This morning, I tried to get a coop student up and running on an older version of the Google App Engine for Eclipse plugin.
The following website and all related links appear to have been wiped off the face of the earth:
https://developers.google.com/eclipse/
Is this just down at the moment? Is it possible to get older versions of the plugin?
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GPE is indeed gone. It was not up to date and many parts of it no longer functioned. Over the coming year, even more core functionality was going to break. We wouldn't be doing anyone any favors by letting them invest their time in a broken tool. This is doubly true for new users such as your students. There are some old GPE snapshots floating around here and there, but those don't really work with GCP in 2018.
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While it is correct that the Google Plugin for Eclipse has been removed from Google's documentation, it is still available.
To install it, in Eclipse Neon, click "Install new Software" and add in this URL.
http://dl.google.com/eclipse/plugin/4.6
Next, click through the dialogues to allow the installation, and restart Eclipse.
While Google has chosen to stop supporting this, I personally feel that the new Cloud Tools for Eclipse plugin is just not ready. Also, in the early days, JDO was what many people were using on top of the data layer, and since GAE has been around for 10+ years, many of us have a lot of infrastructure built on top of this that is costly to change. While it's still possible, in theory, to run the DataNucleus enhancer manually, it's a huge pain that requires in-depth knowledge of the inner workings of GAE and DataNucleus and knowledge of which dependencies go together. It may have been well-documented in the past, but today it is not.
But be warned, one of our engineers recently lost the ability to deploy the project to Google App Engine using the GPE and was forced to use the gcloud tool, which doesn't seem to have sensible defaults, like deploying to a non-default version and instead will deploy straight to default, well, by default. So we're writing a script around that command that will pass in --no-promote so it doesn't immediately start migrating traffic... Visit the gcloud reference for app deploy for more details. Good luck!
For more information on the install process, please see How to install Google Plugin for Eclipse on mkyong.com.
That official documentation is most likely gone for good, the plugin was deprecated in favour of the Google Cloud Tools for Eclipse. From Migrating from the Google Plugin for Eclipse:
You can check the snapshots of the docs on the Wayback Machine, and maybe still find the matching code repositories, if they haven't been removed as well.
But it's probably a good idea to switch to the supported tools sooner than later, especially since they're just getting started.
Related: Migrating GAE project to Java 8 - get XML validation error after adding runtime property to appengine-web.xml