本文主要和大家分享详解Apache Commons Digester，希望能帮助到大家。
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 cl
ass 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
<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
* @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<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
* @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<String, Object> vars = new HashMap<String, Object>();
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：
<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
* @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
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