Here we describe the process for making MIBs manageable through the SNMP protocol adaptor of the Java DMK. In our example, we demonstrate this process on a subset of the MIB-II defined by RFC 1213.
Once you have defined the MIB you want to manage in your SNMP agent you need to generate its MBean representation using the mibgen tool. This tool generates MBeans that represent the whole MIB, each of its groups and nested groups, and each of its table entries. This command-line tool and its output are fully described in the Java Dynamic Management Kit 5.0 Tools Reference.
The mibgen tool only generates the MBean structure to represent the MIB, it is up to the developer to implement the MIB functionality inside the generated classes. Our example gives only a simple implementation of the MIB-II for demonstration purposes. However, this shows you the way to extend the generated classes to provide your own implementation.
The mibgen tool handles all three SNMP protocols identically, and the MIBs implemented are entirely protocol neutral. Consequently, code generated by mibgen for previous versions works perfectly in the SNMPv3 framework.
To run the mibgen tool for our example, go to the examplesDir/Snmp/Agent directory and type the following command:
$ mibgen -d . mib_II_subset.txt
This generates the following files in the current directory:
The MBean (by inheritance) for the whole MIB: RFC1213_MIB.java
The MBean and its helper class for the Snmp group: Snmp.java, SnmpMBean.java, SnmpMeta.java
The MBean and its helper class for the System group: System.java, SystemMBean.java, SystemMeta.java
The MBean and its helper class for the Interfaces group: Interfaces.java, InterfacesMBean.java, InterfacesMeta.java
The class representing the Interfaces table, and the MBean representing entries in the table: TableIfTable.java, IfEntry.java, IfEntryMBean.java, IfEntryMeta.java
Classes representing enumerated types used in these groups: EnumSnmpEnableAuthenTraps.java, EnumIfOperStatus.java, EnumIfAdminStatus.java, EnumIfType.java
The OID table for SNMP managers wanting to access this MIB: RFC1213_MIBOidTable.java
The MBean with the name of the MIB is a central administrative class for managing the other MBeans that represent the MIB groups and table entries. All of the other MBeans contain the SNMP variables as attributes of their management interface. The mibgen tool generates standard MBeans for the MIB, so attributes are implemented with individual getter and setter methods.
These MBeans are just skeletons, meaning that the attribute implementations only return a default value. You must implement the getters and setters of the attributes to read and write data with the correct semantic meaning of the SNMP variable.
Because SNMP does not support actions in MIBs, the only operations in these MBeans are checkers associated with the SNMP “Set” request in writeable variables. Again, these are skeleton methods that you must implement to do the checks that you require before the corresponding “Set” operation. You can add operations and expose them in the MBean interface, but the SNMP manager cannot access them. However, other managers can call these operations if they are connected through another protocol.
Our example only implements a fraction of the attributes, those that are used in this tutorial. The others are simply initialized with a plausible value. Using DEFVAL statements in our MIB, we could force mibgen to generate MBeans with user-defined default values for attributes. As this is not done in our example, mibgen provides a plausible default value according to the variable type.
Our implementations of MIB behavior are contained in the classes with the Impl suffix. These implementation classes extend those that are generated by mibgen so that we can regenerate them without overwriting our customizations.
Here is a summary of the implementation shown in the agent example:
InterfacesImpl.java - adds a notification listener to the IfTable object, then creates two table entries with plausible values and adds them to the table; this class is associated with:
TableEntryListenerImpl.java - the listener for table notifications when entries are added or removed: it prints out the values of a new table entry and prints a message when an entry is removed
IfEntryImpl.java - implements a table entry and provides an internal method for switching the OperStatus variable that triggers a trap (see Example 18–4); this method is not exposed in the MBean interface, so it is only available to the code of this agent application
SnmpImpl.java - initializes and implements variables of the SNMP group; many of these are state variables of the SNMP agent, so we call the getter methods of the SNMP adaptor object to return the information
SystemImpl.java - initializes the System group variables with realistic values
The SnmpImpl.java and SystemImpl.java files provide code that you can reuse when you need to implement these common SNMP groups.
Compile all the classes in the examplesDir/Snmp/Agent directory. The classpath must contain the current directory (.):
$ javac -classpath classpath -d . *.java
We are now ready to look at the implementation of SNMPv1/v2 and SNMPv3 agents and run the example applications.