Java如何使用Jackson格式化LocalDate对象？

我们有一个Recording具有Java 8java.time.LocalDate属性的类。我们需要从JSON字符串反序列化此属性并将其序列化。为此，我们可以使用@JsonDeserialize和@JsonSerialize注释来注释该类的LocalDate属性Recording。

@JsonDeserialize(using = LocalDateDeserializer.class)
@JsonSerialize(using = LocalDateSerializer.class)
private LocalDate releaseDate;

要使用注释，我们需要创建一个类来反序列化和序列化值。要创建一个反序列化器类，我们创建一个extends类StdDeserializer。序列化程序类扩展了StdSerializer该类。下面是LocalDateSerializerandLocalDateDeserializer类的定义。

package org.nhooo.example.jackson;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;

import java.io.IOException;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class LocalDateSerializer extends StdSerializer<LocalDate> {

    public LocalDateSerializer() {
        super(LocalDate.class);
    }

    @Override
    public void serialize(LocalDate value, JsonGenerator generator, SerializerProvider provider) throws IOException {
        generator.writeString(value.format(DateTimeFormatter.ISO_LOCAL_DATE));
    }
}

package org.nhooo.example.jackson;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;

import java.io.IOException;
import java.time.LocalDate;

public class LocalDateDeserializer extends StdDeserializer<LocalDate> {

    protected LocalDateDeserializer() {
        super(LocalDate.class);
    }

    @Override
    public LocalDate deserialize(JsonParser parser, DeserializationContext context) throws IOException {
        return LocalDate.parse(parser.readValueAs(String.class));
    }
}

让我们创建一个简单的类，将Recording对象转换为JSON字符串并应用LocalDateSerializer该类中定义的日期格式程序。

package org.nhooo.example.jackson;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.time.LocalDate;
import java.time.Month;

public class RecordingToJson {
    public static void main(String[] args) {
        Recording recording = new Recording();
        recording.setId(1L);
        recording.setTitle("Twist and Shout");
        recording.setReleaseDate(LocalDate.of(1964, Month.FEBRUARY, 3));

        ObjectMapper mapper = new ObjectMapper();
        try {
            String json = mapper.writeValueAsString(recording);
            System.out.println("JSON = " + json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

    }
}

上面的代码片段的输出是：

JSON = {"id":1,"title":"Twist and Shout","releaseDate":"1964-02-03","artist":null,"label":null}

这是Recording该类的完整定义。

package org.nhooo.example.jackson;

import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;

import java.time.LocalDate;
import java.util.Objects;

public class Recording {
    private Long id;
    private String title;

    @JsonDeserialize(using = LocalDateDeserializer.class)
    @JsonSerialize(using = LocalDateSerializer.class)
    private LocalDate releaseDate;

    private Artist artist;
    private Label label;

    public Recording() {
    }

    public Recording(Long id, String title, LocalDate releaseDate) {
        this.id = id;
        this.title = title;
        this.releaseDate = releaseDate;
    }

    // 吸气剂和二传手
}

Maven依赖

<!-- http://repo1.maven.org/maven2/com/fasterxml/jackson/core/jackson-databind/2.8.6/jackson-databind-2.8.6.jar -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.8.6</version>
</dependency>