POI-處理大Excel文件（xlsx）

上次簡述了使用poi讀取大xls文件，這裏說下讀取xlsx格式的文件的方法

環境模擬

先準備一個大的excel文件（xlsx大小5M），再將jvm的heap縮小到100m(JVM 參數 -Xmx100m)用於模擬OOM
並使用參數在OOM時dump內存 -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d://dump.hprof

使用XSSF讀取

在gradle中引入解析xlsx須要的jar包

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

以後讀取xlsx文件

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

運行以後

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)

一樣報出了OOM，緣由也是處理xlsx時會將數據徹底讀入內存，致使內存溢出。

使用EventApi流式讀取

POI也爲xlsx提供了流式讀取的方式，用於減少內存的使用

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);

        // 得到第一個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;
    }

    /**
     * 處理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;
        }

        //元素開始時的handler
        public void startElement(String uri, String localName, String name,
                                 Attributes attributes) throws SAXException {
            // c => 單元格
            if(name.equals("c")) {
                System.out.print(attributes.getValue("r") + " - ");
                // 獲取單元格類型
                String cellType = attributes.getValue("t");
                if(cellType != null && cellType.equals("s")) {
                    nextIsString = true;
                } else {
                    nextIsString = false;
                }
            }
            lastContents = "";
        }

        //元素結束時的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 => 單元格內容
            if(name.equals("v")) {
                System.out.println(lastContents);
            }
        }

        //讀取元素間內容時的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

一樣的使用這種方法能夠流式讀取打的xlsx文件，可是隻限於讀取內部的數據，並且沒法進行修改操做。以後會介紹寫大文件的方法