Apache Commons Digester 二（规则模块绑定-RulesModule、异步解析-asyncParse、xml变量Substitutor、带参构造方法）

前言

上一篇对Digester做了基本介绍，也已经了解了Digester的基本使用方法，接下来将继续学习其相关特性，本篇主要涉及以下几个内容：

1. 规则模块绑定，通过定义一个RulesModule接口实现类来完成规则的预先绑定，运行时重复使用
2. 异步解析xml
3. 解析xml中的变量，如${sys.user}
4. 使用带参数的构造方法创建对象，参数来自xml节点数据

规则模块预先绑定 - RulesModule接口

在此之前，我们使用Digester的基本流程都是每次在程序运行时绑定规则，然后解析；

事实上，我们可以改变Digester的解析流程，启动的时候预先定义规则集，然后在运行的时候重复使用预先定义的规则；

可能这样说比较空泛，可以看一下如下一个Web应用场景，应该就会有一个比较深刻的理解了；

servlet场景例子

熟悉Web开发的应该都知道servlet了，这里就不细说了，假设有一个EmployeeServlet，如下所示：

由于servlet是单例的，而且Digester不是线程安全的，所以我们会在每次请求的的时候，new出一个Digester对象，来保证线程安全，写法如下：

public class EmployeeServlet
  extends HttpServlet
{

    public void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException
    {
        Digester digester = new Digester();
        digester.setNamespaceAware( true );
        digester.setXIncludeAware( true );
        digester.addObjectCreate( "employee", Employee.class );
        digester.addCallMethod( "employee/firstName", "setFirstName", 0 );
        digester.addCallMethod( "employee/lastName", "setLastName", 0 );

        digester.addObjectCreate( "employee/address", Address.class );
        digester.addCallMethod( "employee/address/type", "setType", 0 );
        digester.addCallMethod( "employee/address/city", "setCity", 0 );
        digester.addCallMethod( "employee/address/state", "setState", 0 );
        digester.addSetNext( "employee/address", "addAddress" );

        Employee employee = digester.parse( openStream( req.getParameter( "employeeId" ) ) );
        ...
}

我们可以很容易发现以上程序的缺点：代码没有复用，每次请求都需重复绑定规则；
不过，我们可以使用RuleSet来解决代码没有复用的问题，如下所示，定义一个EmployeeRuleSet规则集实现RuleSet接口：

public class EmployeeRuleSet
  implements RuleSet
{

    public void addRuleInstances( Digester digester )
    {
        digester.addObjectCreate( "employee", Employee.class );
        digester.addCallMethod( "employee/firstName", "setFirstName", 0 );
        digester.addCallMethod( "employee/lastName", "setLastName", 0 );

        digester.addObjectCreate( "employee/address", Address.class );
        digester.addCallMethod( "employee/address/type", "setType", 0 );
        digester.addCallMethod( "employee/address/city", "setCity", 0 );
        digester.addCallMethod( "employee/address/state", "setState", 0 );
        digester.addSetNext( "employee/address", "addAddress" );
    }

}

然后在servlet中这样使用：

public class EmployeeServlet
  extends HttpServlet
{

    private final RuleSet employeeRuleSet = new EmployeeRuleSet();

    public void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException
    {
        Digester digester = new Digester();
        digester.setNamespaceAware( true );
        digester.setXIncludeAware( true );

        employeeRuleSet.addRuleInstances( digester );

        Employee employee = digester.parse( openStream( req.getParameter( "employeeId" ) ) );
        ...
    }

}

很显然这样做是没有错误的（其实，个人觉得还不如直接写一个私有方法，添加规则，哈哈），但是有如下缺点：

1. RuleSet实际上并不是配置，只是给digester绑定下规则而已；
2. digester对象与客户端耦合度比较高，直接由客户端创建；
3. 每次解析调用前，都需要重复绑定规则
4. 规则绑定的时候，语义性很差，可读性不好；

那么，最佳实践是什么呢，答案是使用RulesModule接口，帮助我们启动时预先绑定规则，然后运行的时候，重复使用预先绑定的规则即可，如下所示：

定义一个RulesModule接口实现类：

class EmployeeModule
    extends AbstractRulesModule
{

    @Override
    protected void configure()
    {
        forPattern( "employee" ).createObject().ofType( Employee.class );
        forPattern( "employee/firstName" ).setBeanProperty();
        forPattern( "employee/lastName" ).setBeanProperty();

        forPattern( "employee/address" ).createObject().ofType( Address.class ).then().setNext( "addAddress");
        forPattern( "employee/address/type" ).setBeanProperty();
        forPattern( "employee/address/city" ).setBeanProperty();
        forPattern( "employee/address/state" ).setBeanProperty();
    }

}

然后在servlet这样使用：

public class EmployeeServlet
    extends HttpServlet
{

    private final DigesterLoader loader = newLoader( new EmployeeModule() )
        .setNamespaceAware( true )
        .setXIncludeAware( true );

    public void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException
    {
        Digester digester = loader.newDigester()

        Employee employee = digester.parse( openStream( req.getParameter("employeeId") ) );
        ...
    }

}

好处显而易见：

1. RulesModule规则绑定的API语义化很强，使用简便，可读性高；
2. 规则绑定的配置移到了启动阶段来完成；
3. digester对象不是由客户端来创建，而是通过DigesterLoader创建；

FromXmlRulesModule

除了自己编写类实现RulesModule接口外，digester自身提供了一个FromXmlRulesModule类，就已经实现了RulesModule接口，我们可以这样使用：

            DigesterLoader loader = DigesterLoader.newLoader( new FromXmlRulesModule()
            {

                @Override
                protected void loadRules()
                {
                    loadXMLRules( DigesterLoaderMain.class.getResource( "myrule.xml" ) );
                }

            } );
　　　　　　　...
　　　　　　Digester digester = loader.newDigester(); // myrule.xml already parsed
　　　　　　...
　　　　　　Digester newDigester = loader.newDigester(); // myrule.xml won't be parsed again!

完整例子

假设有一个xml如下，待解析

<employee>
    <firstName>Pi</firstName>
    <lastName>Chen</lastName>
    <address>
        <type>CITY</type>
        <city>HangZhou</city>
        <state>2</state>
    </address>
</employee>

开始编码，首先，定义一个RulesModule接口实现类：

package apache.commons.digester3.example.rulesbinder.module;

import org.apache.commons.digester3.binder.AbstractRulesModule;

import apache.commons.digester3.example.rulesbinder.pojo.Address;
import apache.commons.digester3.example.rulesbinder.pojo.Employee;
/**
 *
 *
 * @author    http://www.cnblogs.com/chenpi/
 * @version   2017年6月5日
 */
public class EmployeeModule extends AbstractRulesModule {

    @Override
    protected void configure() {
        forPattern("employee").createObject().ofType(Employee.class);
        forPattern("employee/firstName").setBeanProperty();
        forPattern("employee/lastName").setBeanProperty();

        forPattern("employee/address").createObject().ofType(Address.class).then().setNext("addAddress");
        forPattern("employee/address/type").setBeanProperty();
        forPattern("employee/address/city").setBeanProperty();
        forPattern("employee/address/state").setBeanProperty();
    }

}

编写客户端类：

package apache.commons.digester3.example.rulesbinder;

import java.io.IOException;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.binder.DigesterLoader;
import org.xml.sax.SAXException;

import apache.commons.digester3.example.rulesbinder.module.EmployeeModule;
import apache.commons.digester3.example.rulesbinder.pojo.Address;
import apache.commons.digester3.example.rulesbinder.pojo.Employee;
import apache.commons.digester3.example.simpletest.ExampleMain;
/**
 *
 *
 * @author    http://www.cnblogs.com/chenpi/
 * @version   2017年6月5日
 */
public class DigesterLoaderMain {

    private static DigesterLoader dl = DigesterLoader.newLoader(new EmployeeModule())
            .setNamespaceAware(false);
    public static void main(String[] args) {
        try {

            Digester digester = dl.newDigester();
            Employee employee = digester.parse(ExampleMain.class.getClassLoader().getResourceAsStream("employee.xml"));

            System.out.print(employee.getFirstName() + " ");
            System.out.print(employee.getLastName() + ", ");
            for (Address a : employee.getAddressList()) {
                System.out.print(a.getType() + ", ");
                System.out.print(a.getCity() + ", ");
                System.out.println(a.getState());
            }

        } catch (IOException e) {

            e.printStackTrace();
        } catch (SAXException e) {

            e.printStackTrace();
        }
    }
}

结果打印：

Pi Chen, CITY, HangZhou, 2

异步解析XML

异步解析的话，直接调用asyncParse方法即可，不过需要特别注意，因为digester对象并不是线程安全的，如下是一个简单的API使用示例：

承接上一个例子，使用同样的xml和RulesModule实现类;

客户端类：

package apache.commons.digester3.example.rulesbinder;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.binder.DigesterLoader;
import apache.commons.digester3.example.rulesbinder.module.EmployeeModule;
import apache.commons.digester3.example.rulesbinder.pojo.Address;
import apache.commons.digester3.example.rulesbinder.pojo.Employee;
import apache.commons.digester3.example.simpletest.ExampleMain;
/**
 *
 * @author    http://www.cnblogs.com/chenpi/
 * @version   2017年6月5日
 */
public class AsyncParseMain {
    private static DigesterLoader dl = DigesterLoader.newLoader(new EmployeeModule())
            .setNamespaceAware(false).setExecutorService(Executors.newSingleThreadExecutor());
    public static void main(String[] args) {
        try {

            Digester digester = dl.newDigester();
            Future<Employee> future = digester.asyncParse(ExampleMain.class.getClassLoader().getResourceAsStream("employee.xml"));

            Employee employee = future.get();

            System.out.print(employee.getFirstName() + " ");
            System.out.print(employee.getLastName() + ", ");
            for (Address a : employee.getAddressList()) {
                System.out.print(a.getType() + ", ");
                System.out.print(a.getCity() + ", ");
                System.out.println(a.getState());
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

xml变量解析-Substitutor抽象类

这个比较简单，定义一个VariableSubstitutor实现类，用户转换属性和body中定义的变量值；

假设有一个xml如下所示，（其中${type}为变量）：

<employee>
    <firstName>Pi</firstName>
    <lastName>Chen</lastName>
    <address>
        <type>${type}</type>
        <city>HangZhou</city>
        <state>2</state>
    </address>
</employee>

那么可以这样解析如上xml：

package apache.commons.digester3.example.rulesbinder;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.Substitutor;
import org.apache.commons.digester3.binder.DigesterLoader;
import org.apache.commons.digester3.substitution.MultiVariableExpander;
import org.apache.commons.digester3.substitution.VariableSubstitutor;
import org.xml.sax.SAXException;

import apache.commons.digester3.example.rulesbinder.module.EmployeeModule;
import apache.commons.digester3.example.rulesbinder.pojo.Address;
import apache.commons.digester3.example.rulesbinder.pojo.Employee;
import apache.commons.digester3.example.simpletest.ExampleMain;

/**
 *
 *
 * @author http://www.cnblogs.com/chenpi/
 * @version 2017年6月5日
 */
public class SubstitutionMain
{
    private static DigesterLoader dl = DigesterLoader.newLoader(new EmployeeModule())
        .setNamespaceAware(false);

    public static void main(String[] args)
    {

        try
        {
            // set up the variables the input xml can reference
            Map<String, Object> vars = new HashMap<String, Object>();
            vars.put("user.name", "me");
            vars.put("type", "boss");

            // map ${varname} to the entries in the var map
            MultiVariableExpander expander = new MultiVariableExpander();
            expander.addSource("$", vars);

            // allow expansion in both xml attributes and element text
            Substitutor substitutor = new VariableSubstitutor(expander);

            Digester digester = dl.newDigester();
            digester.setSubstitutor(substitutor);

            Employee employee = digester
                .parse(ExampleMain.class.getClassLoader().getResourceAsStream("employee$.xml"));

            System.out.print(employee.getFirstName() + " ");
            System.out.print(employee.getLastName() + ", ");
            for (Address a : employee.getAddressList())
            {
                System.out.print(a.getType() + ", ");
                System.out.print(a.getCity() + ", ");
                System.out.println(a.getState());
            }

        }
        catch (IOException e)
        {

            e.printStackTrace();
        }
        catch (SAXException e)
        {

            e.printStackTrace();
        }
    }
}

带参构造方法使用示例

简单地说，就是在使用ObjectCreateRule规则的时候，能够传递xml中的值（属性值、body值）给构造方法使用；

如下是一个待解析的xml：

<root>
  <bean super="false">
    <rate>9.99</rate>
  </bean>
</root>

那么可以这样解析：

package apache.commons.digester3.example.rulesbinder;

import java.io.IOException;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.ObjectCreateRule;
import org.apache.commons.digester3.binder.DigesterLoader;
import org.xml.sax.SAXException;

import apache.commons.digester3.example.rulesbinder.module.EmployeeModule;
import apache.commons.digester3.example.rulesbinder.pojo.Address;
import apache.commons.digester3.example.rulesbinder.pojo.Employee;
import apache.commons.digester3.example.rulesbinder.pojo.MyBean;
import apache.commons.digester3.example.simpletest.ExampleMain;

/**
 *
 *
 * @author http://www.cnblogs.com/chenpi/
 * @version 2017年6月5日
 */
public class ConstructorParamsMain
{

    public static void main(String[] args)
    {
        try
        {

            ObjectCreateRule createRule = new ObjectCreateRule(MyBean.class);
            createRule.setConstructorArgumentTypes(Double.class, Boolean.class);

            Digester digester = new Digester();
            digester.addRule("root/bean", createRule);
            digester.addCallParam("root/bean", 1, "super");
            digester.addCallParam("root/bean/rate", 0);

            MyBean myBean = digester.parse(ConstructorParamsMain.class.getClassLoader()
                .getResourceAsStream("constructor-params.xml"));

            System.out.println(myBean.getRate());
            System.out.println(myBean.isSuper_());

        }
        catch (IOException e)
        {

            e.printStackTrace();
        }
        catch (SAXException e)
        {

            e.printStackTrace();
        }
    }
}

结果打印：

9.99
false

参考资料

http://commons.apache.org/proper/commons-digester/

代码参考

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