Strange byte[] behavior reading from a URL

问题

In the end, my ultimate goals are:

Read from a URL (what this question is about)
Save the retrieved [PDF] content to a BLOB field in a DB (already have that nailed down)
Read from the BLOB field and attach that content to an email
All without going to a filesystem

The goal with the following method is to get a byte[] that can be used downstream as an email attachment (to avoid writing to disk):

public byte[] retrievePDF() {

         HttpClient httpClient = new HttpClient();

         GetMethod httpGet = new GetMethod("http://website/document.pdf");
         httpClient.executeMethod(httpGet);
         InputStream is = httpGet.getResponseBodyAsStream();

         byte[] byteArray = new byte[(int) httpGet.getResponseContentLength()];

         is.read(byteArray, 0, byteArray.length);

        return byteArray;
}

For a particular PDF, the getResponseContentLength() method returns 101,689 as the length. The strange part is that if I set a break-point and interrogate the byteArray variable, it has 101,689 byte elements, however, after byte #3744 the remaining bytes of the array are all zeroes (0). The resulting PDF is then not readable by a PDF-reader client, like Adobe Reader.

Why would that happen?

Retrieving this same PDF via browser and saving to disk, or using a method like the following (which I patterned after an answer to this StackOverflow post), results in a readable PDF:

public void retrievePDF() {
    FileOutputStream fos = null;
    URL url;
    ReadableByteChannel rbc = null;

    url = new URL("http://website/document.pdf");

    DataSource urlDataSource = new URLDataSource(url);

    /* Open a connection, then set appropriate time-out values */
    URLConnection conn = url.openConnection();
    conn.setConnectTimeout(120000);
    conn.setReadTimeout(120000);

    rbc = Channels.newChannel(conn.getInputStream());

    String filePath = "C:\\temp\\";
    String fileName = "testing1234.pdf";
    String tempFileName = filePath + fileName;

    fos = new FileOutputStream(tempFileName);
    fos.getChannel().transferFrom(rbc, 0, 1 << 24);
    fos.flush();

    /* Clean-up everything */
    fos.close();
    rbc.close();
}

For both approaches, the size of the resulting PDF is 101,689-bytes when doing a Right-click > Properties... in Windows.

Why would the byte array essentially "stop" part-way through?

回答1:

InputStream.read reads up to byteArray.length bytes but might not read exactly that much. It returns how many bytes it read. You should call it repeatedly to fully read the data, like this:

int bytesRead = 0;
while (true) {
    int n = is.read(byteArray, bytesRead, byteArray.length);
    if (n == -1) break;
    bytesRead += n;
}

回答2:

Check the return value of InputStream.read. It's not going to read all at one go. You have to write a loop. Or, better yet, use Apache Commons IO to copy the stream.

回答3:

101689 = 2^16 + 36153 so it would look like, that there is a 16 bit limitation on buffer size. The difference between 36153 and 3744 maybe stems from the header part having been read in an extra small 1K buffer or so, and already containing some bytes.

来源：https://stackoverflow.com/questions/12717756/strange-byte-behavior-reading-from-a-url

标签

java

http

url

bytearray

channel