java - 如何将 HashMap 序列化为 ArrayList 为 JSON？

Question

我有一个具有 HashMap 的模型，如下所示:

private Map<String, String>     attrMap     = new HashMap<String, String>();

并像这样初始化它:

attrMap.add("name", "value of name");
attrMap.add("content", "value of content");

但我想将此字段序列化为对象的ArrayList，如下所示:

[{name: "value of name"}, {content: "value of content"}]

更新1

有没有办法在序列化过程中调用函数，如下所示:

@JsonSerializer(serializeAttrMap)
private Map<String, String>     attrMap     = new HashMap<String, String>();

public String serializeAttrMap() {
    ArrayList<String> entries = new ArrayList<>(this.attrMap.size());
    for(Map.Entry<String,String> entry : attrMap.entrySet())
        entries.add(String.format("{%s: \"%s\"}",
            entry.getKey(), entry.getValue()));
    return Arrays.toString(entries.toArray());
}

更新2

我使用此类来序列化attrMap，但出现无法启动期望字段名称的对象错误。

import java.io.IOException;
import java.util.Map;

import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonProcessingException;
import org.codehaus.jackson.map.JsonSerializer;
import org.codehaus.jackson.map.SerializerProvider;

public class AttrMapSerializer extends JsonSerializer<Map<String, String>> {

        @Override
        public void serialize(Map<String, String> attributes, JsonGenerator generator, SerializerProvider provider) throws IOException, JsonProcessingException {
            for (Map.Entry<String, String> attribute : attributes.entrySet())
            {
                generator.writeStartObject();
                generator.writeObjectField("name", attribute.getKey());
                generator.writeObjectField("content", attribute.getValue());
                generator.writeEndObject();
            }
        }
    }

我是 Jackson 的初学者

Answer 1

以下构造将创建所需的输出:

@Test
public void testJackson() throws JsonProcessingException {
    // Declare the map
    Map<String, String> attrMap = new HashMap<>();

    // Put the data in the map
    attrMap.put("name", "value of name");
    attrMap.put("content", "value of content");

    // Use an object mapper
    final ObjectMapper objectMapper = new ObjectMapper();

    // Collect to a new object structure
    final List<ObjectNode> collected = attrMap.entrySet()
            .stream()
            .map(entry -> objectMapper.createObjectNode().put(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());

    // The output
    final String json = objectMapper.writeValueAsString(collected);

    System.out.println(json); // -> [{"name":"value of name"},{"content":"value of content"}]
}

它结合使用 Jackson 中的 ObjectNode 类和一些 Java 8 流来收集新数据。

编辑:在OP提供更多信息后，他们要求另一种方法，我添加了这个替代方案。

另一种方法是简单地在属性上使用@JacksonSerializer。

// This is the serializer
public static class AttrMapSerializer extends JsonSerializer<Map<String, String>> {
    @Override
    public void serialize(
            final Map<String, String> value,
            final JsonGenerator jgen, final SerializerProvider provider) throws IOException {

        // Iterate the map entries and write them as fields
        for (Map.Entry<String, String> entry : value.entrySet()) {
            jgen.writeStartObject();
            jgen.writeObjectField(entry.getKey(), entry.getValue());
            jgen.writeEndObject();
        }
    }
}

// This could be the POJO
public static class PojoWithMap {
    private Map<String, String> attrMap = new HashMap<>();

    // This instructs the ObjectMapper to use the specified serializer
    @JsonSerialize(using = AttrMapSerializer.class)
    public Map<String, String> getAttributes() {
        return attrMap;
    }
}

public static void main(String... args) throws JsonProcessingException {
    final PojoWithMap pojoWithMap = new PojoWithMap();
    pojoWithMap.getAttributes().put("name", "value of name");
    pojoWithMap.getAttributes().put("content", "value of content");


    final String json = new ObjectMapper().writeValueAsString(pojoWithMap);
    System.out.println(json); // ->
}

这样，序列化就被外部化为序列化器，并且 POJO 完好无损。