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Developing Java Applications for Google App Engine

For anybody building and deploying Java Web applications, whether internal business applications or public facing applications for general usage, hosting the application is a primary consideration. Google App Engine (GAE) provides a hosting environment that is potentially suitable for both types of applications. Before choosing GAE as the deployment environment a number of decisions, both technical and commercial must be made.
From a technical view, key questions include are what version of Java Standard Edition (JSE) is supported?, is all of enterprise Java supported (JEE)?, are there any special configuration files required ? and many others too. Business considerations include what is cost of the service, what is the support available, the reliability, vendor lock-in etc.
GAE runtime environment uses Java 6 so supports developing applications using Java 5 or 6. The runtime environment has some restrictions which enable it to provide scaling and reliability. A GAE application must not :
  • Write to the filesystem- Google provides a datastore as an alternative
  • Open a socket – Google provides a URLFetch service as an alternative
  • Spawn a new thread
In addition to these high level restrictions then Google publish a white list of the available Java classes from Java SE that can be used. From enterprise Java GAE supports the full Servlet and JSP API’s enabling developers to write applications using Frameworks built on these technologies, such as Spring MVC, Struts etc.
Whilst initially appearing somewhat restrictive, GAE does provide a large number of features and facilities for developers to not only host but also build scalable Java Web applications. I will detail these in further posts. Understanding the limitations/restrictions of Cloud Computing providers is vital when considering their adoption as well as the definite benefits they can bring. If you would like to know more about GAE and other Cloud Computing providers such as Amazon EC2 and Azure, why not consider attending the Learning Tree Cloud Computing Course.

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