ShallowEtagHeaderFilter use specified contentlength

Prior to this commit, the ShallowEtagHeaderFilter did not use the
content length given by the content generator to set the
ByteArrayOutputStream's buffer size.
This can lead to performance issues for large content since the buffer
grows as the content is being written.

This commit adds a new ByteArrayOutputStream variant called
ResizableByteArrayOutputStream. This implementation has public methods
for modifying the internal buffer size and does not synchronize on
buffer access.
This commit also make use of this new variant in
ShallowEtagHeaderFilter.

Issue: SPR-8271

spring-core/src/test/java/org/springframework/util/ResizableByteArrayOutputStreamTests.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2002-2014 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.util;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+/**
+ * @author Brian Clozel
+ */
+public class ResizableByteArrayOutputStreamTests {
+
+	private ResizableByteArrayOutputStream baos;
+
+	private String helloString;
+
+	private byte[] helloBytes;
+
+	private static final int INITIAL_SIZE = 32;
+
+	@Before
+	public void setUp() throws Exception {
+		this.baos = new ResizableByteArrayOutputStream(INITIAL_SIZE);
+		this.helloString = "Hello World";
+		this.helloBytes = helloString.getBytes("UTF-8");
+	}
+
+	@Test
+	public void resize() throws Exception {
+		assertEquals(INITIAL_SIZE, this.baos.buffer.length);
+		this.baos.write(helloBytes);
+		int size = 64;
+		this.baos.resize(size);
+		assertEquals(size, this.baos.buffer.length);
+		assertByteArrayEqualsString(helloString, this.baos);
+	}
+
+	@Test
+	public void autoGrow() {
+		assertEquals(INITIAL_SIZE, this.baos.buffer.length);
+		for(int i= 0; i < 33; i++) {
+			this.baos.write(0);
+		}
+		assertEquals(64, this.baos.buffer.length);
+	}
+
+	@Test
+	public void grow() throws Exception {
+		assertEquals(INITIAL_SIZE, this.baos.buffer.length);
+		this.baos.write(helloBytes);
+		this.baos.grow(100);
+		assertEquals(this.helloString.length() + 100, this.baos.buffer.length);
+		assertByteArrayEqualsString(helloString, this.baos);
+	}
+
+	@Test
+	public void write() throws Exception{
+		this.baos.write(helloBytes);
+		assertByteArrayEqualsString(helloString, this.baos);
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void failResize() throws Exception{
+		this.baos.write(helloBytes);
+		this.baos.resize(5);
+	}
+
+	private void assertByteArrayEqualsString(String expected, ResizableByteArrayOutputStream actual) {
+		String actualString = new String(actual.buffer, 0, actual.count());
+		assertEquals(expected, actualString);
+	}
+
+}