Polish OutputCapture and its JUnit Jupiter extension

- Polish Javadoc
- Improve error message in OutputCapture
- Use ExtensionContext.Store in OutputCaptureExtension

See gh-17049

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

@@ -23,7 +23,7 @@ package org.springframework.boot.test.system;
  * standard JUnit assertions. For example: <pre class="code">
  * assertThat(output).contains("started"); // Checks all output
  * assertThat(output.getErr()).contains("failed"); // Only checks System.err
- * assertThat(output.getOut()).contains("ok"); // Only checks System.put
+ * assertThat(output.getOut()).contains("ok"); // Only checks System.out
  * </pre>
  *
  * @author Madhura Bhave

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

@@ -38,6 +38,8 @@ import org.springframework.util.ClassUtils;
  * @author Madhura Bhave
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Sam Brannen
+ * @since 2.2.0
  * @see OutputCaptureExtension
  * @see OutputCaptureRule
  */
@@ -125,7 +127,12 @@ class OutputCapture implements CapturedOutput {
 	}
 
 	private String get(Predicate<Type> filter) {
-		Assert.state(!this.systemCaptures.isEmpty(), "No system captures found. Check that you have used @ExtendWith.");
+		Assert.state(!this.systemCaptures.isEmpty(),
+				"No system captures found. When using JUnit 4, ensure that you have "
+						+ "registered the OutputCaptureRule via @ClassRule or @Rule "
+						+ "and that the field is public. "
+						+ "When using JUnit Jupiter, ensure that you have registered "
+						+ "the OutputCaptureExtension via @ExtendWith.");
 		StringBuilder builder = new StringBuilder();
 		for (SystemCapture systemCapture : this.systemCaptures) {
 			systemCapture.append(builder, filter);

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

@@ -22,19 +22,21 @@ import org.junit.jupiter.api.extension.BeforeAllCallback;
 import org.junit.jupiter.api.extension.BeforeEachCallback;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ExtensionContext.Namespace;
+import org.junit.jupiter.api.extension.ExtensionContext.Store;
 import org.junit.jupiter.api.extension.ParameterContext;
 import org.junit.jupiter.api.extension.ParameterResolutionException;
 import org.junit.jupiter.api.extension.ParameterResolver;
 
 /**
- * JUnit 5 {@code @Extension} to capture {@link System#out System.out} and
- * {@link System#err System.err}. Can be used on a test class via
- * {@link ExtendWith @ExtendWith}. This extension provides {@link ParameterResolver
- * parameter resolution} for a {@link CapturedOutput} instance which can be used to assert
- * that the correct output was written.
+ * JUnit Jupiter {@code @Extension} to capture {@link System#out System.out} and
+ * {@link System#err System.err}. Can be registered for an entire test class or for an
+ * individual test method via {@link ExtendWith @ExtendWith}. This extension provides
+ * {@linkplain ParameterResolver parameter resolution} for a {@link CapturedOutput}
+ * instance which can be used to assert that the correct output was written.
  * <p>
- * To use, add {@link ExtendWith @ExtendWith} and inject the {@link CapturedOutput} as an
- * argument to your test class constructor or test method:
+ * To use with {@link ExtendWith @ExtendWith}, inject the {@link CapturedOutput} as an
+ * argument to your test class constructor, test method, or lifecycle methods:
  *
  * <pre class="code">
  * &#064;ExtendWith(OutputCaptureExtension.class)
@@ -42,7 +44,15 @@ import org.junit.jupiter.api.extension.ParameterResolver;
  *
  *     &#064;Test
  *     void test(CapturedOutput output) {
+ *         System.out.println("ok");
  *         assertThat(output).contains("ok");
+ *         System.err.println("error");
+ *     }
+ *
+ *     &#064;AfterEach
+ *     void after(CapturedOutput output) {
+ *         assertThat(output.getOut()).contains("ok");
+ *         assertThat(output.getErr()).contains("error");
  *     }
  *
  * }
@@ -51,36 +61,35 @@ import org.junit.jupiter.api.extension.ParameterResolver;
  * @author Madhura Bhave
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Sam Brannen
  * @since 2.2.0
  * @see CapturedOutput
  */
 public class OutputCaptureExtension
 		implements BeforeAllCallback, AfterAllCallback, BeforeEachCallback, AfterEachCallback, ParameterResolver {
 
-	private final OutputCapture outputCapture = new OutputCapture();
-
 	OutputCaptureExtension() {
 		// Package private to prevent users from directly creating an instance.
 	}
 
 	@Override
 	public void beforeAll(ExtensionContext context) throws Exception {
-		this.outputCapture.push();
+		getOutputCapture(context).push();
 	}
 
 	@Override
 	public void afterAll(ExtensionContext context) throws Exception {
-		this.outputCapture.pop();
+		getOutputCapture(context).pop();
 	}
 
 	@Override
 	public void beforeEach(ExtensionContext context) throws Exception {
-		this.outputCapture.push();
+		getOutputCapture(context).push();
 	}
 
 	@Override
 	public void afterEach(ExtensionContext context) throws Exception {
-		this.outputCapture.pop();
+		getOutputCapture(context).pop();
 	}
 
 	@Override
@@ -90,9 +99,16 @@ public class OutputCaptureExtension
 	}
 
 	@Override
-	public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext)
-			throws ParameterResolutionException {
-		return this.outputCapture;
+	public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+		return getOutputCapture(extensionContext);
+	}
+
+	private OutputCapture getOutputCapture(ExtensionContext context) {
+		return getStore(context).getOrComputeIfAbsent(OutputCapture.class);
+	}
+
+	private Store getStore(ExtensionContext context) {
+		return context.getStore(Namespace.create(getClass()));
 	}
 
 }