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BerkeleyDB-Core-JAVA-GSG.pdf	669.52 KB	-rw-r--r--
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Code Editor : bindAPI.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>Using the BIND APIs</title>
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    <link rel="up" href="DBEntry.html" title="Chapter 8. Database Records" />
    <link rel="previous" href="usingDbt.html" title="Reading and Writing Database Records" />
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  </head>
  <body>
    <div class="navheader">
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Using the BIND APIs</th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="usingDbt.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 8. Database Records</th>
          <td width="20%" align="right"> <a accesskey="n" href="dbtJavaUsage.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="bindAPI"></a>Using the BIND APIs</h2>
          </div>
        </div>
        <div></div>
      </div>
      <p>Except for Java String and boolean types, efficiently moving data in
    and out of Java byte arrays for storage in a database can be a nontrivial
    operation. To help you with this problem, DB provides the Bind APIs. 
    While these APIs are described in detail in the

<span>
    <i class="citetitle">Berkeley DB Collections Tutorial</i>,
    </span>

this section provides a brief introduction to using the Bind APIs with:</p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>Single field numerical and string objects</p>
            <p>Use this if you want to store a single numerical or string object,
        such as <tt class="classname">Long</tt>, <tt class="classname">Double</tt>, or
        <tt class="classname">String</tt>.</p>
          </li>
          <li>
            <p>Complex objects that implement Java serialization.</p>
            <p>Use this if you are storing objects that implement
            <tt class="classname">Serializable</tt> and if you do not need to sort them.
        </p>
          </li>
          <li>
            <p>Non-serialized complex objects.</p>
            <p>If you are storing objects that do not implement serialization,
        you can create your own custom tuple bindings. Note that you should
        use custom tuple bindings even if your objects are serializable if
        you want to sort on that data.</p>
          </li>
        </ul>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="bindPrimitive"></a>Numerical and String Objects</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>You can use the Bind APIs to store primitive data in a <tt class="classname">DatabaseEntry</tt>
      object. That is, you can store a single field containing one of the following types:</p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                <tt class="classname">String</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Character</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Boolean</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Byte</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Short</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Integer</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Long</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Float</tt>
              </p>
            </li>
            <li>
              <p>
                <tt class="classname">Double</tt>
              </p>
            </li>
          </ul>
        </div>
        <p>
          To store primitive data using the Bind APIs:
      </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>Create an <tt class="classname">EntryBinding</tt> object.</p>
              <p>When you do this, you use <tt class="classname">TupleBinding.getPrimitiveBinding()</tt>
          to return an appropriate binding for the conversion.</p>
            </li>
            <li>
              <p>Use the <tt class="classname">EntryBinding</tt> object to place
          the numerical object on the <tt class="classname">DatabaseEntry</tt>.</p>
            </li>
          </ol>
        </div>
        <p>Once the data is stored in the DatabaseEntry, you can put it to
      the database in whatever manner you wish. For example:</p>
        <a id="java_dbt6"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.EntryBinding;
import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;

...

Database myDatabase = null;
// Database open omitted for clarity.

// Need a key for the put.
try {
    String aKey = "myLong";
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));

// Now build the DatabaseEntry using a TupleBinding
    Long myLong = new Long(123456789l);
    DatabaseEntry theData = new DatabaseEntry();
    EntryBinding myBinding = TupleBinding.getPrimitiveBinding(Long.class);
    myBinding.objectToEntry(myLong, theData);

// Now store it
    myDatabase.put(null, theKey, theData);
} catch (Exception e) {
    // Exception handling goes here
}</pre>
        <p>Retrieval from the <tt class="classname">DatabaseEntry</tt> object is
      performed in much the same way:</p>
        <a id="java_dbt7"></a>
        <pre class="programlisting">package db.GettingStarted;
      
import com.sleepycat.bind.EntryBinding;
import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;

...

Database myDatabase = null;
// Database open omitted for clarity

try {
    // Need a key for the get
    String aKey = "myLong";
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    
    // Need a DatabaseEntry to hold the associated data.
    DatabaseEntry theData = new DatabaseEntry();

// Bindings need only be created once for a given scope
    EntryBinding myBinding = TupleBinding.getPrimitiveBinding(Long.class);

// Get it
    OperationStatus retVal = myDatabase.get(null, theKey, theData, 
                                            LockMode.DEFAULT);
    String retKey = null;
    if (retVal == OperationStatus.SUCCESS) {
        // Recreate the data.
        // Use the binding to convert the byte array contained in theData
        // to a Long type.
        Long theLong = (Long) myBinding.entryToObject(theData);
        retKey = new String(theKey.getData(), "UTF-8");
        System.out.println("For key: '" + retKey + "' found Long: '" + 
                            theLong + "'.");
    } else {
        System.out.println("No record found for key '" + retKey + "'.");
    }
} catch (Exception e) {
    // Exception handling goes here
} </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="object2dbt"></a>Serializable Complex Objects</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>Frequently your application requires you to store and manage
      objects for your record data and/or keys. You may need to do this if you
      are caching objects created by another process. You may also want to do
      this if you want to store multiple data values on a record. When used
      with just primitive data, or with objects containing a single data member,
      DB database records effectively represent a single row in a two-column table.
      By storing a complex object in the record, you can turn each record into
      a single row in an <span class="emphasis"><em>n</em></span>-column table, where
      <span class="emphasis"><em>n</em></span> is the number of data members contained by the
      stored object(s).</p>
        <p>In order to store objects in a 
      DB database, you must convert them to and from a <tt class="literal">byte</tt> array.
      The first instinct for many Java programmers is to do this using Java
      serialization. While this is functionally a correct solution, the result
      is poor space-performance because this causes the class information
      to be stored on every such database record. This information can be quite large
      and it is redundant — the class information does not vary for serialized objects of the same type.
      </p>
        <p>
        In other words, directly using serialization to place your objects into byte
        arrays means that you will be storing a great deal of unnecessary information in
        your database, which ultimately leads to larger databases and more expensive disk
        I/O.
      </p>
        <p>The easiest way for you to solve this problem is to use the Bind
      APIs to perform the serialization for you. Doing so causes the extra
      object information to be saved off to a unique <tt class="classname">Database</tt>
      dedicated for that purpose. This means that you do not have to duplicate
      that information on each record in the <tt class="classname">Database</tt>
      that your application is using to store its information.</p>
        <p>
        Note that when you use the Bind APIs to perform serialization, you still
        receive all the benefits of serialization. You can still use arbitrarily
        complex object graphs, and you still receive built-in class evolution
        through the serialVersionUID (SUID) scheme. All of the Java
        serialization rules apply without modification. For example, you can
        implement Externalizable instead of Serializable.
      </p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="bindCaveats"></a>Usage Caveats</h4>
              </div>
            </div>
            <div></div>
          </div>
          <p>Before using the Bind APIs to perform serialization, you may
        want to consider writing your own custom tuple bindings. Specifically,
        avoid serialization if:
        </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                If you need to sort based on the objects your are storing. 
                The sort order is meaningless for the byte arrays that you
                obtain through serialization. Consequently, you should not use serialization for keys if you
                care about their sort order. You should
                also not use serialization for record data if your
                <tt class="classname">Database</tt> supports duplicate records and you care about sort order.
        </p>
              </li>
              <li>
                <p>
                You want to minimize the size of your byte arrays. Even when using the Bind APIs to perform the
                serialization the resulting <tt class="literal">byte</tt> array may be larger than necessary. You can achieve
                more compact results by building your own custom tuple binding.
            </p>
              </li>
              <li>
                <p>
                You want to optimize for speed. In general, custom tuple bindings are faster than serialization at
                moving data in and out of <tt class="literal">byte</tt> arrays.
            </p>
              </li>
            </ul>
          </div>
          <p>
            For information on building your own custom tuple binding, see <a href="bindAPI.html#customTuple">Custom Tuple Bindings</a>.
        </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="objectSerial"></a>Serializing Objects</h4>
              </div>
            </div>
            <div></div>
          </div>
          <p>To store a serializable complex object using the
        Bind APIs:</p>
          <div class="orderedlist">
            <ol type="1">
              <li>
                <p>
                Implement java.io.Serializable in the class whose instances that
                you want to store.
            </p>
              </li>
              <li>
                <p>Open (create) your databases. You need two. The first is the
            database that you use to store your data. The second is used to
            store the class information.</p>
              </li>
              <li>
                <p>Instantiate a class catalog. You do this with
            <tt class="classname">com.sleepycat.bind.serial.StoredClassCatalog</tt>,
            and at that time you must provide a handle to an open database
            that is used to store the class information.</p>
              </li>
              <li>
                <p>Create an entry binding that uses <tt class="methodname">com.sleepycat.bind.serial.SerialBinding</tt>.</p>
              </li>
              <li>
                <p>Instantiate an instance of the object that you want to
            store, and place it in a <tt class="classname">DatabaseEntry</tt>
            using the entry binding that you created in the previous step.</p>
              </li>
            </ol>
          </div>
          <p>
            For example, suppose you want to store a long, double, and a
            String as a record's data. Then you might create a class that
            looks something like this:
        </p>
          <a id="java_dbt8"></a>
          <pre class="programlisting">package db.GettingStarted;

import java.io.Serializable;

public class MyData implements Serializable {
    private long longData;
    private double doubleData;
    private String description;

MyData() {
        longData = 0;
        doubleData = 0.0;
        description = null;
    }

public void setLong(long data) {
        longData = data;
    }

public void setDouble(double data) {
        doubleData = data;
    }

public void setDescription(String data) {
        description = data;
    }

public long getLong() {
        return longData;
    }

public double getDouble() {
        return doubleData;
    }

public String getDescription() {
        return description;
    }
}</pre>
          <p>You can then store instances of this class as follows:</p>
          <a id="java_dbt9"></a>
          <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.EntryBinding;
import com.sleepycat.bind.serial.StoredClassCatalog;
import com.sleepycat.bind.serial.SerialBinding;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseType;
...

// The key data.
String aKey = "myData";

// The data data
MyData data2Store = new MyData();
data2Store.setLong(123456789l);
data2Store.setDouble(1234.9876543);
data2Store.setDescription("A test instance of this class");

try {
    // Open the database that you will use to store your data
    DatabaseConfig myDbConfig = new DatabaseConfig();
    myDbConfig.setAllowCreate(true);
    myDbConfig.setSortedDuplicates(true);
    myDbConfig.setType(DatabaseType.BTREE);
    Database myDatabase = new Database("myDb", null, myDbConfig);

// Open the database that you use to store your class information.
    // The db used to store class information does not require duplicates
    // support.
    myDbConfig.setSortedDuplicates(false);
    Database myClassDb = new Database("classDb", null, myDbConfig);

// Instantiate the class catalog
    StoredClassCatalog classCatalog = new StoredClassCatalog(myClassDb);

// Create the binding
    EntryBinding dataBinding = new SerialBinding(classCatalog, 
                                                 MyData.class);

// Create the DatabaseEntry for the key
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));

// Create the DatabaseEntry for the data. Use the EntryBinding object
    // that was just created to populate the DatabaseEntry
    DatabaseEntry theData = new DatabaseEntry();
    dataBinding.objectToEntry(data2Store, theData);

// Put it as normal
    myDatabase.put(null, theKey, theData);
    
    // Database and environment close omitted for brevity 
} catch (Exception e) {
    // Exception handling goes here
}</pre>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="objectDeserial"></a>Deserializing Objects</h4>
              </div>
            </div>
            <div></div>
          </div>
          <p>Once an object is stored in the database, you can retrieve the
        <tt class="classname">MyData</tt> objects from the retrieved
        <tt class="classname">DatabaseEntry</tt> using the Bind APIs in much the
        same way as is described above. For example:</p>
          <a id="java_dbt10"></a>
          <pre class="programlisting">package db.GettingStarted;

...

// The key data.
String aKey = "myData";

try {
    // Open the database that stores your data
    DatabaseConfig myDbConfig = new DatabaseConfig();
    myDbConfig.setAllowCreate(false);
    myDbConfig.setType(DatabaseType.BTREE);
    Database myDatabase = new Database("myDb", null, myDbConfig);

// Open the database that stores your class information.
    Database myClassDb = new Database("classDb", null, myDbConfig);

// Instantiate the class catalog
    StoredClassCatalog classCatalog = new StoredClassCatalog(myClassDb);

// Create the binding
    EntryBinding dataBinding = new SerialBinding(classCatalog, 
                                                 MyData.class);

// Create DatabaseEntry objects for the key and data
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    DatabaseEntry theData = new DatabaseEntry();

// Do the get as normal
    myDatabase.get(null, theKey, theData, LockMode.DEFAULT);

// Recreate the MyData object from the retrieved DatabaseEntry using
    // the EntryBinding created above
    MyData retrievedData = (MyData) dataBinding.entryToObject(theData);
 
    // Database and environment close omitted for brevity
} catch (Exception e) {
    // Exception handling goes here
}</pre>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="customTuple"></a>Custom Tuple Bindings</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
        If you want to store complex objects in your database, then you can use
        tuple bindings to do this. While they are more work to write and
        maintain than if you were to use serialization, the
        <tt class="literal">byte</tt> array conversion is faster. In addition, custom
        tuple bindings should allow you to create <tt class="literal">byte</tt> arrays
        that are smaller than those created by serialization. Custom tuple
        bindings also allow you to optimize your BTree comparisons, whereas
        serialization does not.
      </p>
        <p>
        For information on using serialization to store complex objects, see
        <a href="bindAPI.html#object2dbt">Serializable Complex Objects</a>.
      </p>
        <p>To store complex objects using a custom tuple binding:</p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>Implement the class whose instances that you want to store.
          Note that you do not have to implement the Serializable interface.</p>
            </li>
            <li>
              <p>Write a tuple binding using the <tt class="classname">com.sleepycat.bind.tuple.TupleBinding</tt>
          class.</p>
            </li>
            <li>
              <p>Open (create) your database. Unlike serialization, you only
          need one.</p>
            </li>
            <li>
              <p>Create an entry binding that uses the tuple binding that you
          implemented in step 2.</p>
            </li>
            <li>
              <p>Instantiate an instance of the object that you want to store,
          and place it in a <tt class="classname">DatabaseEntry</tt> using the
          entry binding that you created in the previous step.</p>
            </li>
          </ol>
        </div>
        <p>
        For example, suppose you want to your keys to be instances of the
        following class:
      </p>
        <a id="java_dbt11"></a>
        <pre class="programlisting">package db.GettingStarted;

public class MyData2 {
    private long longData;
    private Double doubleData;
    private String description;

public MyData2() {
        longData = 0;
        doubleData = new Double(0.0);
        description = "";
    }

public void setLong(long data) {
        longData = data;
    }

public void setDouble(Double data) {
        doubleData = data;
    }

public void setString(String data) {
        description = data;
    }

public long getLong() {
        return longData;
    }

public Double getDouble() {
        return doubleData;
    }

public String getString() {
        return description;
    }
} </pre>
        <p>In this case, you need to write a tuple binding for the
      <tt class="classname">MyData2</tt> class. When you do this, you must
      implement the <tt class="methodname">TupleBinding.objectToEntry()</tt>
      and <tt class="methodname">TupleBinding.entryToObject()</tt> abstract methods.
      Remember the following as you implement these methods:</p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>You use <tt class="methodname">TupleBinding.objectToEntry()</tt> to convert
          objects to <tt class="literal">byte</tt> arrays. You use
          <tt class="classname">com.sleepycat.bind.tuple.TupleOutput</tt> to write
          primitive data types to the <tt class="literal">byte</tt> array. Note that
          <tt class="classname">TupleOutput</tt> provides methods that allows
          you to work with numerical types (<tt class="literal">long</tt>,
          <tt class="literal">double</tt>, <tt class="literal">int</tt>, and so forth) and
          not the corresponding <tt class="literal">java.lang</tt> numerical
          classes.</p>
            </li>
            <li>
              <p>The order that you write data to the <tt class="literal">byte</tt>
          array in <tt class="methodname">TupleBinding.objectToEntry()</tt> is the order that
          it appears in the array. So given the <tt class="classname">MyData2</tt>
          class as an example, if you write <tt class="literal">description</tt>,
          <tt class="literal">doubleData</tt>, and then <tt class="literal">longData</tt>,
          then the resulting byte array will contain these data elements in
          that order. This means that your records will sort based on the
          value of the <tt class="literal">description</tt> data member and then
          the <tt class="literal">doubleData</tt> member, and so forth. If you
          prefer to sort based on, say, the <tt class="literal">longData</tt> data
          member, write it to the byte array first.</p>
            </li>
            <li>
              <p>You use <tt class="methodname">TupleBinding.entryToObject()</tt> to convert
          the <tt class="literal">byte</tt> array back into an instance of your
          original class. You use <tt class="classname">com.sleepycat.bind.tuple.TupleInput</tt>
          to get data from the <tt class="literal">byte</tt> array.</p>
            </li>
            <li>
              <p>The order that you read data from the <tt class="literal">byte</tt>
          array must be exactly the same as the order in which it was written.</p>
            </li>
          </ul>
        </div>
        <p>For example:</p>
        <a id="java_dbt12"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.bind.tuple.TupleInput;
import com.sleepycat.bind.tuple.TupleOutput;

public class MyTupleBinding extends TupleBinding {

// Write a MyData2 object to a TupleOutput
    public void objectToEntry(Object object, TupleOutput to) {

MyData2 myData = (MyData2)object;

// Write the data to the TupleOutput (a DatabaseEntry).
        // Order is important. The first data written will be
        // the first bytes used by the default comparison routines.
        to.writeDouble(myData.getDouble().doubleValue());
        to.writeLong(myData.getLong());
        to.writeString(myData.getString());
    }

// Convert a TupleInput to a MyData2 object
    public Object entryToObject(TupleInput ti) {

// Data must be read in the same order that it was
        // originally written.
        Double theDouble = new Double(ti.readDouble());
        long theLong = ti.readLong();
        String theString = ti.readString();

MyData2 myData = new MyData2();
        myData.setDouble(theDouble);
        myData.setLong(theLong);
        myData.setString(theString);

return myData;
    }
} </pre>
        <p>In order to use the tuple binding, instantiate the binding and
      then use:</p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p><tt class="methodname">MyTupleBinding.objectToEntry()</tt> to
          convert a MyData2 object to a <tt class="classname">DatabaseEntry</tt>.</p>
            </li>
            <li>
              <p><tt class="methodname">MyTupleBinding.entryToObject()</tt> to convert
          a <tt class="classname">DatabaseEntry</tt> to a <tt class="classname">MyData2</tt>
          object.</p>
            </li>
          </ul>
        </div>
        <p>For example:</p>
        <a id="java_dbt13"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.db.DatabaseEntry;
 
...

TupleBinding keyBinding = new MyTupleBinding();

MyData2 theKeyData = new MyData2();
theKeyData.setLong(123456789l);
theKeyData.setDouble(new Double(12345.6789));
theKeyData.setString("My key data");

DatabaseEntry myKey = new DatabaseEntry();

try {
    // Store theKeyData in the DatabaseEntry
    keyBinding.objectToEntry(theKeyData, myKey);

...
    // Database put and get activity omitted for clarity
    ...

// Retrieve the key data
    theKeyData = (MyData2) keyBinding.entryToObject(myKey);
} catch (Exception e) {
    // Exception handling goes here
}</pre>
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