Polish "Polish OutputCapture and its JUnit Jupiter extension"

See gh-17049

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

@@ -39,7 +39,6 @@ import org.springframework.util.ClassUtils;
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @author Sam Brannen
- * @since 2.2.0
  * @see OutputCaptureExtension
  * @see OutputCaptureRule
  */
@@ -128,11 +127,7 @@ class OutputCapture implements CapturedOutput {
 
 	private String get(Predicate<Type> filter) {
 		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.");
+				"No system captures found. Please check your output capture registration.");
 		StringBuilder builder = new StringBuilder();
 		for (SystemCapture systemCapture : this.systemCaptures) {
 			systemCapture.append(builder, filter);

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

@@ -111,7 +111,8 @@ 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. Please check your output capture registration.");
 		StringBuilder builder = new StringBuilder();
 		for (SystemCapture systemCapture : this.systemCaptures) {
 			systemCapture.append(builder, filter);

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

@@ -20,47 +20,76 @@ import org.junit.jupiter.api.extension.AfterAllCallback;
 import org.junit.jupiter.api.extension.AfterEachCallback;
 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;
 
 /**
- * Internal JUnit 5 {@code @Extension} to capture {@link System#out System.out} and
- * {@link System#err System.err}.
+ * 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 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)
+ * class MyTest {
+ *
+ *     &#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");
+ *     }
+ *
+ * }
+ * </pre>
  *
  * @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
@@ -70,9 +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()));
 	}
 
 }