jacksonJSON – unmarshal上的“没有单字符串构造函数/工厂方法”错误
最简单的情况会给我带来麻烦。 我第一次遇到它。 我能够解组稍微复杂的json，但这个简单的失败了。
什么会导致这个以及为什么jackson只用一根弦就麻烦了？
一个包含用户角色名称的简单类。
public class UpdateUserRole { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } 控制器内部
public void updateUserRole(@PathVariable Long id, @RequestBody UpdateUserRoleReq req) { }
一旦jackson看到这一点，它就会抛出这个错误
org.springframework.http.converter.HttpMessageNotReadableException: Could not read JSON: Can not instantiate value of type [simple type, class com.mycompany.UpdateUserRoleReq] from String value ('{"name":"admin_1407445357682"}'); no single-String constructor/factory method; nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class com.mycompany.UpdateUserRoleReq] from String value ('{"name":"admin_1407445357682"}'); no single-String constructor/factory method at org.springframework.http.converter.json.MappingJackson2HttpMessageConverter.readJavaType(MappingJackson2HttpMessageConverter.java:228) at org.springframework.http.converter.json.MappingJackson2HttpMessageConverter.read(MappingJackson2HttpMessageConverter.java:220) at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters(AbstractMessageConverterMethodArgumentResolver.java:138) at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.readWithMessageConverters(RequestResponseBodyMethodProcessor.java:183) at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.resolveArgument(RequestResponseBodyMethodProcessor.java:98) at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:79) at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:157) at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:124) at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:689) at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:938) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:870) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961) at org.springframework.web.servlet.FrameworkServlet.doPut(FrameworkServlet.java:874) at javax.servlet.http.HttpServlet.service(HttpServlet.java:649) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837) at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.springframework.boot.actuate.trace.WebRequestTraceFilter.doFilterInternal(WebRequestTraceFilter.java:110) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.springframework.boot.actuate.autoconfigure.EndpointWebMvcAutoConfiguration$ApplicationContextFilterConfiguration$1.doFilterInternal(EndpointWebMvcAutoConfiguration.java:257) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
通过严格遵守错误消息，我能够解决一个非常类似的问题。
我在单字符串构造函数的情况下添加了等效项。
public class UpdateUserRole { private String name; public UpdateUserRole() {} public UpdateUserRole(String name) { this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
我是我的情况。 我发送了双重转义的JSON字符串，例如“{\”name \“：\”abc \“，\”age \“：21}”我的String构造函数只完全接收JSON字符串。 其他属性（年龄）根本没有更新。 一旦我将输入更改为{“name”：“abc”，“age”：21}。 jackson能够自动将JSON输入解析为对象。
您必须添加包含单个参数的构造方法。 试一试。