json工具类ObjectMapper的详细使用记录

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 configureFile newState = new File("my-stuff.json");mapper.writeValue(newState, value); // writes JSON serialization of MyValue instance// or, readMyValue 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;
@Testpublic  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：

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：

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）。

@JsonIgnoreprivate String address;