Write an InputStream to an HttpServletResponse

问题

I have an InputStream that I want written to a HttpServletResponse. There's this approach, which takes too long due to the use of byte[]

InputStream is = getInputStream();
int contentLength = getContentLength();

byte[] data = new byte[contentLength];
is.read(data);

//response here is the HttpServletResponse object
response.setContentLength(contentLength);
response.write(data);

I was wondering what could possibly be the best way to do it, in terms of speed and efficiency.

回答1:

Just write in blocks instead of copying it entirely into Java's memory first. The below basic example writes it in blocks of 10KB. This way you end up with a consistent memory usage of only 10KB instead of the complete content length. Also the enduser will start getting parts of the content much sooner.

response.setContentLength(getContentLength());
byte[] buffer = new byte[10240];

try (
    InputStream input = getInputStream();
    OutputStream output = response.getOutputStream();
) {
    for (int length = 0; (length = input.read(buffer)) > 0;) {
        output.write(buffer, 0, length);
    }
}

As creme de la creme with regard to performance, you could use NIO Channels and a directly allocated ByteBuffer. Create the following utility/helper method in some custom utility class, e.g. Utils:

public static long stream(InputStream input, OutputStream output) throws IOException {
    try (
        ReadableByteChannel inputChannel = Channels.newChannel(input);
        WritableByteChannel outputChannel = Channels.newChannel(output);
    ) {
        ByteBuffer buffer = ByteBuffer.allocateDirect(10240);
        long size = 0;

        while (inputChannel.read(buffer) != -1) {
            buffer.flip();
            size += outputChannel.write(buffer);
            buffer.clear();
        }

        return size;
    }
}

Which you then use as below:

response.setContentLength(getContentLength());
Utils.stream(getInputStream(), response.getOutputStream());

回答2:

BufferedInputStream in = null;
BufferedOutputStream out = null;
OutputStream os;
os = new BufferedOutputStream(response.getOutputStream());
in = new BufferedInputStream(new FileInputStream(file));
out = new BufferedOutputStream(os);
byte[] buffer = new byte[1024 * 8];
int j = -1;
while ((j = in.read(buffer)) != -1) {
    out.write(buffer, 0, j);
}

回答3:

I think that is very close to the best way, but I would suggest the following change. Use a fixed size buffer(Say 20K) and then do the read/write in a loop.

For the loop do something like

byte[] buffer=new byte[20*1024];
outputStream=response.getOutputStream();
while(true) {
  int readSize=is.read(buffer);
  if(readSize==-1)
    break;
  outputStream.write(buffer,0,readSize);
}

ps: Your program will not always work as is, because read don't always fill up the entire array you give it.

来源：https://stackoverflow.com/questions/10142409/write-an-inputstream-to-an-httpservletresponse

标签

java

java-ee

servlets

inputstream

outputstream