Allow OutputCaptureRule to be used with AssertJ

Update `OutputCaptureRule` so that it now implements
`CapturedOutput` and can be used directly with AssertJ.

Closes gh-17512

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/system/OutputCaptureRule.java

@@ -21,6 +21,7 @@ import java.util.List;
 
 import org.hamcrest.Matcher;
 import org.junit.Assert;
+import org.junit.Rule;
 import org.junit.rules.TestRule;
 import org.junit.runner.Description;
 import org.junit.runners.model.Statement;
@@ -29,12 +30,28 @@ import static org.hamcrest.Matchers.allOf;
 
 /**
  * JUnit {@code @Rule} to capture output from System.out and System.err.
+ * <p>
+ * To use add as a {@link Rule @Rule}:
+ *
+ * <pre class="code">
+ * public class MyTest {
+ *
+ *     &#064;Rule
+ *     public OutputCaptureRule output = new OutputCaptureRule();
+ *
+ *     &#064;Test
+ *     public void test() {
+ *         assertThat(output).contains("ok");
+ *     }
+ *
+ * }
+ * </pre>
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @since 2.2.0
  */
-public class OutputCaptureRule implements TestRule {
+public class OutputCaptureRule implements TestRule, CapturedOutput {
 
 	private final OutputCapture delegate = new OutputCapture();
 
@@ -73,6 +90,21 @@ public class OutputCaptureRule implements TestRule {
 		OutputCaptureRule.this.delegate.reset();
 	}
 
+	@Override
+	public String getAll() {
+		return this.delegate.getAll();
+	}
+
+	@Override
+	public String getOut() {
+		return this.delegate.getOut();
+	}
+
+	@Override
+	public String getErr() {
+		return this.delegate.getErr();
+	}
+
 	@Override
 	public String toString() {
 		return this.delegate.toString();

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/system/OutputCaptureRuleTests.java

@@ -29,12 +29,18 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class OutputCaptureRuleTests {
 
 	@Rule
-	public OutputCaptureRule outputCapture = new OutputCaptureRule();
+	public OutputCaptureRule output = new OutputCaptureRule();
 
 	@Test
 	public void toStringShouldReturnAllCapturedOutput() {
 		System.out.println("Hello World");
-		assertThat(this.outputCapture.toString()).contains("Hello World");
+		assertThat(this.output.toString()).contains("Hello World");
+	}
+
+	@Test
+	public void captureShouldBeAssertable() {
+		System.out.println("Hello World");
+		assertThat(this.output).contains("Hello World");
 	}
 
 }

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/system/OutputCaptureRule.java

@@ -34,7 +34,7 @@ import static org.hamcrest.Matchers.allOf;
  * @author Andy Wilkinson
  * @since 2.2.0
  */
-public class OutputCaptureRule implements TestRule {
+public class OutputCaptureRule implements TestRule, CapturedOutput {
 
 	private final OutputCapture delegate = new OutputCapture();
 
@@ -64,6 +64,21 @@ public class OutputCaptureRule implements TestRule {
 		};
 	}
 
+	@Override
+	public String getAll() {
+		return this.delegate.getAll();
+	}
+
+	@Override
+	public String getOut() {
+		return this.delegate.getOut();
+	}
+
+	@Override
+	public String getErr() {
+		return this.delegate.getErr();
+	}
+
 	@Override
 	public String toString() {
 		return this.delegate.toString();