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Code Editor : keyCreator.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>Implementing Key 
        Creators
        
        </title>
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    <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="indexes.html" title="Chapter 10. Secondary Databases" />
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    <div class="navheader">
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Implementing Key 
        Creators
        
        </th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="indexes.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 10. Secondary Databases</th>
          <td width="20%" align="right"> <a accesskey="n" href="secondaryProps.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="keyCreator"></a>Implementing Key 
        <span>Creators</span>
        
        </h2>
          </div>
        </div>
        <div></div>
      </div>
      <p>
        You must provide every secondary database with a 
            <span>class</span>
            
        that creates keys from primary records. You identify this 
            <span>class</span>
            
        
        <span>
            using the <tt class="methodname">SecondaryConfig.setKeyCreator()</tt> 
            method.
        </span>
        
     </p>
      <p>
        You can create keys using whatever data you want. Typically you will
        base your key on some information found in a record's data, but you
        can also use information found in the primary record's key. How you build
        your keys is entirely dependent upon the nature of the index that you
        want to maintain.
    </p>
      <p>
        You implement a key creator by writing a class that implements the
        <tt class="classname">SecondaryKeyCreator</tt> interface. This interface
        requires you to implement the <tt class="methodname">SecondaryKeyCreator.createSecondaryKey()</tt>
        method.
   </p>
      <p>
        One thing to remember when implementing this method is that you will
        need a way to extract the necessary information from the data's
        <tt class="classname">DatabaseEntry</tt> and/or the key's
        <tt class="classname">DatabaseEntry</tt> that are provided on calls to this
        method. If you are using complex objects, then you are probably using the
        Bind APIs to perform this conversion. The easiest thing to do is to
        instantiate the <tt class="classname">EntryBinding</tt> or
        <tt class="classname">TupleBinding</tt> that you need to perform the
        conversion, and then provide this to your key creator's constructor.
        The Bind APIs are introduced in <a href="bindAPI.html">Using the BIND APIs</a>.
    </p>
      <p>
        <tt class="methodname">SecondaryKeyCreator.createSecondaryKey()</tt> returns a
        boolean. A return value of <tt class="literal">false</tt> indicates that 
        no secondary key exists, and therefore no record should be added to the secondary database for that primary record.
        If a record already exists in the secondary database, it is deleted.
    </p>
      <p>
        For example, suppose your primary database uses the following class
        for its record data:
    </p>
      <a id="java_index3"></a>
      <pre class="programlisting">package db.GettingStarted;

public class PersonData {
    private String userID;
    private String surname;
    private String familiarName;

public PersonData(String userID, String surname, String familiarName) {
        this.userID = userID;
        this.surname = surname;
        this.familiarName = familiarName;
    }

public String getUserID() {
        return userID;
    }

public String getSurname() {
        return surname;
    }

public String getFamiliarName() {
        return familiarName;
    }
} </pre>
      <p>
        Also, suppose that you have created a custom tuple binding,
        <tt class="classname">PersonDataBinding</tt>, that you use to convert
        <tt class="classname">PersonData</tt> objects to and from
        <tt class="classname">DatabaseEntry</tt> objects. (Custom tuple bindings are
        described in <a href="bindAPI.html#customTuple">Custom Tuple Bindings</a>.)
    </p>
      <p>
        Finally, suppose you want a secondary database that is keyed based
        on the person's full name.
    </p>
      <p>
        Then in this case you might create a key creator as follows:
    </p>
      <a id="java_index4"></a>
      <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.db.SecondaryKeyCreator;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.SecondaryDatabase;

import java.io.IOException;

public class FullNameKeyCreator implements SecondaryKeyCreator {
 
    private TupleBinding theBinding;

public FullNameKeyCreator(TupleBinding theBinding1) {
            theBinding = theBinding1;
    }

public boolean createSecondaryKey(SecondaryDatabase secDb,
                                      DatabaseEntry keyEntry, 
                                      DatabaseEntry dataEntry,
                                      DatabaseEntry resultEntry) {

try {
            PersonData pd = 
                (PersonData) theBinding.entryToObject(dataEntry);
                String fullName = pd.getFamiliarName() + " " + 
                    pd.getSurname();
                resultEntry.setData(fullName.getBytes("UTF-8"));
        } catch (IOException willNeverOccur) {}
        return true;
    }
} </pre>
      <p>Finally, you use this key creator as follows:</p>
      <a id="java_index5"></a>
      <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.tuple.TupleBinding;

import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.DatabaseType;
import com.sleepycat.db.SecondaryDatabase;
import com.sleepycat.db.SecondaryConfig;

import java.io.FileNotFoundException;

...
Database myDb = null;
SecondaryDatabase mySecDb = null;
try {
    // Primary database open omitted for brevity
...

TupleBinding myDataBinding = new MyTupleBinding();
    FullNameKeyCreator fnkc = new FullNameKeyCreator(myDataBinding);

SecondaryConfig mySecConfig = new SecondaryConfig();
    mySecConfig.setKeyCreator(fnkc);
    mySecConfig.setType(DatabaseType.BTREE);

//Perform the actual open
    String secDbName = "mySecondaryDatabase";
    mySecDb = new SecondaryDatabase(secDbName, null, myDb, mySecConfig);
} catch (DatabaseException de) {
    // Exception handling goes here
} catch (FileNotFoundException fnfe) {
    // Exception handling goes here
} finally {
    try {
        if (mySecDb != null) {
            mySecDb.close();
        }

if (myDb != null) {
            myDb.close(); 
        }
    } catch (DatabaseException dbe) {
        // Exception handling goes here
    }
}</pre>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="multikeys"></a>Working with Multiple Keys</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
                    Until now we have only discussed indexes as if there is
                    a one-to-one relationship between the secondary key and
                    the primary database record. In fact, it is possible to
                    generate multiple keys for any given record, provided
                    that you take appropriate steps in your key creator
                    to do so.
            </p>
        <p>
                    For example, suppose you had a database that contained
                    information about books. Suppose further that you
                    sometimes want to look up books by author. Because
                    sometimes books have multiple authors, you may want to
                    return multiple secondary keys for every book that you
                    index.
            </p>
        <p>
                    To do this, you write a key creator that implements
                    <tt class="classname">SecondaryMultiKeyCreator</tt>
                    instead of
                    <tt class="classname">SecondaryKeyCreator</tt>. The key
                    difference between the two is that 
                    <tt class="classname">SecondaryKeyCreator</tt>
                    uses a single <tt class="classname">DatabaseEntry</tt>
                    object as the result, while  
                    <tt class="classname">SecondaryMultiKeyCreator</tt>
                    returns a set of <tt class="classname">DatabaseEntry</tt>
                    objects (using <tt class="classname">java.util.Set</tt>).
                    Also, you assign the 
                    <tt class="classname">SecondaryMultiKeyCreator</tt>
                    implementation using 
                    <tt class="methodname">SecondaryConfig.setMultiKeyCreator()</tt>
                    instead of
                    <tt class="methodname">SecondaryConfig.setKeyCreator()</tt>.
             </p>
        <p>
                     For example:
             </p>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.SecondaryDatabase;
import com.sleepycat.db.SecondaryMultiKeyCreator;

import java.util.HashSet;
import java.util.Set;

public class MyMultiKeyCreator implements SecondaryMultiKeyCreator {

// Constructor not implemented. How this is implemented depends on
    // how you want to extract the data for your keys.
    MyMultiKeyCreator() {
        ...
    }

// Abstract method that we must implement
    public void createSecondaryKeys(SecondaryDatabase secDb,
        DatabaseEntry keyEntry,    // From the primary
        DatabaseEntry dataEntry,   // From the primary
        Set results)               // Results set
        throws DatabaseException {

try {
            // Create your keys, adding each to the set

// Creation of key 'a' not shown
            results.add(a)

// Creation of key 'b' not shown
            results.add(b)

} catch (IOException willNeverOccur) {}
    }
} </pre>
      </div>
    </div>
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          <td width="40%" align="left"><a accesskey="p" href="indexes.html">Prev</a> </td>
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            <a accesskey="u" href="indexes.html">Up</a>
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        </tr>
        <tr>
          <td width="40%" align="left" valign="top">Chapter 10. Secondary Databases </td>
          <td width="20%" align="center">
            <a accesskey="h" href="index.html">Home</a>
          </td>
          <td width="40%" align="right" valign="top"> Secondary Database Properties</td>
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Code Editor : keyCreator.html