As the aim was to integrate many existing Apache projects into Apache Geronimo, the server had a very modular architecture with a lightweight and compact kernel into which other services could be plugged in order to provide additional functionality. Other containers that implemented parts of the J2EE specifications could then be added on top of this, and they would be able to use the services that were available. The various components and services could be stopped when they were not required, to save on system resources. In newer versions, this was improved upon and you could install or uninstall plugins at runtime to add or remove functionality. The latest version even has the ability to generate custom-server assemblies from the Administration Console of the server.
Apache Geronimo leverages a lot of popular open source projects to provide Java EE 5 compliance. Some of the projects used are:
Apache Tomcat
Apache Tomcat is an open source JavaServer Pages(JSP) and Java Servlet container from the Apache Software Foundation. It is fast, lightweight, and very popular, and is used widely in the industry. It is considered as a benchmark for Servlet and JSP containers. Apache Tomcat is used by Apache Geronimo as its web container. It is thereby used to host the web applications that you deploy in Apache Geronimo. Apache Tomcat is hosted at http://tomcat.apache.org.
Apache OpenEJB
Enterprise JavaBeans (EJB) technology provides the server-side component architecture for Java Platform. EJB technology enables the rapid and simplified development of distributed, transactional, secure, and portable applications based on Java technology.
Apache OpenEJB is a free, open source EJB container. It is both embeddable and lightweight. The current version, OpenEJB 3.x, supports EJB 3.0 and is also backward compatible with previous EJB versions. It also has a few experimental EJB 3.1 features. Apache OpenEJB is used by Apache Geronimo as its EJB container. Therefore, all EJB applications that you deploy in Apache Geronimo will in turn be deployed into Apache OpenEJB by the deployment subsystem. Apache OpenEJB is a complete rewrite of OpenEJB 2.1, which was included in Apache Geronimo 1.x. Thus, it is more robust and has a higher order of performance than its predecessor. Apache OpenEJB can be made to work with different JPA providers. However, it provides Container Managed Persistence (CMP) of EJB 2.1 support, through a wrapper over OpenJPA. Apache OpenEJB is hosted at http://openejb.apache.org.
Apache Axis 1.x
Apache Axis is an open source web services engine from the Apache Software Foundation. This version of Apache Axis is used to provide support for JAX-RPC web services. Apache Axis is hosted at http://ws.apache.org/axis.
Apache Axis2
Apache Axis2 is a rewrite of the Apache Axis web services engine. Apache Axis2 is used to provide support for JAX-WS, SOAP 1.2, SAAJ, and all the other specifications of web services, which are required for Java EE 5 compliance. Apache Axis2 is hosted at http://ws.apache.org/axis2/.
Apache CXF
Apache CXF is an open source services framework. It is also available as the web services container in Geronimo. It can be used as an alternative to Apache Axis2. Apache CXF is hosted at http://cxf.apache.org.
Jetty from Mortbay
Jetty is a fast and lightweight open source web server and Servlet container. It is easily embeddable and, like Tomcat, is widely used. Jetty can also be used as the web container in Apache Geronimo, as an alternative to Tomcat. This project is hosted at http://www.mortbay.org/jetty and is distributed under the Apache License, Version 2.0.
Apache ActiveMQ
Apache ActiveMQ is a popular and powerful open source message broker. It is fast, lightweight, embeddable, and supports many cross language clients and protocols. It supports the JMS specification. It is used as a message broker in Apache Geronimo. Apache Geronimo 2.1.4 uses ActiveMQ 4.1.2. Apache ActiveMQ is hosted at http://activemq.apache.org.
Apache XBean
Apache XBean is a plugin-based server framework. It provides functionality to discover, download, and install server plugins from a remote repository. It also supports Inversion of Control(IoC), lifecycle and class loader management, JMX, and Spring Integration. Apache XBean is a subproject of the Apache Geronimo project, and is hosted at http://geronimo.apache.org/xbean/.
Apache Derby
Apache Derby is a free, open source, lightweight, and embeddable RDBMS, implemented entirely in Java. Derby is integrated in Apache Geronimo as the default database, and the Administration Console has built-in portlets for basic Apache Derby administration tasks. Apache Derby is a part of the Apache DB project, and is hosted at http://db.apache.org/derby.
Apache Scout
Apache Scout is an open source implementation of the Java API for XML Registries (JAXR). It provides a registry where web services can be registered by their providers, and users can look for the services they need in the registry. Apache Scout is hosted at http://ws.apache.org/scout/.
Apache OpenJPA
Apache OpenJPA is an open source Java Persistence API (JPA) container that can either be used as a standalone POJO persistence layer or integrated into any EJB 3.0 container. OpenJPA was donated by BEA to the Apache Software Foundation, and is based on BEA's Kodo JPA Engine. It is used as the default JPA provider for Apache Geronimo and Apache OpenEJB. The version used in Apache Geronimo 2.1.4 is OpenJPA 1.2. Apache OpenJPA is hosted at http://openjpa.apache.org/.
Apache log4j & SLF4J
Apache log4j is the logging framework that is used for logging in Apache Geronimo, and SLF4J is the logging adapter that is used so that the source is not tied down to any logging framework. Apache log4j is hosted at http://logging.apache.org/, and SLF4J is hosted at http://www.slf4j.org/
Apache Commons
The Commons is an Apache project focused on all aspects of reusable Java components. Some of the components are:
BeanUtils: Easy-to-use wrappers around the Java reflection and introspection APIsFileUpload: A file upload capability for servlets and web applicationsIO: Collection of I/O utilities, and so on
Apache Commons is hosted at http://commons.apache.org.
Apache GShell
Apache GShell is a framework for building rich command-line applications. GShell is used as a bootstrap launcher for Apache Geronimo. It provides a number of built-in commands, and many optional ones. GShell is extensible, and the version in Apache Geronimo has been extended with many Apache Geronimo specific commands. Apache GShell is a subproject of the Apache Geronimo project, and is hosted at http://cwiki.apache.org/GSHELL.
Apache Yoko
Apache Yoko is a robust and high-performance CORBA server written in Java that can run on any Java Virtual Machine (JVM). It provides the ORB and CORBA naming service that is used by Apache Geronimo. It is also used by Geronimo to expose EJBs as CORBA services. Apache Yoko is a subproject of the Apache Geronimo project, and is hosted at http://cwiki.apache.org/YOKO/.
TranQL
TranQL is an open source framework for building persistence engines. The TranQL project is hosted at the Codehaus. It's also a framework for building J2CA connectors. TranQL provides J2CA connector implementations that wrap drivers for several popular relational databases. The TranQL project is hosted at http://tranql.codehaus.org.
These open source projects are plugged into the Geronimo system via wrapper GBeans or as dependant libraries. You can configure Geronimo servers that contain all or a subset of these projects. Apache Geronimo also provides you with more choice when choosing the parts that will make up your application server than any other application server. For example, you can have a server with Apache Tomcat as the web container and Axis2 as the web services container, or you can have a server with Jetty and CXF. So one key differentiator at the time Apache Geronimo was released was that it was not monolithic like many other Java EE application servers. However, most application server vendors are now re-architecting their servers to also be modular and lightweight.