Use ExtensionContext.Store in OutputCaptureExtension

This commit refactors OutputCaptureExtension so that state is stored in
the ExtensionContext.Store instead of in the extension.

This is more idiomatic for JUnit Jupiter extensions and allows for
potential concurrent use of the extension.

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

@@ -22,6 +22,8 @@
 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;
@@ -66,30 +68,28 @@
 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
@@ -100,8 +100,16 @@ public boolean supportsParameter(ParameterContext parameterContext,
 
 	@Override
 	public Object resolveParameter(ParameterContext parameterContext,
-			ExtensionContext extensionContext) throws ParameterResolutionException {
-		return this.outputCapture;
+			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()));
 	}
 
 }