# SpringBoot - 使用 ObjectMapper 完成 json 和 Java Object 互相轉換
### ObjectMapper 是一款非常好用的 json 轉換工具，可以幫助我們完成 json 和 Java 的 Object 的互相轉換

### 什麼是 Serialize 和 Deserialize？
- Serialize : 將 Java Object 轉換成 json
- Deserialize : 將 json 轉換成 Java Object

### 在 Spring Boot 裡使用 ObjectMapper
ObjectMapper 是由 Jackson library 所提供的一個功能，所以只要在 maven 中加入 spring-boot-starter-web 的 dependency 就可以了

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

### Json 和 Java Object、List、Map 的互轉

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

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;


public class JSONUtils {
    private static final Logger log = LogManager.getLogger(JSONUtils.class);

    /**
     * 將物件轉成JSon String
     * @param value
     * @return
     */
    public static String toJSONString(Object value)
    {
        try
        {
            ObjectMapper mapper = new ObjectMapper();
            return mapper.writeValueAsString(value);
        }
        catch (Exception e)
        {
            log.debug("toJSONString fail, Object=" + value, e);
            return null;
        }
    }

    /**
     * 將JSon String轉成物件
     * @param jsonString
     * @param object
     * @return
     */
    public static <T> T fromJSONString(String jsonString, Class<T> object)
    {
        if (jsonString == null || object == null)
        {
            log.warn("fromJSONString format error");
            return null;
        }

        try
        {
            ObjectMapper mapper = new ObjectMapper();
            return mapper.readValue(jsonString, object);
        }
        catch (Exception e)
        {
            log.debug("fromJSONString fail, jsonString=" + jsonString + ", object=" + object, e);
            return null;
        }
    }

    /**
     * 將Map<String, Object>轉成JSon String
     *
     */
    public static String readMapToJSON(HashMap<String, Object> map)
    {
        if (map == null || map.isEmpty() )
        {
            log.warn("readMaptoJSON format error");
            return null;
        }

        try
        {
            ObjectMapper mapper = new ObjectMapper();
            return mapper.writeValueAsString(map);
        }
        catch (Exception e)
        {
            log.debug("readMaptoJSON fail, map= " + map, e);
            return null;
        }
    }
    
    /**
     * 將JSon String轉成Map<String, Object>
     * 
     */
    public static Map<String, Object> readJSONtoMap(String jsonString)
    {
        if (jsonString == null || jsonString.trim().length() == 0)
        {
            log.warn("readJSONtoMap format error");
            return null;
        }

        try
        {
            ObjectMapper mapper = new ObjectMapper();
            TypeReference<HashMap<String, Object>> typeRef = new TypeReference<HashMap<String, Object>>(){};
            return mapper.readValue(jsonString, typeRef);
        }
        catch (Exception e)
        {
            log.debug("readJSONtoMap fail, jsonString=" + jsonString, e);
            return null;
        }
    }
    
    
    /**
     * 將List<Object>轉成JSon String
     * 
     */
    public static String readListToJSON(List<Object> list)
    {
        if (list == null || list.isEmpty() )
        {
            log.warn("readListToJSON format error");
            return null;
        }

        try
        {
            ObjectMapper mapper = new ObjectMapper();
            return mapper.writeValueAsString(list);
        }
        catch (Exception e)
        {
            log.debug("readListToJSON fail, list=" + list, e);
            return null;
        }
    }
    
    /**
     * 將JSon String轉成 List<Object>
     * 
     */
    public static List< Object> readJSONtoList(String jsonString)
    {
        if (jsonString == null || jsonString.trim().length() == 0)
        {
            log.warn("readJSONtoList format error");
            return null;
        }

        try
        {
            ObjectMapper mapper = new ObjectMapper();
            TypeReference<List< Object>> typeRef = new TypeReference<List< Object>>(){};
            return mapper.readValue(jsonString, typeRef);
        }
        catch (Exception e)
        {
            log.debug("readJSONtoList fail, jsonString=" + jsonString, e);
            return null;
        }
    }
     
}