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A Java Content Repository (JCR) is a database with a standardized Java Application Programming Interface (API). A JCR allows different applications to access one centralized database. Other technologies that are comparable to JCR are: JDBC, Hibernate and WebDAV.

Contrary to relational databases, a JCR has a hierarchical tree structure. The tree consists of nodes and properties. The properties are the leaves of the tree and represent the actual pieces of database content. A property is of one of the following types: string, binary, date, a number type, boolean, name, path or reference.

In addition, a node has attributes that are related to the node itself. Examples are: the name of the node, its path in the tree, its depth in the tree and its type. So, a node has attributes and a node contains properties.

Notice that the tree structure of the repository is seen from the interface point of view. Internally the content may be stored in a relational database. However, for the outside world it acts as a tree-structured database.

Node Types

Every node has one primary node type. The primary node type defines the properties and child nodes that a node is allowed or required to have. Every node has a special property called jcr:primaryType that records the name of its primary node type. In addition to its primary node type, a node may also have one or more mixin types. A mixin node type can mandate more child nodes and properties in addition to those enforced by its primary node type.

Multi-value Properties

In some cases, a property may have more than one value. A property that may have more than one value is referred to as a multi-value property. The property definition determines whether a particular property is a multi-value property.

Standardized by JCP

JCR has been standardized by the Java Community Process (JCP) in JSR 170 (API version 1.0) and JSR 283 (API version 2.0). Depending on the API version and compliance level, a JCR may support the following:

• Read-only or read/write access.
• Different types of nodes and properties.
• Simple or full versioning and multi languages.
• Different query languages like XPath, SQL, SQL2, AQM, JQOM.
• Locking.
• Observation of content changes.
• Export and import of content by XML files.
• JTA (XA transactions).

See also: http://jcp.org