问题描述
我有这堂课
public static class SomeClass {
public SomeClass(String field) {
this.field = field;
}
private final String field;
public String getField() {
return field;
}
}
我也有此测试(已编辑)
@Test
public void testStringifyMapOfObjects() {
Map<String, SomeClass> original = Maps.newTreeMap();
original.put("first", new SomeClass("a"));
original.put("second", new SomeClass("b"));
String encoded = JsonUtil.toJson(original);
Map<String, SomeClass> actual = JsonUtil.fromJson(encoded, Map.class);
Assert.assertEquals("{'first':{'field':'a'},'second':{'field':'b'}}", encoded.replaceAll("\\s", "").replaceAll("\"", "'"));
Assert.assertEquals(original.get("first"), actual.get("first"));
}
测试失败
junit.framework.AssertionFailedError: expected:<eu.ec.dgempl.eessi.facade.transport.test.TestToolTest$SomeClass@6e3ed98c> but was:<{field=a}>
at junit.framework.Assert.fail(Assert.java:47)
at junit.framework.Assert.failNotEquals(Assert.java:277)
at junit.framework.Assert.assertEquals(Assert.java:64)
at junit.framework.Assert.assertEquals(Assert.java:71)
at eu.ec.dgempl.eessi.facade.transport.test.TestToolTest.testStringifyMapOfObjects(TestToolTest.java:90)
我可以制作json来正确地将对象序列化为地图的值，还是应该使用其他东西?
Can I make json to properly serialize objects as the values of the map or should I use something else?
已编辑
public class JsonUtil {
private static final org.slf4j.Logger LOG = org.slf4j.LoggerFactory.getLogger(JsonUtil.class);
public static <T> String toJson(T data) {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(Feature.INDENT_OUTPUT, true);
try {
return mapper.writeValueAsString(data);
} catch (IOException e) {
LOG.warn("can't format a json object from [" + data + "]", e);
return null;
}
//
// return Json.stringify(Json.toJson(data));
}
public static <T> T fromJson(String description, Class<T> theClass) {
try {
JsonNode parse = new ObjectMapper().readValue(description, JsonNode.class);
T fromJson = new ObjectMapper().treeToValue(parse, theClass);
return fromJson;
} catch (JsonParseException e) {
// throw new RuntimeException("can't parse a json object of type " + theClass.getName() + " from [" + description + "]", e);
LOG.warn("can't parse a json object from [" + description + "]", e);
return null;
} catch (JsonMappingException e) {
// throw new RuntimeException("can't parse a json object of type " + theClass.getName() + " from [" + description + "]", e);
LOG.warn("can't parse a json object from [" + description + "]", e);
return null;
} catch (IOException e) {
// throw new RuntimeException("can't parse a json object of type " + theClass.getName() + " from [" + description + "]", e);
LOG.warn("can't parse a json object from [" + description + "]", e);
return null;
}
}
}
推荐答案
您正在遇到与Java泛型相关的问题.总而言之，当将数据反序列化为不可修改的类型(即在运行时无法获得实际类型信息的类型)时，您需要使用超类型令牌.通过阅读以下SO帖子，您可以了解有关什么是超类型令牌(以及为什么需要使用一个超令牌)的更多详细信息:
You are running into a problem related to Java generics. To summarize, when deserializing data into a non-reifiable type (aka a type for which actual type information is not available at runtime) you need to use a supertype token. You can get more detail about what a supertype token is (and why you need to use one) by reading these SO posts:
- Pass parameterized type to method as argument
- Error using Jackson and JSON
- Deserialize JSON to ArrayList using Jackson
还有杰克逊(Jackson)文档中的信息:
And also from the Jackson documentation:
- Data Binding With Generics
- TypeReference Javadoc
基本问题是，当您使用典型的通用对象时，该对象的实际类型参数在运行时不可用.因此，杰克逊不知道要实例化哪个实际类并将数据反序列化到.
解决该问题的最简单方法是在JSON实用程序类中添加一个重载，该重载接受类型引用(与Class<T>相对).例如:
The easiest way to get around the problem would be adding an overload to your JSON utility class, that accepts a type reference (as opposed to a Class<T>). For example:
public static <T> T fromJson(String json, TypeReference<T> typeRef) {
if(json == null || typeRef == null) return null;
return new ObjectMapper().readValue(json, typeRef);
}
要这样使用:
Map<String, SomeClass> actual = JsonUtil.fromJson(
encoded,
new TypeReference<Map<String, SomeClass>>(){});
这篇关于我如何从json恢复对象映射?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！