通过Fegin远程调用 ，返回JPA Page 对象报错

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of org.springframework.data.domain.Page: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: java.io.PushbackInputStream@5ebf9e24; line: 2, column: 12] (through reference chain: com.sysware.cloud.plugin.bean.Response["data"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:270) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:1456) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1012) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:216) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:504) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:104) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:357) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3814) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2938) ~[jackson-databind-2.8.10.jar:2.8.10]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:235) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:223) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:96) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.cloud.netflix.feign.support.SpringDecoder.decode(SpringDecoder.java:59) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at org.springframework.cloud.netflix.feign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:45) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:165) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:133) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76) ~[feign-core-9.5.0.jar:?]
    at feign.hystrix.HystrixInvocationHandler$1.run(HystrixInvocationHandler.java:108) ~[feign-hystrix-9.5.0.jar:?]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:302) ~[hystrix-core-1.5.12.jar:1.5.12]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:298) ~[hystrix-core-1.5.12.jar:1.5.12]
    at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:46) ~[rxjava-1.2.0.jar:1.2.0]
    ... 26 more

大概意思是：不能构造实例org.springframework.data.domain.Page ：抽象类型要么需要映射到具体类型，要么有自定义反序列化器，要么用额外的类型信息进行实例化 。

解释下啊，第一种办法就是page是一个接口，反序列化需要配置映射到具体的类型在Page类上面加@JsonDeserialize注解，指定反序列化的具体实现，但是Page类并不是我们自定义的类，所以在Page接口上加注解应该是不现实的

　　　　　第二种办法就是，需要自定义反序列化器，大概就是说序列化和反序列化是使用默认的readOjbect ,和 writeObject ，但是通过默认的readOject解析不了，需要用户自定义解析Page的解析。

　　　　　第三方式就是，自定义抽象类型的实现类，用实现类接受。

我们查找到Page的实现类，框架本身已经提供PageImpl,所以我们换成PageImpl接受返回值。

然而，换成PageImpl接受报如下错误：

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of org.springframework.data.domain.PageImpl: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)
 at [Source: java.io.PushbackInputStream@6e88a853; line: 3, column: 5] (through reference chain: com.sysware.cloud.plugin.bean.Response["data"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:270) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:1456) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1012) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1206) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:314) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:504) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:104) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:357) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3814) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2938) ~[jackson-databind-2.8.10.jar:2.8.10]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:235) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:223) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:96) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.cloud.netflix.feign.support.SpringDecoder.decode(SpringDecoder.java:59) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at org.springframework.cloud.netflix.feign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:45) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:165) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:133) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76) ~[feign-core-9.5.0.jar:?]
    at feign.hystrix.HystrixInvocationHandler$1.run(HystrixInvocationHandler.java:108) ~[feign-hystrix-9.5.0.jar:?]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:302) ~[hystrix-core-1.5.12.jar:1.5.12]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:298) ~[hystrix-core-1.5.12.jar:1.5.12]
    at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:46) ~[rxjava-1.2.0.jar:1.2.0]
    ... 26 more

大概意思是说：不能构造实例 org.springframework.data.domain.PageImpl : 没有合适的构造函数（此处还在查资料jackson反序列化和构造函数的关系），然后查阅PageImpl的源码，发现确实没有无参的构造函数

    public PageImpl(List<T> content, Pageable pageable, long total) {
        super(content, pageable);
        this.pageable = pageable;
        this.total = !content.isEmpty() && pageable != null && (long)(pageable.getOffset() + pageable.getPageSize()) > total?(long)(pageable.getOffset() + content.size()):total;
    }

    public PageImpl(List<T> content) {
        this(content, (Pageable)null, null == content?0L:(long)content.size());
    }

然后我们通过自定义类继承PageImpl ，并添加无参构造函数，代码如下(这是最终版，其实刚开始的时候Sort上没有加@JsonDeserialize(using=CustomSortDeserializer.class))，后面说加这个注解的作用。

package com.sysware.cloud.plugin.commons.http;

import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import lombok.Data;
import org.springframework.data.domain.PageImpl;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by tianwenqing on 2018/7/9.
 */
@Data
public class RestResponsePage<T> extends PageImpl<T> {
    public RestResponsePage(List<T> content, Pageable pageable, long total) {
        super(content, pageable, total);
    }

    private int number;
    private int size;
    private int totalPages;
    private int numberOfElements;
    private long totalElements;
    private boolean previousPage;
    private boolean first;
    private boolean nextPage;
    private boolean last;
    private List<T> content;
    @JsonDeserialize(using=CustomSortDeserializer.class)
    private Sort sort;

    public RestResponsePage(List<T> content) {
        super(content);
    }

    public RestResponsePage() {
        super(new ArrayList<T>());
    }

    public PageImpl<T> pageImpl() {
        return new PageImpl<T>(getContent(), new PageRequest(getNumber(),
                getSize(), getSort()), getTotalElements());
    }
}

package com.sysware.cloud.plugin.commons.http;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.node.ArrayNode;
import org.springframework.data.domain.Sort;

import java.io.IOException;

/**
 * Created by tianwenqing on 2018/7/9.
 */
public class CustomSortDeserializer extends JsonDeserializer<Sort> {
    @Override
    public Sort deserialize(JsonParser jp, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        ArrayNode node = jp.getCodec().readTree(jp);
        Sort.Order[] orders = new Sort.Order[node.size()];
        int i = 0;
        for(JsonNode obj : node){
            orders[i] =  new Sort.Order(Sort.Direction.valueOf(obj.get("direction").asText()), obj.get("property").asText());
            i++;
        }
        Sort sort = new Sort(orders);
        return sort;
    }
}

上面的的两个类就是最终代码，然后FeginClient端接受的时候用RestResponsePage替代Page，即可。

下面介绍@JsonDeserialize(using=CustomSortDeserializer.class)

刚开始创建完RestResponsePage类，没有加@JsonDeserialize(using=CustomSortDeserializer.class) 直接引用，结果又报下面的错：

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of org.springframework.data.domain.Sort out of START_ARRAY token
 at [Source: java.io.PushbackInputStream@5f88be0f; line: 84, column: 14] (through reference chain: com.sysware.cloud.plugin.bean.Response["data"]->com.sysware.cloud.test.aggregation.RestResponsePage["sort"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:270) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.reportMappingException(DeserializationContext.java:1234) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1122) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1075) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromArray(BeanDeserializerBase.java:1370) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:174) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:150) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:504) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:104) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:357) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:504) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:104) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:357) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3814) ~[jackson-databind-2.8.10.jar:2.8.10]
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2938) ~[jackson-databind-2.8.10.jar:2.8.10]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:235) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:223) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:96) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
    at org.springframework.cloud.netflix.feign.support.SpringDecoder.decode(SpringDecoder.java:59) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at org.springframework.cloud.netflix.feign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:45) ~[spring-cloud-netflix-core-1.4.2.RELEASE.jar:1.4.2.RELEASE]
    at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:165) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:133) ~[feign-core-9.5.0.jar:?]
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76) ~[feign-core-9.5.0.jar:?]
    at feign.hystrix.HystrixInvocationHandler$1.run(HystrixInvocationHandler.java:108) ~[feign-hystrix-9.5.0.jar:?]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:302) ~[hystrix-core-1.5.12.jar:1.5.12]
    at com.netflix.hystrix.HystrixCommand$2.call(HystrixCommand.java:298) ~[hystrix-core-1.5.12.jar:1.5.12]
    at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:46) ~[rxjava-1.2.0.jar:1.2.0]
    ... 26 more

大概意思是说不能反序列化Sort属性，所以我们使用自定义反序列化器序列化Sort，至于为什么不能反序列化，google找到如下解释，还没有看太懂

If you look at Sort, you'll see that it implements Iteratable for Order. And if you examine Order defined in the same class, you'll see the fields in my example JSON. So when Sort is serialized, it is serialized as a list of Order, which is why deserializing into Sort needs a custom deserializer implementation, and also why deserializing without a custom implementation fails (because it's trying to deserialize an array into a single object).

During deserialization, when a setter annotated with @JsonDeserializer is encountered as each property is deserialized, Jackson uses the supplied deserializer to deserialize that specific property.