下载的PDF虽然包含一些数据但看起来是空的 [英] Downloaded PDF looks empty although it contains some data

问题描述

我正在尝试使用JavaScript实现PDF文件下载功能。

作为对 POST 请求的回复，我得到一个PDF文件，在Chrome DevTools控制台中它看起来像（ oResult 数据容器，片段）：

I'm trying to implement a PDF file download functionality with JavaScript.
As a response to a POST request I get a PDF file, in Chrome DevTools console it looks like (the oResult data container, fragment):

％PDF-1.4↵％ ↵40obj↵<>stream↵x

"%PDF-1.4↵%����↵4 0 obj↵<>stream↵x��

现在我我试图初始化下载过程：

Now I'm trying to initialize the download process:

let blob = new Blob([oResult], {type: "application/pdf"});

let link = document.createElement('a');

link.href = window.URL.createObjectURL(blob);
link.download = "tstPDF";

link.click();

结果，点击一个按钮我得到 tstPDF.pdf ，它包含正确的页数，但它自己的PDF是为空，没有显示内容，虽然它是6 KB。

As a result, upon a click on a button I get tstPDF.pdf, it contains the correct number of pages, but the PDF it self is empty, no content is displayed, although it is 6 KB.

当我测试Java服务器端模块时e，生成PDF，一切正常，它通过 ServletOutputStream 发送 InputStream 。因此我假设问题出在客户端的某个地方，可能是 MIME ， BLOB ，编码或类似的。

When I test the Java server-side module, which generates the PDF, everything is working fine, it sends InputStream via ServletOutputStream. Thus I assume that the issue is somewhere on a client side, perhaps something with MIME, BLOB, encoding, or similar.

您是否知道为什么生成的PDF不显示任何数据？

Do you have any idea why the generated PDF doesn't display any data?

推荐答案

我解决了这个问题。
问题在于数据从服务器传递到客户端的方式。
确保服务器以 Base64 编码发送数据至关重要，否则客户端无法将PDF字符串反序列化为二进制格式。下面，您可以找到完整的解决方案。

I solved the issue. The problem was in a way the data is delivered from the server to the client. It is critical to assure that the server sends the data in Base64 encoding, otherwise the client side can't deserialize the PDF string back to the binary format. Below, you can find the full solution.

服务器端：

OutputStream pdfStream = PDFGenerator.pdfGenerate(data);

String pdfFileName = "test_pdf";

// represent PDF as byteArray for further serialization
byte[] byteArray = ((java.io.ByteArrayOutputStream) pdfStream).toByteArray();

// serialize PDF to Base64
byte[] encodedBytes = java.util.Base64.getEncoder().encode(byteArray);

response.reset();
response.addHeader("Pragma", "public");
response.addHeader("Cache-Control", "max-age=0");
response.setHeader("Content-disposition", "attachment;filename=" + pdfFileName);
response.setContentType("application/pdf");

// avoid "byte shaving" by specifying precise length of transferred data
response.setContentLength(encodedBytes.length);

// send to output stream
ServletOutputStream servletOutputStream = response.getOutputStream();

servletOutputStream.write(encodedBytes);
servletOutputStream.flush();
servletOutputStream.close();

客户端：

let binaryString = window.atob(data);

let binaryLen = binaryString.length;

let bytes = new Uint8Array(binaryLen);

for (let i = 0; i < binaryLen; i++) {
    let ascii = binaryString.charCodeAt(i);
    bytes[i] = ascii;
}

let blob = new Blob([bytes], {type: "application/pdf"});

let link = document.createElement('a');

link.href = window.URL.createObjectURL(blob);
link.download = pdfFileName;

link.click();

参考主题：

• 如何将response.outputStream中生成的PDF转换为Base64编码

从JavaScript下载字节文件

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