为什么在使用Spring HATEOAS Jackson序列化器时收到警告 [英] Why do I get warnings when using Spring HATEOAS Jackson serializers
问题描述
Spring HATEOAS 定义和 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,实现似乎在嵌入资源的能力方面受到限制.在尝试解决此限制时,我尝试使用上述Jackson混合中的注释在我自己的类中使用Spring HATEOAS序列化器:
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;
}
}
这可行,但是导致以下WARN
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期望结果:
{
"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
} ]
}
}
那么为什么要发出警告,为什么在发出警告的情况下仍然可以起作用? 解决方案
使用@JsonDeserialize(using = Jackson2HalModule.HalResourcesDeserializer.class)
时,Jackson尝试创建Jackson2HalModule.HalResourcesSerializer
的实例,该实例不起作用,因为它没有默认构造函数.>
Spring HATEOAS(后来)直接 注册了杰克逊的一些东西,包括Jackson2HalModule.HalResourcesSerializer
.现在,序列化可以按预期工作,因为已成功注册了序列化器的实例.
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 to bind the serializers to types like so:
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();
}
This mixin causes types that extend Resources<T>
to get serialized and deserialized using Jackson2HalModule.HalResourcesSerializer.class
. However, the implementation seems limited in its ability to embed resources. In trying to address this limitation, I tried using annotations from the above Jackson mixin to use the Spring HATEOAS serializers in my own class:
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;
}
}
This works, but leads to the following WARN
s getting logged:
[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
Despite these warnings, serialization yields the expected, desired result:
{
"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
} ]
}
}
So why the warnings, and why does it work despite the warnings? The module and serializer are public.
When you use @JsonDeserialize(using = Jackson2HalModule.HalResourcesDeserializer.class)
, then Jackson tries to create an instance of Jackson2HalModule.HalResourcesSerializer
, which doesn't work because it has no default constructor.
Spring HATEOAS (later) directly registers some Jackson stuff, including a Jackson2HalModule.HalResourcesSerializer
. Now serialization works as expected because an instance of the serializer has successfully been registered.
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