Bagaimana Mengendalikan Markah Pesanan Byte (BOM) Semasa Membaca Fail CSV di Java?

Tanda pesanan bait menyebabkan isu dengan pembacaan fail CSV dalam Java

Tanda pesanan bait (BOM) boleh hadir pada permulaan beberapa CSV fail, tetapi tidak semuanya. Apabila hadir, BOM dibaca bersama baris pertama fail, menyebabkan isu semasa membandingkan rentetan.

Begini cara untuk menangani masalah ini:

Penyelesaian:

Laksanakan kelas pembalut, UnicodeBOMInputStream, yang mengesan kehadiran BOM Unicode pada permulaan input aliran. Jika BOM dikesan, kaedah skipBOM() boleh digunakan untuk mengalih keluarnya.

Berikut ialah contoh kelas UnicodeBOMInputStream:

import java.io.IOException;
import java.io.InputStream;
import java.io.PushbackInputStream;

public class UnicodeBOMInputStream extends InputStream {

    private PushbackInputStream in;
    private BOM bom;
    private boolean skipped = false;

    public UnicodeBOMInputStream(InputStream inputStream) throws IOException {
        if (inputStream == null)
            throw new NullPointerException("Invalid input stream: null is not allowed");

        in = new PushbackInputStream(inputStream, 4);

        byte[] bom = new byte[4];
        int read = in.read(bom);

        switch (read) {
            case 4:
                if ((bom[0] == (byte) 0xFF) &&
                        (bom[1] == (byte) 0xFE) &&
                        (bom[2] == (byte) 0x00) &&
                        (bom[3] == (byte) 0x00)) {
                    this.bom = BOM.UTF_32_LE;
                    break;
                } else if ((bom[0] == (byte) 0x00) &&
                        (bom[1] == (byte) 0x00) &&
                        (bom[2] == (byte) 0xFE) &&
                        (bom[3] == (byte) 0xFF)) {
                    this.bom = BOM.UTF_32_BE;
                    break;
                }
            case 3:
                if ((bom[0] == (byte) 0xEF) &&
                        (bom[1] == (byte) 0xBB) &&
                        (bom[2] == (byte) 0xBF)) {
                    this.bom = BOM.UTF_8;
                    break;
                }
            case 2:
                if ((bom[0] == (byte) 0xFF) &&
                        (bom[1] == (byte) 0xFE)) {
                    this.bom = BOM.UTF_16_LE;
                    break;
                } else if ((bom[0] == (byte) 0xFE) &&
                        (bom[1] == (byte) 0xFF)) {
                    this.bom = BOM.UTF_16_BE;
                    break;
                }
            default:
                this.bom = BOM.NONE;
                break;
        }

        if (read > 0)
            in.unread(bom, 0, read);
    }

    public BOM getBOM() {
        return bom;
    }

    public UnicodeBOMInputStream skipBOM() throws IOException {
        if (!skipped) {
            in.skip(bom.bytes.length);
            skipped = true;
        }
        return this;
    }

    @Override
    public int read() throws IOException {
        return in.read();
    }

    @Override
    public int read(byte[] b) throws IOException {
        return in.read(b, 0, b.length);
    }

    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        return in.read(b, off, len);
    }

    @Override
    public long skip(long n) throws IOException {
        return in.skip(n);
    }

    @Override
    public int available() throws IOException {
        return in.available();
    }

    @Override
    public void close() throws IOException {
        in.close();
    }

    @Override
    public synchronized void mark(int readlimit) {
        in.mark(readlimit);
    }

    @Override
    public synchronized void reset() throws IOException {
        in.reset();
    }

    @Override
    public boolean markSupported() {
        return in.markSupported();
    }

    private enum BOM {
        NONE, UTF_8, UTF_16_LE, UTF_16_BE, UTF_32_LE, UTF_32_BE
    }
}

Penggunaan:

Gunakan pembungkus UnicodeBOMInputStream sebagai berikut:

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;

public class CSVReaderWithBOM {

    public static void main(String[] args) throws Exception {
        FileInputStream fis = new FileInputStream("test.csv");
        UnicodeBOMInputStream ubis = new UnicodeBOMInputStream(fis);

        System.out.println("Detected BOM: " + ubis.getBOM());

        System.out.print("Reading the content of the file without skipping the BOM: ");
        InputStreamReader isr = new InputStreamReader(ubis);
        BufferedReader br = new BufferedReader(isr);

        System.out.println(br.readLine());

        br.close();
        isr.close();
        ubis.close();
        fis.close();

        fis = new FileInputStream("test.csv");
        ubis = new UnicodeBOMInputStream(fis);
        isr = new InputStreamReader(ubis);
        br = new BufferedReader(isr);

        ubis.skipBOM();

        System.out.print("Reading the content of the file after skipping the BOM: ");
        System.out.println(br.readLine());

        br.close();
        isr.close();
        ubis.close();
        fis.close();
    }
}

Pendekatan ini membolehkan anda membaca fail CSV dengan atau tanpa BOM dan mengelakkan isu perbandingan rentetan yang disebabkan oleh BOM yang terdapat dalam baris pertama fail.

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