jackson的简单应用

Jackson是我使用的第二种json序列化工具，第一种是阿里的fastjson

1.Jackson的导入

            com.fasterxml.jackson.core
            jackson-databind
            2.5.3

2.核心对象是ObejecjMapper

        TestBean testBean = new TestBean("haha");
        TestBean testBean1 = new TestBean("haha1");
        System.out.println("testbean1"+testBean);
        System.out.println("testbean2"+testBean1);
        l.add(testBean1);
        l.add(testBean);
        ObjectMapper objectMapper = new ObjectMapper();
        String ja = "{\"name\":\"haha\"}";
        String json = objectMapper.writeValueAsString(l);
        System.out.println(json);
        TestBean testBean2 = objectMapper.readValue(ja, TestBean.class);
        System.out.println(testBean2);
        List beanList = objectMapper.readValue(json, new TypeReference>() {});
        System.out.println(beanList);

将对象转换成json字符串

String json = objectMapper.writeValueAsString(l);
输出
[{
    "name":"haha"},{
    "name":"haha1"}]

将json字符串转换为对象

TestBean testBean2 = objectMapper.readValue(ja, TestBean.class);
输出
TestBean(name=haha)

嵌套对象的反序列化

List beanList = objectMapper.readValue(json, new TypeReference>() {});
输出
[TestBean(name=haha), TestBean(name=haha1)]

Jackson 具有从多种数据源读取数据并且转换为json字符串的功能，如文件，流，字符串。

3.注解方式

@JsonProperty注解指定一个属性用于JSON映射，默认情况下映射的JSON属性与注解的属性名称相同，不过可以使用该注解的value值修改JSON属性名，该注解还有一个index属性指定生成JSON属性的顺序，如果有必要的话。可以将属性与json对应起来，专门用于某些接口返回的字符串不符合驼峰命名方式
@JsonIgnore 属性注解，无视某些属性的序列化与反序列化
@JsonIgnoreProperties注解是类注解。在序列化为JSON的时候，@JsonIgnoreProperties({“prop1”, “prop2”})会忽略pro1和pro2两个属性。在从JSON反序列化为Java类的时候，@JsonIgnoreProperties(ignoreUnknown=true)会忽略所有没有Getter和Setter的属性。该注解在Java类和JSON不完全匹配的时候很有用。
@JsonIgnoreType也是类注解，会排除所有指定类型的属性。

@JsonProperty("na")
private String name;

[{
    "na":"haha"},{
    "na":"haha1"}]

Java8增加了一套全新的日期时间类，Jackson对此也有支持。这些支持是以Jackson模块形式提供的，所以首先就是注册这些模块。导入类库之后，Jackson也可以自动搜索所有模块，不需要我们手动注册。

ObjectMapper mapper = new ObjectMapper()
                .registerModule(new JavaTimeModule())
                .registerModule(new ParameterNamesModule())
                .registerModule(new Jdk8Module());

mapper.findAndRegisterModules();

@Data
@NoArgsConstructor
@AllArgsConstructor
@JsonRootName("Person")
public class Person {
     
    @JsonProperty("Name")
    private String name;
    @JsonProperty("NickName")
    private String nickname;
    @JsonProperty("Age")
    private int age;
    @JsonProperty("IdentityCode")
    private String identityCode;
    @JsonProperty
    @JsonFormat(pattern = "yyyy-MM-DD")
    private LocalDate birthday;
}

注册了时间模块之后看看代码中如何写

    static void java8DateTime() throws IOException {
        Person p1 = new Person("yitian", "易天", 25, "10000", LocalDate.of(1994, 1, 1));
        ObjectMapper mapper = new ObjectMapper()
                .registerModule(new JavaTimeModule());
        //mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        String text = mapper.writeValueAsString(p1);
        System.out.println(text);

        Person p2 = mapper.readValue(text, Person.class);
        System.out.println(p2);
    }

这时输出的是

{
    "birthday":[1994,1,1],"Name":"yitian","NickName":"易天","Age":25,"IdentityCode":"10000"}
Person(name=yitian, nickname=易天, age=25, identityCode=10000, birthday=1994-01-01)

不符合规范，取消注释的代码

{
    "birthday":"1994-01-01","Name":"yitian","NickName":"易天","Age":25,"IdentityCode":"10000"}
Person(name=yitian, nickname=易天, age=25, identityCode=10000, birthday=1994-01-01)

Spring Boot集成

自动配置

Spring Boot对Jackson的支持非常完善，只要我们引入相应类库，Spring Boot就可以自动配置开箱即用的Bean。Spring自动配置的ObjectMapper（或者XmlMapper）作了如下配置，基本上可以适应大部分情况。

手动配置

Spring Boot自动配置非常方便，但不是万能的。在必要的时候，我们需要手动配置Bean来替代自动配置的Bean

@Configuration
public class JacksonConfig {
     
    @Bean
    @Primary
    @Qualifier("xml")
    public XmlMapper xmlMapper(Jackson2ObjectMapperBuilder builder) {
        XmlMapper mapper = builder.createXmlMapper(true)
                .build();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        return mapper;
    }

    @Bean
    @Qualifier("json")
    public ObjectMapper jsonMapper(Jackson2ObjectMapperBuilder builder) {
        ObjectMapper mapper = builder.createXmlMapper(false)
                .build();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        return mapper;
    }
}

然后在需要的地方进行依赖注入。需要注意为了区分ObjectMapper和XmlMapper，需要使用@Qualifier注解进行标记。

@Controller
public class MainController {
     
    private ObjectMapper jsonMapper;
    private XmlMapper xmlMapper;
    private Person person = new Person("yitian", 10000, LocalDate.of(1994, 1, 1));

    public MainController(@Autowired @Qualifier("json") ObjectMapper jsonMapper, @Autowired @Qualifier("xml") XmlMapper xmlMapper) {
        this.jsonMapper = jsonMapper;
        this.xmlMapper = xmlMapper;
    }