final修饰的list改为不可变集合，CellFormatResultWrapper改为静态内部类

easyexcel-core/src/main/java/com/alibaba/excel/analysis/csv/CsvExcelReadExecutor.java

@@ -3,11 +3,7 @@ package com.alibaba.excel.analysis.csv;
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.nio.file.Files;
-import java.util.ArrayList;
-import java.util.Iterator;
-import java.util.LinkedHashMap;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 import com.alibaba.excel.analysis.ExcelReadExecutor;
 import com.alibaba.excel.context.csv.CsvReadContext;
@@ -49,7 +45,7 @@ public class CsvExcelReadExecutor implements ExcelReadExecutor {
 
     @Override
     public List<ReadSheet> sheetList() {
-        return sheetList;
+        return (this.sheetList != null ? Collections.unmodifiableList(this.sheetList) : Collections.emptyList());
     }
 
     @Override

easyexcel-core/src/main/java/com/alibaba/excel/analysis/v07/XlsxSaxAnalyser.java

@@ -3,13 +3,7 @@ package com.alibaba.excel.analysis.v07;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.Iterator;
-import java.util.List;
-import java.util.Map;
-import java.util.Optional;
-import java.util.UUID;
+import java.util.*;
 
 import javax.xml.parsers.ParserConfigurationException;
 import javax.xml.parsers.SAXParser;
@@ -211,7 +205,7 @@ public class XlsxSaxAnalyser implements ExcelReadExecutor {
 
     @Override
     public List<ReadSheet> sheetList() {
-        return sheetList;
+        return (this.sheetList != null ? Collections.unmodifiableList(this.sheetList) : Collections.emptyList());
     }
 
     private void parseXmlSource(InputStream inputStream, ContentHandler handler) {

easyexcel-core/src/main/java/com/alibaba/excel/metadata/format/DataFormatter.java

@@ -851,7 +851,7 @@ public class DataFormatter {
      * non-cachable wrapper around a
      * {@link CellFormatResult}
      */
-    private final class CellFormatResultWrapper extends Format {
+    private static final class CellFormatResultWrapper extends Format {
         private final CellFormatResult result;
 
         private CellFormatResultWrapper(CellFormatResult result) {