Spring HttpMessageNotWritableException 无法写入 JSON，NullPointerException
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【中文标题】Spring HttpMessageNotWritableException 无法写入 JSON，NullPointerException【英文标题】：Spring HttpMessageNotWritableException Could not write JSON, NullPointerException 【发布时间】：2019-05-15 13:09:16 【问题描述】：我正在使用 Spring Boot 2.0.3、Spring Data REST、Spring HATEOAS、Hibernate。 我遇到了一个奇怪的问题，有时会在特定条件下发生。
我正在从 Angular 应用程序调用 REST 端点，有时由于 Jackson 在创建响应期间引发的异常而导致调用失败。
我有一个由 Angular 应用程序调用的@RepositoryRestController（这是在某些时候引发异常的端点）：
@RepositoryRestController
@PreAuthorize("isAuthenticated()")
public class FrameController
@PostMapping(path = "/frames/property/groupBy")
public ResponseEntity<?> groupByProperty(@PathVariable("property") String property, @RequestBody(required = true) List<Filter> filters, Pageable pageable)
Page<String> pageResult = frameService.groupByProperty(property, filters, pageable);
log.info("Grouped result, size:,total results: , ", pageResult.getSize(), pageResult.getTotalElements(), pageResult.getContent());
return ResponseEntity.ok(pagedResourcesAssembler.toResource(pageResult, stringResourceAssembler));
这是从控制器调用的服务：
@Service
@Transactional
@PreAuthorize("isAuthenticated()")
public class FrameService
public Page<String> groupByProperty(@NotNull String property, @NotNull List<Filter> filters, Pageable pageable)
String where = "WHERE 1=1";
String groupBy = " GROUP BY " + property;
//BUILDING QUERY HERE
TypedQuery<String> query = entityManager.createQuery("SELECT CAST(" + property + " as string) FROM Frame f " + where + groupBy + orderBy, String.class);
TypedQuery<Long> queryCount = entityManager.createQuery("SELECT COUNT(*) FROM Frame f " + where + groupBy, Long.class);
/**
* 3. SETTING REAL VALUES IN THE QUERY
*/
//SETTING PARAMETERS HERE
/**
* 4. PAGINATION
*/
query.setFirstResult((int) pageable.getOffset());
query.setMaxResults(pageable.getPageSize());
long totalResult = queryCount.getResultList().size();
return AppUtils.listToPageable(query.getResultList(), pageable, totalResult);
服务每次都有效，我从来没有例外。
我还公开了一个 RestRepository：
@Transactional
@PreAuthorize("isAuthenticated()")
public interface ContactRepository extends JpaRepository<Contact, Long>
@Query(value = "SELECT c FROM Contact c WHERE :searchString IS NULL OR (companyName LIKE CONCAT(:searchString,'%')) OR (CONCAT(lastName,' ',firstName) LIKE CONCAT(:searchString,'%'))")
public Page<Contact> autocomplete(@Param("searchString") String searchString, Pageable pageable);
经过大量测试后，我发现仅当我在调用端点 autocomplete of ContactRepository 之前调用端点 /frames/property/groupBy 时才会引发我要显示的异常。
这是我得到的例外：
14/12/2018 09:51:58,550 INFO http-nio-8082-exec-5 FrameController:110 - Grouped result, size:40,total results: 8, [47, 49, 50, 51, 52, 53, 55, 58]
14/12/2018 09:51:58,560 ERROR http-nio-8082-exec-5 ApplicationExceptionHandler:133 -
org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.springframework.hateoas.PagedResources["_embedded"])
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:296) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:102) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:272) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:224) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.hateoas.mvc.ResourceProcessorHandlerMethodReturnValueHandler.handleReturnValue(ResourceProcessorHandlerMethodReturnValueHandler.java:114) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:119) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:877) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:783) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) [spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) [spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974) [spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:877) [spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851) [spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:101) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:101) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at cloud.test.server.filters.StoreFilter.doFilterInternal(StoreFilter.java:34) [classes/:?]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.filterAndRecordMetrics(WebMvcMetricsFilter.java:158) [spring-boot-actuator-2.0.3.RELEASE.jar:2.0.3.RELEASE]
at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.filterAndRecordMetrics(WebMvcMetricsFilter.java:126) [spring-boot-actuator-2.0.3.RELEASE.jar:2.0.3.RELEASE]
at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:111) [spring-boot-actuator-2.0.3.RELEASE.jar:2.0.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.boot.actuate.web.trace.servlet.HttpTraceFilter.doFilterInternal(HttpTraceFilter.java:90) [spring-boot-actuator-2.0.3.RELEASE.jar:2.0.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:320) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:127) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:91) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:119) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:137) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:170) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilterInternal(BasicAuthenticationFilter.java:158) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at cloud.test.server.filters.JwtAuthenticationTokenFilter.doFilterInternal(JwtAuthenticationTokenFilter.java:141) [classes/:?]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at cloud.test.server.filters.TenantFilter.doFilterInternal(TenantFilter.java:30) [classes/:?]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:116) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:66) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:105) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:215) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:178) [spring-security-web-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:357) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:270) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1468) [tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.31.jar:8.5.31]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_181]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_181]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.31.jar:8.5.31]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_181]
Caused by: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.springframework.hateoas.PagedResources["_embedded"])
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:391) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:351) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:727) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1396) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:913) ~[jackson-databind-2.9.6.jar:2.9.6]
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:286) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
... 113 more
Caused by: java.lang.NullPointerException
at org.springframework.data.rest.core.support.RepositoryRelProvider.getCollectionResourceRelFor(RepositoryRelProvider.java:52) ~[spring-data-rest-core-3.0.8.RELEASE.jar:3.0.8.RELEASE]
at org.springframework.hateoas.core.DelegatingRelProvider.getCollectionResourceRelFor(DelegatingRelProvider.java:50) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.hateoas.hal.HalEmbeddedBuilder.getDefaultedRelFor(HalEmbeddedBuilder.java:125) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.hateoas.hal.HalEmbeddedBuilder.add(HalEmbeddedBuilder.java:78) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.hateoas.hal.Jackson2HalModule$EmbeddedMapper.map(Jackson2HalModule.java:904) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer.serialize(Jackson2HalModule.java:338) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at org.springframework.hateoas.hal.Jackson2HalModule$HalResourcesSerializer.serialize(Jackson2HalModule.java:308) ~[spring-hateoas-0.24.0.RELEASE.jar:?]
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1396) ~[jackson-databind-2.9.6.jar:2.9.6]
at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:913) ~[jackson-databind-2.9.6.jar:2.9.6]
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:286) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
... 113 more
14/12/2018 09:51:58,572 DEBUG http-nio-8082-exec-5 ApplicationExceptionHandler:263 -
总结一下，如果我调用 /frames/property/groupBy 端点甚至 100 次一切正常，但是如果我只调用端点 autocomplete 一次然后我再次调用 /frames/property/groupBy 端点，我会立即出现异常。
这很奇怪，即使我在代码内部进行了调试，我也不清楚出了什么问题。你有什么提示吗？会不会是 Spring/Jackson 的错误？
####################### 更新################### ####
真的在里面调试，终于找到引发异常的那一行了。
就是org.springframework.data.rest.core.support.RepositoryRelProvider中的这段代码：
@Override
public String getCollectionResourceRelFor(Class<?> type)
return mappings.getObject().getMetadataFor(type).getRel();
实际上我的类型是String 而mappings.getObject().getMetadataFor(type) 为null，所以当它尝试调用getRel() 时会引发异常。
这是一个错误吗？
###################### 更新 2 ################### #####
我发现问题出在我使用暴露存储库的方法时：
@Transactional
@PreAuthorize("isAuthenticated()")
public interface ContactRepository extends JpaRepository<Contact, Long>
@Query(value = "SELECT c FROM Contact c WHERE :searchString IS NULL OR (companyName LIKE CONCAT(:searchString,'%')) OR (CONCAT(lastName,' ',firstName) LIKE CONCAT(:searchString,'%'))")
public Page<Contact> autocomplete(@Param("searchString") String searchString, Pageable pageable);
事实上，在这种情况下，RepositorySearchController.executeQueryMethod() 它检测到参数是一个字符串，并调用ResourceMetadata metadata = mappings.getMetadataFor(domainType.getType());，在PersistentEntitiesResourceMappings 中将一个空值放入ResourceMetadata 的缓存中。
不确定这是否只是效果错误的原因，但如果我将端点移动到调用该存储库的控制器中，那么一切都很好。
【问题讨论】：
我发现这篇 SO 帖子有助于解决上述问题，***.com/questions/21346387/…，它描述了创建自定义 ResourceAssemblerSupport 【参考方案1】：Jackson 尝试从它的 getter 中序列化一个字段。由于某种原因，getter 方法可能会抛出 NullPointerException。
如果您希望Jackson 忽略此字段，您可以使用@JsonIgnore 注释getter 或使用@JsonIgnoreProperties 注释类并指定属性名称。
【讨论】：
不清楚哪个类在我的情况下放置了“@JsonIgnore”注释。我不认为我的某个 bean 中有 null。 如果我需要这个字段怎么办？ @nbirla以上是关于Spring HttpMessageNotWritableException 无法写入 JSON，NullPointerException的主要内容，如果未能解决你的问题，请参考以下文章
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