如何使用Jackson对对象数组进行反序列化

Jackson数据绑定留档表示Jackson支持反序列化“所有支持类型的数组”，但我无法弄清楚这一点的确切语法。

对于单个对象，我会这样做：

//json input{"id" : "junk","stuff" : "things"}
//JavaMyClass instance = objectMapper.readValue(json, MyClass.class);

现在对于一个数组，我想这样做：

//json input[{"id" : "junk","stuff" : "things"},{"id" : "spam","stuff" : "eggs"}]
//JavaList<MyClass> entries = ?

有没有人知道有没有一个神奇的失踪命令？如果没有，那么解决方案是什么？

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最佳答案

首先创建一个映射器：

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

作为数组：

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

如列表：

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

另一种指定List类型的方法：

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

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

这个解决方案似乎对我来说是最好的。

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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();}
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对于通用实现：

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);}
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首先创建一个线程安全的ObjectReader实例。

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

然后使用它：

List<MyClass> result = objectReader.readValue(inputStream);
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这是一个可以转换json2Object或Object2json的实用程序，无论你的pojo（实体T）

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;}
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您还可以创建一个扩展ArrayList的类：

public static class MyList extends ArrayList<Myclass> {}

然后像这样使用它： 

List<MyClass> list = objectMapper.readValue(json, MyList.class);
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我无法使用这个答案，因为我的linter不允许未经检查的转换。

这里有一个你可以使用的替代方案。我觉得它实际上是一个更清洁的解决方案。

public <T> List<T> parseJsonArray(String json, Class<T> clazz) throws JsonProcessingException {var tree = objectMapper.readTree(json);var list = new ArrayList<T>();for (JsonNode jsonNode : tree) {list.add(objectMapper.treeToValue(jsonNode, clazz));}return list;}
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试试这个

List<MyClass> list = mapper.readerForListOf(MyClass.class).readValue(json)

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