- Berkeley DB Reference Guide:
- Java API
Java programming notes
The Java API closely parallels the Berkeley DB C++ and C interfaces. If you
are currently using either of those APIs, there will be very little to
surprise you in the Java API. We have even taken care to make the names
of classes, constants, methods and arguments identical, where possible,
across all three APIs.
- The Java runtime does not automatically close Berkeley DB objects on
finalization. There are several reasons for this. One is that
finalization is generally run only when garbage collection occurs, and
there is no guarantee that this occurs at all, even on exit. Allowing
specific Berkeley DB actions to occur in ways that cannot be replicated seems
wrong. Second, finalization of objects may happen in an arbitrary
order, so we would have to do extra bookkeeping to make sure that
everything was closed in the proper order. The best word of advice is
to always do a close() for any matching open() call. Specifically, the
Berkeley DB package requires that you explicitly call close on each individual
Db and Dbc object that you opened. Your database
activity may not be synchronized to disk unless you do so.
- Some methods in the Java API have no return type, and throw a
DbException when an severe error arises. There are some notable
methods that do have a return value, and can also throw an exception.
Db.get and Dbc.get both return 0 when a get succeeds,
return Db.DB_NOTFOUND when the key is not found, and throw an error
when there is a severe error. This approach allows the programmer to
check for typical data-driven errors by watching return values without
special casing exceptions.
An object of type DbDeadlockException is thrown when a deadlock
An object of type DbMemoryException is thrown when the system
cannot provide enough memory to complete the operation (the ENOMEM
system error on UNIX).
An object of type DbRunRecoveryException, a subclass of
DbException, is thrown when there is an error that requires a
recovery of the database using db_recover.
- There is no class corresponding to the C++ DbMpoolFile class in the Berkeley DB
Java API. There is a subset of the memp_XXX methods in the DbEnv
class. This has been provided to allow you to perform certain
administrative actions on underlying memory pools opened as a consequence
of DbEnv.open. Direct access to other memory pool functionality
is not appropriate for the Java environment.
- Berkeley DB always turns on the Db.DB_THREAD flag because
threads are expected in Java.
- If there are embedded null strings in the curslist argument for
Db.join, they will be treated as the end of the list of
cursors, even if you may have allocated a longer array. Fill in all
the strings in your array unless you intend to cut it short.
- The callback installed for DbEnv.set_errcall will run in the same
thread as the caller to DbEnv.set_errcall. Make sure that thread
remains running until your application exits or until DbEnv.close
- If you are using custom class loaders in your application, make sure
that the Berkeley DB classes are loaded by the system class loader, not a
custom class loader. This is due to a JVM bug that can cause an access
violation during finalization (see the bug 4238486 in Sun Microsystem's
Java Bug Database).
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