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<title>Using the JE JE Collections API</title>
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<link rel="start" href="index.html" title="Berkeley DB Java Edition Collections Tutorial" />
<link rel="up" href="collectionOverview.html" title="Appendix A.  API Notes and Details" />
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<p>Library Version 12.2.7.5</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">
Using the JE JE Collections API
</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="collectionOverview.html">Prev</a> </td>
<th width="60%" align="center">Appendix A. 
API Notes and Details
</th>
<td width="20%" align="right"> <a accesskey="n" href="UsingStoredCollections.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="UsingCollectionsAPI"></a>
Using the JE JE Collections API
</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="UsingCollectionsAPI.html#UsingTransactions">
Using Transactions
</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="UsingCollectionsAPI.html#TransactionRollback">
Transaction Rollback
</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="UsingCollectionsAPI.html#AccessMethodRestrictions">
Access Method Restrictions
</a>
</span>
</dt>
</dl>
</div>
<p>
An
<a class="ulink" href="../../java/com/sleepycat/je/Environment.html" target="_top">Environment</a>
manages the resources for one or more data stores. A
<a class="ulink" href="../../java/com/sleepycat/je/Database.html" target="_top">Database</a>
object
represents a single database and is created via a method on the
environment object.
<a class="ulink" href="../../java/com/sleepycat/je/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
objects represent an index associated with a primary database.
Primary and secondary databases are then used to create stored
collection objects, as described in
<a class="xref" href="UsingStoredCollections.html" title="Using Stored Collections">
Using Stored Collections
</a>.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="UsingTransactions"></a>
Using Transactions
</h3>
</div>
</div>
</div>
<p>
Once you have an environment, one or more databases, and one or
more stored collections, you are ready to access (read and write)
stored data. For a transactional environment, a transaction must be
started before accessing data, and must be committed or aborted
after access is complete. The JE JE Collections API provides several
ways of managing transactions.
</p>
<p>
The recommended technique is to use the
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
class along with your own implementation of the
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
interface.
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
will call your
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
implementation class to perform the data access or work of the
transaction. This technique has the following benefits:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Transaction exceptions will be handled transparently and
retries will be performed when deadlocks are detected.
</p>
</li>
<li>
<p>
The transaction will automatically be committed if your
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html#doWork()" target="_top">TransactionWorker.doWork()</a>
method returns normally, or will be
aborted if <code class="methodname">doWork()</code> throws an exception.
</p>
</li>
<li>
<p>
<code class="classname">TransactionRunner</code> can be used for non-transactional
environments as well, allowing you to write your application
independently of the environment.
</p>
</li>
</ul>
</div>
<p>
If you don't want to use
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>,
the alternative is to use the
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>
class.
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Obtain a CurrentTransaction instance by calling the
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html#getInstance(com.sleepycat.db.Environment)" target="_top">CurrentTransaction.getInstance</a>
method. The instance returned
can be used by all threads in a program.
</p>
</li>
<li>
<p>
Use
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html#beginTransaction(com.sleepycat.db.TransactionConfig)" target="_top">CurrentTransaction.beginTransaction()</a>,
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html#commitTransaction()" target="_top">CurrentTransaction.commitTransaction()</a>
and
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html#abortTransaction()" target="_top">CurrentTransaction.abortTransaction()</a>
to directly begin, commit and abort transactions.
</p>
</li>
</ol>
</div>
<p>
If you choose to use CurrentTransaction directly you must handle
the
<a class="ulink" href="../../java/com/sleepycat/je/LockConflictException.html" target="_top">LockConflictException</a>
exception and perform retries yourself. Also note that
CurrentTransaction may only be used in a transactional
environment.
</p>
<p>
The JE JE Collections API supports transaction auto-commit.
If no transaction is active and a write operation is requested for
a transactional database, auto-commit is used automatically.
</p>
<p>
The JE JE Collections API also supports transaction
dirty-read via the
<a class="ulink" href="../../java/com/sleepycat/collections/StoredCollections.html" target="_top">StoredCollections</a>
class. When dirty-read is enabled for a collection, data will be
read that has been modified by another transaction but not
committed. Using dirty-read can improve concurrency since reading
will not wait for other transactions to complete. For a
non-transactional container, dirty-read has no effect. See
<a class="ulink" href="../../java/com/sleepycat/collections/StoredCollections.html" target="_top">StoredCollections</a>
for how to create a dirty-read collection.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="TransactionRollback"></a>
Transaction Rollback
</h3>
</div>
</div>
</div>
<p>
When a transaction is aborted (or rolled back) the application
is responsible for discarding references to any data objects that
were modified during the transaction. Since the JE JE Collections API
treats data by value, not by reference, neither the data
objects nor the JE JE Collections API objects contain status
information indicating whether the data objects are 1- in sync with
the database, 2- dirty (contain changes that have not been written
to the database), 3- stale (were read previously but have become
out of sync with changes made to the database), or 4- contain
changes that cannot be committed because of an aborted
transaction.
</p>
<p>
For example, a given data object will reflect the current state
of the database after reading it within a transaction. If the
object is then modified it will be out of sync with the database.
When the modified object is written to the database it will then be
in sync again. But if the transaction is aborted the object will
then be out of sync with the database. References to objects for aborted
transactions
should no longer be used. When these objects are needed later they
should be read fresh from the database.
</p>
<p>
When an existing stored object is to be updated, special care
should be taken to read the data, then modify it, and then write it
to the database, all within a single transaction. If a stale data
object (an object that was read previously but has since been
changed in the database) is modified and then written to the
database, database changes may be overwritten unintentionally.
</p>
<p>
When an application enforces rules about concurrent access to
specific data objects or all data objects, the rules described here
can be relaxed. For example, if the application knows that a
certain object is only modified in one place, it may be able to
reliably keep a current copy of that object. In that case, it is
not necessary to reread the object before updating it. That said,
if arbitrary concurrent access is to be supported, the safest
approach is to always read data before modifying it within a single
transaction.
</p>
<p>
Similar concerns apply to using data that may have become stale.
If the application depends on current data, it should be read fresh
from the database just before it is used.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="AccessMethodRestrictions"></a>
Access Method Restrictions
</h3>
</div>
</div>
</div>
<p>
The BTREE access method is always used for JE Databases. Sorted
duplicates — more then one record for a single key — are
optional.
</p>
<p>
The restrictions imposed by the access method on the database
model are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
If duplicates are allowed then more than one value may be
associated with the same key. This means that the data store cannot
be strictly considered a map — it is really a multi-map. See
<a class="xref" href="UsingStoredCollections.html" title="Using Stored Collections">
Using Stored Collections
</a>
for implications on the use of the collection interfaces.
</p>
</li>
<li>
<p>
If duplicate keys are allowed for a data store then the data
store may not have secondary indices.
</p>
</li>
<li>
<p>
With sorted duplicates, all values for the same key must be
distinct.
</p>
</li>
</ul>
</div>
<p>
See
<a class="xref" href="UsingStoredCollections.html" title="Using Stored Collections">
Using Stored Collections
</a>
for more information on how access methods impact the use of stored
collections.
</p>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="collectionOverview.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="collectionOverview.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="UsingStoredCollections.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Appendix A. 
API Notes and Details
 </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> 
Using Stored Collections
</td>
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