Apache POI is an open-source J**A library for working with files in Microsoft Office formats, including Excel, Word, and PowerPoint. It provides read and write operations to these files, allowing us to create, modify, and parse these files through j**a**.
The core classes of the POI library mainly include the following:
Workbook: Represents the entire workbook, i.e., an excel file. Workbooks can be created through the WorkbookFactory factory class.
createsheet(): Creates a new sheet.
getsheetat(): Gets a worksheet based on the index value.
getsheet(): Get a worksheet based on the name.
write(): Writes the workbook to an output stream, such as a file stream.
sheet: Represents a worksheet. A workbook can consist of multiple worksheets.
createrow(): Creates a new row.
getrow(): Gets a row based on the index value.
GetPhysicalNumberofRows(): Gets the number of rows that actually exist.
flushrows(): Flushes recently written rows to disk to free up memory.
row: indicates a row. A worksheet can consist of multiple rows.
createcell(): Creates a new cell.
getcell(): Gets a cell based on the index value.
getlastcellnum(): Gets the index value of the last cell in the row.
cell: represents a cell. Located at the intersection in the worksheet, the data is stored.
setCellValue(): Sets the value of the cell.
getcelltype(): Get the type of the cell.
getstringcellvalue(): Get the string value of the cell.
In addition to the above core classes, POI also provides some other helper classes for handling formats, styles, formulas, etc., such as cellstyle, dataformat, and formulaevaluator.
It is important to note that when dealing with large amounts of data, Poi provides two implementation classes: xssfworkbook and sxssfworkbook. The xssfworkbook class is suitable for small data files, while the sxssfworkbook class is suitable for large data files, which avoids memory overflow by using disks for temporary storage.
To avoid memory overflows, we can use batching, paging, or multithreading. Here's an example of using j**a** and a text description:
import org.apache.poi.ss.usermodel.*;import j**a.io.fileoutputstream;import j**a.io.ioexception;import j**a.util.list;public class excelexportexample rowcount++;batchcount++;When a certain number is accumulated, write to excel and clear the temporary data to avoid memory overflow if (batchcount >= batch size) try (fileoutputstream outputstream = new fileoutputstream(filepath)) workbookclose();Close the workbook } public static void main(string args) throws ioexception private static list generatedata(int count) }
In this example, we first create a new excel file and create a worksheet in it. We then iterate through the list of data and create a row based on each piece of data and insert the data into the appropriate cells. At the same time, we use:sxssfsheetto represent worksheets to support the processing of large amounts of data.
At each traversal to the set batch size, we flush the written data to an excel file and empty the temporary data to avoid memory overflow. Finally, after all the data is written, we write the excel file to disk and close the workbook.
Note that in the examplegeneratedata()The method is a pseudo-method, and you need to implement the logic of data generation according to your actual business needs. You can take data from a database, file, or other data source and convert it into an array of strings.
In this way, we can safely and efficiently export large amounts of data to excel files without memory overflows.