如何将原始PDF文件字符串转Base64？解码后PDF无法生成求助

问题背景 #

尝试将从其他服务获取的原始PDF字符串转换为Base64，生成的Base64格式看似正确，但解码后无法生成可正常打开的PDF文件。已尝试UTF-8编码等多种转换方法，因无法获取实际PDF文件，只能处理该原始字符串。

原始代码 #

public String get() throws CustomException, IOException {

    String testPDf = "%PDF-1.4\n" +
        "%âãÏÓ\n" +
        "1 0 obj\n" +
        "<< /Type /Catalog\n" +
        "   /Pages 2 0 R\n" +
        ">>\n" +
        "endobj\n" +
        "2 0 obj\n" +
        "<< /Type /Pages\n" +
        "   /Kids [3 0 R]\n" +
        "   /Count 1\n" +
        ">>\n" +
        "endobj\n" +
        "3 0 obj\n" +
        "<< /Type /Page\n" +
        "   /Parent 2 0 R\n" +
        "   /Resources << /Font << /F1 << /Type /Font\n" +
        "                                /Subtype /Type1\n" +
        "                                /BaseFont /Helvetica\n" +
        "                             >> >>\n" +
        "   /MediaBox [0 0 612 792]\n" +
        "   /Contents 4 0 R\n" +
        ">>\n" +
        "endobj\n" +
        "4 0 obj\n" +
        "<< /Length 55 >>\n" +
        "stream\n" +
        "BT\n" +
        "/F1 18 Tf\n" +
        "100 100 Td\n" +
        "(Hello, World!) Tj\n" +
        "ET\n" +
        "endstream\n" +
        "endobj\n" +
        "xref\n" +
        "0 5\n" +
        "0000000000 65535 f\n" +
        "0000000018 00000 n\n" +
        "0000000077 00000 n\n" +
        "0000000175 00000 n\n" +
        "0000000451 00000 n\n" +
        "trailer\n" +
        "<< /Size 5\n" +
        "   /Root 1 0 R\n" +
        ">>\n" +
        "startxref\n" +
        "561\n" +
        "%%EOF";

    testPDf = testPDf.replaceAll("\n", "");

    testPDf = toBase64(testPDf);

    return testPDf;
}

public static String toBase64(String str) throws UnsupportedEncodingException {
    byte[] bytes = str.getBytes("UTF-8");
    String encoded = Base64.getEncoder().encodeToString(bytes);
    return encoded;
}

错误原因 #

1. 破坏PDF结构：代码中通过replaceAll("\n", "")删除了所有换行符，但PDF是结构化的二进制文件，其xref表、startxref的偏移量均基于原始字节流位置计算，去掉换行后会导致解析器无法识别文件结构。
2. 错误的编码方式：PDF并非纯UTF-8文本，包含二进制字节（比如文件头的%âãÏÓ是PDF的二进制标记），用UTF-8编码转换字符串会改变原始字节，导致数据损坏。

解决方法 #

1. 保留原始换行符：不要去除PDF字符串中的换行，维持文件的原始结构。
2. 使用单字节编码还原原始字节：采用ISO-8859-1编码（单字节编码，可无损保留所有原始字节）将字符串转换为字节数组，再进行Base64编码。

修正后的代码 #

public String get() throws CustomException, IOException {

    String testPDf = "%PDF-1.4\n" +
        "%âãÏÓ\n" +
        "1 0 obj\n" +
        "<< /Type /Catalog\n" +
        "   /Pages 2 0 R\n" +
        ">>\n" +
        "endobj\n" +
        "2 0 obj\n" +
        "<< /Type /Pages\n" +
        "   /Kids [3 0 R]\n" +
        "   /Count 1\n" +
        ">>\n" +
        "endobj\n" +
        "3 0 obj\n" +
        "<< /Type /Page\n" +
        "   /Parent 2 0 R\n" +
        "   /Resources << /Font << /F1 << /Type /Font\n" +
        "                                /Subtype /Type1\n" +
        "                                /BaseFont /Helvetica\n" +
        "                             >> >>\n" +
        "   /MediaBox [0 0 612 792]\n" +
        "   /Contents 4 0 R\n" +
        ">>\n" +
        "endobj\n" +
        "4 0 obj\n" +
        "<< /Length 55 >>\n" +
        "stream\n" +
        "BT\n" +
        "/F1 18 Tf\n" +
        "100 100 Td\n" +
        "(Hello, World!) Tj\n" +
        "ET\n" +
        "endstream\n" +
        "endobj\n" +
        "xref\n" +
        "0 5\n" +
        "0000000000 65535 f\n" +
        "0000000018 00000 n\n" +
        "0000000077 00000 n\n" +
        "0000000175 00000 n\n" +
        "0000000451 00000 n\n" +
        "trailer\n" +
        "<< /Size 5\n" +
        "   /Root 1 0 R\n" +
        ">>\n" +
        "startxref\n" +
        "561\n" +
        "%%EOF";

    // 移除错误的换行符删除操作
    testPDf = toBase64(testPDf);

    return testPDf;
}

public static String toBase64(String str) throws UnsupportedEncodingException {
    // 使用ISO-8859-1编码还原原始字节
    byte[] bytes = str.getBytes("ISO-8859-1");
    String encoded = Base64.getEncoder().encodeToString(bytes);
    return encoded;
}

内容的提问来源于stack exchange，提问作者BobbySin