当字段为可选时，杰克逊不会从 json 读回数据

Question

我正在使用 Java 8 来执行此任务。我还关注 JDK8 数据类型的依赖性工作。

        <dependency>
            <groupId>com.fasterxml.jackson.datatype</groupId>
            <artifactId>jackson-datatype-jdk8</artifactId>
            <version>2.6.3</version>
        </dependency>

我有一个 class 看起来像

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonProperty;

import java.util.Optional;

public class Person {
    private String firstName;
    private String lastName;
    private int age;
    private Optional<Address> address;
    private Optional<String> phone;

    private Person() {
    }

    public Person(String firstName, String lastName, int age) {
        this(firstName, lastName, age, Optional.empty(), Optional.empty());
    }

    public Person(String firstName, String lastName, int age,
                  Optional<Address> address, Optional<String> phone) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.address = address;
        this.phone = phone;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public int getAge() {
        return age;
    }

    @JsonIgnore
    public Optional<Address> getAddress() {
        return address;
    }

    @JsonIgnore
    public Optional<String> getPhone() {
        return phone;
    }

    @JsonProperty("address")
    private Address getAddressForJson(){
        return address.orElse(null);
    }

    @JsonProperty("phone")
    private String getPhoneForJson() {
        return phone.orElse(null);
    }
}

和

public class Address {
    private String street;
    private String city;
    private String state;
    private int zip;
    private String country;

    public Address(String street, String city, String state, int zip, String country) {
        this.street = street;
        this.city = city;
        this.state = state;
        this.zip = zip;
        this.country = country;
    }

    public String getStreet() {
        return street;
    }

    public String getCity() {
        return city;
    }

    public String getState() {
        return state;
    }

    public int getZip() {
        return zip;
    }

    public String getCountry() {
        return country;
    }
}

我编写了一个测试，将有效的 Person 对象写入文件，然后将其读回 Person 对象。我的测试是

@Test
    public void writeAndReadPersonAsJsonOnFile() throws Exception {
        Address address = new Address("1 Infinite Loop", "Cupertino", "CA", 95014, "USA");
        String phone = "1-800-My-Apple";
        Person person = new Person("john", "doe", 21, Optional.of(address), Optional.of(phone));
        ObjectMapper objectMapper = registerJdkModuleAndGetMapper();
        File file = temporaryFolder.newFile("person.json");
        objectMapper.writeValue(file, person);

        assertTrue(file.exists());
        assertTrue(file.length() > 0);

        Person personFromFile = objectMapper.readValue(file, Person.class);
        assertEquals(person, personFromFile);

    }

    private ObjectMapper registerJdkModuleAndGetMapper() {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.registerModule(new Jdk8Module());
        return objectMapper;
    }

作为测试的一部分创建的 file 具有以下内容

{
    "firstName": "john",
    "lastName": "doe",
    "age": 21,
    "address": {
        "street": "1 Infinite Loop",
        "city": "Cupertino",
        "state": "CA",
        "zip": 95014,
        "country": "USA"
    },
    "phone": "1-800-My-Apple"
}

但是当回读时，我得到 personFromFile 看起来像下面的

personFromFile = {Person@1178} 
 firstName = "john"
 lastName = "doe"
 age = 21
 address = null
 phone = null

如您所见，address 和 phone 它们都是空的，即使它们存在于文件中。

这里有什么问题？

更新代码库是 https://github.com/101bits/java8-optional-json。这也包含失败的测试

Answer 1

尝试用@JsonCreator 标记其中一个构造函数，以告诉 Jackson 使用哪个构造函数。注意：这还需要你用 @JsonProperty

标记每个构造函数的参数

当您希望 Jackson 使用构造函数或工厂方法构造对象而不是让 Jackson 使用 setter 或 public（非最终）字段时，您 should use the @JsonCreator annotation

此外，在您为 Person 和 Address

覆盖 "equals" 之前，您的测试不会通过

public class Person {
    private String firstName;
    private String lastName;
    private int age;
    private Optional<Address> address;
    private Optional<String> phone;

    public Person(String firstName, String lastName, int age) {
        this(firstName, lastName, age, Optional.empty(), Optional.empty());
    }

    @JsonCreator
    public Person(
            @JsonProperty("firstName") String firstName,
            @JsonProperty("lastName") String lastName,
            @JsonProperty("age") int age,
            @JsonProperty("address") Optional<Address> address,
            @JsonProperty("phone") Optional<String> phone) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.address = address;
        this.phone = phone;
    }

更新：Pull Request 通过测试

Answer 2

据我所知，optional 不会被序列化，因此，如果您使用默认的 java 序列化，则在反序列化时您不会获得值。但是，如果您使用序列化，应该没问题。

请参阅此 link 了解更多详情： Why java.util.Optional is not Serializable, how to serialize the object with such fields

当字段为可选时，杰克逊不会从 json 读回数据

jackson does not read back data from json when fields are Optional

java

json

optional

jackson

java-8