Add support for JUnit 5

This commit adds support for JUnit 5 and its Jupiter programming
model. A new Jupiter Extension implementation,
RestDocumentationExtension, is provided. This extension can be applied
to a test class to allow it to use Spring REST Docs to document a
RESTful API.

Closes gh-296

spring-restdocs-core/src/main/java/org/springframework/restdocs/RestDocumentationExtension.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2014-2017 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
+ *
+ *      http://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.restdocs;
+
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.extension.AfterEachCallback;
+import org.junit.jupiter.api.extension.BeforeEachCallback;
+import org.junit.jupiter.api.extension.Extension;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.api.extension.ParameterResolutionException;
+import org.junit.jupiter.api.extension.ParameterResolver;
+import org.junit.jupiter.api.extension.TestExtensionContext;
+
+/**
+ * A JUnit Jupiter {@link Extension} used to automatically manage the
+ * {@link RestDocumentationContext}.
+ *
+ * @author Andy Wilkinson
+ */
+public class RestDocumentationExtension implements Extension, BeforeEachCallback,
+		AfterEachCallback, RestDocumentationContextProvider, ParameterResolver {
+
+	private final ManualRestDocumentation delegate = new ManualRestDocumentation();
+
+	@Override
+	public void beforeEach(TestExtensionContext context) throws Exception {
+		Class<?> testClass = context.getTestClass().orElseThrow(
+				() -> new IllegalStateException("No test class was available"));
+		Method testMethod = context.getTestMethod().orElseThrow(
+				() -> new IllegalStateException("No test method was available"));
+		this.delegate.beforeTest(testClass, testMethod.getName());
+	}
+
+	@Override
+	public void afterEach(TestExtensionContext context) throws Exception {
+		this.delegate.afterTest();
+	}
+
+	@Override
+	public RestDocumentationContext beforeOperation() {
+		return this.delegate.beforeOperation();
+	}
+
+	@Override
+	public boolean supports(ParameterContext parameterContext,
+			ExtensionContext extensionContext) throws ParameterResolutionException {
+		return RestDocumentationContextProvider.class
+				.isAssignableFrom(parameterContext.getParameter().getType());
+	}
+
+	@Override
+	public Object resolve(ParameterContext parameterContext,
+			ExtensionContext extensionContext) throws ParameterResolutionException {
+		return this;
+	}
+
+}