Converter registration of easyexcel

In the use of easyexcel Error in template export

Caused by: com.alibaba.excel.exception.ExcelWriteDataConvertException: Can not find 'Converter' support class Timestamp.
	at com.alibaba.excel.write.executor.AbstractExcelWriteExecutor.doConvert(AbstractExcelWriteExecutor.java:323)
	at com.alibaba.excel.write.executor.AbstractExcelWriteExecutor.convert(AbstractExcelWriteExecutor.java:277)
	at com.alibaba.excel.write.executor.ExcelWriteFillExecutor.doFill(ExcelWriteFillExecutor.java:258)
	at com.alibaba.excel.write.executor.ExcelWriteFillExecutor.fill(ExcelWriteFillExecutor.java:127)
	at com.alibaba.excel.write.ExcelBuilderImpl.fill(ExcelBuilderImpl.java:82)
	at com.alibaba.excel.ExcelWriter.fill(ExcelWriter.java:107)
	at com.alibaba.excel.ExcelWriter.fill(ExcelWriter.java:95)
	at com.xQuant.platform.common.excel.utils.ExportData2ExcelUtil.lambda$export2Excel$0(ExportData2ExcelUtil.java:95)
	at java.util.HashMap.forEach(HashMap.java:1289)
	at com.xQuant.platform.common.excel.utils.ExportData2ExcelUtil.export2Excel(ExportData2ExcelUtil.java:88)
	at com.xQuant.platform.common.excel.utils.ExportData2ExcelUtil.export2Excel(ExportData2ExcelUtil.java:115)
	at com.S3003.cams.query.bo.CommonQueryBO.genExcel(CommonQueryBO.java:406)
	... 79 common frames omitted

It can be seen from the error message that , There is no converter .
Create the following converter by referring to the official source code

import com.alibaba.excel.converters.Converter;
import com.alibaba.excel.enums.CellDataTypeEnum;
import com.alibaba.excel.metadata.GlobalConfiguration;
import com.alibaba.excel.metadata.data.ReadCellData;
import com.alibaba.excel.metadata.data.WriteCellData;
import com.alibaba.excel.metadata.property.ExcelContentProperty;
import com.alibaba.excel.util.DateUtils;
import com.alibaba.excel.util.WorkBookUtil;
import org.apache.poi.ss.usermodel.DateUtil;

import java.sql.Timestamp;
import java.text.ParseException;
import java.util.Date;

public class EasyTimestampConverter implements Converter<Timestamp>  {
    

    @Override
    public Class<?> supportJavaTypeKey() {
    
        return Timestamp.class;
    }

    @Override
    public CellDataTypeEnum supportExcelTypeKey() {
    
        return CellDataTypeEnum.STRING;
    }

    @Override
    public Timestamp convertToJavaData(ReadCellData<?> cellData, ExcelContentProperty contentProperty,
                                  GlobalConfiguration globalConfiguration) throws ParseException {
    
        Date javaDate;
        if (contentProperty == null || contentProperty.getDateTimeFormatProperty() == null) {
    
            javaDate = DateUtil.getJavaDate(cellData.getNumberValue().doubleValue(),
                    globalConfiguration.getUse1904windowing(), null);
        } else {
    
            javaDate = DateUtil.getJavaDate(cellData.getNumberValue().doubleValue(),
                    contentProperty.getDateTimeFormatProperty().getUse1904windowing(), null);
        }
        if (javaDate != null){
    
            return new Timestamp(javaDate.getTime());
        }else {
    
            return null;
        }
    }


    @Override
    public WriteCellData<?> convertToExcelData(Timestamp value, ExcelContentProperty contentProperty,
                                               GlobalConfiguration globalConfiguration) throws Exception {
    
        WriteCellData<?> cellData = new WriteCellData<>(value);
        String format = null;
        if (contentProperty != null && contentProperty.getDateTimeFormatProperty() != null) {
    
            format = contentProperty.getDateTimeFormatProperty().getFormat();
        }
        WorkBookUtil.fillDataFormat(cellData, format, DateUtils.defaultDateFormat);
        return cellData;
    }
}

And then execute excel Register during operation

try (ExcelWriter excelWriter = EasyExcel.write(outputFile.getPath()).withTemplate(excelTemplateFile).registerConverter(new EasyTimestampConverter()).build()) {
    
  // ... excel Operation logic 
}         

According to the truth , It should be OK at this time . however , Error is still reported when the program is executed .
Only source code analysis …
Check the error source code com.alibaba.excel.write.executor.AbstractExcelWriteExecutor#doConvert

private WriteCellData<?> doConvert(CellWriteHandlerContext cellWriteHandlerContext) {
    
     ExcelContentProperty excelContentProperty = cellWriteHandlerContext.getExcelContentProperty();

     Converter<?> converter = null;
     if (excelContentProperty != null) {
    
         converter = excelContentProperty.getConverter();
     }
     if (converter == null) {
    
         // csv is converted to string by default
         if (writeContext.writeWorkbookHolder().getExcelType() == ExcelTypeEnum.CSV) {
    
             cellWriteHandlerContext.setTargetCellDataType(CellDataTypeEnum.STRING);
         }
         converter = writeContext.currentWriteHolder().converterMap().get(
             ConverterKeyBuild.buildKey(cellWriteHandlerContext.getOriginalFieldClass(),
                 cellWriteHandlerContext.getTargetCellDataType()));
     }
     if (cellWriteHandlerContext.getOriginalValue() == null && !(converter instanceof NullableObjectConverter)) {
    
         return new WriteCellData<>(CellDataTypeEnum.EMPTY);
     }
     if (converter == null) {
    
         throw new ExcelWriteDataConvertException(cellWriteHandlerContext,
             "Can not find 'Converter' support class " + cellWriteHandlerContext.getOriginalFieldClass()
                 .getSimpleName() + ".");
     }
     WriteCellData<?> cellData;
     try {
    
         cellData = ((Converter<Object>)converter).convertToExcelData(
             new WriteConverterContext<>(cellWriteHandlerContext.getOriginalValue(), excelContentProperty,
                 writeContext));
     } catch (Exception e) {
    
         throw new ExcelWriteDataConvertException(cellWriteHandlerContext,
             "Convert data:" + cellWriteHandlerContext.getOriginalValue() + " error, at row:"
                 + cellWriteHandlerContext.getRowIndex(), e);
     }
     if (cellData == null || cellData.getType() == null) {
    
         throw new ExcelWriteDataConvertException(cellWriteHandlerContext,
             "Convert data:" + cellWriteHandlerContext.getOriginalValue()
                 + " return is null or return type is null, at row:"
                 + cellWriteHandlerContext.getRowIndex());
     }
     return cellData;
 }

One of the most critical behaviors

converter = writeContext.currentWriteHolder().converterMap().get(
             ConverterKeyBuild.buildKey(cellWriteHandlerContext.getOriginalFieldClass(),
                 cellWriteHandlerContext.getTargetCellDataType()));

stay debug When ,converterMap() It includes its own defined converter , But the corresponding key incorrect . When is the converter defined by yourself stored in convertMap What about the middle ？ Continue to view source code , Last in com.alibaba.excel.write.metadata.holder.AbstractWriteHolder#AbstractWriteHolder The corresponding logic is found .

if (writeBasicParameter.getCustomConverterList() != null
     && !writeBasicParameter.getCustomConverterList().isEmpty()) {
    
     for (Converter<?> converter : writeBasicParameter.getCustomConverterList()) {
    
         getConverterMap().put(ConverterKeyBuild.buildKey(converter.supportJavaTypeKey()), converter);
     }
 }

You can see that the mapping is built when finding the converter and storing the converter key Values in different ways .（ Calculate or calculate bug Well ？？）
And then we studied DefaultConverterLoader 了 , Discovery can be achieved by loadDefaultWriteConverter Method to get the corresponding Map

public static Map<ConverterKey, Converter<?>> loadDefaultWriteConverter() {
    
    return defaultWriteConverter;
}

Can be obtained Map Then add the converter by yourself ( In the use of EasyExcel Add the following code to the class of )

static {
    
   EasyTimestampConverter converter = new EasyTimestampConverter();
   DefaultConverterLoader.loadDefaultWriteConverter().put(ConverterKeyBuild.buildKey(converter.supportJavaTypeKey(), converter.supportExcelTypeKey()), converter);
}