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HTML
223 lines
9 KiB
HTML
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Using Transactions</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Collections Tutorial" />
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<link rel="up" href="BasicProgram.html" title="Chapter 2. The Basic Program" />
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<link rel="prev" href="implementingmain.html" title="Implementing the Main Program" />
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<link rel="next" href="addingdatabaseitems.html" title="Adding Database Items" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.2</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">
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Using Transactions
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="implementingmain.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The Basic Program
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="addingdatabaseitems.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="usingtransactions"></a>
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Using Transactions
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</h2>
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</div>
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</div>
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</div>
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<p>
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DB transactional applications have standard
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transactional characteristics: recoverability, atomicity and
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integrity (this is sometimes also referred to generically as <span class="emphasis"><em>ACID
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properties</em></span>). The DB Java Collections API provides these
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transactional capabilities using a <span class="emphasis"><em>transaction-per-thread</em></span>
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model. Once a transaction is begun, it is implicitly associated
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with the current thread until it is committed or aborted. This
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model is used for the following reasons.
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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The transaction-per-thread model is commonly used in other Java
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APIs such as J2EE.
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</p>
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</li>
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<li>
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<p>
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Since the Java collections API is used for data access, there
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is no way to pass a transaction object to methods such
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as
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<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/util/Map.html#put(K, V)" target="_top">Map.put</a>.
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</p>
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</li>
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</ul>
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</div>
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<p>
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The DB Java Collections API provides two transaction APIs. The
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lower-level API is the
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<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>
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class. It provides a way to get the transaction for the current
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thread, and to begin, commit and abort transactions. It also
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provides access to the Berkeley DB core API
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<a class="ulink" href="../../java/com/sleepycat/db/Transaction.html" target="_top">Transaction</a>
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object. With
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<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>,
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just as in the
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<span>com.sleepycat.db</span>
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API, the application is responsible
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for beginning, committing and aborting transactions, and for
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handling deadlock exceptions and retrying operations. This API may
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be needed for some applications, but it is not used in the
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example.
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</p>
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<p>
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The example uses the higher-level
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
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and
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
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APIs, which are build on top of
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<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>.
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<code class="methodname">TransactionRunner.run()</code> automatically begins a transaction and
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then calls the <code class="methodname">TransactionWorker.doWork()</code> method, which is
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implemented by the application.
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</p>
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<p>
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The <code class="methodname">TransactionRunner.run()</code> method automatically detects
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deadlock exceptions and performs retries by repeatedly calling the
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<code class="methodname">TransactionWorker.doWork()</code> method until the operation succeeds
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or the maximum retry count is reached. If the maximum retry count
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is reached or if another exception (other than
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<span>
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>)
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</span>
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is thrown by <code class="methodname">TransactionWorker.doWork()</code>, then the transaction
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will be automatically aborted. Otherwise, the transaction will be
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automatically committed.
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</p>
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<p>
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Using this high-level API, if <code class="methodname">TransactionRunner.run()</code>
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throws an exception, the application can assume that the operation
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failed and the transaction was aborted; otherwise, when an
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exception is not thrown, the application can assume the operation
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succeeded and the transaction was committed.
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</p>
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<p>
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The <code class="methodname">Sample.run()</code> method creates a <code class="classname">TransactionRunner</code>
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object and calls its <code class="methodname">run()</code> method.
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</p>
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<a id="cb_sample1"></a>
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<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.collections.TransactionRunner;
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import com.sleepycat.collections.TransactionWorker;</code></strong>
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...
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public class Sample
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{
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private SampleDatabase db;
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...
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<strong class="userinput"><code> private void run()
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throws Exception
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{
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TransactionRunner runner =
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new TransactionRunner(db.getEnvironment());
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runner.run(new PopulateDatabase());
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runner.run(new PrintDatabase());
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}
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...
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private class PopulateDatabase implements TransactionWorker
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{
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public void doWork()
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throws Exception
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{
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}
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}
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private class PrintDatabase implements TransactionWorker
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{
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public void doWork()
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throws Exception
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{
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}
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}</code></strong>
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} </pre>
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<p>
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The <code class="methodname">run()</code> method is called by <code class="methodname">main()</code> and was outlined
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in the previous section. It first creates a
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<code class="classname">TransactionRunner</code>, passing the database environment to its
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constructor.
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</p>
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<p>
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It then calls <code class="methodname">TransactionRunner.run()</code> to execute two
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transactions, passing instances of the application-defined
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<code class="classname">PopulateDatabase</code> and
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<code class="classname">PrintDatabase</code> nested classes.
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These classes implement the <code class="methodname">TransactionWorker.doWork()</code> method
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and will be fully described in the next two sections.
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</p>
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<p>
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For each call to <code class="methodname">TransactionRunner.run()</code>, a separate
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transaction will be performed. The use of two transactions in the
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example — one for populating the database and another for printing
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its contents — is arbitrary. A real-life application should be
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designed to create transactions for each group of operations that
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should have ACID properties, while also
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taking into account the impact of transactions on performance.
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</p>
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<p>
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The advantage of using <code class="classname">TransactionRunner</code> is that deadlock
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retries and transaction begin, commit and abort are handled
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automatically. However, a <code class="classname">TransactionWorker</code> class must be
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implemented for each type of transaction. If desired, anonymous
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inner classes can be used to implement the <code class="classname">TransactionWorker</code>
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interface.
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</p>
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</div>
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|
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<div class="navfooter">
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<hr />
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|||
|
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<table width="100%" summary="Navigation footer">
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|||
|
|
<tr>
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|||
|
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<td width="40%" align="left"><a accesskey="p" href="implementingmain.html">Prev</a> </td>
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|
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<td width="20%" align="center">
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<a accesskey="u" href="BasicProgram.html">Up</a>
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|||
|
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</td>
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|||
|
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<td width="40%" align="right"> <a accesskey="n" href="addingdatabaseitems.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">
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Implementing the Main Program
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</td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top">
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Adding Database Items
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</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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