#68 clearCellValue workaround for every setCellFormula() call (#70)

streaming

jxls-poi/src/main/java/org/jxls/transform/poi/PoiCellData.java

@@ -1,5 +1,8 @@
 package org.jxls.transform.poi;
 
+import java.util.Date;
+import java.util.Map;
+
 import org.apache.poi.ss.formula.FormulaParseException;
 import org.apache.poi.ss.usermodel.Cell;
 import org.apache.poi.ss.usermodel.CellStyle;
@@ -9,15 +12,14 @@
 import org.apache.poi.ss.usermodel.RichTextString;
 import org.apache.poi.ss.usermodel.Sheet;
 import org.apache.poi.ss.util.CellReference;
+import org.apache.poi.xssf.usermodel.XSSFCell;
 import org.jxls.common.CellRef;
 import org.jxls.common.Context;
 import org.jxls.util.Util;
+import org.openxmlformats.schemas.spreadsheetml.x2006.main.CTCell;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.util.Date;
-import java.util.Map;
-
 /**
  * Cell data wrapper for POI cell
  * 
@@ -244,6 +246,7 @@ private void updateFormulaCellContents(Cell cell) {
                 cell.setCellValue((String) evaluationResult);
             } else {
                 cell.setCellFormula((String) evaluationResult);
+                clearCellValue(cell); // This call is especially important for streaming.
             }
         } catch (FormulaParseException e) {
             try {
@@ -257,6 +260,19 @@ private void updateFormulaCellContents(Cell cell) {
         }
     }
 
+    // protected so any user can change this piece of code
+    protected void clearCellValue(org.apache.poi.ss.usermodel.Cell poiCell) {
+        if (poiCell instanceof XSSFCell) {
+            CTCell cell = ((XSSFCell) poiCell).getCTCell(); // POI internal access, but there's no other way
+            // Now do the XSSFCell.setFormula code that was done before POI commit https://github.com/apache/poi/commit/1253a29
+            // After setting the formula in attribute f we clear the value attribute v if set. This causes a recalculation
+            // and prevents wrong formula results.
+            if (cell.isSetV()) {
+                cell.unsetV();
+            }
+        }
+    }
+
     private void updateCellStyle(Cell cell, CellStyle cellStyle) {
         cell.setCellStyle(cellStyle);
     }