps or sets. By default this works out of the box as easy as accessing any other type of
configuration data, e.g.

[source, java]
-----------------------------------------------
Configuration config = ConfigurationProvider.getConfiguration();

// Without any content specification, a list of String is returned.
List<String> simpleList = config.get("my.list.config.entry", List.class);

// Using a TypeLiteral allows to use every convertible sub type supported by the system.
List<Integer> intList = config.get("my.list.config.entry", new TypeLiteral<List<Integer>>(){});
-----------------------------------------------

Configuration in that case, by default, is a simple comma-separated list of entries, e.g.

[source, properties]
-----------------------------------------------
my.list.config.entry=1,34454,23,344545,3445
-----------------------------------------------

Additionally the module allows adding additional meta-entries, which allows to tweak some of the
inner-workings, e.g.

* using your own +PropertyConverter+ implementation for parsing entries.
* specifying a custom separator String to be used to split the items (default is {{','}}.
* specifying a custom separator String to be used to split key/value paris when parsing map entries.
* specifying the implementation type of the collection instance to be returned.
* specifying if the resulting collection should be returned as a modifiable collection.

=== Supported Types

This module supports the following types:

* +java.util.Collection+
* +java.util.List+
* +java.util.ArrayList+
* +java.util.LinkedList+
* +java.util.Set+
* +java.util.SortedSet+
* +java.util.TreeSet+
* +java.util.HashSet+
* +java.util.Map+
* +java.util.SortedMap+
* +java.util.HashMap+
* +java.util.TreeMap+

Hereby the type is determined primarly by the parameter type accessed, e.g.
+config.get("mylist", ArrayList.class)+ will always return an +ArrayList+
as result.

==== Configuring the target implementation type

Tamaya Collections allows you to configure the target collection type by adding the
following meta-configuration entry (shown for the +mylist+ entry). Hereby the package part +java.util.+
can be ommitted:

[ source, properties]
-----------------------------------------------
mylist=a,b,c
_mylist.collection-type=LinkedList
-----------------------------------------------

When calling +config.get("mylist", ArrayList.class)+ this parameter does not have any effect, so you will still
get an +ArrayList+ as a result. However when you call +config.get("mylist", List.class)+ you will
get a +LinkedList+ as implementation type.

This mechanism similarly applies to all kind of collections, so you can use it similarly to define the implementation
type returned when accessing +List+, +Map+ or +Collection+.


=== Collecting Configuration Entries instead of Overriding

By default Tamaya applies always an overriding +CombinationPolicy+, where only the configuration entry for
the most significant configuration entry is used. In case of collections (and maybe also other use cases),
overriding is not always the mechanism of choice. E.g. when you want to have all entries added to your
configuration to be *combined* to a new entry containing all values provided by any property sources.

Therefore Tamaya Collections also provides a more sophistiated +CombinationPolicy+ (automatically configured)
that allows to adapt the way how configuration entries are combined. All you must do is declaring
the mechanism to be applied by an according meta-configuration parameter, e.g. for +my.list+ your config may
look as follows:

[source, properties]
-----------------------------------------------
# from PropertSource 1
my.list=1,2,3

# from PropertSource 2
my.list=4,5,6

# without any additional meta-info these entries would be combined to
my.list=4,5,6
-----------------------------------------------

With Tamaya Collections you can now configure the combination policy as follows:

[source, properties]
-----------------------------------------------
# use one of the default policies: override / collect
_my.list.combination-policy=collect

# use an custom CombinationPolicy to combine the values
_my.list.combination-policy=com.mycomp.app.MyCombincationPolicy
-----------------------------------------------

So declaring the +collect+ policy the resulting raw output of the entry looks as follows:

[source, properties]
-----------------------------------------------
# result when applying the collect policy:
my.list=1,2,3,4,5,6
-----------------------------------------------

The customizable policy mechanism of Tamaya Collections also honors the +item-separator+ meta-configuration
parameter explained later in this document.


=== Format of Collection Configuration

By default collections are modelled as simple String values, that are tokenized into individual parts using a
defined +item-separator+ (by default +','+). So a given configuration entry of +1,2,3+ is mapped to +"1","2","3".
If the target context type is something different than String the smae conversion logic is used as when mapping
configuration parameters directly to non-String target types (implemented as +PropertyConverter+ classes, manahed
within the current +ConfigurationContext+. The procedure is identical for all collection types, including +Map+ types,
with the difference that each token in the list is parsed once more for separating it into a +key+ and a +value+.
The default separator for map entries hereby is +"::"+. Map keys, as of now, are always of type +String+, whereas
for values the same logic is applied as for non-map collection types.

[source, properties]
-----------------------------------------------
# a list, using the default format
list=1,2,3,4,5,6

# a map, using the default format
map=a::b, c::d
-----------------------------------------------

==== Trimming of entries

By default all tokens parsed are trimmed _before_ adding them to the final collection. In case of map entries this is
also the case for key/value entries. So the following configuration results in the identical values for
+list1,list2+ and +map1,map2+:

[source, properties]
-----------------------------------------------
# a list, using the default format
list1=1,2,3,4,5,6
list2=1, 2, 3, 4, 5, 6

# a map, using the default format
map1=a::b, c::d
map2=a :: b, c :: d
-----------------------------------------------

Nevertheless truncation can be controlled by the usage of brackets, e.g. the last list or map entry will have a single
space character as value:

[source, properties]
-----------------------------------------------
# a list, with a ' ' value at the end
list3=1, 2, 3, 4, 5, [ ]

# a map, with a ' ' value for key '0'
map3=1 :: a, 2 :: b, 0::[ ]
-----------------------------------------------

Hereby +\[+ escapes the sequence.


==== Customizing the format

The item and entry separators (by default +','+ and +"::"+) can be customized by setting corresponding meta-data
entries as follows, resulting in the same values as in the prevoius listing:

[source, properties]
-----------------------------------------------
# a list, with a ' ' value at the end
list3=1__2__3__ 4__ 5__[ ]
_list3.item-separator=__

# a map, with a ' ' value for key '0'
map3=1->a, 2->b, 0->[ ]
_map3.map-entry-separator=->
-----------------------------------------------

Of course these settings also can be combined:

[source, properties]
-----------------------------------------------
# a reformatted map
redefined-map=0==none | 1==single | 2==any
_redefined-map.map-entry-separator===
_redefined-map.item-separator=|
-----------------------------------------------