Split up JUnit 5 OutputCapture class

Split the JUnit 5 `OutputCapture` class into separate `OutputExtension`
and `CapturedOutput` classes. The JUnit 5 callback methods are now
contained only in the `OutputExtension` class so no longer pollute the
public API that users will interact with.

The `CapturedOutput` class has also been updated to capture System.err
and System.out separately to allow distinct assertions if required.

Closes gh-17029

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/extension/CapturedOutputTests.java

@@ -0,0 +1,180 @@
+/*
+ * Copyright 2012-2019 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
+ *
+ *      https://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.boot.test.extension;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.util.NoSuchElementException;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Tests for {@link CapturedOutput}.
+ *
+ * @author Phillip Webb
+ */
+class CapturedOutputTests {
+
+	private PrintStream originalOut;
+
+	private PrintStream originalErr;
+
+	private TestPrintStream systemOut;
+
+	private TestPrintStream systemErr;
+
+	private CapturedOutput output = new CapturedOutput();
+
+	@BeforeEach
+	void replaceSystemStreams() {
+		this.originalOut = System.out;
+		this.originalErr = System.err;
+		this.systemOut = new TestPrintStream();
+		this.systemErr = new TestPrintStream();
+		System.setOut(this.systemOut);
+		System.setErr(this.systemErr);
+	}
+
+	@AfterEach
+	void restoreSystemStreams() {
+		System.setOut(this.originalOut);
+		System.setErr(this.originalErr);
+	}
+
+	@Test
+	void pushWhenEmptyStartsCapture() {
+		System.out.print("A");
+		this.output.push();
+		System.out.print("B");
+		assertThat(this.output).isEqualTo("B");
+	}
+
+	@Test
+	void pushWhenHasExistingStartesNewCapture() {
+		System.out.print("A");
+		this.output.push();
+		System.out.print("B");
+		this.output.push();
+		System.out.print("C");
+		assertThat(this.output).isEqualTo("BC");
+	}
+
+	@Test
+	void popWhenEmptyThrowsException() {
+		assertThatExceptionOfType(NoSuchElementException.class)
+				.isThrownBy(this.output::pop);
+	}
+
+	@Test
+	void popWhenHasExistingEndsCapture() {
+		this.output.push();
+		System.out.print("A");
+		this.output.pop();
+		System.out.print("B");
+		assertThat(this.systemOut.toString()).isEqualTo("AB");
+	}
+
+	@Test
+	void captureAlsoWritesToSystemOut() {
+		this.output.push();
+		System.out.print("A");
+		assertThat(this.systemOut.toString()).isEqualTo("A");
+	}
+
+	@Test
+	void captureAlsoWritesToSystemErr() {
+		this.output.push();
+		System.err.print("A");
+		assertThat(this.systemErr.toString()).isEqualTo("A");
+	}
+
+	@Test
+	void lengthReturnsCapturedLength() {
+		this.output.push();
+		System.out.print("ABC");
+		assertThat(this.output.length()).isEqualTo(3);
+	}
+
+	@Test
+	void charAtReturnsCapturedCharAt() {
+		this.output.push();
+		System.out.print("ABC");
+		assertThat(this.output.charAt(1)).isEqualTo('B');
+	}
+
+	@Test
+	void subSequenceReturnsCapturedSubSequence() {
+		this.output.push();
+		System.out.print("ABC");
+		assertThat(this.output.subSequence(1, 3)).isEqualTo("BC");
+	}
+
+	@Test
+	void getAllReturnsAllCapturedOutput() {
+		this.output.push();
+		System.out.print("A");
+		System.err.print("B");
+		System.out.print("C");
+		assertThat(this.output.getAll()).isEqualTo("ABC");
+	}
+
+	@Test
+	void toStringReturnsAllCapturedOutput() {
+		this.output.push();
+		System.out.print("A");
+		System.err.print("B");
+		System.out.print("C");
+		assertThat(this.output.toString()).isEqualTo("ABC");
+	}
+
+	@Test
+	void getErrReturnsOnlyCapturedErrOutput() {
+		this.output.push();
+		System.out.print("A");
+		System.err.print("B");
+		System.out.print("C");
+		assertThat(this.output.getErr()).isEqualTo("B");
+	}
+
+	@Test
+	void getOutReturnsOnlyCapturedOutOutput() {
+		this.output.push();
+		System.out.print("A");
+		System.err.print("B");
+		System.out.print("C");
+		assertThat(this.output.getOut()).isEqualTo("AC");
+	}
+
+	private static class TestPrintStream extends PrintStream {
+
+		TestPrintStream() {
+			super(new ByteArrayOutputStream());
+		}
+
+		@Override
+		public String toString() {
+			return this.out.toString();
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/extension/OutputExtensionExtendWithTests.java

@@ -23,21 +23,21 @@ import org.junit.jupiter.api.extension.ExtensionContext;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
- * Tests for {@link OutputCapture} when used via {@link ExtendWith @ExtendWith}.
+ * Tests for {@link OutputExtension} when used via {@link ExtendWith @ExtendWith}.
  *
  * @author Madhura Bhave
  */
-@ExtendWith(OutputCapture.class)
-@ExtendWith(OutputCaptureExtendWithTests.BeforeAllExtension.class)
-class OutputCaptureExtendWithTests {
+@ExtendWith(OutputExtension.class)
+@ExtendWith(OutputExtensionExtendWithTests.BeforeAllExtension.class)
+class OutputExtensionExtendWithTests {
 
 	@Test
-	void captureShouldReturnOutputCapturedBeforeTestMethod(OutputCapture output) {
+	void captureShouldReturnOutputCapturedBeforeTestMethod(CapturedOutput output) {
 		assertThat(output).contains("Before all").doesNotContain("Hello");
 	}
 
 	@Test
-	void captureShouldReturnAllCapturedOutput(OutputCapture output) {
+	void captureShouldReturnAllCapturedOutput(CapturedOutput output) {
 		System.out.println("Hello World");
 		System.err.println("Error!!!");
 		assertThat(output).contains("Before all").contains("Hello World")

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/extension/OutputExtensionRegisterExtensionTests.java

@@ -21,15 +21,15 @@ import org.junit.jupiter.api.extension.RegisterExtension;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
- * Tests for {@link OutputCapture} when used via
+ * Tests for {@link OutputExtension} when used via
  * {@link RegisterExtension @RegisterExtension}.
  *
  * @author Madhura Bhave
  */
-class OutputCaptureRegisterExtensionTests {
+class OutputExtensionRegisterExtensionTests {
 
 	@RegisterExtension
-	OutputCapture output = new OutputCapture();
+	CapturedOutput output = OutputExtension.capture();
 
 	@Test
 	void captureShouldReturnAllCapturedOutput() {