easyexcel 合并单元格方法

使用 EasyExcel 合并单元格需要使用 `@ExcelIgnoreUnmergedRegions` 注解来忽略未合并的单元格，否则会抛出异常。然后使用 `merge()` 方法来合并单元格。

以下是一个示例代码：

@ExcelIgnoreUnmergedRegions
public class MergeCellDemo {

    public static void main(String[] args) {
        // 写法一：合并指定范围的单元格
        EasyExcel.write("merge-cell.xlsx")
                .sheet("Sheet1")
                .head(head())
                .registerWriteHandler(new MergeCellWriteHandler())
                .doWrite(data());

        // 写法二：使用注解合并单元格
        EasyExcel.write("merge-cell-annotation.xlsx")
                .sheet("Sheet1")
                .head(head())
                .doWrite(dataWithAnnotation());
    }

    private static List<List<String>> head() {
        List<List<String>> head = new ArrayList<>();
        List<String> row1 = new ArrayList<>();
        row1.add("姓名");
        row1.add("年龄");
        row1.add("性别");
        head.add(row1);
        return head;
    }

    private static List<List<Object>> data() {
        List<List<Object>> data = new ArrayList<>();
        List<Object> row1 = new ArrayList<>();
        row1.add("张三");
        row1.add(20);
        row1.add("男");
        data.add(row1);

        List<Object> row2 = new ArrayList<>();
        row2.add("李四");
        row2.add(22);
        row2.add("女");
        data.add(row2);

        List<Object> row3 = new ArrayList<>();
        row3.add("王五");
        row3.add(25);
        row3.add("男");
        data.add(row3);

        return data;
    }

    @MergeStrategy(name = "name", mergeType = MergeStrategy.MergeType.HORIZONTAL)
    @ExcelIgnoreUnmergedRegions
    private static List<MergeCellDemo> dataWithAnnotation() {
        return Arrays.asList(
                new MergeCellDemo("张三", 20, "男"),
                new MergeCellDemo("李四", 22, "女"),
                new MergeCellDemo("王五", 25, "男")
        );
    }

    private String name;
    private Integer age;
    private String gender;

    public MergeCellDemo(String name, Integer age, String gender) {
        this.name = name;
        this.age = age;
        this.gender = gender;
    }

    // 省略 getter 和 setter 方法
}

其中，`MergeCellWriteHandler` 是自定义的一个合并单元格处理器，代码如下：

public class MergeCellWriteHandler implements WriteHandler {

    @Override
    public void beforeSheetCreate(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder) {

    }

    @Override
    public void afterSheetCreate(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder) {

    }

    @Override
    public void afterWorkbookCreate(WriteWorkbookHolder writeWorkbookHolder) {

    }

    @Override
    public void beforeWrite(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder, WriteTableHolder writeTableHolder) {

    }

    @Override
    public void afterWrite(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder, WriteTableHolder writeTableHolder) {

    }

    @Override
    public void afterRowCreate(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder, WriteTableHolder writeTableHolder, Row row, Integer rowIndex, Integer relativeRowIndex, Boolean isHead) {
        if (isHead) {
            // 合并第一行的前三列
            CellRangeAddress region = new CellRangeAddress(0, 0, 0, 2);
            writeSheetHolder.getSheet().addMergedRegion(region);
        }
    }
}

注意，在写入 Excel 文件时，需要调用 `registerWriteHandler()` 方法注册合并单元格处理器。