将HashMap序列化为JSON字符串，同时避免使用Java中的某些字段 [英] Serialize HashMap to a JSON string while avoiding certain fields in Java

问题描述

我有如下地图：

Map map = new HashMap<String, String>();
map.put("key1", "value1");
map.put("key2", "value2");
map.put("key3", "value3");

我需要将此地图转换为JSON字符串。我知道这可以使用Jackson完成，如下所示：

I need to convert this map into a JSON string. I know that this can be done using Jackson as below:

new ObjectMapper().writeValueAsString(map);

问题是，我不想映射 key3 到字符串。输出字符串应如下所示：

The problem is, I do not want to map key3 to the String. The output String should be as below:

{"key1":"value1","key2":"value2"}

在将序列化为字符串时，有什么方法可以避免HashMap中的某些字段？此外，我想在序列化时向String添加某些字段。例如，我需要添加一个名为key4的字段，其值为value4。因此，最终字符串应为：

Is there any way to avoid certain fields from the HashMap while serializing it to a String? Moreover, I want to add certain fields to the String while serializing. For instance, I need to add a field called "key4" with a value "value4". Thus, the final String should be:

{"key1":"value1","key2":"value2","key4":"value4"}

我如何使用杰克逊这样做？或者在Java中还有其他方法吗？

How do I do this using Jackson? Or is there any other way to do this in Java?

推荐答案

如果要序列化 HashMap 像这样，您应该实现自定义序列化程序。下面是一个示例：

If you want to serialize a HashMap like this, you should implement a custom serializer. Here is one example:

public class CustomSerializer extends StdSerializer<Map> {
    protected CustomSerializer() {
        super(Map.class);
    }

    @Override
    public void serialize(Map map, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException {
        jsonGenerator.writeStartObject();
        for (Object key : map.keySet()) {
            if(!"key3".equals(key)){
                jsonGenerator.writeStringField((String) key, (String) map.get(key));
            }
        }
        jsonGenerator.writeStringField("key4","value4");
        jsonGenerator.writeEndObject();
    }
}

public class Main {

    public static void main(String[] args) throws Exception{

        Map<String, String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value2");
        map.put("key3","value3");

        ObjectMapper mapper = new ObjectMapper();
        SimpleModule serializerModule = new SimpleModule("SerializerModule", new Version(1, 0, 0, null, "mt", "customSerializerTest"));
        serializerModule.addSerializer(new CustomSerializer());
        mapper.registerModule(serializerModule);

        System.out.println(mapper.writeValueAsString(map));
    }
}

Output:
{"key1":"value1","key2":"value2","key4":"value4"}

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