Spring and Jackson returning a response with status 200 OK but incorrect JSON after an exception

Question

I have a simple controller that is returning an instance of MyClass1 . MyClass1 contains a list of MyClass2 and use the @JsonDeserialize annotation to tell Jackson to use a custom serializer "MySerializer".

public class MyClass1 {

    @JsonSerialize(contentUsing = MySerializer.class)
    private List<MyClass2> class2;

    public List<MyClass2> getClass2() {
        return class2;
    }

    public void setClass2(List<MyClass2> propertyTest2) {
        this.class2 = propertyTest2;
    }

}

public class MyClass2 {

    private String myString;

    public String getMyString() {
        return myString;
    }

    public void setMyString(String myString) {
        this.myString = myString;
    }
}

public class MySerializer extends StdSerializer<MyClass2> {

    public MySerializer() {
        this(null);
    }

    protected MySerializer(Class<MyClass2> t) {
        super(t);
    }

    @Override
    public void serialize(MyClass2 value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        gen.writeStartObject();
        gen.writeObjectField("myObject", value);
        gen.writeEndObject();
    }

}

@RestController
public class MyController {

    @GetMapping(value = "/api", produces = "application/json;charset=UTF-8")
    public MyClass1 myMethod() {

        MyClass1 class1 = new MyClass1();
        MyClass2 class2 = new MyClass2();
        class2.setMyString("test string");
        List<MyClass2> class2List = new ArrayList<>();
        class2List.add(class2);
        class1.setClass2(class2List);

        return class1;
    }

}

So with this code whenever I send a get request to /api I have the response:

{
    "class2": [
        {
            "myObject": {
                "myString": "test string"
            }
        }
    ]
}

However if I update the code of MyClass1 and add a property errorProperty in order to throw an exception:

public class MyClass1 {

    @JsonSerialize(contentUsing = MySerializer.class)
    private List<MyClass2> class2;

    public List<MyClass2> getClass2() {
        return class2;
    }

    public void setClass2(List<MyClass2> propertyTest2) {
        this.class2 = propertyTest2;
    }

    public int getMyError() {
        String error = null;
        error.toLowerCase();
        return 1;
    }
}

I have the following response from Spring Boot (with a status of 200 OK):

{
    "class2": [
        {
            "myObject": {
                "myString": "test string"
            }
        }
    ]
}{
    "timestamp": "2022-05-11T16:55:09.312+00:00",
    "status": 200,
    "error": "OK",
    "path": "/api"
}

I would like to understand why I'm having a response 200 ok because I would have expected a 500 internal server error since an exception was raised. And also the response has 2 jsons in it so I don't think it is a correct response. Is it an issue in the way I'm using the @JsonSerialize feature or a bug from jackson.

Also here is the stacktrace:

java.lang.NullPointerException: null
    at com.example.test_spring.serializeronproperty.MyClass1.getMyError(MyClass1.java:22) ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_291]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_291]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_291]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_291]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:689) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1518) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:1007) ~[jackson-databind-2.13.2.1.jar:2.13.2.1]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:456) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:104) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:290) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:183) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:78) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:135) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:655) ~[tomcat-embed-core-9.0.62.jar:4.0.FR]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.19.jar:5.3.19]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:764) ~[tomcat-embed-core-9.0.62.jar:4.0.FR]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.19.jar:5.3.19]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.19.jar:5.3.19]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.19.jar:5.3.19]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.19.jar:5.3.19]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197) ~[tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:360) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:890) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1743) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.62.jar:9.0.62]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_291]

Answer 1

The way you are using @JsonSerialize is fine and is not causing your issue.

Spring Boot would return the serialized 500 error response you are expecting if you were to force the NullPointerException somewhere outside the model object, such as the controller, but the odd behavior you are seeing is because of logic in your getter. Simply moving that logic out of your getter will get your expected result.

This odd behavior is happening during spring-webmvc's ServletInvocableHandlerMethod 's invokeAndHandle() method when it is attempting to serialize MyClass1 and is unable to resolve the NullPointerException being forced in its getMyError() method.

No exceptions were raised during the processing of the webRequest and so it is serializing the response with a 200 OK response. I believe an assumption is being made that best practices are being used and the response can carry itself out without meeting anymore logic as we should be dealing with a model object without further logic to process (outside your typical getters and setters). During the serialization of this response, unexpected logic is found in the getter by Jackson and an exception (where one is not anticipated) is raised, causing the odd behavior. I believe spring-boot then intercepts the exception and serializes it and appends it to the response.