Finding out whether process model instance is completed from java code

No. Plug-ins can use injected services in their constructor that are automatically injected by Appian by simply having them as arguments in your constructor. Check the examples under Shared Components. They have the source code, they will be good training.

For example: "Process Management Services" has ResumeProcesses.java which in its constructor states that Appian should inject ProcessExecutionService automatically, so all the creator needed was to put that in the constructor and store that reference in a variable.

The author designed a slightly more complex architecture than what you normally need but what she did basically translates into this (see how she can use the "pes" variable in her run method just because she declared it as a variable that'll be populated automatically by Appian in the constructor)

@ProcessManagement
public class ResumeProcesses extends extends AppianSmartService{

private ProcessExecutionService pes;

          public ResumeProcesses(ProcessExecutionService pes) {
this.pes = pes
          }

          @Override
          public void run() throws SmartServiceException {
                    Integer[] resultCodeIds;

                    if (dryRun) {
                              resultCodeIds = new ProcessManagerDryRun(pes).resumeProcesses(processIds);
                    } else {
                              resultCodeIds = pes.resumeProcesses(processIds);
                    }

                    setResultCodeList(resultCodeIds);
          }

No. Plug-ins can use injected services in their constructor that are automatically injected by Appian by simply having them as arguments in your constructor. Check the examples under Shared Components. They have the source code, they will be good training.

For example: "Process Management Services" has ResumeProcesses.java which in its constructor states that Appian should inject ProcessExecutionService automatically, so all the creator needed was to put that in the constructor and store that reference in a variable.

The author designed a slightly more complex architecture than what you normally need but what she did basically translates into this (see how she can use the "pes" variable in her run method just because she declared it as a variable that'll be populated automatically by Appian in the constructor)

@ProcessManagement
public class ResumeProcesses extends extends AppianSmartService{

private ProcessExecutionService pes;

          public ResumeProcesses(ProcessExecutionService pes) {
this.pes = pes
          }

          @Override
          public void run() throws SmartServiceException {
                    Integer[] resultCodeIds;

                    if (dryRun) {
                              resultCodeIds = new ProcessManagerDryRun(pes).resumeProcesses(processIds);
                    } else {
                              resultCodeIds = pes.resumeProcesses(processIds);
                    }

                    setResultCodeList(resultCodeIds);
          }