java - SpringBoot， Controller 通过ResponseBodyAdvice返回数据出现错误(类无法转换为字符串)

Question

我想要使用 ResponseBodyAdvice 格式化 Controller 的结果。 例如， Controller 方法返回一个字符串(“hello”)，ResponseBodyAdvice 将其格式化为 Json 数据 {code:1, data:"hello"}。

这是我的 Controller 方法代码:

@PostMapping("login")
public String login(String username, String pwd) throws Exception {
    System.out.println(username + pwd);
    return "hhh";
}

这是我的 ResponseBodyAdvice:

@ControllerAdvice
public class GlobalControllerAdvice implements ResponseBodyAdvice {

    @Override
    public boolean supports(MethodParameter methodParameter, Class aClass) {
        return true;
    }

    @Override
    public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType, Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
        return TResult.success(o); // result is my class, it occur error
        // return JSON.toJSONString(TResult.success(o)); // result is string, it's ok
    }
}

错误消息是:

java.lang.ClassCastException: cn.tianyu.blog_api.util.TResult cannot be cast to java.lang.String
at org.springframework.http.converter.StringHttpMessageConverter.getContentLength(StringHttpMessageConverter.java:43) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.http.converter.AbstractHttpMessageConverter.addDefaultHeaders(AbstractHttpMessageConverter.java:259) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:210) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:275) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:180) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:119) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:877) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:783) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:877) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851) ~[spring-webmvc-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.8.RELEASE.jar:5.0.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:800) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1471) [tomcat-embed-core-8.5.32.jar:8.5.32]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.32.jar:8.5.32]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_144]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_144]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.32.jar:8.5.32]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_144]

结果是

TResult.class

public class TResult {

private Integer code;
private String message;
private Object data;

public TResult() {
}

public TResult(Integer code, String message) {
    this.code = code;
    this.message = message;
}

public static TResult success() {
    TResult result = new TResult();
    result.setResultCode(TResultCode.SUCCESS);
    return result;
}

public static TResult success(Object data) {
    TResult result = new TResult();
    result.setResultCode(TResultCode.SUCCESS);
    result.setData(data);
    return result;
}

注意红色箭头方向的返回类型。 最终结果类型是application/json，我设置了一个断点，方法“ResponseBodyAdvice.beforeBodyWrite()”被调用了两次。第一次发生这个错误。第二次组装错误消息。

请帮忙!非常感谢!

Answer 1

现在我知道原因了。 Spring会通过方法返回值使用相应的HttpMessageConvert。 当方法返回String.class时，String将使用StringHttpMessageConvert.class来处理结果。 但是StringHttpMessageConvert只支持String.class，我将其更改为TResult.class(在GlobalControllerAdvice.class中，参见问题)，因此出现错误。 这里有两种方法来解决它: 1:重写StringHttpMessageConvert，这种方式比较复杂。 2:不要改变GlobalControllerAdvice.class中的返回类，只改变返回值(json格式)，我就是这样的

这是代码:

public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType, Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
    TResult result = TResult.success(o);
    // just change the value
    if (o instanceof String) {
        return JSON.toJSONString(result);
    }
    return result;
}