To call an SAP function module from a Java application, you first need SAP JCo (Java Connector); you can download it from
SAP's site, though you need an account to do so.
To actually open the connection, use the tip code shown here.
Class jcoClass = Class.forName("com.sap.mw.jco.JCO", true, this.getClass().getClassLoader());
Client client = JCO.createClient(clientNumber, user, passWord, lang, host, systemId);
//all parameters are strings
client.connect();
Repository repository = new JCO.Repository("SAP", client);
IFunctionTemplate functionTemplate = repository.getFunctionTemplate
("YOUR_FUNCTIONAL_MODULE_NAME");
Function function = functionTemplate.getFunction();
ParameterList parameterList = function.getImportParameterList();
parameterList.setValue("value1", "INPUT_PARAM_NAME");
parameterList.setValue("value2", "INPUT_PARAM_NAME2");
client.execute(function);