前言

Digester规则的定义除了可以在代码中直接new规则添加到 Digester对象外，还可以用xml配置规则，如下所示：

<digester-rules>  <pattern value="*/foo">    <object-create-rule pattern="bar" classname="Foobar" />    <set-properties-rule pattern="bar" />  </pattern></digester-rules>

显然，xml的规则配置目前已经不是主流方法了，今后的大趋势肯定是用注解，所以本文将主要关注如何在javabean上使用注解来配置Digester规则；

Digester规则注解

简单描述下Digester 有哪几类注解，详细信息建议看api文档，其实也比较简单了，熟悉Digester规则的话，基本不用看也知道；

类型注解

@ObjectCreate 绑定Digester的ObjectCreateRule规则
@FactoryCreate 绑定Digester的FactoryCreateRule规则

属性注解

@BeanPropertySetter 绑定Digester的BeanPropertySetterRule规则
@SetProperty 绑定Digester的SetPropertiesRule规则

方法注解

@CallMethod ----> org.apache.commons.digester3.CallMethodRule
@SetNext ----> org.apache.commons.digester3.SetNextRule
@SetRoot ----> org.apache.commons.digester3.SetRootRule
@SetTop ----> org.apache.commons.digester3.SetTopRule

参数注解

@CallParam ----> org.apache.commons.digester3.rule.CallParamRule

Digester注解例子

看了半天，可能也还没有清晰直观的认识，直接看个例子，基本也就差不多了

如下是我们要解析的xml文件：

<rss version="2.0">    <channel>        <title>Apache</title>        <link>http://www.apache.org</link>        <description>The Apache Software Foundation</description>        <language>en-US</language>        <rating>(PICS-1.1 "http://www.rsac.org/ratingsv01.html"            2 gen true comment "RSACi North America Server"            for "http://www.rsac.org" on "1996.04.16T08:15-0500"            r (n 0 s 0 v 0 l 0))        </rating>        <image>            <title>Apache</title>            <url>http://jakarta.apache.org/images/jakarta-logo.gif</url>            <link>http://jakarta.apache.org</link>            <width>505</width>            <height>480</height>            <description>The Jakarta project. Open source, serverside java.            </description>        </image>        <item>            <title>Commons Attributes 2.1 Released</title>            <link>http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040815.1            </link>            <description>The Apache Commons team is happy to announce the release                of Commons Attributes 2.1.                This is the first release of the new Commons-Attributes code.            </description>        </item>        <item>            <title>Cloudscape Becomes Apache Derby</title>            <link>http://jakarta.apache.org/site/news/elsewhere-2004-2ndHalf.html#20040803.1            </link>            <description>IBM has submitted a proposal to the Apache DB project                for a Java-based package to be called ‘Derby‘.</description>        </item>        <item>            <title>Commons BeanUtils 1.7 Released</title>            <link>http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040802.1            </link>            <description />        </item>        <item>            <title>Commons JXPath 1.2 Released</title>            <link>http://jakarta.apache.org/site/news/news-2004-2ndHalf.html#20040801.2            </link>            <description />        </item>    </channel></rss>

首先，观察分析要解析的xml文档，创建javabean对象，加上Digester元注解，

先创建一个Channel对象，如下所示，感觉也不需要过多解析，很直观了，注意下@SetNext注解，是根据方法参数对象里定义的注解匹配的

package apache.commons.digester3.example.annotations;import java.util.ArrayList;import java.util.List;import org.apache.commons.digester3.annotations.rules.BeanPropertySetter;import org.apache.commons.digester3.annotations.rules.ObjectCreate;import org.apache.commons.digester3.annotations.rules.SetNext;/** *  *  * @author http://www.cnblogs.com/chenpi/ * @version 2017年6月7日 */@ObjectCreate(pattern = "rss/channel")public class Channel{    private final List<Item> items = new ArrayList<Item>();    private Image image;        @BeanPropertySetter(pattern = "rss/channel/title")    private String title;    @BeanPropertySetter(pattern = "rss/channel/link")    private String link;    @BeanPropertySetter(pattern = "rss/channel/description")    private String description;    @BeanPropertySetter(pattern = "rss/channel/language")    private String language;    @SetNext    public void setImage(Image image)    {        this.image = image;    }    @SetNext    public void addItem(Item item)    {        this.items.add(item);    }    /**     * @return the title     */    public String getTitle()    {        return title;    }    /**     * @param title the title to set     */    public void setTitle(String title)    {        this.title = title;    }    /**     * @return the link     */    public String getLink()    {        return link;    }    /**     * @param link the link to set     */    public void setLink(String link)    {        this.link = link;    }    /**     * @return the description     */    public String getDescription()    {        return description;    }    /**     * @param description the description to set     */    public void setDescription(String description)    {        this.description = description;    }    /**     * @return the language     */    public String getLanguage()    {        return language;    }    /**     * @param language the language to set     */    public void setLanguage(String language)    {        this.language = language;    }    /**     * @return the items     */    public List<Item> getItems()    {        return items;    }    /**     * @return the image     */    public Image getImage()    {        return image;    }}

在定义两个对象Image和Item，如下所示，至此，我们就已经把xml中所有元素需要匹配的规则定义完了

package apache.commons.digester3.example.annotations;import org.apache.commons.digester3.annotations.rules.BeanPropertySetter;import org.apache.commons.digester3.annotations.rules.ObjectCreate;/** *  *  * @author http://www.cnblogs.com/chenpi/ * @version 2017年6月7日 */@ObjectCreate(pattern = "rss/channel/item")public class Item{    @BeanPropertySetter(pattern = "rss/channel/item/description")    private String description;    @BeanPropertySetter(pattern = "rss/channel/item/link")    private String link;    @BeanPropertySetter(pattern = "rss/channel/item/title")    private String title;    /**     * @return the description     */    public String getDescription()    {        return description;    }    /**     * @param description the description to set     */    public void setDescription(String description)    {        this.description = description;    }    /**     * @return the link     */    public String getLink()    {        return link;    }    /**     * @param link the link to set     */    public void setLink(String link)    {        this.link = link;    }    /**     * @return the title     */    public String getTitle()    {        return title;    }    /**     * @param title the title to set     */    public void setTitle(String title)    {        this.title = title;    }}

package apache.commons.digester3.example.annotations;import org.apache.commons.digester3.annotations.rules.BeanPropertySetter;import org.apache.commons.digester3.annotations.rules.ObjectCreate;/** *  *  * @author http://www.cnblogs.com/chenpi/ * @version 2017年6月7日 */@ObjectCreate(pattern = "rss/channel/image")public class Image{    @BeanPropertySetter(pattern = "rss/channel/image/description")    private String description;    @BeanPropertySetter(pattern = "rss/channel/image/width")    private int width;    @BeanPropertySetter(pattern = "rss/channel/image/height")    private int height;    @BeanPropertySetter(pattern = "rss/channel/image/link")    private String link;    @BeanPropertySetter(pattern = "rss/channel/image/title")    private String title;    @BeanPropertySetter(pattern = "rss/channel/image/url")    private String url;    /**     * @return the description     */    public String getDescription()    {        return description;    }    /**     * @param description the description to set     */    public void setDescription(String description)    {        this.description = description;    }    /**     * @return the width     */    public int getWidth()    {        return width;    }    /**     * @param width the width to set     */    public void setWidth(int width)    {        this.width = width;    }    /**     * @return the height     */    public int getHeight()    {        return height;    }    /**     * @param height the height to set     */    public void setHeight(int height)    {        this.height = height;    }    /**     * @return the link     */    public String getLink()    {        return link;    }    /**     * @param link the link to set     */    public void setLink(String link)    {        this.link = link;    }    /**     * @return the title     */    public String getTitle()    {        return title;    }    /**     * @param title the title to set     */    public void setTitle(String title)    {        this.title = title;    }    /**     * @return the url     */    public String getUrl()    {        return url;    }    /**     * @param url the url to set     */    public void setUrl(String url)    {        this.url = url;    }    // getters and setters}

规则与javabean绑定完后，就可以开始解析了，注意我们这里使用DigesterLoader来创建Digester实例对象，而DigesterLoader实例是通过FromAnnotationsRuleModule创建的，该类允许我们从一个注解的类上加载规则集：

/* * File Name: Main.java * Description:  * Author: http://www.cnblogs.com/chenpi/ * Create Date: 2017年6月7日 */package apache.commons.digester3.example.annotations;import org.apache.commons.digester3.Digester;import org.apache.commons.digester3.annotations.FromAnnotationsRuleModule;import org.apache.commons.digester3.binder.DigesterLoader;/** *  * @author http://www.cnblogs.com/chenpi/ * @version 2017年6月7日 */public class Main{    public static void main(String[] args)    {        try        {            DigesterLoader loader = DigesterLoader.newLoader(new FromAnnotationsRuleModule()            {                @Override                protected void configureRules()                {                    bindRulesFrom(Channel.class);                }            });            Digester digester = loader.newDigester();            Channel channel = digester                .parse(Main.class.getClassLoader().getResourceAsStream("rss.xml"));            System.out.println(channel.getTitle());            System.out.println(channel.getImage().getDescription());        }        catch (Exception e)        {            e.printStackTrace();        }    }}

结语

至此，Digester 的学习就告一段落了，除了使用xml定义规则（有注解了，感觉没必要在使用xml配置规则）和Digester插件模块（感觉很少会使用，不过参考代码里有例子）没详细说明外，基本涵盖了所有内容；

有兴趣的还可以深入研究，看看源码，相对而言也不难；

参考资料

http://commons.apache.org/proper/commons-digester/guide/annotations.html

参考代码

https://github.com/peterchenhdu/apache-commons-digester-example

Apache Commons Digester 三（规则注解）

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