多次调用ObjectMapper readValue，直到抛出StackOverflowException

Question

我试图为我的值类构建一个自定义反序列化器类。但是ObjectMapper. readValue会被多次调用，直到抛出StackOverflow异常。以下只是一个例子：

@JsonDeserialize(using = UserDeserializer.class)
@Getter
public class User {
    private String name;
}

public class UserDeserializer extends StdDeserializer<User> {
    public UserDeserializer() {
        super(User.class);
    }

    @Override
    public User deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        return ctxt.readValue(p, User.class);
    }
}

public class UserDeserializerTest {
    @Test
    public void whenUserIsDeserializedFromJsonThenNameShouldEqualBob() {
        String = "{ \"name\" : \"bob\" }";
        ObjectMapper mapper = new ObjectMapper();
        User user = mapper.readValue(json, User.class);
        assertThat(user.getName).isEqualTo("bob");
    }
}

Maven依赖：

<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.6.1</version>
    </dependency>
    <dependency>
        <groupId>org.assertj</groupId>
        <artifactId>assertj-core</artifactId>
        <version>1.7.1</version>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.16.2</version>
    </dependency>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.12</version>
        <scope>test</scope>
    </dependency>
</dependencies>

引发以下StackOverflowException：

Exception in thread "main" java.lang.StackOverflowError
...

// The following lines are repeated.

    at com.fasterxml.jackson.databind.DeserializationContext.readValue(DeserializationContext.java:754)
    at com.fasterxml.jackson.databind.DeserializationContext.readValue(DeserializationContext.java:746)
    at example.UserDeserializer.deserialize(UserDeserializer.java:25)
    at example.UserDeserializer.deserialize(UserDeserializer.java:15)

Process finished with exit code 1

我尝试通过模块注册反序列化程序并删除@JsonAnnotation，但仍然引发相同的异常：

SimpleModule module = new SimpleModule("User Simple Module");
module.addDeserializer(User.class, new UserDeserializer());
ObjectMapper mapper = new ObjectMapper();
mapper.addModule(module);
mapper.readValue(json, User.class);

然后我尝试在我的UserDeserializer中创建一个新的ObjectMapper，并从我的User类和测试通行证中删除@JsonDeseriize。

@Override
public User deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
    return new ObjectMapper().readValue(p, User.class);
}

在执行上述工作时，我希望使用@JsonDeseriize注释在每个类的基础上注册反序列化器，而不是注册全局自定义对象映射器。我这样想是有原因的。

我尝试了其他代码组合，例如：

// Add the 'as' keyword to the annotation
@JsonDeserialise(using = UserDeserialize.class, as = User.class)

// Locally assigning a mapper from the JsonParser inside the UserDeserializer class
ObjectMapper mapper = (ObjectMapper) p.getCodec();
mapper.readValue(p, User.class);

// Constructing a new JSON Parser inside the deserializer
ObjectMapper mapper = (ObjectMapper) p.getCodec();
JsonFactory factory = mapper.getFactory();
JsonParser parser = factory.createParser(p.getText());
return (User) parser.readValueAs(handledType());

// Reading the Json into a Tree
ObjectMapper mapper = (ObjectMapper) p.getCodec();
ObjectNode root = (ObjectNode) p.readValueAsTree();
return mapper.readValue(root.traverse(), User.class);

但是将JSON读成一棵树会抛出：

Exception in thread "main" java.lang.RuntimeException: java.lang.IllegalStateException: No ObjectCodec defined for parser, needed for deserialization
    at example.User.main(User.java:46)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.IllegalStateException: No ObjectCodec defined for parser, needed for deserialization
    at com.fasterxml.jackson.core.JsonParser._codec(JsonParser.java:1565)
    at com.fasterxml.jackson.core.JsonParser.readValueAsTree(JsonParser.java:1559)
    at example.UserDeserializer.deserialize(UserDeserializer.java:37)
    at example.UserDeserializer.deserialize(UserDeserializer.java:17)
    at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:3674)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1996)
    at example.UserDeserializer.deserialize(UserDeserializer.java:38)
    at example.UserDeserializer.deserialize(UserDeserializer.java:17)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3702)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2714)
    at example.User.main(User.java:36)
    ... 5 more

我还尝试在我的UserDeserializer中实现上下文解析器，但仍然得到与以前相同的StackOverflow Excema：

private UserDeserializer(Class<?> vc) {
    super(vc);
}

public JsonDeserializer<User> createContextual(DeserializationContext ctxt, BeanProperty property) throws JsonMappingException {
    return new UserDeserializer(ctxt.getContextualType().getRawClass());
}

@Override
public User deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
    return (User) new ObjectMapper().readValue(p, handledType());
}

我希望这只是PEBKAC错误的一个糟糕案例，在您的帮助/指导下，可以随时找到解决方案。

问候

Answer 1

我同意KDM，请尝试以下反序列化程序：

    public class UserDeserializer extends JsonDeserializer<User> {

    @Override
    public User deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode node = p.readValueAsTree();
        User user = new User();
        user.setName(node.get("name").asText());
        return user;
    }
}