Java heap retention in oracle.jdbc.driver.T4CXAResource$XidListEntry

Issue

• Heap dump analysis shows significant memory retained by InternalManagedConnectionPool where each managed connection has an associated TxConnectionListener which retains an OracleXAResource which, in turn, owns a Hashtable of transaction identifiers where each entry contains:
  • key: org.jboss.jca.core.tx.jbossts.XidWrapperImpl
  • value: oracle.jdbc.xa.OracleXAResource$XidListEntry

Class Name                                                                                   | Retained Heap | Shallow Heap | Percentage
-----------------------------------------------------------------------------------------------------------------------------------------
|- org.jboss.jca.core.connectionmanager.listener.TxConnectionListener @ 0x769449578          |    38,931,808 |           80 |      1.75%
|  |- org.jboss.jca.adapters.jdbc.xa.XAManagedConnection @ 0x7694495c8                       |    38,931,104 |           80 |      1.75%
|  |  |- oracle.jdbc.xa.client.OracleXAResource @ 0x76941b1d0                                |    38,633,848 |           64 |      1.73%
|  |  |  |- java.util.Hashtable @ 0x768fcb888                                                |    38,633,640 |           56 |      1.73%
|  |  |  |  '- java.util.Hashtable$Entry[417791] @ 0x7ca2e5898                               |    38,633,584 |    1,671,184 |      1.73%
|  |  |  |     |- java.util.Hashtable$Entry @ 0x7b4d6ebf8                                    |       128,000 |           32 |      0.01%
|  |  |  |     |  | ...
|  |  |  |     |  |- org.jboss.jca.core.tx.jbossts.XidWrapperImpl @ 0x7dcdaeae8              |           144 |           40 |      0.00%
|  |  |  |     |  |- oracle.jdbc.xa.OracleXAResource$XidListEntry @ 0x7db342560              |            24 |           24 |      0.00%
-----------------------------------------------------------------------------------------------------------------------------------------

• In some cases, it is observed that there are thousands of distinct entries containing unique global/branch transaction ID pairs for transactions which have completed hours or days prior to collection of the heap dump (i.e. these persist indefinitely)
  • Individually, the Xid instances retain little memory but the accumulation over time (rate of accumulation varies with application/load) becomes significant