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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Java API</dl></h3></td>
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<p>
<h3 align=center>Overview of the Java API</h3>
<p>The Java API is a Java framework that extends the well known
<a href="http://java.sun.com/j2se/1.3/docs/guide/collections/">Java Collections</a> design pattern such that collections can now be
stored, updated and queried in a transactional manner.  The Java API is
a layer on top of Berkeley DB.</p>
<p>Together the Java API and Berkeley DB provide an embedded data management
solution with all the benefits of a full transactional storage and the
simplicity of a well known Java API.  Java programmers who need fast,
scalable, transactional data management for their projects can quickly
adopt and deploy the Java API with confidence.</p>
<p>This framework was first known as
<a href="http://greybird-db.sourceforge.net/">Greybird DB</a> written
by Mark Hayes.  Sleepycat Software has collaborated with Mark to
permanently incorporate his excellent work into our distribution and
support it as an ongoing part of Berkeley DB.  The repository of source code
that remains at Sourceforge at version 0.9.0 is considered the last
version before incorporation and will remain intact but will not be
updated to reflect changes made as part of Berkeley DB.</p>
<h3>What does The Java API add to Berkeley DB?</h3>
<p>Berkeley DB has always provided a Java API which can be roughly
described as a map and cursor interface, where the keys and values
are represented as byte arrays.  This API is a Java (JNI) interface to
the C API and it closely modeled the Berkeley DB C API's interface.  The Java API
is a layer on top of that thin JNI mapping of the C API to Berkeley DB that adds
significant new functionality in several ways.</p>
<p><ul type=disc>
<li>An implementation of the Java Collections interfaces (Map, Set, List and
Iterator) is provided.
<li>Transactions are supported using the conventional Java
transaction-per-thread model, where the current transaction is implicitly
associated with the current thread.
<li>Transaction runner utilities are provided that automatically perform
transaction retry and exception handling.
<li>Keys and values are represented as Java objects rather than byte arrays.
Bindings are used to map between Java objects and the stored byte
arrays.
<li>The tuple data format is provided as the simplest data representation,
and is useful for keys as well as simple compact values.
<li>The serial data format is provided for storing arbitrary Java objects
without writing custom binding code.  Java serialization is extended to
store the class descriptions separately, making the data records much
more compact than with standard Java serialization.
<li>Custom data formats and bindings can be easily added.  XML data format
and XML bindings could easily be created using this feature.
<li>In addition to secondary indices, foreign key indices are provided with
integrity constraints.
<li>The Java API API insulates the application from minor differences in the
use of the Berkeley DB Data Store, Concurrent Data Store and Transactional Data
Store products.  This allows for development with one and deployment with
another without significant changes to code.
</ul>
<p>Note that the Java API does not support caching of programming
language objects nor keep track of their stored status.  This is in
contrast to "persistent object" approaches such as those defined by
<a href="http://www.odmg.org/">ODMG</a> and JDO
(<a href="http://www.jcp.org/en/jsr/detail?id=12">JSR 12</a>).
Such approaches have benefits but also require sophisticated object
caching.  For simplicity the Java API treats data objects by value, not
by reference, and does not perform object caching of any kind.  Since
the Java API is a thin layer, its reliability and performance
characteristics are roughly equivalent to those of Berkeley DB, and database
tuning is accomplished in the same way as for any Berkeley DB database.</p>
<h3>Choices to make</h3>
<p>There are several important choices to make when developing an
application using Java API.</p>
<ol>
<p><li>Choose the Berkeley DB Environment
<p>Depending on your application's concurrency and transactional
requirements you may choose one of the three Berkeley DB Environments:
Data Store, Concurrent Data Store, or Transactional Data Store.  For
details on creating and configuring the environment see
<a href="../../java/com/sleepycat/db/DbEnv.html">DbEnv</a>
.</p>
<p><li>Choose the Berkeley DB Access Method
<p>For each Berkeley DB datastore, or data store as it is called within
the Java API, you may choose from any of the four Berkeley DB access methods --
BTREE, HASH, RECNO or QUEUE (
<a href="../../java/com/sleepycat/db/Db.html#DB_BTREE">Db.DB_BTREE</a>
,
<a href="../../java/com/sleepycat/db/Db.html#DB_HASH">Db.DB_HASH</a>
,
<a href="../../java/com/sleepycat/db/Db.html#DB_RECNO">Db.DB_RECNO</a>
 or
<a href="../../java/com/sleepycat/db/Db.html#DB_QUEUE">Db.DB_QUEUE</a>
) -- and a number of other
database options.
Your choice depends on several factors such as whether you need ordered
keys, unique keys, record number access, etc.  For more information on
access methods see the 
<a href="cs_bdb.html">com.sleepycat.bdb</a>
 package
description.</p>
<p><li>Choose the Data Format for Keys and Values
<p>For each database you may choose a data format for the keys and
values.  For example, the tuple data format is useful for keys because it
has a deterministic sort order.  The serial format is useful for values
if you want to store arbitrary Java objects.  In some cases a custom data
format may be appropriate.  For details on choosing a data format see the
<a href="cs_bdb_bind.html">com.sleepycat.bdb.bind</a>
 package description.</p>
<p><li>Choose the Binding for Keys and Values
<p>With the serial data format you do not have to create a binding for
each Java class that is stored since Java serialization is used.  But for
other formats a binding must be defined that translates between stored
byte arrays and Java objects.  For details see the
<a href="cs_bdb_bind.html">com.sleepycat.bdb.bind</a>
 package description.</p>
<p><li>Choose Secondary Indices and Foreign Key Indices
<p>Any data store that has unique keys may have any number of indices.
An index has keys that are derived from data values in the primary data
store.  This allows lookup and iteration of objects in the data store by
its index keys.  A foreign key index is a special type of index where
the index keys are also the primary keys of another data store.  For each
index you must define how the index keys are derived from the data values
using a 
<a href="../../java/com/sleepycat/bdb/bind/KeyExtractor.html">KeyExtractor</a>
.  For details see
the 
<a href="../../java/com/sleepycat/bdb/DataIndex.html">DataIndex</a>
,
<a href="../../java/com/sleepycat/bdb/ForeignKeyIndex.html">ForeignKeyIndex</a>
 and
<a href="../../java/com/sleepycat/bdb/bind/KeyExtractor.html">KeyExtractor</a>
 classes.</p>
<p><li>Choose the Collection Interface for each Data Store
<p>The standard Java Collection interfaces are used for accessing data
stores and data indices.  The Map and Set interfaces may be used for any
type of data store, while the List interface may only be used for data
stores with record number access.  The Iterator interface is used through
the Map, Set and List interfaces.  For more information on collection
interfaces see the 
<a href="cs_bdb_collection.html">com.sleepycat.bdb.collection</a>
 package.</p>
</ol>
<h3>Things to keep in mind</h3>
<p>A single 
<a href="../../java/com/sleepycat/bdb/DataStore.html">DataStore</a>
 object should
be created for each database, and all data stores in an application should
normally be used with a single 
<a href="../../java/com/sleepycat/db/DbEnv.html">DbEnv</a>
object.</p>
<p>However, any number of bindings and collections may be created for
the same data store.  This allows multiple views of the same stored data.
For example, a data store may be viewed as a Map of keys to values, a Set
of keys, a Collection of values, or a List of values. String values, for
example, may be used with the built-in binding to the String class, or
with a custom binding to another class that represents the string values
differently.</p>
<p>It is sometimes desirable to use a Java class that encapsulates both a
data key and a data value.  For example, a Part object might contain both
the part number (key) and the part name (value).  Using the Java API this type
of object is called an "entity".  An entity binding is used to translate
between the Java object and the stored data key and value.  Entity bindings
may be used with all Collection types.</p>
<p>Please be aware the Java API collection classes provided do not conform
completely to the interface contracts defined in the
<code>java.util</code> package.  For example, all iterators must be
explicitly closed and the <code>size()</code> method is not available.
The differences between Java API collections and standard Java collections
are documented in the
<a href="cs_bdb_collection.html">com.sleepycat.bdb.collection</a>
 package description.</p>
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