Suppress unwanted output to System.err when pretty printing content

Previously, when the XML pretty printer attempted to pretty print
the content of a request or response it would output an error to
System.err if the content was not valid XML. While, benign, this
output was distracting.

This commit updates the XML pretty printer to suppress its output to
System.err by configuring it with an ErrorHandler and an ErrorListener
that swallow any errors of which they are notified.

Closes gh-153

spring-restdocs-core/src/test/java/org/springframework/restdocs/operation/preprocess/PrettyPrintingContentModifierTests.java

@@ -16,9 +16,12 @@
 
 package org.springframework.restdocs.operation.preprocess;
 
+import org.junit.Rule;
 import org.junit.Test;
+import org.springframework.restdocs.test.OutputCapture;
 
 import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.Matchers.isEmptyString;
 import static org.junit.Assert.assertThat;
 
 /**
@@ -29,6 +32,9 @@ import static org.junit.Assert.assertThat;
  */
 public class PrettyPrintingContentModifierTests {
 
+	@Rule
+	public OutputCapture outputCapture = new OutputCapture();
+
 	@Test
 	public void prettyPrintJson() throws Exception {
 		assertThat(new PrettyPrintingContentModifier().modifyContent(
@@ -56,7 +62,9 @@ public class PrettyPrintingContentModifierTests {
 	@Test
 	public void nonJsonAndNonXmlContentIsHandledGracefully() throws Exception {
 		String content = "abcdefg";
+		this.outputCapture.expect(isEmptyString());
 		assertThat(new PrettyPrintingContentModifier().modifyContent(content.getBytes(),
 				null), equalTo(content.getBytes()));
+
 	}
 }

spring-restdocs-core/src/test/java/org/springframework/restdocs/test/OutputCapture.java

@@ -0,0 +1,154 @@
+/*
+ * Copyright 2012-2015 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.
+ * 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.restdocs.test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.PrintStream;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.hamcrest.Matcher;
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+
+import static org.hamcrest.Matchers.allOf;
+import static org.junit.Assert.assertThat;
+
+/**
+ * JUnit {@code @Rule} to capture output from System.out and System.err.
+ *
+ * @author Phillip Webb
+ * @author Andy Wilkinson
+ */
+public class OutputCapture implements TestRule {
+
+	private CaptureOutputStream captureOut;
+
+	private CaptureOutputStream captureErr;
+
+	private ByteArrayOutputStream capturedOutput;
+
+	private List<Matcher<? super String>> matchers = new ArrayList<Matcher<? super String>>();
+
+	@Override
+	public Statement apply(final Statement base, Description description) {
+		return new Statement() {
+			@Override
+			public void evaluate() throws Throwable {
+				captureOutput();
+				try {
+					base.evaluate();
+				}
+				finally {
+					try {
+						if (!OutputCapture.this.matchers.isEmpty()) {
+							assertThat(getOutputAsString(),
+									allOf(OutputCapture.this.matchers));
+						}
+					}
+					finally {
+						releaseOutput();
+					}
+				}
+			}
+		};
+	}
+
+	private void captureOutput() {
+		this.capturedOutput = new ByteArrayOutputStream();
+		this.captureOut = new CaptureOutputStream(System.out, this.capturedOutput);
+		this.captureErr = new CaptureOutputStream(System.err, this.capturedOutput);
+		System.setOut(new PrintStream(this.captureOut));
+		System.setErr(new PrintStream(this.captureErr));
+	}
+
+	private String getOutputAsString() {
+		flush();
+		return this.capturedOutput.toString();
+	}
+
+	private void releaseOutput() {
+		System.setOut(this.captureOut.getOriginal());
+		System.setErr(this.captureErr.getOriginal());
+		this.capturedOutput = null;
+	}
+
+	private void flush() {
+		try {
+			this.captureOut.flush();
+			this.captureErr.flush();
+		}
+		catch (IOException ex) {
+			// ignore
+		}
+	}
+
+	/**
+	 * Verify that the output is matched by the supplied {@code matcher}. Verification is
+	 * performed after the test method has executed.
+	 *
+	 * @param matcher the matcher
+	 */
+	public final void expect(Matcher<? super String> matcher) {
+		this.matchers.add(matcher);
+	}
+
+	private static final class CaptureOutputStream extends OutputStream {
+
+		private final PrintStream original;
+
+		private final OutputStream copy;
+
+		private CaptureOutputStream(PrintStream original, OutputStream copy) {
+			this.original = original;
+			this.copy = copy;
+		}
+
+		@Override
+		public void write(int b) throws IOException {
+			this.copy.write(b);
+			this.original.write(b);
+			this.original.flush();
+		}
+
+		@Override
+		public void write(byte[] b) throws IOException {
+			write(b, 0, b.length);
+		}
+
+		@Override
+		public void write(byte[] b, int off, int len) throws IOException {
+			this.copy.write(b, off, len);
+			this.original.write(b, off, len);
+		}
+
+		private PrintStream getOriginal() {
+			return this.original;
+		}
+
+		@Override
+		public void flush() throws IOException {
+			this.copy.flush();
+			this.original.flush();
+		}
+
+	}
+
+}