Overview
P3 runs a pure instance of Equinox plug-in system right within Jira seamlessly integrated.
Thus P3 is essentially Equinox with access to Atlassian’s public Java API for Jira API and the Data Center underlying capabilities provided by Jira to scale and run Equinox on a cluster.
Equinox is a popular and powerful plugin framework sponsored by the Eclipse Foundation and the reference implementation of the industry-standard OSGi specifications.
Atlassian’s UPM and P3 share a lot of concepts as they are both OSGi plug-in frameworks but also there are a lot of big differences at times.
P3 is not a replacement for Atlassian’s P2 plug-in framework. In fact, P3 does not support the concept of UPM plug-in modules which is native to Atlassian's products: you cannot create modules specific to Jira like tab panels, web panels, JQL functions, etc. directly from Equinox.
You are free to build whatever you can imagine if it is compatible with Equinox. And this is really powerful indeed. P3 allows using a bunch of technologies in an easier way. Of course, in the end, everything is Java code running on the JVM so using P3 or P2 does not restrict what you can do with Java. the difference is how classes are loaded and what they “can view”.
Next steps