org.jbpm.context.exe.ContextInstance serves as the central interface to work with process variables. You can obtain the ContextInstance from a ProcessInstance like this :

ProcessInstance processInstance = ...;
ContextInstance contextInstance = (ContextInstance) processInstance.getInstance(ContextInstance.class);

The most basic operations are

void ContextInstance.setVariable(String variableName, Object value);
void ContextInstance.setVariable(String variableName, Object value, Token token);
Object ContextInstance.getVariable(String variableName);
Object ContextInstance.getVariable(String variableName, Token token);

The variable names are java.lang.String. By default, jBPM supports the following value types:

Also an untyped null value can be stored persistently.

All other types can be stored in the process variables without any problem. But it will cause an exception when you try to save the process instance.

To configure jBPM for storing hibernate persistent objects in the variables, see Storing hibernate persistent objects.