This page describes how the SNMP4J-Agent MOFactory concept can be used to apply customised columnar objects to SNMP table implementations.

Step-by-step guide

This guide assumes that you have generated your MIB instrumentation code with AgenPro, but you can do the same with manual written code too.

1. Generate the MIB module instrumentation code with AgenPro and activate factoryColumn property (set its value to yes) for your entire MIB module (or all affected tables). This will create the code to create MOColumn instances for read-only columns using the MOFactory provided to your MIB module.
   This step is important, because we will create our own MOFactory implementation to create our own MOColumn subclasses. 
   The generated code for each table will look like (sample taken from the Snmp4jAgentTutorialMib.java):

   @SuppressWarnings(value={"unchecked"}) private void createSnmp4jAgentTutorialFileTreeEUEntry(MOFactory moFactory) { // Index definition snmp4jAgentTutorialFileTreeEUEntryIndexes = new MOTableSubIndex[] { moFactory.createSubIndex(oidSnmp4jAgentTutorialFileTreeEUIndex, SMIConstants.SYNTAX_OBJECT_IDENTIFIER, 0, 128) }; snmp4jAgentTutorialFileTreeEUEntryIndex = moFactory.createIndex(snmp4jAgentTutorialFileTreeEUEntryIndexes, false, new MOTableIndexValidator() { public boolean isValidIndex(OID index) { boolean isValidIndex = true; //--AgentGen BEGIN=snmp4jAgentTutorialFileTreeEUEntry::isValidIndex //--AgentGen END return isValidIndex; } }); // Columns MOColumn[] snmp4jAgentTutorialFileTreeEUEntryColumns = new MOColumn[10]; snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEUPath] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEUPath, SMIConstants.SYNTAX_OCTET_STRING, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY), tcModuleSNMPv2Tc, tcDefDisplayString); snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEUType] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEUType, SMIConstants.SYNTAX_INTEGER, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY)); snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEUCreationTime] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEUCreationTime, SMIConstants.SYNTAX_OCTET_STRING, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY), tcModuleSNMPv2Tc, tcDefDateAndTime); snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEULastModified] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEULastModified, SMIConstants.SYNTAX_OCTET_STRING, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY), tcModuleSNMPv2Tc, tcDefDateAndTime); snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEULastAccessed] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEULastAccessed, SMIConstants.SYNTAX_OCTET_STRING, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY), tcModuleSNMPv2Tc, tcDefDateAndTime); snmp4jAgentTutorialFileTreeEUEntryColumns[idxSnmp4jAgentTutorialFileTreeEUSizeInBytes] = moFactory.createColumn(colSnmp4jAgentTutorialFileTreeEUSizeInBytes, SMIConstants.SYNTAX_GAUGE32, moFactory.createAccess(MOAccessImpl.ACCESSIBLE_FOR_READ_ONLY)); ... }

2. As you can see in the above code, the moFactory member of the generated MIB module class is used to create the actual MOColumn instances. To create our custom MOColumn instances we need our own MOFactory instance first:

   myMOFactory = new DefaultMOFactory() {   @Override public <V extends Variable> MOColumn<V> createColumn(int columnID, int syntax, MOAccess access, V defaultValue, boolean mutableInService) { return new MOMutableColumn<V>(columnID, syntax, access, defaultValue, mutableInService) { @Override public V getValue(MOTableRow row, int column, SubRequest subRequest) { if (subRequest.getScope() instanceof MOContextScope) { MOContextScope moContextScope = (MOContextScope)subRequest.getScope(); if ("myContext".equals(moContextScope.getContext())) { ... } } return (V) row.getValue(column); } } } public <V extends Variable> MOColumn<V> createColumn(int columnID, int syntax, MOAccess access, V defaultValue, boolean mutableInService, String tcModuleName, String textualConvention) { TextualConvention<V> tc = getTextualConvention(tcModuleName, textualConvention); if (tc != null) { // Here you might need to create a MOColumn proxy object to apply your interception code  // to columns based on textual conventions too: return tc.createColumn(columnID, syntax, access, defaultValue, mutableInService); } return createColumn(columnID, syntax, access, defaultValue, mutableInService); } }

3. Now you need to use the created MOFactory with your instance(s) of the MIB module. Basically you have to change the MIB module creation to the following. In fact, there are several locations where this can be done when using AgenPro generated code:

   1. When creating the MIB module (see below)

   2. When creating the Module instance that encapsulates all MIB modules generated for an agent

   3. By overriding the Agent.getFactory() method and returning your custom MOFactory

   Snmp4jAgentTutorialMib snmp4jAgentTutorialMib = new Snmp4jAgentTutorialMib(myMOFactory);

4. For GET type requests overriding the MOColumn.getValue method is sufficient. If you also want to control the processing of SET requests, you need also to override the following methods defined in MOMutableColumn as needed:
   1. prepare
   2. commit
   3. undo
   4. cleanup

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