英文:
How to take user images, store them, and render them later?
问题
我正在尝试构建一个应用程序,该应用程序从HTML表单中获取用户图像,使用Java Servlet将该图像存储在数据库中,然后使用另一个HTML标签将图像呈现在屏幕上。用于接受图像的HTML表单如下:
<form action="../servlets/submitrequest" method="post" enctype="multipart/form-data">
<input type="number" name="cost" placeholder="Cost" required/><br/>
<p id="img">Image of receipt: <input type="file" name="image" required/><br/></p>
<input type="submit" id="submit" name="submit" value="Submit" onClick="success();"/><br/>
</form>
保存文件的Servlet的相关部分如下:
Random r = new Random();
String dir = "C://tmp/reciepts/" + r.nextInt() + ".png";
for (Part part : request.getParts()) {
if (part.getName().equalsIgnoreCase("image"))
part.write(dir);
}
然后,Servlet将dir
保存为数据库中的图像文件路径(PostgreSQL)。然而,当我尝试在Web浏览器上显示图像URL时,Chrome抛出以下错误:
Not allowed to load local resource: file:///C://tmp/images/1154006734.png
根据这里的解释,这是因为Chrome出于安全原因无法读取本地文件。因此,我开始寻找将图像保存到webcontent文件夹的方法,以便它成为Web应用程序内容的一部分。我遵循了这个示例,并将我的Servlet代码更改为:
String dir = request.getServletContext().getRealPath("/") + File.separator + "reciepts";
File fileSaveDir = new File(dir);
if (!fileSaveDir.exists()) {
fileSaveDir.mkdir();
}
request.getPart("image").write(dir);
即使在执行了这些更改后,我仍然收到相同的错误:
Not allowed to load local resource: file:///C:/Users/Colby/eclipse-workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/Project_1//reciepts/image.file
因此,我的问题是:我应该在哪里存储用户上传的文件,以便Web浏览器和应用程序本身都可以访问?
英文:
I'm trying to build an application that takes a user image from an html form, uses a java servlet to store said image in a database, and then renders the image onto the screen with another html tag. The HTML form to accept the image is:
<form action="../servlets/submitrequest" method="post" enctype="multipart/form-data">
<input type="number" name="cost" placeholder="Cost" required/><br/>
<p id="img">Image of receipt: <input type="file" name="image" required/><br/></p>
<input type="submit" id="submit" name="submit" value="Submit" onClick="success();"/><br/>
</form>
The relevent part of my servlet that saves the file is:
Random r = new Random();
String dir = "C://tmp/reciepts/" + r.nextInt() + ".png";
for(Part part: request.getParts()) {
if(part.getName().equalsIgnoreCase("image"))
part.write(dir);
}
The servlet then saves dir
as the image filepath in the database (postgresql). However when I tried to take the image URL and display it on the web browser Chrome throws the following error:
Not allowed to load local resource: file:///C://tmp/images/1154006734.png
Which according to <a href="https://stackoverflow.com/questions/39007243/cannot-open-local-file-chrome-not-allowed-to-load-local-resource">this</a> is caused because chrome can't read local files because of security reasons. Fair enough. So I started looking for ways to save it to the webcontent folder, so it would be a part of the content of the web application. I followed <a href="https://stackoverflow.com/questions/50345697/getting-error-while-uploading-image-with-form-elements">this</a> example and changed my servlet's code to:
String dir = request.getServletContext().getRealPath("/") + File.separator + "reciepts";
File fileSaveDir = new File(dir);
if (!fileSaveDir.exists()) {
fileSaveDir.mkdir();
}
request.getPart("image").write(dir);
Even after doing this I still received the same error:
Not allowed to load local resource: file:///C:/Users/Colby/eclipse-workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/Project_1//reciepts/image.file
So my question is this: where do I store files uploaded by the user where they can be accessed by both the web browser and the application itself?
答案1
得分: 0
你需要将servlet的doGet方法更改为以流的方式返回二进制数据。
response.setContentType("application/pdf");
try (
ServletOutputStream output = response.getOutputStream();
InputStream input = getServletContext().getResourceAsStream("myLocalFile.pdf");
){
// 将缓冲流传输到输出流
byte[] buffer = new byte[2048];
int bytesRead = -1;
while ((bytesRead = input.read(buffer)) != -1) {
output.write(buffer, 0, bytesRead);
}
}
然后在你的HTML中,不要使用<img src...>
指向一个文件,而是指向上述的servlet。
注意
当你的网络应用被远程用户访问时,他们无法直接了解或访问你的服务器文件系统。
英文:
You have to change the doGet of a servlet to stream back the binary data.
response.setContentType("application/pdf");
try (
ServletOutputStream output = response.getOutputStream();
InputStream input = getServletContext().getResourceAsStream("myLocalFile.pdf");
){
//transfer buffered stream to output stream
byte[] buffer = new byte[2048];
int bytesRead = -1;
while ((bytesRead = input.read(buffer)) != -1) {
output.write(buffer, 0, bytesRead);
}
}
Then in your HTML rather than having <img src...>
pointing to a file
have it pointing to the above servlet.
note
Of course when your web application is run from a remote user, then will have no knowledge or access to your server's file system directly.
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