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。