Content repository API for Java
Content Repository API for Java (JCR) is a specification for a Java platform application programming interface (API) to access content repositories in a uniform manner.[dead link][not in citation given] The content repositories are used in content management systems to keep the content data and also the metadata used in content management systems (CMS) such as versioning metadata. The specification was developed under the Java Community Process as JSR-170 (Version 1). and as JSR-283 (version 2) The main Java package is
A JCR is a type of object database tailored to storing, searching, and retrieving hierarchical data. The JCR API grew out of the needs of content management systems, which require storing documents and other binary objects with associated metadata; however, the API is applicable to many additional types of applications. In addition to object storage, the JCR provides: APIs for versioning of data; transactions; observation of changes in data; and import or export of data to XML in a standard way.
Data structure 
The data in a JCR consists of a tree of nodes with associated properties. Data is stored in the properties, which may hold simple values such as numbers and strings or binary data of arbitrary length. Nodes may optionally have one or more types associated with them which dictate the kinds of properties, number and type of child nodes, and certain behavioral characteristics of the nodes. Nodes may point to other nodes via a special reference type property. In this way nodes in a JCR offer both referential integrity and object-oriented concept of inheritance. Additional node types include the referenceable node type which allows the user to reference said node through use of a universally unique identifier. Another popular type is the versionable type. This makes the repository track a document's history and store copies of each version of the document.
A JCR can export portions of its tree to XML in two standard formats and can import hierarchies directly from XML. JSR 283 compliant implementations must support a standardized form of SQL for queries and a query object model QOM. JSR 283 deprecates the XPath query language defined in JSR 170. The Apache Jackrabbit reference implementation of JCR also supports the integration of the Apache Lucene search engine to give full text searches of data in the repository.
Available implementations 
Both JSRs are led by David Nüscheler of Day Software. Day has both a commercial JCR implementation called Content Repository Extreme (CRX) and is involved in the open source Apache Jackrabbit JCR, which had its 2.0 release in January 2010. Jahia, Hippo CMS and Magnolia are Enterprise Content Management systems built on the JCR API, using Jackrabbit as its repository by default, but able to plug in any other JSR-170 certified repository implementation. JSR-170 is also supported by Oracle Beehive, Oracle's enterprise collaboration software. The eXo JCR by ObjectWeb is an open source repository with paid commercial support and documentation. Many other commercial vendors are working on JCR compatibility and products. Alfresco is an open source content management system which provides a JCR API onto their repository. GX released the new community version of its Web enterprise content management (ECM) platform based natively on Apache Jackrabbit and JSR.
ModeShape is a new open source JCR implementation that lets applications access existing information in external systems through the JCR API. ModeShape repositories can also federate content from multiple sources into one, unified repository.
JCRDev.com provides a common platform for JCR Developers. It is an online community built especially for people working on Java Content Repository.
PHP adaptions have been started by TYPO3, translating the Java API to PHP. The PHPCR interfaces are also implemented by Jackalope, a PHP frontend talking to the Jackrabbit backend through the WebDAV interface, and Midgard, a cross-language content repository.
- JCR: A Practitioner's Perspective, by Alexandru Popescu and Joseph Ottinger
- Java API for XML Registries - Another Java API for Registries and Repositories
- JSR-170 (HTML version of the specification)
- Apache Jackrabbit home page
- JCR links to articles and applications based on JCR.
- eXo JCR Implementation
- TheServerSide.com: eXo Platform v2, Portal, JCR, ECM, Groupware and Business Intelligence: Part 2
- ModeShape open source project
- phpcr on github
- Jackalope: JCR for PHP
- Bergius, Henri (2011-12-23). "Midgard2 PHPCR provider hits 1.0".
See also 
- Content Management Interoperability Services (CMIS), another content repository API