最近公司做了一个CMS项目，其中涉及到图片上传后页面展现，如果图片过大就会带来过多的带宽消耗，同时也会影响客户体验；
解决方案：利用浏览器缓存，HTTP - 304来解决；
import java.io.Closeable;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;
import java.io.RandomAccessFile;
import java.net.URLDecoder;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.zip.GZIPOutputStream;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@SuppressWarnings("serial")
public class FileServlet extends HttpServlet {
// Constants ----------------------------------------------------------------------------------
private static final int DEFAULT_BUFFER_SIZE = 10240; // ..bytes = 10KB.
private static final long DEFAULT_EXPIRE_TIME = 604800000L; // ..ms = 1 week.
private static final String MULTIPART_BOUNDARY = "MULTIPART_BYTERANGES";
// Properties ---------------------------------------------------------------------------------
// private String basePath;
// Actions ------------------------------------------------------------------------------------
/**
* Initialize the servlet.
* @see HttpServlet#init().
*/
public void init() throws ServletException {
/**
* 按照我们的业务，不需要这个init的自配置，直接去掉了
*/
// Get base path (path to get all resources from) as init parameter.
// this.basePath = getServletContext().getRealPath(getInitParameter("basePath"));
// Validate base path.
// if (this.basePath == null) {
// throw new ServletException("FileServlet init param 'basePath' is required.");
// } else {
// File path = new File(this.basePath);
// if (!path.exists()) {
// throw new ServletException("FileServlet init param 'basePath' value '"
// + this.basePath + "' does actually not exist in file system.");
// } else if (!path.isDirectory()) {
// throw new ServletException("FileServlet init param 'basePath' value '"
// + this.basePath + "' is actually not a directory in file system.");
// } else if (!path.canRead()) {
// throw new ServletException("FileServlet init param 'basePath' value '"
// + this.basePath + "' is actually not readable in file system.");
// }
// }
}
/**
* Process HEAD request. This returns the same headers as GET request, but without content.
* @see HttpServlet#doHead(HttpServletRequest, HttpServletResponse).
*/
protected void doHead(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
// Process request without content.
processRequest(request, response, false);
}
/**
* Process GET request.
* @see HttpServlet#doGet(HttpServletRequest, HttpServletResponse).
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
// Process request with content.
processRequest(request, response, true);
}
/**
* Process the actual request.
* @param request The request to be processed.
* @param response The response to be created.
* @param content Whether the request body should be written (GET) or not (HEAD).
* @throws IOException If something fails at I/O level.
*/
private void processRequest
(HttpServletRequest request, HttpServletResponse response, boolean content)
throws IOException
{
// Validate the requested file ------------------------------------------------------------
// Get requested file by path info.
String requestedFile = (String) request.getAttribute("filepath");
// Check if file is actually supplied to the request URL.
if (requestedFile == null) {
// Do your thing if the file is not supplied to the request URL.
// Throw an exception, or send 404, or show default/warning page, or just ignore it.
response.sendError(HttpServletResponse.SC_NOT_FOUND);
return;
}
// URL-decode the file name (might contain spaces and on) and prepare file object.
File file = new File(requestedFile);
// Check if file actually exists in filesystem.
if (!file.exists()) {
// Do your thing if the file appears to be non-existing.
// Throw an exception, or send 404, or show default/warning page, or just ignore it.
response.sendError(HttpServletResponse.SC_NOT_FOUND);
return;
}
// Prepare some variables. The ETag is an unique identifier of the file.
String fileName = file.getName();
long length = file.length();
long lastModified = file.lastModified();
String eTag = fileName + "_" + length + "_" + lastModified;
long expires = System.currentTimeMillis() + DEFAULT_EXPIRE_TIME;
// Validate request headers for caching ---------------------------------------------------
// If-None-Match header should contain "*" or ETag. If so, then return 304.
String ifNoneMatch = request.getHeader("If-None-Match");
if (ifNoneMatch != null && matches(ifNoneMatch, eTag)) {
response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
response.setHeader("ETag", eTag); // Required in 304.
response.setDateHeader("Expires", expires); // Postpone cache with 1 week.
return;
}
// If-Modified-Since header should be greater than LastModified. If so, then return 304.
// This header is ignored if any If-None-Match header is specified.
long ifModifiedSince = request.getDateHeader("If-Modified-Since");
if (ifNoneMatch == null && ifModifiedSince != -1 && ifModifiedSince + 1000 > lastModified) {
response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
response.setHeader("ETag", eTag); // Required in 304.
response.setDateHeader("Expires", expires); // Postpone cache with 1 week.
return;
}
// Validate request headers for resume ----------------------------------------------------
// If-Match header should contain "*" or ETag. If not, then return 412.
String ifMatch = request.getHeader("If-Match");
if (ifMatch != null && !matches(ifMatch, eTag)) {
response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
return;
}
// If-Unmodified-Since header should be greater than LastModified. If not, then return 412.
long ifUnmodifiedSince = request.getDateHeader("If-Unmodified-Since");
if (ifUnmodifiedSince != -1 && ifUnmodifiedSince + 1000 <= lastModified) {
response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
return;
}
// Validate and process range -------------------------------------------------------------
// Prepare some variables. The full Range represents the complete file.
Range full = new Range(0, length - 1, length);
List<Range> ranges = new ArrayList<Range>();
// Validate and process Range and If-Range headers.
String range = request.getHeader("Range");
if (range != null) {
// Range header should match format "bytes=n-n,n-n,n-n...". If not, then return 416.
if (!range.matches("^bytes=\\d*-\\d*(,\\d*-\\d*)*$")) {
response.setHeader("Content-Range", "bytes */" + length); // Required in 416.
response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
return;
}
// If-Range header should either match ETag or be greater then LastModified. If not,
// then return full file.
String ifRange = request.getHeader("If-Range");
if (ifRange != null && !ifRange.equals(eTag)) {
try {
long ifRangeTime = request.getDateHeader("If-Range"); // Throws IAE if invalid.
if (ifRangeTime != -1 && ifRangeTime + 1000 < lastModified) {
ranges.add(full);
}
} catch (IllegalArgumentException ignore) {
ranges.add(full);
}
}
// If any valid If-Range header, then process each part of byte range.
if (ranges.isEmpty()) {
for (String part : range.substring(6).split(",")) {
// Assuming a file with length of 100, the following examples returns bytes at:
// 50-80 (50 to 80), 40- (40 to length=100), -20 (length-20=80 to length=100).
long start = sublong(part, 0, part.indexOf("-"));
long end = sublong(part, part.indexOf("-") + 1, part.length());
if (start == -1) {
start = length - end;
end = length - 1;
} else if (end == -1 || end > length - 1) {
end = length - 1;
}
// Check if Range is syntactically valid. If not, then return 416.
if (start > end) {
response.setHeader("Content-Range", "bytes */" + length); // Required in 416.
response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
return;
}
// Add range.
ranges.add(new Range(start, end, length));
}
}
}
// Prepare and initialize response --------------------------------------------------------
// Get content type by file name and set default GZIP support and content disposition.
String contentType = getServletContext().getMimeType(fileName);
boolean acceptsGzip = false;
String disposition = "inline";
// If content type is unknown, then set the default value.
// For all content types, see: http://www.w3schools.com/media/media_mimeref.asp
// To add new content types, add new mime-mapping entry in web.xml.
if (contentType == null) {
contentType = "application/octet-stream";
}
// If content type is text, then determine whether GZIP content encoding is supported by
// the browser and expand content type with the one and right character encoding.
if (contentType.startsWith("text")) {
String acceptEncoding = request.getHeader("Accept-Encoding");
acceptsGzip = acceptEncoding != null && accepts(acceptEncoding, "gzip");
contentType += ";charset=UTF-8";
}
// Else, expect for images, determine content disposition. If content type is supported by
// the browser, then set to inline, else attachment which will pop a 'save as' dialogue.
else if (!contentType.startsWith("image")) {
String accept = request.getHeader("Accept");
disposition = accept != null && accepts(accept, contentType) ? "inline" : "attachment";
}
// Initialize response.
response.reset();
response.setBufferSize(DEFAULT_BUFFER_SIZE);
response.setHeader("Content-Disposition", disposition + ";filename=\"" + fileName + "\"");
response.setHeader("Accept-Ranges", "bytes");
response.setHeader("ETag", eTag);
response.setDateHeader("Last-Modified", lastModified);
response.setDateHeader("Expires", expires);
// Send requested file (part(s)) to client ------------------------------------------------
// Prepare streams.
RandomAccessFile input = null;
OutputStream output = null;
try {
// Open streams.
input = new RandomAccessFile(file, "r");
output = response.getOutputStream();
if (ranges.isEmpty() || ranges.get(0) == full) {
// Return full file.
Range r = full;
response.setContentType(contentType);
response.setHeader("Content-Range", "bytes " + r.start + "-" + r.end + "/" + r.total);
if (content) {
if (acceptsGzip) {
// The browser accepts GZIP, so GZIP the content.
response.setHeader("Content-Encoding", "gzip");
output = new GZIPOutputStream(output, DEFAULT_BUFFER_SIZE);
} else {
// Content length is not directly predictable in case of GZIP.
// So only add it if there is no means of GZIP, else browser will hang.
response.setHeader("Content-Length", String.valueOf(r.length));
}
// Copy full range.
copy(input, output, r.start, r.length);
}
} else if (ranges.size() == 1) {
// Return single part of file.
Range r = ranges.get(0);
response.setContentType(contentType);
response.setHeader("Content-Range", "bytes " + r.start + "-" + r.end + "/" + r.total);
response.setHeader("Content-Length", String.valueOf(r.length));
response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT); // 206.
if (content) {
// Copy single part range.
copy(input, output, r.start, r.length);
}
} else {
// Return multiple parts of file.
response.setContentType("multipart/byteranges; boundary=" + MULTIPART_BOUNDARY);
response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT); // 206.
if (content) {
// Cast back to ServletOutputStream to get the easy println methods.
ServletOutputStream sos = (ServletOutputStream) output;
// Copy multi part range.
for (Range r : ranges) {
// Add multipart boundary and header fields for every range.
sos.println();
sos.println("--" + MULTIPART_BOUNDARY);
sos.println("Content-Type: " + contentType);
sos.println("Content-Range: bytes " + r.start + "-" + r.end + "/" + r.total);
// Copy single part range of multi part range.
copy(input, output, r.start, r.length);
}
// End with multipart boundary.
sos.println();
sos.println("--" + MULTIPART_BOUNDARY + "--");
}
}
} finally {
// Gently close streams.
close(output);
close(input);
}
}
// Helpers (can be refactored to public utility class) ----------------------------------------
/**
* Returns true if the given accept header accepts the given value.
* @param acceptHeader The accept header.
* @param toAccept The value to be accepted.
* @return True if the given accept header accepts the given value.
*/
private static boolean accepts(String acceptHeader, String toAccept) {
String[] acceptValues = acceptHeader.split("\\s*(,|;)\\s*");
Arrays.sort(acceptValues);
return Arrays.binarySearch(acceptValues, toAccept) > -1
|| Arrays.binarySearch(acceptValues, toAccept.replaceAll("/.*$", "/*")) > -1
|| Arrays.binarySearch(acceptValues, "*/*") > -1;
}
/**
* Returns true if the given match header matches the given value.
* @param matchHeader The match header.
* @param toMatch The value to be matched.
* @return True if the given match header matches the given value.
*/
private static boolean matches(String matchHeader, String toMatch) {
String[] matchValues = matchHeader.split("\\s*,\\s*");
Arrays.sort(matchValues);
return Arrays.binarySearch(matchValues, toMatch) > -1
|| Arrays.binarySearch(matchValues, "*") > -1;
}
/**
* Returns a substring of the given string value from the given begin index to the given end
* index as a long. If the substring is empty, then -1 will be returned
* @param value The string value to return a substring as long for.
* @param beginIndex The begin index of the substring to be returned as long.
* @param endIndex The end index of the substring to be returned as long.
* @return A substring of the given string value as long or -1 if substring is empty.
*/
private static long sublong(String value, int beginIndex, int endIndex) {
String substring = value.substring(beginIndex, endIndex);
return (substring.length() > 0) ? Long.parseLong(substring) : -1;
}
/**
* Copy the given byte range of the given input to the given output.
* @param input The input to copy the given range to the given output for.
* @param output The output to copy the given range from the given input for.
* @param start Start of the byte range.
* @param length Length of the byte range.
* @throws IOException If something fails at I/O level.
*/
private static void copy(RandomAccessFile input, OutputStream output, long start, long length)
throws IOException
{
byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
int read;
if (input.length() == length) {
// Write full range.
while ((read = input.read(buffer)) > 0) {
output.write(buffer, 0, read);
}
} else {
// Write partial range.
input.seek(start);
long toRead = length;
while ((read = input.read(buffer)) > 0) {
if ((toRead -= read) > 0) {
output.write(buffer, 0, read);
} else {
output.write(buffer, 0, (int) toRead + read);
break;
}
}
}
}
/**
* Close the given resource.
* @param resource The resource to be closed.
*/
private static void close(Closeable resource) {
if (resource != null) {
try {
resource.close();
} catch (IOException ignore) {
// Ignore IOException. If you want to handle this anyway, it might be useful to know
// that this will generally only be thrown when the client aborted the request.
}
}
}
// Inner classes ------------------------------------------------------------------------------
/**
* This class represents a byte range.
*/
protected class Range {
long start;
long end;
long length;
long total;
/**
* Construct a byte range.
* @param start Start of the byte range.
* @param end End of the byte range.
* @param total Total length of the byte source.
*/
public Range(long start, long end, long total) {
this.start = start;
this.end = end;
this.length = end - start + 1;
this.total = total;
}
}
}