Module org.apache.sis.metadata

# Package org.apache.sis.metadata

package org.apache.sis.metadata
Root package for various metadata implementations.

## Foreword

Many metadata standards exist, including Dublin core, ISO 19115 and the Image I/O metadata defined in `javax​.imageio​.metadata`. The SIS implementation focuses on ISO 19115 (including its ISO 19115-2 extension), but the classes are designed in a way that allow the usage of different standards. This genericity goal should be keep in mind in the discussion below.

## How Metadata are defined

A metadata standard is defined by a set of Java interfaces belonging to a specific package and its sub-packages. For example, the ISO 19115 standard is defined by the GeoAPI interfaces defined in the `org​.opengis​.metadata` package and sub-packages. That standard is identified in SIS by the `MetadataStandard​.ISO_19115` constant. Other standards are defined as well, for example the `MetadataStandard​.ISO_19123` constant stands for the standards defined by the interfaces in the `org​.opengis​.coverage` package and sub-packages.

For each interface, the collection of declared getter methods defines its properties (or attributes). If a `UML` annotation is attached to the getter method, the identifier declared in that annotation is taken as the property name. This is typically the name defined by the International Standard from which the interface is derived. Otherwise (if there is no `UML` annotation) the property name is inferred from the method name like what the Java Beans framework does.

The implementation classes, if they exist, are defined in different packages than the interfaces. For example, the ISO 19115 interfaces, declared in `org​.opengis​.metadata`, are implemented by SIS in `org​.apache​.sis​.metadata​.iso`. The sub-packages hierarchy is the same, and the names of implementation classes are the name of the implemented interfaces prefixed with `Abstract` or `Default`.

### Notes

- The `Abstract` prefix means that the class is abstract in the sense of the implemented standard. It it not necessarily abstract in the sense of Java. Because incomplete metadata are common in practice, sometimes we wish to instantiate an "abstract" class despite the lack of knowledge about the exact sub-type.
- The properties are determined by the getter methods declared in the interfaces. Getter methods declared in the implementation classes are ignored.
- Setter methods, if any, can be declared in the implementation classes without the need for declarations in the interfaces. In other words, interfaces are assumed read-only unless a specific implementation provide setter methods.
- The implementation is not required to exist as source code. They can be generated on the fly with `Proxy`. This is the approach taken by the `org​.apache​.sis​.metadata​.sql` package for generating metadata implementations backed by the content of a database.

## How Metadata are handled

Metadata objects in SIS are mostly containers: they provide getter and setter methods for manipulating the values associated to properties (for example the `title` property of a `Citation` object), but provide few logic. The package `org​.apache​.sis​.metadata​.iso` and its sub-packages are the main examples of such containers.

In addition, the metadata modules provide support methods for handling the metadata objects through Java Reflection. This is an approach similar to Java Beans, in that users are encouraged to use directly the API of Plain Old Java objects (actually interfaces) every time their type is known at compile time, and fallback on the reflection technic when the type is known only at runtime.

Using Java reflection, a metadata can be viewed in many different ways:

- As a `Map`
  The `MetadataStandard` class provides various methods returning a view of an arbitrary metadata implementation as a `Map`, where the key are the property names and the values are the return values, types or descriptions of getter methods. The map is writable if the underlying metadata implementation has setter methods, otherwise attempts to set a value throw an `UnmodifiableMetadataException`.
- As a `TreeTable`
  The metadata are organized as a tree. For example, the `Citation` metadata contains one or many `ResponsibleParty` elements, each of them containing a `Contact` element, which contains a `Telephone` element, etc. For each node, there is many information that can be displayed in columns:
  - A description of the element.
  - The type of values (`String`, `double`, etc).
  - The range of valid values (if the type is numeric), or an enumeration of valid values (if the type is a code list).
  - The value stored in the element, or the default value.
- As a table record in a database (using `org​.apache​.sis​.metadata​.sql`)
  It is possible to establish the following mapping between metadata and a SQL database:
  - Each metadata interface maps to a table of the same name in the database.
  - Each property in the above interface maps to a column of the same name in the above table.
  - Each instance of the above interface is a record in the above table.
  Using Java reflection, it is possible to generate implementations of the metadata interfaces where each call to a getter method is translated into a SQL query for the above database.

## How Metadata are marshalled

The ISO 19115-3 standard defines how ISO 19115-1 metadata shall be represented in XML. The SIS library supports XML marshalling and unmarshalling with JAXB annotations.

Only the implementation classes defined in the `org​.apache​.sis​.metadata​.iso` packages and sub-packages are annotated for JAXB marshalling. If a metadata is implemented by another package (for example `org​.apache​.sis​.metadata​.sql`), then it shall be converted to an annotated class before to be marshalled. All SIS annotated classes provide a copy constructor for this purpose. A shallow copy is sufficient; JAXB adapters will convert the elements on-the-fly when needed.

The annotated classes can be given to a JAXB `Marshaller`. For best results, it shall be a marshaller obtained from the `MarshallerPool`, otherwise some XML outputs may be incomplete (missing namespaces for instance). The `XML` class provides convenience methods for this purpose.

Since:
  0.3

- Related Packages
  Package
  Description
  org.apache.sis.metadata.iso
  Root package for ISO 19115 metadata about resources (data or services).
  org.apache.sis.metadata.sql
  An implementation of Metadata interfaces fetching the data from an SQL database.
- Class
  Description
  AbstractMetadata
  Provides basic operations using Java reflection for metadata implementations.
  InvalidMetadataException
  Thrown when a metadata is in a invalid state or has illegal property values.
  KeyNamePolicy
  The name of the keys included in a `Map` of metadata.
  MetadataCopier
  Performs deep copies of given metadata instances.
  MetadataStandard
  Enumeration of some metadata standards.
  ModifiableMetadata
  Base class of metadata having an editable content.
  ModifiableMetadata.State
  Whether the metadata is still editable or has been made final.
  TitleProperty
  Identifies the name of a property to use for summarizing in one line the content of a metadata object.
  TypeValuePolicy
  The kind of values in the `MetadataStandard​.asTypeMap(…)`.
  UnmodifiableMetadataException
  Thrown on attempt to set a read-only value in a metadata object.
  ValueExistencePolicy
  Whatever `MetadataStandard​.asValueMap(…)` shall contain entries for null, nil or empty values.