| Current Path : /usr/share/doc/db4-devel-4.7.25/gsg/JAVA/ |
| Current File : //usr/share/doc/db4-devel-4.7.25/gsg/JAVA/DBEntry.html |
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Chapter 8. Database Records</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.62.4" />
<link rel="home" href="index.html" title="Getting Started with Berkeley DB" />
<link rel="up" href="baseapi.html" title="Part II. Programming with the Base API" />
<link rel="previous" href="CoreJavaUsage.html" title="Database Example" />
<link rel="next" href="usingDbt.html" title="Reading and Writing Database Records" />
</head>
<body>
<div class="navheader">
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Chapter 8. Database Records</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="CoreJavaUsage.html">Prev</a> </td>
<th width="60%" align="center">Part II. Programming with the Base API</th>
<td width="20%" align="right"> <a accesskey="n" href="usingDbt.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="DBEntry"></a>Chapter 8. Database Records</h2>
</div>
</div>
<div></div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="DBEntry.html#usingDbEntry">Using Database Records</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="usingDbt.html">Reading and Writing Database Records</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="usingDbt.html#databaseWrite">Writing Records to the Database</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#databaseRead">Getting Records from the Database</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#recordDelete">Deleting Records</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#datapersist">Data Persistence</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="bindAPI.html">Using the BIND APIs</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="bindAPI.html#bindPrimitive">Numerical and String Objects</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bindAPI.html#object2dbt">Serializable Complex Objects</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bindAPI.html#customTuple">Custom Tuple Bindings</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="dbtJavaUsage.html">Database Usage Example</a>
</span>
</dt>
</dl>
</div>
<p>
DB records contain two parts — a key and some data. Both the key
and its corresponding data are
encapsulated in
<span><tt class="classname">DatabaseEntry</tt> class objects.</span>
Therefore, to access a DB record, you need two such
<span>objects,</span> one for the key and
one for the data.
</p>
<p>
<tt class="classname">DatabaseEntry</tt> can hold any kind of data from simple
Java primitive types to complex Java objects so long as that data can be
represented as a Java <tt class="literal">byte</tt> array. Note that due to
performance considerations, you should not use Java serialization to convert
a Java object to a <tt class="literal">byte</tt> array. Instead, use the Bind APIs
to perform this conversion (see
<a href="bindAPI.html">Using the BIND APIs</a> for more
information).
</p>
<p>
This chapter describes how you can convert both Java primitives and Java
class objects into and out of <tt class="literal">byte</tt> arrays. It also
introduces storing and retrieving key/value pairs from a database. In
addition, this chapter describes how you can use comparators to influence
how DB sorts its database records.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="usingDbEntry"></a>Using Database Records</h2>
</div>
</div>
<div></div>
</div>
<p>
Each database record is comprised of two
<span><tt class="classname">DatabaseEntry</tt> objects</span>
— one for the key and another for the data.
<span>The key and data information are passed to-
and returned from DB using
<tt class="classname">DatabaseEntry</tt> objects as <tt class="literal">byte</tt>
arrays. Using <tt class="classname">DatabaseEntry</tt>s allows DB to
change the underlying byte array as well as return multiple values (that
is, key and data). Therefore, using <tt class="classname">DatabaseEntry</tt> instances
is mostly an exercise in efficiently moving your keys and your data in
and out of <tt class="literal">byte</tt> arrays.</span>
</p>
<p>
For example, to store a database record where both the key and the
data are Java <tt class="classname">String</tt> objects, you instantiate a
pair of <tt class="classname">DatabaseEntry</tt> objects:
</p>
<a id="java_dbt1"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.DatabaseEntry;
...
String aKey = "key";
String aData = "data";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
} catch (Exception e) {
// Exception handling goes here
}
// Storing the record is described later in this chapter </pre>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Notice that we specify <tt class="literal">UTF-8</tt> when we retrieve the
<tt class="literal">byte</tt> array from our <tt class="classname">String</tt>
object. Without parameters, <tt class="methodname">String.getBytes()</tt> uses the
Java system's default encoding. You should never use a system's default
encoding when storing data in a database because the encoding can change.
</p>
</div>
<p>
When the record is retrieved from the database, the method that you
use to perform this operation populates two <tt class="classname">DatabaseEntry</tt>
instances for you, one for the key and another for the data. Assuming Java
<tt class="classname">String</tt> objects, you retrieve your data from the
<tt class="classname">DatabaseEntry</tt> as follows:
</p>
<a id="java_dbt2"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.DatabaseEntry;
...
// theKey and theData are DatabaseEntry objects. Database
// retrieval is described later in this chapter. For now,
// we assume some database get method has populated these
// objects for us.
// Use DatabaseEntry.getData() to retrieve the encapsulated Java
// byte array.
byte[] myKey = theKey.getData();
byte[] myData = theData.getData();
String key = new String(myKey, "UTF-8");
String data = new String(myData, "UTF-8"); </pre>
<p>
There are a large number of mechanisms that you can use to move data in
and out of <tt class="literal">byte</tt> arrays. To help you with this
activity, DB provides the bind APIs. These APIs allow you to
efficiently store both primitive data types and complex objects in
<tt class="literal">byte</tt> arrays.
</p>
<p>
The next section describes basic database put and get operations. A
basic understanding of database access is useful when describing database
storage of more complex data such as is supported by the bind APIs. Basic
bind API usage is then described in <a href="bindAPI.html">Using the BIND APIs</a>.
</p>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="CoreJavaUsage.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="baseapi.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="usingDbt.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Database Example </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Reading and Writing Database Records</td>
</tr>
</table>
</div>
</body>
</html>
Copyright 2K16 - 2K18 Indonesian Hacker Rulez
;?>)