POI-处理大Excel文件（xlsx）

上次简述了使用poi读取大xls文件，这里说下读取xlsx格式的文件的方法

环境模拟

先准备一个大的excel文件（xlsx大小5M），再将jvm的heap缩小到100m(JVM 参数 -Xmx100m)用于模拟OOM
并使用参数在OOM时dump内存 -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d://dump.hprof

使用XSSF读取

在gradle中引入解析xlsx须要的jar包

compile 'org.apache.poi:poi:3.15'
    compile 'org.apache.poi:poi-ooxml:3.15'
    compile 'xerces:xercesImpl:2.11.0'

以后读取xlsx文件

public static void main(String [] args) throws IOException {
        InputStream is = new FileInputStream("d://large.xlsx");
        Workbook wb = new XSSFWorkbook(is);
    }

运行以后

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:3236)
    at java.io.ByteArrayOutputStream.toByteArray(ByteArrayOutputStream.java:178)
    at org.apache.poi.openxml4j.util.ZipInputStreamZipEntrySource$FakeZipEntry.<init>(ZipInputStreamZipEntrySource.java:136)
    at org.apache.poi.openxml4j.util.ZipInputStreamZipEntrySource.<init>(ZipInputStreamZipEntrySource.java:56)
    at org.apache.poi.openxml4j.opc.ZipPackage.<init>(ZipPackage.java:99)
    at org.apache.poi.openxml4j.opc.OPCPackage.open(OPCPackage.java:342)
    at org.apache.poi.util.PackageHelper.open(PackageHelper.java:37)
    at org.apache.poi.xssf.usermodel.XSSFWorkbook.<init>(XSSFWorkbook.java:285)
    at blog.excel.Xlsx.main(Xlsx.java:17)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

一样报出了OOM，缘由也是处理xlsx时会将数据彻底读入内存，致使内存溢出。

使用EventApi流式读取

POI也为xlsx提供了流式读取的方式，用于减少内存的使用

public class ExampleEventUserModel{
    public void processOneSheet(String filename) throws Exception {
        OPCPackage pkg = OPCPackage.open(filename);
        XSSFReader r = new XSSFReader( pkg );
        SharedStringsTable sst = r.getSharedStringsTable();

        XMLReader parser = fetchSheetParser(sst);

        // 得到第一个sheet
        InputStream sheet2 = r.getSheet("rId1");
        InputSource sheetSource = new InputSource(sheet2);
        parser.parse(sheetSource);
        sheet2.close();
    }

    public XMLReader fetchSheetParser(SharedStringsTable sst) throws SAXException {
        XMLReader parser =
                XMLReaderFactory.createXMLReader(
                        "org.apache.xerces.parsers.SAXParser"
                );
        ContentHandler handler = new SheetHandler(sst);
        parser.setContentHandler(handler);
        return parser;
    }

    /**
     * 处理sax的handler
     */
    private static class SheetHandler extends DefaultHandler {
        private SharedStringsTable sst;
        private String lastContents;
        private boolean nextIsString;

        private SheetHandler(SharedStringsTable sst) {
            this.sst = sst;
        }

        //元素开始时的handler
        public void startElement(String uri, String localName, String name,
                                 Attributes attributes) throws SAXException {
            // c => 单元格
            if(name.equals("c")) {
                System.out.print(attributes.getValue("r") + " - ");
                // 获取单元格类型
                String cellType = attributes.getValue("t");
                if(cellType != null && cellType.equals("s")) {
                    nextIsString = true;
                } else {
                    nextIsString = false;
                }
            }
            lastContents = "";
        }

        //元素结束时的handler
        public void endElement(String uri, String localName, String name)
                throws SAXException {
            if(nextIsString) {
                int idx = Integer.parseInt(lastContents);
                lastContents = new XSSFRichTextString(sst.getEntryAt(idx)).toString();
                nextIsString = false;
            }

            // v => 单元格内容
            if(name.equals("v")) {
                System.out.println(lastContents);
            }
        }

        //读取元素间内容时的handler
        public void characters(char[] ch, int start, int length)
                throws SAXException {
            lastContents += new String(ch, start, length);
        }
    }

    public static void main(String[] args) throws Exception {
        ExampleEventUserModel example = new ExampleEventUserModel();
        example.processOneSheet("d://large.xlsx");
    }

}

不足

一样的使用这种方法能够流式读取打的xlsx文件，可是只限于读取内部的数据，并且没法进行修改操做。以后会介绍写大文件的方法