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Suppress wrapper object when serializing Java object into JSON using Jackson

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忘掉有多难
忘掉有多难 2021-02-14 23:45

I have a web service that returns a list as JSON. It uses Jackson to map a List of Java POJOs into JSON. The problem is that the JSON representation has a wrapper object around

5条回答
  • 傲寒
    傲寒 (楼主)
    2021-02-15 00:39

    You could write custom serializer:

    public class UnwrappingSerializer extends JsonSerializer
{
    @Override
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider)
            throws IOException, JsonProcessingException
    {
        JavaType type = TypeFactory.type(value.getClass());
        getBeanSerializer(type, provider).serialize(value, new UnwrappingJsonGenerator(jgen), provider);
    }

    private synchronized JsonSerializer getBeanSerializer(JavaType type, SerializerProvider provider)
    {
        JsonSerializer result = cache.get(type);
        if (result == null) {
            BasicBeanDescription beanDesc = provider.getConfig().introspect(type);
            result = BeanSerializerFactory.instance.findBeanSerializer(type, provider.getConfig(), beanDesc);
            cache.put(type, result);
        }
        return result;
    }

    private Map> cache = new HashMap>();

    private static class UnwrappingJsonGenerator extends JsonGeneratorDelegate
    {
        UnwrappingJsonGenerator(JsonGenerator d)
        {
            super(d);
        }

        @Override
        public void writeEndObject() throws IOException, JsonGenerationException
        {
            if (depth-- >= yieldDepth) {
                super.writeEndObject();
            }
        }

        @Override
        public void writeFieldName(SerializedString name) throws IOException, JsonGenerationException
        {
            if (depth >= yieldDepth) {
                super.writeFieldName(name);
            }
        }

        @Override
        public void writeFieldName(String name) throws IOException, JsonGenerationException
        {
            if (depth >= yieldDepth) {
                super.writeFieldName(name);
            }
        }

        @Override
        public void writeStartObject() throws IOException, JsonGenerationException
        {
            if (++depth >= yieldDepth) {
                super.writeStartObject();
            }
        }

        private int depth;
        private final int yieldDepth = 2;
    }
}



    It will ignore outer objects on depth lower than specified (2 by default).
    


    Then use it as follows:
    


    public class UnwrappingSerializerTest
{
    public static class BaseT1
    {
        public List getTest()
        {
            return test;
        }

        public void setTest(List test)
        {
            this.test = test;
        }

        private List test;
    }

    @JsonSerialize(using = UnwrappingSerializer.class)
    public static class T1 extends BaseT1
    {
    }

    @JsonSerialize(using = UnwrappingSerializer.class)
    public static class T2
    {
        public BaseT1 getT1()
        {
            return t1;
        }

        public void setT1(BaseT1 t1)
        {
            this.t1 = t1;
        }

        private BaseT1 t1;
    }

    @Test
    public void test() throws IOException
    {
        ObjectMapper om = new ObjectMapper();
        T1 t1 = new T1();
        t1.setTest(Arrays.asList("foo", "bar"));
        assertEquals("[\"foo\",\"bar\"]", om.writeValueAsString(t1));

        BaseT1 baseT1 = new BaseT1();
        baseT1.setTest(Arrays.asList("foo", "bar"));
        T2 t2 = new T2();
        t2.setT1(baseT1);
        assertEquals("{\"test\":[\"foo\",\"bar\"]}", om.writeValueAsString(t2));
    }
}

    


    Notes:
    


      

    • It expects only single field wrapper and will generate invalid JSON on something like {{field1: {...}, field2: {...}}
      • 

    • If you use custom SerializerFactory you probably will need to pass it to the serializer.
      • 

    • It uses separate serializer cache so this also can be an issue.
      • 

    
    
    
             
    
                                                        
    
            
            
    
              
                
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