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：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

异步解析xml

解析xml中的变量，如${sys.user}

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：RuleSet实际上并不是配置，只是给digester绑定下规则而已；

digester对象与客户端耦合度比较高，直接由客户端创建；

每次解析调用前，都需要重复绑定规则

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 EmployeeModuleextends AbstractRulesModule

{

@Overrideprotected 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 EmployeeServletextends 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：RulesModule规则绑定的API语义化很强，使用简便，可读性高；

规则绑定的配置移到了启动阶段来完成；

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( .getResource( "myrule.xml"

完整例子

假设有一个xml如下，待解析

Pi

Chen

CITY

HangZhou

2

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

* @version 2017年6月5日 */public class EmployeeModule extends AbstractRulesModule {

@Overrideprotected 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

* @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

* @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 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)：

Pi

Chen

${type}

HangZhou

2

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

* @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 referenceMap vars = new HashMap();

vars.put("user.name", "me");

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

expander.addSource("$", vars);// allow expansion in both xml attributes and element textSubstitutor 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：

9.99

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

* @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

代码参考