feignClient调用接口异常：Type definition error: [simple type, class com.baomidou.mybatisplus.core.metadata.IPage]; 

feign.codec.DecodeException: Type definition error: [simple type, class com.baomidou.mybatisplus.core.metadata.IPage]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.baomidou.mybatisplus.core.metadata.IPage` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (PushbackInputStream); line: 1, column: 1]

	at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:174)
	at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:134)
	at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:77)
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:102)
	at com.sun.proxy.$Proxy101.getGoodList(Unknown Source)
	at cpm.leyou.search.client.BrandClientTest.getSpu(BrandClientTest.java:53)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
	at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
	at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
	at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
	at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class com.baomidou.mybatisplus.core.metadata.IPage]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.baomidou.mybatisplus.core.metadata.IPage` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (PushbackInputStream); line: 1, column: 1]
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:240)
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:225)
	at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:100)
	at org.springframework.cloud.openfeign.support.SpringDecoder.decode(SpringDecoder.java:60)
	at org.springframework.cloud.openfeign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:45)
	at feign.optionals.OptionalDecoder.decode(OptionalDecoder.java:36)
	at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:170)
	... 35 more
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.baomidou.mybatisplus.core.metadata.IPage` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (PushbackInputStream); line: 1, column: 1]
	at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
	at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)
	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)
	at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:265)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3084)
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:237)
	... 41 more

方案1：将服务端的返回值由IPage<T>转为Page<T>

方案2：feign接口返回结果使用Object接收

@GetMapping("app/spu/page")
CommonResult pageAppFeign(@SpringQueryMap QueryParam param);

查看IPage代码分析得出，因为com.baomidou.mybatisplus.core.metadata.IPage是一个接口，所以不能构建实例。抽象类型需要映射到实现类型，可以通过自定义反序列化或者包含额外的类型信息。

接下来，我们可以通过这个提示，自定义一个反序列化类来处理该问题，具体代码如下：

public class IPageDeserializer extends StdDeserializer<IPage> {
    protected IPageDeserializer(Class<?> vc) {
        super(vc);
    }
 
    /**
     * Method that can be called to ask implementation to deserialize
     * JSON content into the value type this serializer handles.
     * Returned instance is to be constructed by method itself.
     * <p>
     * Pre-condition for this method is that the parser points to the
     * first event that is part of value to deserializer (and which
     * is never JSON 'null' literal, more on this below): for simple
     * types it may be the only value; and for structured types the
     * Object start marker or a FIELD_NAME.
     * </p>
     * <p>
     * The two possible input conditions for structured types result
     * from polymorphism via fields. In the ordinary case, Jackson
     * calls this method when it has encountered an OBJECT_START,
     * and the method implementation must advance to the next token to
     * see the first field name. If the application configures
     * polymorphism via a field, then the object looks like the following.
     * <pre>
     *      {
     *          "@class": "class name",
     *          ...
     *      }
     *  </pre>
     * Jackson consumes the two tokens (the <tt>@class</tt> field name
     * and its value) in order to learn the class and select the deserializer.
     * Thus, the stream is pointing to the FIELD_NAME for the first field
     * after the @class. Thus, if you want your method to work correctly
     * both with and without polymorphism, you must begin your method with:
     * <pre>
     *       if (p.getCurrentToken() == JsonToken.START_OBJECT) {
     *         p.nextToken();
     *       }
     *  </pre>
     * This results in the stream pointing to the field name, so that
     * the two conditions align.
     * <p>
     * Post-condition is that the parser will point to the last
     * event that is part of deserialized value (or in case deserialization
     * fails, event that was not recognized or usable, which may be
     * the same event as the one it pointed to upon call).
     * <p>
     * Note that this method is never called for JSON null literal,
     * and thus deserializers need (and should) not check for it.
     *
     * @param p    Parsed used for reading JSON content
     * @param ctxt Context that can be used to access information about
     *             this deserialization activity.
     * @return Deserialized value
     */
    @Override
    public IPage deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode node = p.getCodec().readTree(p);
        String s  = node.toString();
        ObjectMapper om = new ObjectMapper();
        Page page = om.readValue(s,Page.class);
        return page;
    }
}

在Converter中配置进去即可，代码如下：

@Configuration
public class WebDataConvertConfig extends WebMvcConfigurationSupport {
   
 
    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(mappingJackson2HttpMessageConverter());
        super.configureMessageConverters(converters);
    }
 
    @Bean
    public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
 
        SimpleModule module = new SimpleModule();
        module.addDeserializer(IPage.class, new IPageDeserializer(IPage.class));
        mapper.registerModule(module);
        
        converter.setSupportedMediaTypes(Arrays.asList(MediaType.APPLICATION_JSON,MediaType.APPLICATION_JSON_UTF8,MediaType.APPLICATION_OCTET_STREAM));
        converter.setObjectMapper(mapper);
 
        return converter;
    }
 
 
}