fix: descriptive error for unrecognized cell type attribute

fesod-sheet/src/main/java/org/apache/fesod/sheet/analysis/v07/handlers/CellTagHandler.java

@@ -33,6 +33,7 @@
 import org.apache.fesod.sheet.constant.FesodSheetConstants;
 import org.apache.fesod.sheet.context.xlsx.XlsxReadContext;
 import org.apache.fesod.sheet.enums.CellDataTypeEnum;
+import org.apache.fesod.sheet.exception.ExcelAnalysisException;
 import org.apache.fesod.sheet.metadata.GlobalConfiguration;
 import org.apache.fesod.sheet.metadata.data.ReadCellData;
 import org.apache.fesod.sheet.read.metadata.holder.xlsx.XlsxReadSheetHolder;
@@ -49,8 +50,8 @@ public class CellTagHandler extends AbstractXlsxTagHandler {
     @Override
     public void startElement(XlsxReadContext xlsxReadContext, String name, Attributes attributes) {
         XlsxReadSheetHolder xlsxReadSheetHolder = xlsxReadContext.xlsxReadSheetHolder();
-        xlsxReadSheetHolder.setColumnIndex(PositionUtils.getCol(
-                attributes.getValue(ExcelXmlConstants.ATTRIBUTE_R), xlsxReadSheetHolder.getColumnIndex()));
+        String cellReference = attributes.getValue(ExcelXmlConstants.ATTRIBUTE_R);
+        xlsxReadSheetHolder.setColumnIndex(PositionUtils.getCol(cellReference, xlsxReadSheetHolder.getColumnIndex()));
 
         // t="s" ,it means String
         // t="str" ,it means String,but does not need to be read in the 'sharedStrings.xml'
@@ -59,7 +60,11 @@ public void startElement(XlsxReadContext xlsxReadContext, String name, Attribute
         // t="e" ,it means Error
         // t="n" ,it means Number
         // t is null ,it means Empty or Number
-        CellDataTypeEnum type = CellDataTypeEnum.buildFromCellType(attributes.getValue(ExcelXmlConstants.ATTRIBUTE_T));
+        String cellType = attributes.getValue(ExcelXmlConstants.ATTRIBUTE_T);
+        CellDataTypeEnum type = CellDataTypeEnum.buildFromCellType(cellType);
+        if (type == null) {
+            throw new ExcelAnalysisException("Invalid cell data type: '" + cellType + "' in cell " + cellReference);
+        }
         xlsxReadSheetHolder.setTempCellData(new ReadCellData<>(type));
         xlsxReadSheetHolder.setTempData(new StringBuilder());
 

fesod-sheet/src/main/java/org/apache/fesod/sheet/enums/CellDataTypeEnum.java

@@ -83,8 +83,9 @@ public enum CellDataTypeEnum {
     /**
      * Build data types
      *
-     * @param cellType
-     * @return
+     * @param cellType the raw {@code t} attribute value of a cell
+     * @return the matching type, {@link #EMPTY} when {@code cellType} is empty, or {@code null} when
+     *         {@code cellType} is not a recognized type; callers are expected to handle the {@code null} case.
      */
     public static CellDataTypeEnum buildFromCellType(String cellType) {
         if (StringUtils.isEmpty(cellType)) {

fesod-sheet/src/test/java/org/apache/fesod/sheet/analysis/v07/handlers/CellTagHandlerTest.java

@@ -0,0 +1,61 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.fesod.sheet.analysis.v07.handlers;
+
+import org.apache.fesod.sheet.context.xlsx.XlsxReadContext;
+import org.apache.fesod.sheet.exception.ExcelAnalysisException;
+import org.apache.fesod.sheet.read.metadata.holder.xlsx.XlsxReadSheetHolder;
+import org.apache.fesod.sheet.testkit.Tags;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+import org.xml.sax.helpers.AttributesImpl;
+
+/**
+ * Regression test for <a href="https://github.com/apache/fesod/issues/955">issue #955</a>.
+ *
+ * <p>A cell whose {@code t} attribute is not a recognized type made {@code buildFromCellType} return
+ * {@code null}, which then tripped the {@code ReadCellData} constructor with a confusing
+ * {@code IllegalArgumentException: Type can not be null} that named neither the cell nor the attribute.
+ * The handler must instead throw an {@link ExcelAnalysisException} naming the invalid type and its location.
+ */
+@Tag(Tags.UNIT)
+class CellTagHandlerTest {
+
+    @Test
+    void startElement_throwsDescriptiveError_forUnknownCellType() {
+        XlsxReadContext context = Mockito.mock(XlsxReadContext.class);
+        XlsxReadSheetHolder sheetHolder = Mockito.mock(XlsxReadSheetHolder.class);
+        Mockito.when(context.xlsxReadSheetHolder()).thenReturn(sheetHolder);
+
+        AttributesImpl attributes = new AttributesImpl();
+        attributes.addAttribute("", "r", "r", "CDATA", "B4");
+        attributes.addAttribute("", "t", "t", "CDATA", "unknown");
+
+        ExcelAnalysisException exception = Assertions.assertThrows(
+                ExcelAnalysisException.class, () -> new CellTagHandler().startElement(context, "c", attributes));
+
+        // The message must name the unrecognized type and the exact cell (Excel reference) for diagnostics.
+        String message = exception.getMessage();
+        Assertions.assertTrue(message.contains("'unknown'"), "should name the unrecognized type: " + message);
+        Assertions.assertTrue(message.contains("B4"), "should name the cell reference: " + message);
+    }
+}