Class AgentConfigManager

java.lang.Object
org.snmp4j.agent.AgentConfigManager
All Implemented Interfaces:
Runnable, VariableProvider

public class AgentConfigManager extends Object implements Runnable, VariableProvider
The AgentConfigManager is the main component of a SNMP4J-Agent. It puts together agent configuration and agent components like command processor, message dispatcher, managed objects and server, and standard MIB modules like USM, VACM, etc.
Since:
1.2
Version:
3.0
Author:
Frank Fock
  • Field Details

    • agent

      protected CommandProcessor agent
    • workerPool

      protected org.snmp4j.util.WorkerPool workerPool
    • vacm

      protected VACM vacm
    • usm

      protected org.snmp4j.security.USM usm
    • servers

      protected MOServer[] servers
    • session

      protected org.snmp4j.Session session
    • dispatcher

      protected org.snmp4j.MessageDispatcher dispatcher
    • engineID

      protected org.snmp4j.smi.OctetString engineID
    • proxyForwarder

      protected ProxyForwarder proxyForwarder
    • notificationOriginator

      protected NotificationOriginator notificationOriginator
    • configuration

      protected MOInputFactory configuration
    • persistenceProvider

      protected MOPersistenceProvider persistenceProvider
    • persistenceImportMode

      protected int persistenceImportMode
    • engineBootsProvider

      protected org.snmp4j.cfg.EngineBootsProvider engineBootsProvider
    • counterSupport

      protected org.snmp4j.mp.CounterSupport counterSupport
    • securityProtocols

      protected org.snmp4j.security.SecurityProtocols securityProtocols
    • securityModels

      protected org.snmp4j.security.SecurityModels securityModels
    • snmpv2MIB

      protected SNMPv2MIB snmpv2MIB
    • snmpMpdMib

      protected SnmpMpdMib snmpMpdMib
    • targetMIB

      protected SnmpTargetMIB targetMIB
    • communityMIB

      protected SnmpCommunityMIB communityMIB
    • notificationMIB

      protected SnmpNotificationMIB notificationMIB
    • frameworkMIB

      protected SnmpFrameworkMIB frameworkMIB
    • usmMIB

      protected UsmMIB usmMIB
    • vacmMIB

      protected VacmMIB vacmMIB
    • tlsTmMib

      protected SnmpTlsTmMib tlsTmMib
    • proxyMIB

      protected SnmpProxyMIB proxyMIB
    • snmpUsmDhObjectsMib

      protected SnmpUsmDhObjectsMib snmpUsmDhObjectsMib
    • snmp4jLogMIB

      protected Snmp4jLogMib snmp4jLogMIB
    • snmp4jConfigMIB

      protected Snmp4jConfigMib snmp4jConfigMIB
    • snmp4jProxyMIB

      protected Snmp4jProxyMib snmp4jProxyMIB
    • notificationLogMIB

      protected NotificationLogMib notificationLogMIB
    • notificationLogDefaultLimit

      protected org.snmp4j.smi.UnsignedInteger32 notificationLogDefaultLimit
    • notificaitonLogGlobalLimit

      protected org.snmp4j.smi.UnsignedInteger32 notificaitonLogGlobalLimit
    • notificaitonLogGlobalAge

      protected org.snmp4j.smi.UnsignedInteger32 notificaitonLogGlobalAge
    • moFactory

      protected MOFactory moFactory
    • sysDescr

      protected org.snmp4j.smi.OctetString sysDescr
    • sysOID

      protected org.snmp4j.smi.OID sysOID
    • sysServices

      protected org.snmp4j.smi.Integer32 sysServices
    • defaultContext

      protected org.snmp4j.smi.OctetString defaultContext
    • runState

      protected AgentState runState
    • tableSizeLimit

      protected MOTableSizeLimit<MOTableRow> tableSizeLimit
    • diffieHellmanKickstartParameters

      protected Collection<DHKickstartParameters> diffieHellmanKickstartParameters
    • agentStateListeners

      protected List<AgentStateListener<AgentConfigManager>> agentStateListeners
  • Constructor Details

    • AgentConfigManager

      public AgentConfigManager(org.snmp4j.smi.OctetString agentsOwnEngineID, org.snmp4j.MessageDispatcher messageDispatcher, VACM vacm, MOServer[] moServers, org.snmp4j.util.WorkerPool workerPool, MOInputFactory configurationFactory, MOPersistenceProvider persistenceProvider, org.snmp4j.cfg.EngineBootsProvider engineBootsProvider)
      Creates a SNMP agent configuration which can be run by calling run() later.
      Parameters:
      agentsOwnEngineID - the authoritative engine ID of the agent.
      messageDispatcher - the MessageDispatcher to use. The message dispatcher must be configured outside, i.e. transport mappings have to be added before this constructor is being called.
      vacm - a view access control model. Typically, this parameter is set to null to use the default VACM associated with the VacmMIB.
      moServers - the managed object server(s) that serve the managed objects available to this agent.
      workerPool - the WorkerPool to be used to process incoming request.
      configurationFactory - a MOInputFactory that creates a MOInput stream with containing serialized ManagedObject information with the agent's configuration or null otherwise.
      persistenceProvider - the primary MOPersistenceProvider to be used to load and store persistent MOs.
      engineBootsProvider - the provider of engine boots counter.
    • AgentConfigManager

      public AgentConfigManager(org.snmp4j.smi.OctetString agentsOwnEngineID, org.snmp4j.MessageDispatcher messageDispatcher, VACM vacm, MOServer[] moServers, org.snmp4j.util.WorkerPool workerPool, MOInputFactory configurationFactory, MOPersistenceProvider persistenceProvider, org.snmp4j.cfg.EngineBootsProvider engineBootsProvider, MOFactory moFactory)
      Creates a SNMP agent configuration which can be run by calling run() later.
      Parameters:
      agentsOwnEngineID - the authoritative engine ID of the agent.
      messageDispatcher - the MessageDispatcher to use. The message dispatcher must be configured outside, i.e. transport mappings have to be added before this constructor is being called.
      vacm - a view access control model. Typically, this parameter is set to null to use the default VACM associated with the VacmMIB.
      moServers - the managed object server(s) that serve the managed objects available to this agent.
      workerPool - the WorkerPool to be used to process incoming request.
      configurationFactory - a MOInputFactory that creates a MOInput stream with containing serialized ManagedObject information with the agent's configuration or null otherwise.
      persistenceProvider - the primary MOPersistenceProvider to be used to load and store persistent MOs.
      engineBootsProvider - the provider of engine boots counter.
      moFactory - the MOFactory to be used to create ManagedObjects created by this config manager. If null the DefaultMOFactory will be used.
      Since:
      1.4
    • AgentConfigManager

      public AgentConfigManager(org.snmp4j.smi.OctetString agentsOwnEngineID, org.snmp4j.MessageDispatcher messageDispatcher, VACM vacm, MOServer[] moServers, org.snmp4j.util.WorkerPool workerPool, MOInputFactory configurationFactory, MOPersistenceProvider persistenceProvider, org.snmp4j.cfg.EngineBootsProvider engineBootsProvider, MOFactory moFactory, Collection<DHKickstartParameters> diffieHellmanKickstartParameters)
      Creates a SNMP agent configuration which can be run by calling run() later.
      Parameters:
      agentsOwnEngineID - the authoritative engine ID of the agent.
      messageDispatcher - the MessageDispatcher to use. The message dispatcher must be configured outside, i.e. transport mappings have to be added before this constructor is being called.
      vacm - a view access control model. Typically, this parameter is set to null to use the default VACM associated with the VacmMIB.
      moServers - the managed object server(s) that serve the managed objects available to this agent.
      workerPool - the WorkerPool to be used to process incoming request.
      configurationFactory - a MOInputFactory that creates a MOInput stream with containing serialized ManagedObject information with the agent's configuration or null otherwise.
      persistenceProvider - the primary MOPersistenceProvider to be used to load and store persistent MOs.
      engineBootsProvider - the provider of engine boots counter.
      moFactory - the MOFactory to be used to create ManagedObjects created by this config manager. If null the DefaultMOFactory will be used.
      diffieHellmanKickstartParameters - an optional list of Diffie Hellman kickstart parameters to initialize the USM authentication and privacy keys.
      Since:
      3.0
  • Method Details

    • setContext

      public void setContext(org.snmp4j.security.SecurityModels securityModels, org.snmp4j.security.SecurityProtocols securityProtocols, org.snmp4j.mp.CounterSupport counterSupport)
      Sets context objects needed for common security services that are derived from static instances by default. By setting this to a different set of instances of these objects for each agent, several agents controlled by different AgentConfigManager instances can coexist within the same Java process. Thus, using this method is required before calling run() for concurrent execution of run(), i.e. in a multi-threaded multi-agent architecture.
      Parameters:
      securityModels - the security models supported by this agent.
      securityProtocols - the security protocols supported by this agent.
      counterSupport - the CounterSupport instance that records and returns all event counters of this agent.
      Since:
      3.5.0
    • run

      public void run()
      Initializes, configures, restores agent state, and then launches the SNMP agent depending on its current run state. For example, if initialize() has not yet been called it will be called before the agent is being configured in the next step.

      See also initialize(), configure(), restoreState(), and launch().

      Specified by:
      run in interface Runnable
    • addAgentStateListener

      public void addAgentStateListener(AgentStateListener<AgentConfigManager> agentStateListener)
    • removeAgentStateListener

      public boolean removeAgentStateListener(AgentStateListener<AgentConfigManager> agentStateListener)
    • fireAgentStateChange

      protected void fireAgentStateChange()
    • getState

      public int getState()
      Returns the state of the agent.
      Returns:
      a integer constant from AgentState.STATE_CREATED thru AgentState.STATE_RUNNING.
    • getVACM

      public VACM getVACM()
      Returns the VACM used by this agent config manager.
      Returns:
      the VACM instance of this agent.
      Since:
      1.4
    • getSNMPv2MIB

      public SNMPv2MIB getSNMPv2MIB()
      Returns the SNMPv2-MIB implementation used by this config manager.
      Returns:
      the SNMPv2MIB instance of this agent.
      Since:
      1.4
    • getSnmpMpdMib

      public SnmpMpdMib getSnmpMpdMib()
      Returns the SNMPv2-MPD-MIB implementation used by this config manager.
      Returns:
      the SnmpMpdMib instance of this agent.
      Since:
      2.6
    • getSnmpTargetMIB

      public SnmpTargetMIB getSnmpTargetMIB()
      Returns the SNMP-TARGET-MIB implementation used by this config manager.
      Returns:
      the SnmpTargetMIB instance of this agent.
      Since:
      1.4
    • getSnmpNotificationMIB

      public SnmpNotificationMIB getSnmpNotificationMIB()
      Returns the SNMP-NOTIFICATION-MIB implementation used by this config manager.
      Returns:
      the SnmpNotificationMIB instance of this agent.
      Since:
      1.4
    • getSnmpCommunityMIB

      public SnmpCommunityMIB getSnmpCommunityMIB()
      Returns the SNMP-COMMUNITY-MIB implementation used by this config manager.
      Returns:
      the SnmpCommunityMIB instance of this agent.
      Since:
      1.4
    • getNotificationLogMIB

      public NotificationLogMib getNotificationLogMIB()
      Returns the NOTIFICATION-LOG-MIB implementation used by this config manager.
      Returns:
      the NotificationLogMib instance of this agent.
      Since:
      1.4.2
    • getSnmp4jLogMIB

      public Snmp4jLogMib getSnmp4jLogMIB()
      Returns the SNMP4J-LOG-MIB implementation used by this config manager.
      Returns:
      the Snmp4jLogMib instance of this agent.
      Since:
      1.4.2
    • getSnmp4jConfigMIB

      public Snmp4jConfigMib getSnmp4jConfigMIB()
      Returns the SNMP4J-CONFIG-MIB implementation used by this config manager.
      Returns:
      the Snmp4jConfigMib instance of this agent.
      Since:
      1.4.2
    • getSnmp4jProxyMIB

      public Snmp4jProxyMib getSnmp4jProxyMIB()
      Returns the SNMP4J-CONFIG-MIB implementation used by this config manager.
      Returns:
      the Snmp4jConfigMib instance of this agent.
      Since:
      2.0
    • launch

      protected void launch()
      Launch the agent by registering and lauching (i.e., set to listen mode) transport mappings.
    • fireLaunchNotifications

      protected void fireLaunchNotifications()
      Fire notifications after agent start, i.e. sending a coldStart trap.
    • continueProcessing

      public boolean continueProcessing()
      Continues processing of SNMP requests by coupling message dispatcher and agent. To succeed, the current state of the agent must be AgentState.STATE_SUSPENDED.
      Returns:
      true if the running state could be restored, false otherwise.
    • suspendProcessing

      public void suspendProcessing()
      Suspends processing of SNMP requests. This call decouples message dispatcher and agent. All transport mappings remain unchanged and thus all ports remain opened.
    • shutdown

      public void shutdown()
      Shutdown the agent by closing the internal SNMP session - including the transport mappings provided through the configured MessageDispatcher and then store the agent state to persistent storage (if available).
    • waitUntilPendingModificationsDone

      protected void waitUntilPendingModificationsDone()
      Wait until all pending modifications (external via SNMP request or internal by LockRequest) to any of the servers are done.
      Since:
      3.7.0
    • registerShutdownHook

      public void registerShutdownHook()
      Registers a shutdown hook Thread at the Runtime instance.
    • initSnmp4jLogMIB

      public void initSnmp4jLogMIB()
    • initSnmp4jConfigMIB

      public void initSnmp4jConfigMIB(MOPersistenceProvider[] persistenceProvider)
    • initSnmp4jProxyMIB

      public void initSnmp4jProxyMIB(org.snmp4j.smi.OctetString context)
    • getCommandProcessor

      public CommandProcessor getCommandProcessor()
    • getVacmMIB

      public VacmMIB getVacmMIB()
    • getUsm

      public org.snmp4j.security.USM getUsm()
    • getUsmMIB

      public UsmMIB getUsmMIB()
    • getSecurityProtocols

      public org.snmp4j.security.SecurityProtocols getSecurityProtocols()
    • getSecurityModels

      public org.snmp4j.security.SecurityModels getSecurityModels()
    • initNotificationLogMIB

      protected void initNotificationLogMIB(VACM vacm, SnmpNotificationMIB notifyMIB)
    • initSecurityContext

      protected void initSecurityContext()
    • initSecurityModels

      protected void initSecurityModels(org.snmp4j.cfg.EngineBootsProvider engineBootsProvider)
    • initMessageDispatcherWithMPs

      protected void initMessageDispatcherWithMPs(org.snmp4j.MessageDispatcher mp)
    • registerTransportMappings

      protected void registerTransportMappings()
    • launchTransportMappings

      protected void launchTransportMappings() throws IOException
      Throws:
      IOException
    • launchTransportMappings

      protected static void launchTransportMappings(Collection<? extends org.snmp4j.TransportMapping<?>> transportMappings) throws IOException
      Puts a list of transport mappings into listen mode.
      Parameters:
      transportMappings - a list of TransportMapping instances.
      Throws:
      IOException - if a transport cannot listen to incoming messages.
    • stopTransportMappings

      protected static void stopTransportMappings(Collection<? extends org.snmp4j.TransportMapping<?>> transportMappings) throws IOException
      Closes a list of transport mappings.
      Parameters:
      transportMappings - a list of TransportMapping instances.
      Throws:
      IOException - if a transport cannot be closed.
    • saveState

      public boolean saveState()
      Save the state of the agent persistently - if necessary persistent storage is available.
      Returns:
      true if state has been saved successfully, false is returned if an error occurred or no persistenceProvider is set. The error details can be found in the runState object.
    • restoreState

      public boolean restoreState()
      Restore a previously persistently saved state - if available.
      Returns:
      true if the agent state could be restored successfully, false otherwise.
    • configure

      public void configure()
      Configures components and managed objects using the getConfigurationFactory(). If the factory is null then nothing will be done. In any case, the agent state is advanced to AgentState.STATE_CONFIGURED if no error occurred.
    • initMandatoryMIBs

      protected void initMandatoryMIBs()
    • linkCounterListener

      protected void linkCounterListener()
    • getSupportedSecurityProtocols

      protected org.snmp4j.security.SecurityProtocols getSupportedSecurityProtocols()
      Gets the set of security protocols supported by this agent configuration (only called if initialize() if setContext(SecurityModels, SecurityProtocols, CounterSupport) did not set a non-null SecurityProtocols instance before.
      Returns:
      SecurityProtocols.getInstance() by default after initialization by SecurityProtocols.addDefaultProtocols().
    • createUSM

      protected org.snmp4j.security.USM createUSM()
      Creates the USM used by this agent configuration.
      Returns:
      an USM initialized by the engine boots from the engineBootsProvider and engineID.
    • createTSM

      protected org.snmp4j.security.TSM createTSM()
      Creates the TSM used by this agent configuration.
      Returns:
      an USM initialized by the engine boots from the engineBootsProvider and engineID.
    • getSysServices

      public org.snmp4j.smi.Integer32 getSysServices()
      Gets the system services ID which can be modified by altering its value.
      Returns:
      72 by default.
    • getSysOID

      public org.snmp4j.smi.OID getSysOID()
      Gets the system OID which can be modified by altering its value.
      Returns:
      an OID - by default the SNMP4J root OID is returned.
    • getSysDescr

      public org.snmp4j.smi.OctetString getSysDescr()
      Returns the sysDescr.0 value for this agent which can be modified by altering its value.
      Returns:
      an OctetString describing the node of the form SNMP4J-Agent version [SNMP4J-version] - <os.name> - <os.arch> - <os.version>.
    • getSysUpTime

      public SysUpTime getSysUpTime()
      Gets the sysUpTime.0 instance for the default context.
      Returns:
      a SysUpTime instance.
    • getNotificationOriginator

      public NotificationOriginator getNotificationOriginator()
      Returns the notification originator of this agent configuration. To get the (multi-threaded) NotificationOriginator of the agent, use getAgentNotificationOriginator() instead.
      Returns:
      a NotificationOriginator instance.
    • getAgentNotificationOriginator

      public NotificationOriginator getAgentNotificationOriginator()
      Returns the notification originator of the agent. Use this method to get a NotificationOriginator for sending your notifications.
      Returns:
      the NotificationOriginator instance.
      Since:
      1.4
    • setNotificationOriginator

      public void setNotificationOriginator(NotificationOriginator notificationOriginator)
      Sets the notification originator of this agent configuration.
      Parameters:
      notificationOriginator - a NotificationOriginator instance.
    • getCounterSupport

      public org.snmp4j.mp.CounterSupport getCounterSupport()
    • setCounterSupport

      public void setCounterSupport(org.snmp4j.mp.CounterSupport counterSupport)
    • getConfigurationFactory

      public MOInputFactory getConfigurationFactory()
      Gets the factory that generates a MOInput instance to load initial or reset configuration into the agent.
      Returns:
      a configuration provider factory or null if no configuration is set.
      Since:
      3.5.5
    • setConfigurationFactory

      public void setConfigurationFactory(MOInputFactory configuration)
      Sets the configuration factory that generates a MOInput instance to load initial or reset configuration into the agent. Configuration is a read-only source for MIB data whereas a getPersistenceProvider() provides the mechanism to store and restore MIB data persistently.
      Parameters:
      configuration - a configuration provider factory.
      Since:
      3.5.5
    • getPersistenceProvider

      public MOPersistenceProvider getPersistenceProvider()
      Gets the persistence provider which stores and restores non-volatile (i.e., persistent) MIB data.
      Returns:
      a MOPersistenceProvider instance.
      Since:
      3.5.5
    • setPersistenceProvider

      public void setPersistenceProvider(MOPersistenceProvider persistenceProvider)
      Sets the persistence provider which stores and restores non-volatile (i.e., persistent) MIB data.
      Parameters:
      persistenceProvider - a MOPersistenceProvider instance.
      Since:
      3.5.5
    • initialize

      public void initialize()
      Initialize the agent by creating counter support, SNMP session, engine ID, command processor, worker pool, security models (initSecurityModels(EngineBootsProvider)), message dispatcher with message processing models (initMessageDispatcherWithMPs(MessageDispatcher), mandatory MIBs (initMandatoryMIBs()), link MIBs with each other, link counter listeners (linkCounterListener()), and initialize optional MIBs (initOptionalMIBs(). Advance state to AgentState.STATE_INITIALIZED.
    • createCounterSupport

      protected org.snmp4j.mp.CounterSupport createCounterSupport()
    • setTableSizeLimits

      public void setTableSizeLimits(Properties sizeLimits)
      Sets the table size limits for the tables in this agent. If this method is called while the agent's registration is being changed, a ConcurrentModificationException might be thrown.
      Parameters:
      sizeLimits - a set of properties as defined by MOTableSizeLimit.
      Since:
      1.4
    • setTableSizeLimit

      public void setTableSizeLimit(int sizeLimit)
      Sets the table size limit for the tables in this agent. If this method is called while the agent's registration is being changed, a ConcurrentModificationException might be thrown.
      Parameters:
      sizeLimit - the maximum size (numer of rows) of tables allowed for this agent.
      Since:
      1.4
    • initOptionalMIBs

      protected void initOptionalMIBs()
    • initSnmpUsmDhObjectsMib

      protected void initSnmpUsmDhObjectsMib()
    • getDhKickstartParameters

      protected Collection<DHKickstartParameters> getDhKickstartParameters()
      Overwrite this method to provide Diffie Hellman kick start parameters.
      Returns:
      an empty list of DHKickstartParameters by default.
      Since:
      3.0
    • getDefaultContext

      public org.snmp4j.smi.OctetString getDefaultContext()
      Returns the default context - which is the context that is used by the base agent to register its MIB objects. By default it is null which causes the objects to be registered virtually for all contexts. In that case, subagents for example my not register their own objects under the same subtree(s) in any context. To allow subagents to register their own instances of those MIB modules, an empty OctetString should be used as default context instead.
      Returns:
      null or an OctetString (normally the empty string) denoting the context used for registering default MIBs.
    • getContext

      protected org.snmp4j.smi.OctetString getContext(MOGroup mibGroup, org.snmp4j.smi.OctetString defaultContext)
      This method can be overwritten by a subagent to specify the contexts each MIB module (group) will be registered to.
      Parameters:
      mibGroup - a group of ManagedObjects (i.e., a MIB module).
      defaultContext - the context to be used by default (i.e., the null context)
      Returns:
      the context for which the module should be registered.
    • registerMIBs

      protected void registerMIBs(org.snmp4j.smi.OctetString context) throws DuplicateRegistrationException
      Register the initialized MIB modules in the specified context of the agent.
      Parameters:
      context - the context to register the internal MIB modules. This should be null by default.
      Throws:
      DuplicateRegistrationException - if some of the MIB modules registration regions conflict with already registered regions.
    • unregisterMIBs

      protected void unregisterMIBs(org.snmp4j.smi.OctetString context)
      Unregister the initialized MIB modules from the default context of the agent.
      Parameters:
      context - the context where the MIB modules have been previously registered.
    • setupProxyForwarder

      public void setupProxyForwarder()
    • setupSnmpUsmDhObjectsMib

      public void setupSnmpUsmDhObjectsMib(Collection<DHKickstartParameters> dhKickstartParameters)
    • createNotificationOriginator

      protected NotificationOriginator createNotificationOriginator()
    • createProxyForwarder

      protected ProxyForwarder createProxyForwarder(CommandProcessor agent)
      Creates and registers the default proxy forwarder application (ProxyForwarderImpl).
      Parameters:
      agent - the command processor that uses the proxy forwarder.
      Returns:
      a ProxyForwarder instance.
    • createCommandProcessor

      protected CommandProcessor createCommandProcessor(org.snmp4j.smi.OctetString engineID)
      Creates the command processor.
      Parameters:
      engineID - the engine ID of the agent.
      Returns:
      a new CommandProcessor instance.
    • createSnmpSession

      protected org.snmp4j.Session createSnmpSession(org.snmp4j.MessageDispatcher dispatcher)
      Creates the SNMP session to be used for this agent.
      Parameters:
      dispatcher - the message dispatcher to be associated with the session.
      Returns:
      a SNMP session (a Snmp instance by default).
    • setPersistenceImportMode

      public void setPersistenceImportMode(int importMode)
      Sets the import mode for the MOPersistenceProvider.
      Parameters:
      importMode - one of the import modes defined by ImportMode.
      Since:
      1.4
    • getPersistenceImportMode

      public int getPersistenceImportMode()
      Returns the currently active import mode for the MOPersistenceProvider.
      Returns:
      one of the import modes defined by ImportMode.
      Since:
      1.4
    • getVariable

      public org.snmp4j.smi.Variable getVariable(String name)
      Description copied from interface: VariableProvider
      Gets the variable with the specified name, which might be either a textual name or an instance OID.
      Specified by:
      getVariable in interface VariableProvider
      Parameters:
      name - the name or OID of the variable to return.
      Returns:
      a Variable instance or null if such a variable with the specified name or OID does not exists.