java - inputstream流读到最后为什么没有返回 -1 ?

InputStream fileSource = req.getInputStream();
String tempFile = "/home/joy/桌面/tempFile";
FileOutputStream tempFileOutputStream = new FileOutputStream(tempFile);
byte[] bytes = new byte[10 * 1024];
int i = 0;
while ((i = fileSource.read(bytes, 0, bytes.length)) != -1) {
    System.out.println("已读字节：" + i + ", 剩余字节: " + fileSource.available());
    System.out.println(new String(bytes));
    tempFileOutputStream.write(bytes);
    tempFileOutputStream.flush();
    if (fileSource.available() < bytes.length) {
        bytes = new byte[fileSource.available()];
    }
}
tempFileOutputStream.close();
fileSource.close();

为什么该流读完的时候并没有返回-1，而是返回0，导致无限循环.(上传文档类型文件不会，字节文件就会发生这种情况)