关于java：如何使用Jackson反序列化一组对象


How to use Jackson to deserialise an array of objects

Jackson的数据绑定文档表明，Jackson支持反序列化"所有支持类型的数组"，但我无法确定具体的语法。

对于单个对象，我会这样做：

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//json input
{
   "id" :"junk",
   "stuff" :"things"
}

//Java
MyClass instance = objectMapper.readValue(json, MyClass.class);

对于一个数组，我想这样做：

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//json input
[{
   "id" :"junk",
   "stuff" :"things"
},
{
   "id" :"spam",
   "stuff" :"eggs"
}]

//Java
List<MyClass> entries = ?

号

有人知道是否有魔法丢失的命令吗？如果不是，那么解决方案是什么？

相关讨论

首先创建映射器：

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import com.fasterxml.jackson.databind.ObjectMapper;// in play 2.3
ObjectMapper mapper = new ObjectMapper();

作为数组：

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MyClass[] myObjects = mapper.readValue(json, MyClass[].class);

号

作为列表：

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List<MyClass> myObjects = mapper.readValue(jsonInput, new TypeReference<List<MyClass>>(){});

指定列表类型的另一种方法：

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List<MyClass> myObjects = mapper.readValue(jsonInput, mapper.getTypeFactory().constructCollectionType(List.class, MyClass.class));

。

相关讨论

来自尤金·茨科夫罗波夫

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List<MyClass> myObjects = Arrays.asList(mapper.readValue(json, MyClass[].class))

这个解决方案对我来说似乎是最好的

相关讨论

对于一般实现：

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public static <T> List<T> parseJsonArray(String json,
                                         Class<T> classOnWhichArrayIsDefined)
                                         throws IOException, ClassNotFoundException {
   ObjectMapper mapper = new ObjectMapper();
   Class<T[]> arrayClass = (Class<T[]>) Class.forName("[L" + classOnWhichArrayIsDefined.getName() +";");
   T[] objects = mapper.readValue(json, arrayClass);
   return Arrays.asList(objects);
}

。

相关讨论

首先创建一个线程安全的ObjectReader实例。

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ObjectMapper objectMapper = new ObjectMapper();
ObjectReader objectReader = objectMapper.reader().forType(new TypeReference<List<MyClass>>(){});

然后使用它：

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List<MyClass> result = objectReader.readValue(inputStream);

。

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try {
    ObjectMapper mapper = new ObjectMapper();
    JsonFactory f = new JsonFactory();
    List<User> lstUser = null;
    JsonParser jp = f.createJsonParser(new File("C:\\maven\\user.json"));
    TypeReference<List<User>> tRef = new TypeReference<List<User>>() {};
    lstUser = mapper.readValue(jp, tRef);
    for (User user : lstUser) {
        System.out.println(user.toString());
    }

} catch (JsonGenerationException e) {
    e.printStackTrace();
} catch (JsonMappingException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

。

下面是一个转换json2object或object2json的实用程序，无论您的POJO是什么(实体T)

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import java.io.IOException;
import java.io.StringWriter;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

/**
 *
 * @author TIAGO.MEDICI
 *
 */
public class JsonUtils {

    public static boolean isJSONValid(String jsonInString) {
        try {
            final ObjectMapper mapper = new ObjectMapper();
            mapper.readTree(jsonInString);
            return true;
        } catch (IOException e) {
            return false;
        }
    }

    public static String serializeAsJsonString(Object object) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objMapper = new ObjectMapper();
        objMapper.enable(SerializationFeature.INDENT_OUTPUT);
        objMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
        StringWriter sw = new StringWriter();
        objMapper.writeValue(sw, object);
        return sw.toString();
    }

    public static String serializeAsJsonString(Object object, boolean indent) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objMapper = new ObjectMapper();
        if (indent == true) {
            objMapper.enable(SerializationFeature.INDENT_OUTPUT);
            objMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
        }

        StringWriter stringWriter = new StringWriter();
        objMapper.writeValue(stringWriter, object);
        return stringWriter.toString();
    }

    public static <T> T jsonStringToObject(String content, Class<T> clazz) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper objMapper = new ObjectMapper();
        obj = objMapper.readValue(content, clazz);
        return obj;
    }

    @SuppressWarnings("rawtypes")
    public static <T> T jsonStringToObjectArray(String content) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper mapper = new ObjectMapper();
        obj = mapper.readValue(content, new TypeReference<List>() {
        });
        return obj;
    }

    public static <T> T jsonStringToObjectArray(String content, Class<T> clazz) throws JsonParseException, JsonMappingException, IOException {
        T obj = null;
        ObjectMapper mapper = new ObjectMapper();
        mapper = new ObjectMapper().configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
        obj = mapper.readValue(content, mapper.getTypeFactory().constructCollectionType(List.class, clazz));
        return obj;
    }