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BATCH-1276: Change Common Pattern "Writing a Summary Footer" to use AfterWrite for updating total.

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dhgarrette
2009-06-06 17:50:23 +00:00
parent b716c94eba
commit 73264ff40d
1 changed files with 44 additions and 28 deletions
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72
src/site/docbook/reference/common-patterns.xml
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@@ -195,22 +195,32 @@
the end of the file. This footer serves as a summarization of the file
or provides a checksum.</para>
<para>For example, if a batch job is writing Trade records to a flat
file, and there is a requirement that the total amount from all the
Trades is placed in a footer, then the following ItemWriter
implementation can be used:</para>
<para>For example, if a batch job is writing
<classname>Trade</classname> records to a flat file, and there is a
requirement that the total amount from all the
<classname>Trade</classname>s is placed in a footer, then the following
<classname>ItemWriter</classname> implementation can be used:</para>
<programlisting>public class TradeItemWriter implements ItemWriter&lt;Trade&gt;,
FlatFileFooterCallback {
private ItemWriter&lt;Trade&gt; delegate;
private double totalAmount = 0.0;
private BigDecimal totalAmount = BigDecimal.ZERO;
public void write(List&lt;? extends Trade&gt; items) {
for (Trade trade : items) {
// There must be an amount to total
Assert.notNull(trade.getAmount());
}
delegate.write(items);
for(Trade trade : items) {
totalAmount += trade.getAmount();
}
@AfterWrite
public void updateTotalPrice(List&lt;Trade&gt; trades) {
for (Trade trade : trades) {
totalAmount = totalAmount.add(trade.getAmount());
}
}
@@ -226,9 +236,21 @@
<code>amount</code> from each <classname>Trade</classname> item written.
After the last <classname>Trade</classname> is processed, the framework
will call <methodname>writeFooter</methodname>, which will put that
<code>totalAmount</code> into the file. In order for the method to be
called, the <classname>TradeItemWriter</classname> must be wired into
the <classname>FlatFileItemWriter</classname> as the
<code>totalAmount</code> into the file. Note that the
<property>totalAmount</property> is <emphasis>not</emphasis> updated in
the <methodname>write</methodname> method; it is updated in the
<classname>AfterWrite</classname> method. This is done to ensure that if
a skip occurs in the <methodname>write</methodname> method, that the
<property>totalAmount</property> will be left unchanged. The
<methodname>write</methodname> method also validates that the
<property>amount</property> on each <classname>Trade</classname> is not
null; this guarantees that no exceptions will be thrown by the
<classname>AfterWrite</classname> method.</para>
<para>In order for the <methodname>writeFooter</methodname> method to be
called, the <classname>TradeItemWriter</classname> (which implements
<classname>FlatFileFooterCallback</classname>) must be wired into the
<classname>FlatFileItemWriter</classname> as the
<code>footerCallback</code>:</para>
<programlisting>&lt;bean id="tradeItemWriter" class="..TradeItemWriter"&gt;
@@ -236,9 +258,9 @@
&lt;/bean&gt;
&lt;bean id="flatFileItemWriter" class="org.spr...FlatFileItemWriter"&gt;
&lt;property name="resource" ref="outputResource" /&gt;
&lt;property name="lineAggregator" ref="lineAggregator"/&gt;
<emphasis role="bold">&lt;property name="footerCallback" ref="tradeItemWriter" /&gt;</emphasis>
&lt;property name="resource" ref="outputResource" /&gt;
&lt;property name="lineAggregator" ref="lineAggregator"/&gt;
<emphasis role="bold"> &lt;property name="footerCallback" ref="tradeItemWriter" /&gt;</emphasis>
&lt;/bean&gt;</programlisting>
<para>The way that the <classname>TradeItemWriter</classname> has been
@@ -251,17 +273,17 @@
with the methods <methodname>open</methodname> and
<methodname>update</methodname>:</para>
<programlisting> private static final String TOTAL_AMOUNT_KEY = "total.amount";
<programlisting>private static final String TOTAL_AMOUNT_KEY = "total.amount";
public void open(ExecutionContext executionContext) {
if (executionContext.containsKey(TOTAL_AMOUNT_KEY) {
totalAmount = executionContext.getDouble(TOTAL_AMOUNT_KEY);
}
}
public void open(ExecutionContext executionContext) {
if (executionContext.containsKey(TOTAL_AMOUNT_KEY) {
totalAmount = (BigDecimal) executionContext.get(TOTAL_AMOUNT_KEY);
}
}
public void update(ExecutionContext executionContext) {
executionContext.setDouble(TOTAL_AMOUNT_KEY, totalAmount);
}</programlisting>
public void update(ExecutionContext executionContext) {
executionContext.put(TOTAL_AMOUNT_KEY, totalAmount);
}</programlisting>
<para>The <methodname>update</methodname> method will store the most
current version of <code>totalAmount</code> to the
@@ -272,12 +294,6 @@
for processing, allowing the <classname>TradeItemWriter</classname> to
pick up on restart where it left off the previous time the
<classname>Step</classname> was executed.</para>
<para>It should be noted that it is not always necessary to implement
<classname>ItemStream</classname>. For example, if the
<classname>ItemWriter</classname> is re-runnable, meaning that it
maintains its own state in a transactional resource like a database,
there is no need to maintain state within the writer itself.</para>
</section>
</section>