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<title>Berkeley DB Reference Guide: Handling exceptions</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Java API Tutorial - Basic</dl></h3></td>
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<p>
<h3 align=center>Handling exceptions</h3>
<p>Exception handling was illustrated in the previous sections <a href="main.html">Implementing the main program</a> and <a href="transact.html">Using transactions</a>. This
section examines exception handling in a Java API application in more detail.</p>
<p>There are two exceptions that must be treated specially:
<a href="../../java/com/sleepycat/db/DbRunRecoveryException.html">DbRunRecoveryException</a>
and
<a href="../../java/com/sleepycat/db/DbDeadlockException.html">DbDeadlockException</a>
.</p>
<p>
<a href="../../java/com/sleepycat/db/DbRunRecoveryException.html">DbRunRecoveryException</a>
is thrown
when the only solution is to shut down the application and run recovery. All
applications must catch this exception and follow the recovery procedure.</p>
<p>When
<a href="../../java/com/sleepycat/db/DbDeadlockException.html">DbDeadlockException</a>
is thrown, the
application should normally retry the operation. If a deadlock continues to
occur for some maximum number of retries, the application should give up and
try again later or take other corrective actions. The Java API provides two APIs
for transaction execution.</p>
<p><ul type=disc>
<li>When using the
<a href="../../java/com/sleepycat/bdb/CurrentTransaction.html">CurrentTransaction</a>
class directly, the application must catch
<a href="../../java/com/sleepycat/db/DbDeadlockException.html">DbDeadlockException</a>
and follow the procedure described previously.
<li>When using the
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html">TransactionRunner</a>
class, retries are performed automatically and the application need only handle
the case where the maximum number of retries has been reached. In that case,
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html#run">TransactionRunner.run</a>
will throw
<a href="../../java/com/sleepycat/db/DbDeadlockException.html">DbDeadlockException</a>
.
</ul>
<p>When using the
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html">TransactionRunner</a>
class
there are two other considerations.</p>
<p><ul type=disc>
<li>First, if the application-defined
<a href="../../java/com/sleepycat/bdb/TransactionWorker.html#doWork">TransactionWorker.doWork</a>
method throws an exception the transaction will
automatically be aborted, and otherwise the transaction will automatically be
committed. Applications should design their transaction processing with this
in mind.
<li>Second, please be aware that
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html#run">TransactionRunner.run</a>
unwraps exceptions in order to discover whether a
nested exception is a
<a href="../../java/com/sleepycat/db/DbDeadlockException.html">DbDeadlockException</a>
.
This is particularly important since all Berkeley DB exceptions that occur while
calling a stored collection method are wrapped with a
<a href="../../java/com/sleepycat/bdb/util/RuntimeExceptionWrapper.html">RuntimeExceptionWrapper</a>
. This wrapping is
necessary because Berkeley DB exceptions are checked exceptions, and the Java
collections API does not allow such exceptions to be thrown.
</ul>
<p>When calling
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html#run">TransactionRunner.run</a>
,
the unwrapped (nested) exception will be unwrapped and thrown automatically.
If you are not using
<a href="../../java/com/sleepycat/bdb/TransactionRunner.html">TransactionRunner</a>
or
if you are handling exceptions directly for some other reason, use the
<a href="../../java/com/sleepycat/bdb/util/ExceptionUnwrapper.html#unwrap">ExceptionUnwrapper.unwrap</a>
method to get
the nested exception. For example, this can be used to discover that an
exception is a
<a href="../../java/com/sleepycat/db/DbRunRecoveryException.html">DbRunRecoveryException</a>
as
shown below.</p>
<blockquote><pre><b>
import com.sleepycat.bdb.util.ExceptionUnwrapper;
...
catch (Exception e)
{
e = ExceptionUnwrapper.unwrap(e);
if (e instanceof DbRunRecoveryException)
{
// follow recovery procedure
}
}
</b></pre></blockquote>
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