18.3.2.3 JmxMBeanTableMetaImpl

JmxMBeanTableMetaImpl extends the generated JmxMBeanTableMeta class, and overrides the following JmxMBeanTableMeta methods.

• getEntry(SnmpOid rowOid), so that it returns a JmxMBeanEntryImpl object for the indicated row instead of a JmxMBeanEntry table entry.

• contains(SnmpOid oid, Object userData), so that it tells whether the indicated row is present in the table.

• getNextOid(Object userData), so that it returns the row index, the rowOid, of the first row in the table.

• getNextOid(SnmpOid rowOid,Object userData), so that it returns the index of the row that immediately follows the indicated row in the table.

• createNewEntry(SnmpMibSubRequest req, SnmpOid rowOid, int depth), so that it makes it possible to create new rows from SNMP SET requests. Overriding this method is not necessary when the table is read-only.

• removeTableRow(SnmpMibSubRequest req, SnmpOid rowOid, int depth), so that it makes it possible to delete rows from SNMP SET requests. Overriding this method is not necessary when the table is read-only.

In addition to overriding the above methods, it is the JmxMBeanTableMetaImpl class that implements the caching mechanism. An SnmpUserData factory is used to create a transient request-contextual cache, that will remain active for the duration of the incoming request. After the request has completed, the contextual-cache is garbage-collected.

JmxMBeanTableMetaImpl defines the following subclasses, that implement the caching mechanism.

• The JmxMBeanTableData is a snapshot of the table index. It is computed on demand, and cached in a JmxMBeanTableCache object which holds it through a WeakReference. The JmxMBeanTableData includes a time-stamp which is used to determine whether the cached data is still valid, or if it needs to be recomputed.

• The JmxMBeanTableCache is a weak cache used to avoid constant recomputation of the JmxMBeanTableData. It holds the last computed JmxMBeanTableData in a weak reference, so that the garbage collector might reclaim it at any time, provided that no request is currently working on that most-recently computed copy. When an SNMP request that needs to access the jmxMBeanTable comes in, the JmxMBeanTableData is looked up from the JmxMBeanTableCache, and recomputed if needed. The JmxMBeanTableData object is then put in the request-contextual cache where it remains until the request is completed. The request will always consult that particular copy, thus ensuring the consistency of the returned data. If in the meantime a concurrent request comes in, it might obtain the same JmxMBeanTableData copy as the previous request, or a newly computed object, if the previous one was found to be obsolete. Consequently, the two requests might work on the same, or on different snapshots of the table. The snapshot obtained by a given request never changes.

• The JmxMBeanTableDataItem is a simple class that wraps an ObjectName and a JmxMBeanEntry. The JmxMBeanEntry is lazy-evaluated. The JmxMBeanTableData contains an array of JmxMBeanTableDataItem objects.