json工具类ObjectMapper的详细使用记录

╰半橙微兮° 2022-09-11 01:23 117阅读 0赞

## 1。用于json与其他对象之间转化的工具类： ##

public class JsonUtil { 
        private static final ObjectMapper MAPPER = new ObjectMapper();
        private static final Logger logger = LoggerFactory.getLogger(JsonUtil.class);
    
        static { 
            MAPPER.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        }
    
        public static <T> String toJsonStr(T o) { 
            try { 
                return MAPPER.writeValueAsString(o);//json转化为string
            } catch (JsonProcessingException e) { 
                logger.error(e.getMessage(), e);
            }
            return null;
        }
    
        public static <T> T toJsonObject(String json, Class<T> valueType) { 
            try { 
                return MAPPER.<T>readValue(json, valueType);
            } catch (IOException e) { 
                logger.error(e.getMessage(), e);
            }
            return null;
        }
    
    
        public static <T> List<T> toJsonListObject(String json, Class<T> valueType) { 
            try { 
                JavaType getCollectionType = MAPPER.getTypeFactory().constructParametricType(List.class, valueType);
                List<T> list = MAPPER.readValue(json, getCollectionType);
                return list;
            } catch (IOException e) { 
                logger.error(e.getMessage(), e);
            }
            return null;
        }
    
        public static <T> T toJsonObject(InputStream stream, Class<T> valueType) { 
            try { 
                T object = MAPPER.<T>readValue(stream, valueType);
                return object;
            } catch (IOException e) { 
                logger.error(e.getMessage(), e);
            }
            return null;
        }
    }

将文件与对象关联的解析类：

public class JSON
    { 
        public static final String DEFAULT_FAIL = "\"Parse failed\"";
        private static final ObjectMapper objectMapper = new ObjectMapper();
        private static final ObjectWriter objectWriter = objectMapper.writerWithDefaultPrettyPrinter();
    //.writerWithDefaultPrettyPrinter();用于输出时的格式化
        public static void marshal(File file, Object value) throws Exception
        { 
            try
            { 
                objectWriter.writeValue(file, value);
            }
            catch (JsonGenerationException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static void marshal(OutputStream os, Object value) throws Exception
        { 
            try
            { 
                objectWriter.writeValue(os, value);
            }
            catch (JsonGenerationException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static String marshal(Object value) throws Exception
        { 
            try
            { 
                return objectWriter.writeValueAsString(value);
            }
            catch (JsonGenerationException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static byte[] marshalBytes(Object value) throws Exception
        { 
            try
            { 
                return objectWriter.writeValueAsBytes(value);
            }
            catch (JsonGenerationException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static <T> T unmarshal(File file, Class<T> valueType) throws Exception
        { 
            try
            { 
                return objectMapper.readValue(file, valueType);
            }
            catch (JsonParseException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static <T> T unmarshal(InputStream is, Class<T> valueType) throws Exception
        { 
            try
            { 
                return objectMapper.readValue(is, valueType);
            }
            catch (JsonParseException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static <T> T unmarshal(String str, Class<T> valueType) throws Exception
        { 
            try
            { 
                return objectMapper.readValue(str, valueType);
            }
            catch (JsonParseException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    
        public static <T> T unmarshal(byte[] bytes, Class<T> valueType) throws Exception
        { 
            try
            { 
                if (bytes == null)
                { 
                    bytes = new byte[0];
                }
                return objectMapper.readValue(bytes, 0, bytes.length, valueType);
            }
            catch (JsonParseException e)
            { 
                throw new Exception(e);
            }
            catch (JsonMappingException e)
            { 
                throw new Exception(e);
            }
            catch (IOException e)
            { 
                throw new Exception(e);
            }
        }
    }

ObjectMapper 的一个使用例子：

final ObjectMapper mapper = new ObjectMapper(); // can use static singleton, inject: just make sure to reuse!
      MyValue value = new MyValue();
      // ... and configure
      File newState = new File("my-stuff.json");
      mapper.writeValue(newState, value); // writes JSON serialization of MyValue instance
      // or, read
      MyValue older = mapper.readValue(new File("my-older-stuff.json"), MyValue.class);
    
      // Or if you prefer JSON Tree representation:
      JsonNode root = mapper.readTree(newState);
      // and find values by, for example, using a JsonPointer expression:
      int age = root.at("/personal/age").getValueAsInt();

用ObjectMapper来输出一串json字符串：

import com.fasterxml.jackson.databind.ObjectMapper;
    @Test
        public  void testJson() throws JsonProcessingException { 
           List<Word>blogs= wordMapper.selectAll();
           String newjson = null;
           ObjectMapper objectMapper = new ObjectMapper();
          newjson= objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(blogs);
            System.out.println(newjson);
          newjson=  JsonUtil.toJsonStr(blogs.getClass());
            System.out.println(newjson.getBytes(StandardCharsets.UTF_8));
       
        }

结果：  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAbWFyY2ggb2YgVGltZQ_size_19_color_FFFFFF_t_70_g_se_x_16]  
把json串写入到文件中：

List<Word>blogs= wordMapper.selectAll();
           String newjson = null;
           ObjectMapper objectMapper = new ObjectMapper();
            File newState = new File("my-stuff.json");
            objectMapper.writeValue(newState,blogs);

结果：  
![在这里插入图片描述][bee5bb5c16014c4394bdf3be97c6202d.png]  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAbWFyY2ggb2YgVGltZQ_size_20_color_FFFFFF_t_70_g_se_x_16]  
使用toJsonListObject得到json转化而来的对象：

String newjson = "[{\"id\":1,\"english\":\"apple\",\"chinese\":\"苹果\",\"level\":1,\"frequency\":3,\"img\":\"http://localhost:8107/0.png\",\"usa\":\"an apple\"},\n" +
                   " {\"id\":2,\"english\":\"pear\",\"chinese\":\"梨子\",\"level\":2,\"frequency\":2,\"img\":\"http://localhost:8107/2.png\",\"usa\":\"eat pear\"},\n" +
                   " {\"id\":3,\"english\":\"car\",\"chinese\":\"车\",\"level\":1,\"frequency\":3,\"img\":null,\"usa\":\"cars(复数）buy car(买车）\"},{\"id\":4,\"english\":\"people\",\"chinese\":\"人\",\"level\":1,\"frequency\":3,\"img\":null,\"usa\":\"people(单复一样） we are people\"}]";
    
    
    
    
            List<Word>blog2= JsonUtil.toJsonListObject(newjson,Word.class);
            List<String>str = blog2.stream().map(e->{ 
                String ch = e.getChinese();
                return ch;
            }).collect(Collectors.toList());
            System.out.println(str);

输出  
\[苹果, 梨子, 车, 人\]

## 一些相关注解： ##

@JsonInclude

//Include.Include.ALWAYS 默认  
//Include.NON\_DEFAULT 属性为默认值不序列化  
//Include.NON\_EMPTY 属性为 空（“”） 或者为 NULL 都不序列化  
//Include.NON\_NULL 属性为NULL 不序列化  
@JsonProperty：用于指明属性的名称。  
@JsonProperty(“create\_date”)  
private Date createDate;  
此时将对象序列化以后得到的json串中上面的属性为create\_date"  
@JsonIgnore：用于忽略指定属性，当该注解出现在field、getter、setter或者构造方法中任意一个上时，都意味着忽略所有（即序列化和反序列化都被忽略）；有一种情况，当getter上注解@JsonIgnore而setter上注解@JsonProperty，就会出现“只读”情况（read from input, but is not written output）。

@JsonIgnore
    
    private String address;

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[bee5bb5c16014c4394bdf3be97c6202d.png]: /images/20220829/c261fe815eaf4b4cbce318626f70eaea.png
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAbWFyY2ggb2YgVGltZQ_size_20_color_FFFFFF_t_70_g_se_x_16]: /images/20220829/67e409b436944549ab3c7eb2cf0aa3e6.png