Performance improvements in ShallowEtagHeaderFilter

Prior to this change, the ShallowEtagHeaderFilter would use a
ResizableByteArrayOutputStream to internally write data and calculate
the ETag. While that implementation is faster than the regular
ByteArrayOutputStream (since it has a better strategy for growing the
internal buffer), a lot of buffer copying/writing still happens.

This change adds a new FastByteArrayOutputStream implementation that
internally uses a LinkedList<Byte[]> to store the content. So when
writing bytes to that OutputStream implementation, new byte[] are
added to the list when the previous ones are full. This saves most
of the instantiating/copying operations.

Note that new methods were added in DigestUtils to allow usage of
Streams instead of byte[], which is more efficient in our case.

Fixes #653

Issue: SPR-12081

spring-web/src/main/java/org/springframework/web/filter/ShallowEtagHeaderFilter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.web.filter;
 
 import java.io.IOException;
+import java.io.InputStream;
 import javax.servlet.FilterChain;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
@@ -26,7 +27,6 @@ import org.springframework.http.HttpMethod;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.DigestUtils;
-import org.springframework.util.StreamUtils;
 import org.springframework.web.util.ContentCachingResponseWrapper;
 import org.springframework.web.util.WebUtils;
 
@@ -93,15 +93,12 @@ public class ShallowEtagHeaderFilter extends OncePerRequestFilter {
 
 		HttpServletResponse rawResponse = (HttpServletResponse) responseWrapper.getResponse();
 		int statusCode = responseWrapper.getStatusCode();
-		byte[] body = responseWrapper.getContentAsByteArray();
 
 		if (rawResponse.isCommitted()) {
-			if (body.length > 0) {
-				StreamUtils.copy(body, rawResponse.getOutputStream());
-			}
+			responseWrapper.copyBodyToResponse();
 		}
-		else if (isEligibleForEtag(request, responseWrapper, statusCode, body)) {
-			String responseETag = generateETagHeaderValue(body);
+		else if (isEligibleForEtag(request, responseWrapper, statusCode, responseWrapper.getContentInputStream())) {
+			String responseETag = generateETagHeaderValue(responseWrapper.getContentInputStream());
 			rawResponse.setHeader(HEADER_ETAG, responseETag);
 			String requestETag = request.getHeader(HEADER_IF_NONE_MATCH);
 			if (responseETag.equals(requestETag)) {
@@ -115,20 +112,14 @@ public class ShallowEtagHeaderFilter extends OncePerRequestFilter {
 					logger.trace("ETag [" + responseETag + "] not equal to If-None-Match [" + requestETag +
 							"], sending normal response");
 				}
-				if (body.length > 0) {
-					rawResponse.setContentLength(body.length);
-					StreamUtils.copy(body, rawResponse.getOutputStream());
-				}
+				responseWrapper.copyBodyToResponse();
 			}
 		}
 		else {
 			if (logger.isTraceEnabled()) {
 				logger.trace("Response with status code [" + statusCode + "] not eligible for ETag");
 			}
-			if (body.length > 0) {
-				rawResponse.setContentLength(body.length);
-				StreamUtils.copy(body, rawResponse.getOutputStream());
-			}
+			responseWrapper.copyBodyToResponse();
 		}
 	}
 
@@ -143,11 +134,11 @@ public class ShallowEtagHeaderFilter extends OncePerRequestFilter {
 	 * @param request the HTTP request
 	 * @param response the HTTP response
 	 * @param responseStatusCode the HTTP response status code
-	 * @param responseBody the response body
+	 * @param inputStream the response body
 	 * @return {@code true} if eligible for ETag generation; {@code false} otherwise
 	 */
 	protected boolean isEligibleForEtag(HttpServletRequest request, HttpServletResponse response,
-			int responseStatusCode, byte[] responseBody) {
+			int responseStatusCode, InputStream inputStream) {
 
 		if (responseStatusCode >= 200 && responseStatusCode < 300 &&
 				HttpMethod.GET.name().equals(request.getMethod())) {
@@ -162,13 +153,18 @@ public class ShallowEtagHeaderFilter extends OncePerRequestFilter {
 	/**
 	 * Generate the ETag header value from the given response body byte array.
 	 * <p>The default implementation generates an MD5 hash.
-	 * @param bytes the response body as byte array
+	 * @param inputStream the response body as an InputStream
 	 * @return the ETag header value
 	 * @see org.springframework.util.DigestUtils
 	 */
-	protected String generateETagHeaderValue(byte[] bytes) {
+	protected String generateETagHeaderValue(InputStream inputStream) {
 		StringBuilder builder = new StringBuilder("\"0");
-		DigestUtils.appendMd5DigestAsHex(bytes, builder);
+		try {
+			DigestUtils.appendMd5DigestAsHex(inputStream, builder);
+		}
+		catch (IOException e) {
+			throw new RuntimeException(e);
+		}
 		builder.append('"');
 		return builder.toString();
 	}

spring-web/src/main/java/org/springframework/web/util/ContentCachingResponseWrapper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.web.util;
 
 import java.io.IOException;
+import java.io.InputStream;
 import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.io.UnsupportedEncodingException;
@@ -24,8 +25,7 @@ import javax.servlet.ServletOutputStream;
 import javax.servlet.http.HttpServletResponse;
 import javax.servlet.http.HttpServletResponseWrapper;
 
-import org.springframework.util.ResizableByteArrayOutputStream;
-import org.springframework.util.StreamUtils;
+import org.springframework.util.FastByteArrayOutputStream;
 
 /**
  * {@link javax.servlet.http-HttpServletResponse} wrapper that caches all content written to
@@ -39,7 +39,7 @@ import org.springframework.util.StreamUtils;
  */
 public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {
 
-	private final ResizableByteArrayOutputStream content = new ResizableByteArrayOutputStream(1024);
+	private final FastByteArrayOutputStream content = new FastByteArrayOutputStream(1024);
 
 	private final ServletOutputStream outputStream = new ResponseServletOutputStream();
 
@@ -107,9 +107,7 @@ public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {
 
 	@Override
 	public void setContentLength(int len) {
-		if (len > this.content.capacity()) {
-			this.content.resize(len);
-		}
+		this.content.resize(len);
 	}
 
 	// Overrides Servlet 3.1 setContentLengthLong(long) at runtime
@@ -118,16 +116,12 @@ public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {
 			throw new IllegalArgumentException("Content-Length exceeds ShallowEtagHeaderFilter's maximum (" +
 					Integer.MAX_VALUE + "): " + len);
 		}
-		if (len > this.content.capacity()) {
-			this.content.resize((int) len);
-		}
+		this.content.resize((int) len);
 	}
 
 	@Override
 	public void setBufferSize(int size) {
-		if (size > this.content.capacity()) {
-			this.content.resize(size);
-		}
+		this.content.resize(size);
 	}
 
 	@Override
@@ -142,7 +136,7 @@ public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {
 	}
 
 	/**
-	 * Return the status code as specifed on the response.
+	 * Return the status code as specified on the response.
 	 */
 	public int getStatusCode() {
 		return this.statusCode;
@@ -155,14 +149,24 @@ public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {
 		return this.content.toByteArray();
 	}
 
-	private void copyBodyToResponse() throws IOException {
+	public void copyBodyToResponse() throws IOException {
 		if (this.content.size() > 0) {
-			getResponse().setContentLength(this.content.size());
-			StreamUtils.copy(this.content.toByteArray(), getResponse().getOutputStream());
+			HttpServletResponse rawResponse = (HttpServletResponse) getResponse();
+			if(! rawResponse.isCommitted()){
+				rawResponse.setContentLength(this.content.size());
+			}
+			this.content.writeTo(rawResponse.getOutputStream());
 			this.content.reset();
 		}
 	}
 
+	public int getContentSize(){
+		return this.content.size();
+	}
+
+	public InputStream getContentInputStream(){
+		return this.content.getInputStream();
+	}
 
 	private class ResponseServletOutputStream extends ServletOutputStream {