67 lines
3.8 KiB
XML
67 lines
3.8 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!-- ============================================================================= -->
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<!-- Copyright © 2009 Red Hat, Inc. and others. -->
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<!-- -->
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<!-- The text of and illustrations in this document are licensed by Red Hat under -->
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<!-- a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). -->
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<!-- -->
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<!-- An explanation of CC-BY-SA is available at -->
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<!-- -->
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<!-- http://creativecommons.org/licenses/by-sa/3.0/. -->
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<!-- -->
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<!-- In accordance with CC-BY-SA, if you distribute this document or an adaptation -->
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<!-- of it, you must provide the URL for the original version. -->
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<!-- -->
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<!-- Red Hat, as the licensor of this document, waives the right to enforce, -->
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<!-- and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent -->
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<!-- permitted by applicable law. -->
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<!-- ============================================================================= -->
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<chapter id="last-value-queues">
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<title>最新值队列（Last-Value Queues）</title>
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<para>最新值队列是一种特殊的队列。当一个新消息到达一个最新值队列时，它会将所有与该消息定义的Last-Value相同的旧消息
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抛弃。换句话说，只有最新的消息被保留下来。</para>
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<para>一个典型的用例是股价信息，通常你只关心一支股票的最新价格。</para>
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<section>
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<title>最新值队列的配置</title>
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<para>最新值队列的配置在address-setting内：</para>
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<programlisting>
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<address-setting match="jms.queue.lastValueQueue">
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<last-value-queue>true</last-value-queue>
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</address-setting>
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</programlisting>
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<para>默认的<literal>last-value-queue</literal>值是false。可以使用通配符来匹配地址。
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（参见 <xref linkend="wildcard-syntax"/>）。</para>
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</section>
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<section>
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<title>使用Last-Value参数</title>
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<para>用来标识最新值的参数名是<literal>"_HQ_LVQ_NAME"</literal>
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（相当于核心API中定义的常量<literal>Message.HDR_LAST_VALUE_NAME</literal>）。</para>
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<para>如果两个消息具有相同的Last-Value值，那么较新的消息就会保留，另外一个被丢弃：</para>
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<programlisting>
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// send 1st message with Last-Value property set to STOCK_NAME
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TextMessage message =
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session.createTextMessage("1st message with Last-Value property set");
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message.setStringProperty("_HQ_LVQ_NAME", "STOCK_NAME");
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producer.send(message);
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// send 2nd message with Last-Value property set to STOCK_NAME
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message =
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session.createTextMessage("2nd message with Last-Value property set");
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message.setStringProperty("_HQ_LVQ_NAME", "STOCK_NAME");
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producer.send(message);
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...
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// only the 2nd message will be received: it is the latest with
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// the Last-Value property set
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TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
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System.out.format("Received message: %s\n", messageReceived.getText());
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</programlisting>
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</section>
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<section>
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<title>例子</title>
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<para>参见<xref linkend="examples.last-value-queue"/>。它展示的是在JMS应用中来配置和使用
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最新值队列。</para>
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</section>
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</chapter>
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