Tomcat中Digester的作用

上一篇文章中大致讲了一下 Tomcat的Bootstrap、Catalina及Server之间的关系，这篇文章准备讲讲用来解析conf/server.xml的Digester工具类tomcat源码解析（三）——Digester类源码解析及Rule分析

 

我在看源码时，在Catalina中会看到大量的Digester代码段，这其实是Catalina解析server.xml配置的方式。

    /**
     * Create and configure the Digester we will be using for shutdown.
     * @return the digester to process the stop operation
     */
    protected Digester createStopDigester() {

        // Initialize the digester
        Digester digester = new Digester();
        digester.setUseContextClassLoader(true);

        // Configure the rules we need for shutting down
        digester.addObjectCreate("Server",
                                 "org.apache.catalina.core.StandardServer",
                                 "className");
        digester.addSetProperties("Server");
        digester.addSetNext("Server",
                            "setServer",
                            "org.apache.catalina.Server");

        return digester;

    }

 

为了便于理解，先上样例吧。

我的目的是将player.xml中的数据转换成的Player实例

player.xml

<player age="998" name="balabala">
    <role name="worker"/>
</player>

Player.java和Role.java

/** 玩家 */
public class Player {
    private String name;
    private int    age;
    private Role   role;
}

/** 角色 */
public class Role {
    private String name;
}

期望输出

System.out.println(player);
Player{name='balabala', age=998, role=Role{name='worker'}}

实现代码

public class DigesterTest {

    @Test
    public void test() throws IOException, SAXException {
        InputStream inputStream = this.getClass().getClassLoader().getResourceAsStream("player.xml");
        Digester digester = createPlayerDigester();
        Player player = (Player) digester.parse(inputStream);
        System.out.println(player);
        inputStream.close();
    }

    private Digester createPlayerDigester() {
        Digester digester = new Digester();
        digester.push(new Player());
        digester.addSetProperties("player");

        digester.addObjectCreate("player/role", "com.lance.test.digester.Role", "className");
        digester.addSetProperties("player/role");
        digester.addSetNext("player/role", "setRole", "com.lance.test.digester.Role");

        return digester;
    }
}

先不考虑代码细节，你会发现.xml的内容被解析成player对象，Tomcat的Catalina类也是一个道理，这只是用来解析server.xml等配置文件的。实现原理的细节可以参考上面提供的链接