public class ImmutableUser { private final String name; private final int age; private final String identityCard; public ImmutableUser(String name, int age, String identityCard) { this.name = name; this.age = age; this.identityCard = identityCard; } public String getName() { return name; } public int getAge() { return age; } public String getIdentityCard() { return identityCard; } }或许你会写下这样的代码,结果发现一执行就报错了:no Creators, like default constructor, exist.
@Test public void readImmutableUser() throws JsonProcessingException { String userJson = "{"name":"Tom","age":23,"identityCard":"61012420000101012x"}"; ObjectMapper mapper = new ObjectMapper(); ImmutableUser user = mapper.readValue(userJson, ImmutableUser.class); System.out.println(MAPPER.writeValueAsString(user)); } com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of com.learn.more.entiry.ImmutableUser (no Creators, like default constructor, exist): ......一 使用Jackson注解
import com.fasterxml.jackson.annotation.JsonCreator; import com.fasterxml.jackson.annotation.JsonProperty; // 堆代码 duidaima.com // 与ImmutableUser的区别:使用@JsonProperty、@JsonCreator修饰全参构造器 public class ImmutableUser2 { private final String name; private final int age; private final String identityCard; @JsonCreator public ImmutableUser2(@JsonProperty("name") String name, @JsonProperty("age") int age, @JsonProperty("identityCard") String identityCard) { this.name = name; this.age = age; this.identityCard = identityCard; } public String getName() { return name; } public int getAge() { return age; } public String getIdentityCard() { return identityCard; } }还是上面的测试代码,这次发现执行成功了。需注意,@JsonProperty的value必须填写,且最好与Json串中字段名一致。否则将导致下面的异常。
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Invalid type definition for type com.learn.more.entiry.ImmutableUser2: ..., annotations: ... has no property name (and is not Injectable): can not use as property-based Creator二 使用jackson-module-parameter-names
<dependency> <groupId>com.fasterxml.jackson.module</groupId> <artifactId>jackson-module-parameter-names</artifactId> <version>${jackson.version}</version> </dependency>向ObjectMapper注册ParameterNamesModule,就可以执行成功了。
@Test public void readImmutableUser() throws JsonProcessingException { String userJson = "{"name":"Tom","age":23,"identityCard":"61012420000101012x"}"; ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new ParameterNamesModule()); ImmutableUser user = mapper.readValue(userJson, ImmutableUser.class); System.out.println(MAPPER.writeValueAsString(user)); }三 使用Mixins机制
import com.fasterxml.jackson.annotation.JsonCreator; import com.fasterxml.jackson.annotation.JsonProperty; public class ImmutableUserMixin { @JsonCreator public ImmutableUserMixin(@JsonProperty("name") String name, @JsonProperty("age") int age, @JsonProperty("identityCard") String identityCard) { } }再创建一个自定义的JacksonMixinModule类,将ImmutableUser与ImmutableUserMixin对应关系,设置到SetupContext。
import com.fasterxml.jackson.databind.module.SimpleModule; import com.learn.more.entiry.ImmutableUser; import com.learn.more.entiry.ImmutableUserMixin; public class JacksonMixinModule extends SimpleModule { public JacksonMixinModule() { super(JacksonMixinModule.class.getName()); } // 注册所有使用Mixin机制的类 @Override public void setupModule(SetupContext context) { context.setMixInAnnotations(ImmutableUser.class, ImmutableUserMixin.class); // ...... } }向ObjectMapper注册JacksonMixinModule类。那么,在反序列化ImmutableUser时,将依据ImmutableUserMixin的构造器声明来绑定属性值。
@Test public void readImmutableUserMixin() throws JsonProcessingException { String userJson = "{"name":"Tom","age":23,"identityCard":"61012420000101012x"}"; ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new JacksonMixinModule()); ImmutableUser user = mapper.readValue(userJson, ImmutableUser.class); System.out.println(MAPPER.writeValueAsString(user)); }
可以成功反序列化。