P4J is intended for use in standalone and embedded products such as Perforce's P4WSAD Eclipse plug-in or third-party products. P4J does not support all possible Perforce server commands. In particular, P4J does not support some of the more obscure Perforce admin or job-related commands, but the set of services available through P4J is intended to be useful and sufficient for day-to-day developer work and for most scripted applications.
P4J allows the user to select a protocol for communicating with Perforce servers (see the P4JServerFactory class in the com.perforce.p4j.server package); the details of these protocols are typically fairly opaque, but currently the two major protocol implementations use the Perforce C++ API or the Perforce p4 command-line interpreter respectively.
P4J assumes JDK 5 or later (it makes extensive use of parameterized types, for example), and -- depending on the protocol chosen -- your environment must include a working p4 command line interpreter (which comes with every Perforce installation) or the C++ API (optionally available from Perforce's website). In most client configurations, one or both of these will already be installed in any case -- ask your IT staff.
The main P4J usage model is described in detail elsewhere, but in summary, the vast majority of uses start with the server factory, P4JServerFactory; this factory is used to obtain a P4JServer object that can be used to issue commands to the associated Perforce server. Other important interfaces and objects include the P4JClient (usually obtained through a P4JServer method call), which is used to issue Perforce client-specific commands (such as Perforce client workspace file edit, add, delete, and sync operations); the P4JChangeList interface for Perforce changelists; and the various Perforce file interfaces for interrogating and managing Perforce-managed files.
Note that P4J is thread-safe when each thread uses its own P4JServer object and subsidiary objects; other usage models do not guarantee thread-safety.