Write POJO to csv, error: Can not start an object, expecting field name

[AlwaysNoobCoder]

version: 2.15.4

SequenceWriter sw = csvMapper.writerWithSchemaFor(reg.getMapper(locator).getEntityType()).writeValues(System.out);
handler.streamRead(req, t -> {

try {
	log.info("writing {} {} ", t.getClass().getName(), reg.getMapper(locator).getEntityType().getName());
	sw.write(t);
} catch (IOException e) {
	e.printStackTrace();
} catch (Exception e) {
	e.printStackTrace();
}
});

2024-04-12T16:54:25.922+08:00  INFO 28231 --- [nio-8080-exec-2] c.w.i.s.pub.ws.SEAExportWSHandler        : writing com.wxt.itps.services.pub.models.User com.wxt.itps.services.pub.models.User
com.fasterxml.jackson.core.JsonGenerationException: Can not start an object, expecting field name
	at com.fasterxml.jackson.core.JsonGenerator._reportError(JsonGenerator.java:2849)
	at com.fasterxml.jackson.dataformat.csv.CsvGenerator._verifyValueWrite(CsvGenerator.java:958)
	at com.fasterxml.jackson.dataformat.csv.CsvGenerator.writeStartObject(CsvGenerator.java:585)
	at com.fasterxml.jackson.core.base.GeneratorBase.writeStartObject(GeneratorBase.java:295)
	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:174)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:479)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:399)
	at com.fasterxml.jackson.databind.SequenceWriter.write(SequenceWriter.java:153)
	at com.wxt.itps.services.pub.ws.SEAExportWSHandler.lambda$0(SEAExportWSHandler.java:64)
	at java.base/java.lang.Iterable.forEach(Iterable.java:75)
	at fwx.sea.impl.mybatis.MybatisSEARequestHandler.doStreamingRead(MybatisSEARequestHandler.java:228)
	at com.wxt.itps.services.pub.ws.SEARequestHandlerWrapper.streamRead(SEARequestHandlerWrapper.java:23)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:351)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:196)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:765)
	at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123)
	at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:392)
	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:765)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:717)
	at com.wxt.itps.services.pub.ws.SEARequestHandlerWrapper$$SpringCGLIB$$0.streamRead(<generated>)
	at com.wxt.itps.services.pub.ws.SEAExportWSHandler.handleTextMessage(SEAExportWSHandler.java:60)
	at org.springframework.web.socket.handler.AbstractWebSocketHandler.handleMessage(AbstractWebSocketHandler.java:43)
	at org.springframework.web.socket.handler.WebSocketHandlerDecorator.handleMessage(WebSocketHandlerDecorator.java:75)
	at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.handleMessage(LoggingWebSocketHandlerDecorator.java:56)
	at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.handleMessage(ExceptionWebSocketHandlerDecorator.java:58)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.handleTextMessage(StandardWebSocketHandlerAdapter.java:113)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$1.onMessage(StandardWebSocketHandlerAdapter.java:70)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$1.onMessage(StandardWebSocketHandlerAdapter.java:67)
	at org.apache.tomcat.websocket.WsFrameBase.sendMessageText(WsFrameBase.java:387)
	at org.apache.tomcat.websocket.server.WsFrameServer.sendMessageText(WsFrameServer.java:130)
	at org.apache.tomcat.websocket.WsFrameBase.processDataText(WsFrameBase.java:484)
	at org.apache.tomcat.websocket.WsFrameBase.processData(WsFrameBase.java:284)
	at org.apache.tomcat.websocket.WsFrameBase.processInputBuffer(WsFrameBase.java:130)
	at org.apache.tomcat.websocket.server.WsFrameServer.onDataAvailable(WsFrameServer.java:85)
	at org.apache.tomcat.websocket.server.WsFrameServer.doOnDataAvailable(WsFrameServer.java:184)
	at org.apache.tomcat.websocket.server.WsFrameServer.notifyDataAvailable(WsFrameServer.java:164)
	at org.apache.tomcat.websocket.server.WsHttpUpgradeHandler.upgradeDispatch(WsHttpUpgradeHandler.java:152)
	at org.apache.coyote.http11.upgrade.UpgradeProcessorInternal.dispatch(UpgradeProcessorInternal.java:60)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:57)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63)
	at java.base/java.lang.Thread.run(Thread.java:833)

Any hint on this? I have no clue at all..

[cowtowncoder]

Maybe start by showing what class com.wxt.itps.services.pub.models.User looks like.
And then try creating an instance and writing with simple mapper.writer(schema).writeValueAsString() in a test.

[AlwaysNoobCoder]

I did a test without spring:

public class UserCSVTest {

	public static void main(String[] args) throws JsonProcessingException {
		
		CsvMapper mapper = new CsvMapper();
		
		User user = new User();
		user.setId("1");
		user.setName("foo");
		
		String userCsv = mapper.writerWithSchemaFor(user.getClass()).writeValueAsString(user);
		
		System.out.println(userCsv);
	}

}

it outputs:

,,,,,,,,,1,,,,,,foo,,,,,,,,

the com.wxt.itps.services.pub.models.User:

package com.wxt.itps.services.pub.models;

import fwx.commons.model.MetaData;
import fwx.sea.api.annotations.PrimaryKey;
import fwx.sea.api.validation.SEACreate;
import fwx.sea.api.validation.SEAUpdate;
import java.io.Serializable;
import java.time.Instant;
import org.hibernate.validator.constraints.Length;

@PrimaryKey({"id"})
public class User extends MetaData implements Serializable {

    private String id;

    @Length(max=36, groups = {SEACreate.class, SEAUpdate.class} )
    private String manageGroup;

    @Length(max=32, groups = {SEACreate.class, SEAUpdate.class} )
    private String userType;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String employeeNumber;

    @Length(max=18, groups = {SEACreate.class, SEAUpdate.class} )
    private String citizenId;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String username;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String name;

    @Length(max=128, groups = {SEACreate.class, SEAUpdate.class} )
    private String fullPinyin;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String shortPinyin;

    @Length(max=32, groups = {SEACreate.class, SEAUpdate.class} )
    private String gender;

    @Length(max=512, groups = {SEACreate.class, SEAUpdate.class} )
    private String credential;


    @Length(max=64, groups = {SEACreate.class, SEAUpdate.class} )
    private String officeLocation;


    @Length(max=32, groups = {SEACreate.class, SEAUpdate.class} )
    private String status;

    @Length(max=64, groups = {SEACreate.class, SEAUpdate.class} )
    private String email;

    @Length(max=32, groups = {SEACreate.class, SEAUpdate.class} )
    private String tel;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String mobile;

    @Length(max=36, groups = {SEACreate.class, SEAUpdate.class} )
    private String superior;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String externalId;

    @Length(max=16, groups = {SEACreate.class, SEAUpdate.class} )
    private String syncStatus;

    private Instant lastSyncTime;

    private static final long serialVersionUID = 1L;

    ... // getters and setters..
}

public class MetaData {
    
    private Instant createdAt;
    
    private String creator;
    
    private Instant lastUpdatedAt;
    
    private String lastUpdator;

    public Instant getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(Instant createdAt) {
        this.createdAt = createdAt;
    }

    public String getCreator() {
        return creator;
    }

    public void setCreator(String creator) {
        this.creator = creator;
    }

    public Instant getLastUpdatedAt() {
        return lastUpdatedAt;
    }

    public void setLastUpdatedAt(Instant lastUpdatedAt) {
        this.lastUpdatedAt = lastUpdatedAt;
    }

    public String getLastUpdator() {
        return lastUpdator;
    }

    public void setLastUpdator(String lastUpdator) {
        this.lastUpdator = lastUpdator;
    }

}

and the CsvMapper bean:

	@Bean
	public CsvMapper getCsvMapper() {
		CsvMapper csvMapper = new CsvMapper();
		return csvMapper;
	}

@cowtowncoder It's a great honor to get your personal reply. Can you look at it again?

[AlwaysNoobCoder]

I am still confused why Jackson not show me the real underlying error cause.

I changed my code to following:

handler.streamRead(req, t -> {
        try {
	        log.info("writing {} {} ", t.getClass().getName(), reg.getMapper(locator).getEntityType().getName());
	        sw.write(t);
        } catch (Exception e) {
	        log.error("error when writing object to csv", e);
	        throw new RuntimeException(e.getMessage());           //  break the Iterator#forEache by thorw a RuntimeException
        }
});

then it show me the real error:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Java 8 date/time type `java.time.Instant` not supported by default

The problem is solved, but I am still full of doubts. Maybe it is caused by Java's streaming or concurrency?

[cowtowncoder]

Ok yes, that is odd: Jackson should not suppress the exception. Especially this:

String userCsv = mapper.writerWithSchemaFor(user.getClass()).writeValueAsString(user);

should have thrown the exception as expected, to be caught.

Sometimes what happens are catch clauses that silently catch exceptions but that didn't seem to be the case here.
Another possibility are auto-close statements, where it might be that attempts to close() JsonGenerator could produce secondary errors (which may have been the case for the first exception about not being able to write specific token -- that sounds like it might be secondary problem).

But at least you were able to find the underlying root cause by isolating code. That is usually a good way to figure things out, even if it should not be required.