JAVA基础之HashMap转List

Love The Way You Lie 2022-05-22 01:53 440阅读 0赞

### 前言 ###

今天在开发中，用elasticsearch \+ java实现热门关键字搜索商品列表这样一个功能。。。![惊恐][panic.gif]遇到一个HashMap强转List失败问题。这里记录一下~![大笑][laugh.gif]

### 调用接口 ###

这里我先用postman调了一下通过热门关键字查询商品列表接口

![20180612211106994][]

发现500，服务器内部错误了。。。what?因为es在阿里云服务器上面，本地又不能打断点，debug调试。查看服务器日志，找到问题所在；

### 服务器日志    ###

问题所在如下图：

![20180612210709100][]

噢~~~HashMap转强转List失败了。

### 发现问题 ###

查看了日志，再看了看代码。

@Override
        public Map<String, Object> handleEsGoodsVipPrice(Integer uId, Map<String, Object> map) {
            Map<String, Object> vipUser = vipUserMapper.queryUserVipInfoByUserId(uId);
            // es查询结果得到Map，map中取到esGoods，转为list，处理数据后返回
            List<EsGoods> list = (List<EsGoods>) map.get("esGoods");
    
            if (vipUser != null) {
                // 用户有Vip等级
                // 根据userID得到用户vip等级ID
                Integer gradeId = (Integer) vipUser.get("gradeId");
                list.forEach(bsGoodsRecommendH5 -> {
                    BsGoodsVipPrice goodsVipPrice = goodsVipPriceMapper.queryVipPricesByGoodsIdAndVipId(bsGoodsRecommendH5.getGoodsId(), gradeId);
                    if (goodsVipPrice != null && goodsVipPrice.getVipPrice() != null) {
                        bsGoodsRecommendH5.setVipPrice(goodsVipPrice.getVipPrice());
                    } else {
                        //如果没查到该用户对应vip等级的vip价格  取最低Vip等级的vip价格
                        Long defaultVipPrice = goodsVipPriceMapper.queryDefaultVipPriceByGoodsId(bsGoodsRecommendH5.getGoodsId());
                        bsGoodsRecommendH5.setVipPrice(defaultVipPrice);
                    }
                });
    
            } else {
                // 用户没有vip等级 vip价格默认取最低等级对应的价格
                list.forEach(bsGoodsRecommendH5 -> {
                    Long defaultVipPrice = goodsVipPriceMapper.queryDefaultVipPriceByGoodsId(bsGoodsRecommendH5.getGoodsId());
                    bsGoodsRecommendH5.setVipPrice(defaultVipPrice);
                });
            }
            map.put("esGoods", list);
            return map;
        }

原来就是这里强转出了问题

List<EsGoods> list = (List<EsGoods>) map.get("esGoods");

### 解决方法 ###

呵~~既然hashMap强转list失败了，我就不强转了。先把hashMap先转为json字符串，然后将得到的json字符串转为list就ok了！！！

代码如下：![生气][mad.gif]

//将map.get("esGoods")先转为json字符串，然后将得到的json字符串转为list就ok了！！！
    List<EsGoods> list = JsonUtils.json2ListBean(JsonUtils.toJson(map.get("esGoods")), EsGoods.class);

### 总结 ###

map不能转list？？因为开发时间比较紧，日后再带着这个问题，去寻找答案吧

##### 附上json工具类 #####

import com.fasterxml.jackson.annotation.JsonInclude.Include;
    import com.fasterxml.jackson.core.JsonFactory;
    import com.fasterxml.jackson.core.JsonGenerator;
    import com.fasterxml.jackson.core.JsonParser.Feature;
    import com.fasterxml.jackson.core.type.TypeReference;
    import com.fasterxml.jackson.databind.ObjectMapper;
    import com.wybc.common.exception.BusiException;
    import com.wybc.common.model.E;
    import net.sf.json.JSONArray;
    
    import java.beans.BeanInfo;
    import java.beans.Introspector;
    import java.beans.PropertyDescriptor;
    import java.io.IOException;
    import java.io.StringWriter;
    import java.lang.reflect.Method;
    import java.util.*;
    
    /**
     * Json工具类
     */
    public class JsonUtils {
    
    	private static final ObjectMapper mapper = new ObjectMapper();
    
    	static {
    		mapper.configure(Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
    		mapper.configure(Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);
    		mapper.setSerializationInclusion(Include.NON_NULL);
    	}
    
    	private JsonUtils() {
    	}
    
    	/**
    	 * json字符串转换为类
    	 */
    	public static <T> T toBean(String json, Class<T> clazz) {
    		try {
    			T bean = (T) mapper.readValue(json, clazz);
    			return bean;
    		} catch (IOException e) {
    			e.printStackTrace();
    		}
    		return null;
    	}
    
    	@SuppressWarnings("unchecked")
    	public static HashMap<String, Object> toBean(String json) {
    		return toBean(json, HashMap.class);
    	}
    
    	@SuppressWarnings("unchecked")
    	public static HashMap<String,String> toBeanStr(String json) {
    		return toBean(json, HashMap.class);
    	}
    	
    	@SuppressWarnings("unchecked")
    	public static <T> T toBean(String json, TypeReference<T> tr){
    		try {
    			T bean = (T) mapper.readValue(json, tr);
    			return bean;
    		} catch (IOException e) {
    			e.printStackTrace();
    		}
    		return null;
    	}
    
    	/**
    	 * 对象转换为json字符串
    	 */
    	public static String toJson(Object bean) {
    		String json = null;
    		JsonGenerator gen = null;
    		StringWriter sw = new StringWriter();
    		try {
    			gen = new JsonFactory().createGenerator(sw);
    			mapper.writeValue(gen, bean);
    			json = sw.toString();
    		} catch (IOException e) {
    			throw new BusiException(E.SYSTEM_ERROR);
    		} finally {
    			try {
    				if (gen != null) {
    					gen.close();
    				}
    				if (sw != null) {
    					sw.close();
    				}
    			} catch (IOException e) {
    				e.printStackTrace();
    			}
    		}
    		return json;
    	}
    
    	@SuppressWarnings("unchecked")
    	public static List<Object> toList(String json) {
    		return toBean(json, ArrayList.class);
    	}
    
    	/**
    	 * 对象转换为Map
    	 */
    	public static Map<String, Object> transBean2Map(Object obj) {
    		if (obj == null) {
    			return null;
    		}
    		Map<String, Object> map = new HashMap<String, Object>();
    		try {
    			BeanInfo beanInfo = Introspector.getBeanInfo(obj.getClass());
    			PropertyDescriptor[] propertyDescriptors = beanInfo.getPropertyDescriptors();
    			for (PropertyDescriptor property : propertyDescriptors) {
    				String key = property.getName();
    				if (!key.equals("class")) {
    					Method getter = property.getReadMethod();
    					Object value = getter.invoke(obj);
    					map.put(key, value);
    				}
    			}
    		} catch (Exception e) {
    			System.out.println("transBean2Map Error " + e);
    		}
    		return map;
    	}
    
    	/**
    	 * json字符串转换为List
    	 */
    	public static <T> List<T>json2ListBean(String json,Class<T>cls){
    		JSONArray jArray= JSONArray.fromObject(json);
    	    Collection<T> collection = JSONArray.toCollection(jArray, cls);
    	    List<T> list = new ArrayList<T>();
    	    Iterator<T> it = collection.iterator();
    	    while (it.hasNext()) {
    	        T bean = (T) it.next();
    	        list.add(bean);
    	    }
    	    return list;
    	}
    }

ps:工具能中用到的ObjectMapper类是Jackson库的主要类，如果要了解该类的源码以及使用方法，可自行百度。学习之路任重而道远！

[panic.gif]: https://static-blog.csdn.net/xheditor/xheditor_emot/default/panic.gif
[laugh.gif]: https://static-blog.csdn.net/xheditor/xheditor_emot/default/laugh.gif
[20180612211106994]: /images/20220522/024d752eef5041009880524a9bcd5d50.png
[20180612210709100]: /images/20220522/3988130c9e3c4ab8888d3292b7faf848.png
[mad.gif]: https://static-blog.csdn.net/xheditor/xheditor_emot/default/mad.gif