java反射快速入门（二）

上一遍博文 , 简单介绍java 反射的常用接口,本遍博文, 我会结合项目开发的实际例子讲解下 java反射的使用

现在有个需求, 要将一个对象转换成xml格式, 或者将一串xml转换一个对象, 这时我们循序渐进, 先从最简单的入手

一： 方案①

场景 ： NBA球员信息描述, 实体类如下

package test.reflect2;import org.dom4j.Element;

public class UserXMLDO {

    private Long id;
    private String name;
    private String password;
    private Double height;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Double getHeight() {
        return height;
    }

    public void setHeight(Double height) {
        this.height = height;
    }
}

现在要把传进来的一个userXMLDO对象转成一串xml

public static String toStringXML(UserXMLDO userXMLDO) throws Exception {
        StringBuffer sb = new StringBuffer();
        sb.append("<UserInfo>");
        sb.append(genNodeXmlString("id", userXMLDO.getId()));
        sb.append(genNodeXmlString("name", userXMLDO.getName()));
        sb.append(genNodeXmlString("password", userXMLDO.getPassword()));
        sb.append(genNodeXmlString("height", userXMLDO.getHeight()));
        sb.append("</UserInfo>");
        return sb.toString();
    }
    private static String genNodeXmlString(String nodeName, Object value) {
        StringBuffer sb = new StringBuffer();
        sb.append("<");
        sb.append(nodeName);
        sb.append(">");
        sb.append(value);
        sb.append("</");
        sb.append(nodeName);
        sb.append(">");
        return sb.toString();
    }

再提供一个方法,将xml转成userXMLDO对象

public static UserXMLDO toUserXMLDO(String xml) throws Exception {
        UserXMLDO  userXMLDO = new UserXMLDO();
        Element rootElement = DocumentXmlHelper.read(xml).getRootElement();

        for(Iterator<?> it = rootElement.elementIterator("id"); it.hasNext();) {
            Element element = (Element) it.next();
            userXMLDO.setId(Long.valueOf(element.getText()));
        }

        for(Iterator<?> it = rootElement.elementIterator("name"); it.hasNext();) {
            Element element = (Element) it.next();
            userXMLDO.setName(element.getText());
        }

        for(Iterator<?> it = rootElement.elementIterator("password"); it.hasNext();) {
            Element element = (Element) it.next();
            userXMLDO.setPassword(element.getText());
        }

        for(Iterator<?> it = rootElement.elementIterator("height"); it.hasNext();) {
            Element element = (Element) it.next();
            userXMLDO.setHeight(Double.valueOf(element.getText()));
        }
        return userXMLDO;
    }

上面的方案一,是最容易的想法做法,但此做法显得有点简单粗暴 。比如我要给球员添加一个 体重 属性, 这时你会发现, 要同时修改两个方法

toStringXML()、toUserXMLDO() 的实现

修改如下 ：
　　
private Double weight;

toStringXML() ： sb.append(genNodeXmlString("weight", userXMLDO.getName()));

toUserXMLDO() ：

for(Iterator<?> it = rootElement.elementIterator("weight"); it.hasNext();) { 

　　Element element = (Element) it.next(); 
　　userXMLDO.setHeight(Double.valueOf(element.getText()));
}

添加1个属性,改起来还是可以接受, 添加2个属性,还是可以接受的、....... 再添加100个属性呢？不能接受了吧~

二： 方案② 用反射

添加类型枚举

 package test.reflect2;

 import java.util.HashMap;
 import java.util.Map;

 public enum NodeEnum {

     userName("name", String.class),
     password("password", String.class),
     userId("id", Long.class),
     height("height", Double.class),
     ;

     NodeEnum(String name, Class<?> type) {
         this.name = name;
         this.type = type;
     }

     public static Map<String, Class<?>> getNodeMap() {
         Map<String, Class<?>> map = new HashMap<String, Class<?>>();
         NodeEnum[] nodeEnums = NodeEnum.values();
         for (NodeEnum ne : nodeEnums) {
             map.put(ne.getName(), ne.getType());
         }
         return map;
     }

     private String name;

     private Class<?> type;

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }

     public Class<?> getType() {
         return type;
     }

     public void setType(Class<?> type) {
         this.type = type;
     }

 }

实体类添加

 public class UserXMLDO {

     private static Map<String, Class<?>> nodeEnumMap = null;
     static {
         nodeEnumMap = NodeEnum.getNodeMap();
     }

     private Long id;
     private String name;
     private String password;
     private Double height;
     private Double weight;

     public static String toStringXML(UserXMLDO userXMLDO) throws Exception {
         StringBuffer sb = new StringBuffer();
         sb.append("<UserInfo>");
         Iterator<Entry<String, Class<?>>> it = nodeEnumMap.entrySet().iterator();
         while (it.hasNext()) {
             Entry<String, Class<?>> entry = it.next();
             Object object = userXMLDO.getClass().getDeclaredField(entry.getKey()).get(userXMLDO);
             sb.append(genNodeXmlString(entry.getKey(), object == null ? "" : object));
         }
         sb.append("</UserInfo>");
         return sb.toString();
     }

     public static UserXMLDO toUserXMLDO(String xml) throws Exception {
         UserXMLDO userXMLDO = new UserXMLDO();
         Element rootElement = DocumentXmlHelper.read(xml).getRootElement();

         Iterator<?> it = rootElement.elementIterator();
         while (it.hasNext()) {
             Element e = (Element) it.next();
             setAttrValue(userXMLDO, e.getName(), e.getTextTrim());
         }
         return userXMLDO;
     }

     private static void setAttrValue(UserXMLDO userXMLDO, String nodeName, String value) throws Exception {

         Field f = userXMLDO.getClass().getDeclaredField(nodeName);
         if (nodeEnumMap.get(nodeName).getName().equals(String.class.getName())) {
             f.set(userXMLDO, value);
         } else if (nodeEnumMap.get(nodeName).getName().equals(Long.class.getName())
                 || nodeEnumMap.get(nodeName).getName().equals(Double.class.getName())) {
             f.set(userXMLDO,
                     nodeEnumMap.get(nodeName).getDeclaredMethod("valueOf", String.class).invoke(userXMLDO, value));
         } else {
             throw new Exception("仅支持Long, String 类型");
         }
     }

     private static String genNodeXmlString(String nodeName, Object value) {
         StringBuffer sb = new StringBuffer();
         sb.append("<");
         sb.append(nodeName);
         sb.append(">");
         sb.append(value);
         sb.append("</");
         sb.append(nodeName);
         sb.append(">");
         return sb.toString();
     }

     public Long getId() {
         return id;
     }

     public void setId(Long id) {
         this.id = id;
     }

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }

     public String getPassword() {
         return password;
     }

     public void setPassword(String password) {
         this.password = password;
     }

     public Double getHeight() {
         return height;
     }

     public void setHeight(Double height) {
         this.height = height;
     }

     public Double getWeight() {
         return weight;
     }

     public void setWeight(Double weight) {
         this.weight = weight;
     }
 }

test ：

 public static void main(String[] args) throws Exception {
         UserXMLDO userXMLDO = new UserXMLDO();
         userXMLDO.setId(1L);
         userXMLDO.setName("威少");
         userXMLDO.setPassword("三双无解");
         userXMLDO.setHeight(1.90d);
         userXMLDO.setWeight(85.5d);

         String xml = UserXMLDO.toStringXML(userXMLDO);
         System.out.println(xml);

         UserXMLDO userXMLDO2 = UserXMLDO.toUserXMLDO(xml);
         System.out.println(userXMLDO2);
     }

方案② , 体现出了很多灵活性, 把 可变 与 不可变 code 抽取出来了, 可能这样说, 或许有点难理解, 我就针对这种做法, 举个例子...

如 ： 我要球员添加一个 体重 属性, 这是你会发现, 不用去修改

toStringXML()、toUserXMLDO() 的实现

你只要在  NodeEnum 增加一个类型定义即可 : 

weight("weight", Double.class),

素不素改动很小, 如果换一个新人来接手维护, 每次有新的属性需求添加, 他可以很快添加完成, 不需要去改方法的实现, 这样就相对稳定了~~很巧妙做到“以不变应万变”！！

三、总结

相信比较上述两种方案后, 你会觉得反射有时特别好用, 让代码更加健硕, 但是也要记住, 不要一味盲目, 甚至刻意去使用反射, 因为性能差是反射的一大弊端！要权衡评估下。