为什么在使用 Spring HATEOAS Jackson 序列化程序时会收到警告

Question

Spring HATEOAS defines and registers a HttpMessageConverter with a Jackson module that contains serializers to transform its ResourceSupport and Resources types to HAL JSON representations. The modules uses Jackson mixins 将序列化程序绑定到如下类型：

package org.springframework.hateoas.hal;

// Imports omitted

public abstract class ResourcesMixin<T> extends Resources<T> {

    @Override
    @XmlElement(name = "embedded")
    @JsonProperty("_embedded")
    @JsonSerialize(include = JsonSerialize.Inclusion.NON_EMPTY, using = Jackson2HalModule.HalResourcesSerializer.class)
    @JsonDeserialize(using = Jackson2HalModule.HalResourcesDeserializer.class)
    public abstract Collection<T> getContent();    
}

此 mixin 导致扩展 Resources<T> 的类型使用 Jackson2HalModule.HalResourcesSerializer.class 进行序列化和反序列化。但是，执行seems limited in its ability to embed resources。在尝试解决此限制时，我尝试使用上述 Jackson mixin 中的注释在我自己的 class:

中使用 Spring HATEOAS 序列化程序

package mypackage;

// Imports omitted

public class ModelAResource extends Resource<ModelA> {

    private Collection<Resource<ModelB>> modelBResources;

    public ModelAResource(Model model, Collection<Resource<ModelB>> modelBResources) {
        super(model);
        this.modelBResources = modelBResources;
    }

    @JsonProperty("_embedded")
    @JsonSerialize(include = JsonSerialize.Inclusion.NON_EMPTY, using = Jackson2HalModule.HalResourcesSerializer.class)
    @JsonDeserialize(using = Jackson2HalModule.HalResourcesDeserializer.class)
    public Collection<Resource<ModelB>> getModelBResources() {
        return modelBResources;
    }
}

这有效，但会导致以下 WARN 被记录：

[2015-01-27 23:36:41.469] boot - 12516  WARN [qtp1175418534-17] ---
MappingJackson2HttpMessageConverter: Failed to evaluate serialization for type
[class mypackage.ModelAResource]:
org.springframework.beans.factory.BeanCreationException: Error creating bean 
with name
'org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer':
Instantiation of bean failed; nested exception is 
org.springframework.beans.BeanInstantiationException: Failed to instantiate
[org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer]: No
default constructor found; nested exception is
java.lang.NoSuchMethodException:
org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer<init>()

[2015-01-27 23:36:41.478] boot - 12516  WARN [qtp1175418534-17] ---
MappingJackson2HttpMessageConverter: Failed to evaluate serialization for type
[class mypackage.ModelAResource]:
com.fasterxml.jackson.databind.JsonMappingException: Class
org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer has no 
default (no arg) constructor

尽管有这些警告，序列化仍会产生预期的结果：

{
  "id" : 6,
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/modelA/6"
    },
    "links" : {
      "href" : "http://localhost:8080/modelA/6/modelBs"
    }
  },
  "_embedded" : {
    "modelBs" : [ {
      "_links": {
        "self": {
          "href": "http://localhost:8080/modelA/6/modelBs/1"
        }
      }, 
      "id" : 1
    } ]
  }
}

那么为什么会出现警告，为什么尽管有警告它仍然有效？ module and serializer are public。

Answer 1

当您使用 @JsonDeserialize(using = Jackson2HalModule.HalResourcesDeserializer.class) 时，Jackson 会尝试创建 Jackson2HalModule.HalResourcesSerializer 的实例，但该实例不起作用，因为它没有默认构造函数。

Spring HATEOAS（稍后）直接注册一些 Jackson 的东西，包括 Jackson2HalModule.HalResourcesSerializer。现在序列化按预期工作，因为序列化程序的实例已成功注册。

为什么在使用 Spring HATEOAS Jackson 序列化程序时会收到警告

Why do I get warnings when using Spring HATEOAS Jackson serializers

java

spring

spring-mvc

jackson

spring-hateoas