Add AssertJ support for the HTTP handler

This commit adds AssertJ compatible assertions for the component that
produces the result from the request.

See gh-21178

spring-test/src/main/java/org/springframework/test/util/MethodAssert.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2002-2024 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
+ *
+ *      https://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.test.util;
+
+import java.lang.reflect.Method;
+
+import org.assertj.core.api.AbstractObjectAssert;
+import org.assertj.core.api.Assertions;
+
+import org.springframework.lang.Nullable;
+
+/**
+ * AssertJ {@link org.assertj.core.api.Assert assertions} that can be applied
+ * to a {@link Method}.
+ *
+ * @author Stephane Nicoll
+ * @since 6.2
+ */
+public class MethodAssert extends AbstractObjectAssert<MethodAssert, Method> {
+
+	public MethodAssert(@Nullable Method actual) {
+		super(actual, MethodAssert.class);
+		as("Method %s", actual);
+	}
+
+	/**
+	 * Verify that the actual method has the given {@linkplain Method#getName()
+	 * name}.
+	 * @param name the expected method name
+	 */
+	public MethodAssert hasName(String name) {
+		isNotNull();
+		Assertions.assertThat(this.actual.getName()).as("Method name").isEqualTo(name);
+		return this.myself;
+	}
+
+	/**
+	 * Verify that the actual method is declared in the given {@code type}.
+	 * @param type the expected declaring class
+	 */
+	public MethodAssert hasDeclaringClass(Class<?> type) {
+		isNotNull();
+		Assertions.assertThat(this.actual.getDeclaringClass())
+				.as("Method declaring class").isEqualTo(type);
+		return this.myself;
+	}
+
+}

spring-test/src/main/java/org/springframework/test/web/servlet/assertj/HandlerResultAssert.java

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2002-2024 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
+ *
+ *      https://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.test.web.servlet.assertj;
+
+import java.lang.reflect.Method;
+
+import org.assertj.core.api.AbstractObjectAssert;
+import org.assertj.core.api.Assertions;
+
+import org.springframework.cglib.core.internal.Function;
+import org.springframework.lang.Nullable;
+import org.springframework.test.util.MethodAssert;
+import org.springframework.util.ClassUtils;
+import org.springframework.web.method.HandlerMethod;
+import org.springframework.web.servlet.mvc.method.annotation.MvcUriComponentsBuilder;
+import org.springframework.web.servlet.mvc.method.annotation.MvcUriComponentsBuilder.MethodInvocationInfo;
+
+/**
+ * AssertJ {@link org.assertj.core.api.Assert assertions} that can be applied to
+ * a handler or handler method.
+
+ * @author Stephane Nicoll
+ * @since 6.2
+ */
+public class HandlerResultAssert extends AbstractObjectAssert<HandlerResultAssert, Object> {
+
+	public HandlerResultAssert(@Nullable Object actual) {
+		super(actual, HandlerResultAssert.class);
+		as("Handler result");
+	}
+
+	/**
+	 * Return a new {@linkplain MethodAssert assertion} object that uses
+	 * the {@link Method} that handles the request as the object to test.
+	 * Verify first that the handler is a {@linkplain #isMethodHandler() method
+	 * handler}.
+	 * Example: <pre><code class='java'>
+	 * // Check that a GET to "/greet" is invoked on a "handleGreet" method name
+	 * assertThat(mvc.perform(get("/greet")).handler().method().hasName("sayGreet");
+	 * </code></pre>
+	 */
+	public MethodAssert method() {
+		return new MethodAssert(getHandlerMethod());
+	}
+
+	/**
+	 * Verify that the handler is managed by a method invocation, typically on
+	 * a controller.
+	 */
+	public HandlerResultAssert isMethodHandler() {
+		return isNotNull().isInstanceOf(HandlerMethod.class);
+	}
+
+	/**
+	 * Verify that the handler is managed by the given {@code handlerMethod}.
+	 * This creates a "mock" for the given {@code controllerType} and record the
+	 * method invocation in the {@code handlerMethod}. The arguments used by the
+	 * target method invocation can be {@code null} as the purpose of the mock
+	 * is to identify the method that was invoked.
+	 * Example: <pre><code class='java'>
+	 * // If the method has a return type, you can return the result of the invocation
+	 * assertThat(mvc.perform(get("/greet")).handler().isInvokedOn(
+	 *         GreetController.class, controller -> controller.sayGreet());
+	 * // If the method has a void return type, the controller should be returned
+	 * assertThat(mvc.perform(post("/persons/")).handler().isInvokedOn(
+	 *         PersonController.class, controller -> controller.createPerson(null, null));
+	 * </code></pre>
+	 * @param controllerType the controller to mock
+	 * @param handlerMethod the method
+	 */
+	public <T> HandlerResultAssert isInvokedOn(Class<T> controllerType, Function<T, Object> handlerMethod) {
+		MethodAssert actual = method();
+		Object methodInvocationInfo = handlerMethod.apply(MvcUriComponentsBuilder.on(controllerType));
+		Assertions.assertThat(methodInvocationInfo)
+				.as("Method invocation on controller '%s'", controllerType.getSimpleName())
+				.isInstanceOfSatisfying(MethodInvocationInfo.class, mii ->
+						actual.isEqualTo(mii.getControllerMethod()));
+		return this;
+	}
+
+	/**
+	 * Verify that the handler is of the given {@code type}. For a controller
+	 * method, this is the type of the controller.
+	 * Example: <pre><code class='java'>
+	 * // Check that a GET to "/greet" is managed by GreetController
+	 * assertThat(mvc.perform(get("/greet")).handler().hasType(GreetController.class);
+	 * </code></pre>
+	 * @param type the expected type of the handler
+	 */
+	public HandlerResultAssert hasType(Class<?> type) {
+		isNotNull();
+		Class<?> actualType = this.actual.getClass();
+		if (this.actual instanceof HandlerMethod handlerMethod) {
+			actualType = handlerMethod.getBeanType();
+		}
+		Assertions.assertThat(ClassUtils.getUserClass(actualType)).as("Handler result type").isEqualTo(type);
+		return this;
+	}
+
+	private Method getHandlerMethod() {
+		isMethodHandler(); // validate type
+		return ((HandlerMethod) this.actual).getMethod();
+	}
+
+
+}

spring-test/src/test/java/org/springframework/test/util/MethodAssertTests.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2002-2024 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
+ *
+ *      https://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.test.util;
+
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.ReflectionUtils;
+
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Tests for {@link MethodAssert}.
+ *
+ * @author Stephane Nicoll
+ */
+class MethodAssertTests {
+
+	@Test
+	void isEqualTo() {
+		Method method = ReflectionUtils.findMethod(TestData.class, "counter");
+		assertThat(method).isEqualTo(method);
+	}
+
+	@Test
+	void hasName() {
+		assertThat(ReflectionUtils.findMethod(TestData.class, "counter")).hasName("counter");
+	}
+
+	@Test
+	void hasNameWithWrongName() {
+		Method method = ReflectionUtils.findMethod(TestData.class, "counter");
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(method).hasName("invalid"))
+				.withMessageContainingAll("Method name", "counter", "invalid");
+	}
+
+	@Test
+	void hasNameWithNullMethod() {
+		Method method = ReflectionUtils.findMethod(TestData.class, "notAMethod");
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(method).hasName("name"))
+				.withMessageContaining("Expecting actual not to be null");
+	}
+
+	@Test
+	void hasDeclaringClass() {
+		assertThat(ReflectionUtils.findMethod(TestData.class, "counter")).hasDeclaringClass(TestData.class);
+	}
+
+	@Test
+	void haDeclaringClassWithWrongClass() {
+		Method method = ReflectionUtils.findMethod(TestData.class, "counter");
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(method).hasDeclaringClass(Method.class))
+				.withMessageContainingAll("Method declaring class",
+						TestData.class.getCanonicalName(), Method.class.getCanonicalName());
+	}
+
+	@Test
+	void hasDeclaringClassWithNullMethod() {
+		Method method = ReflectionUtils.findMethod(TestData.class, "notAMethod");
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(method).hasDeclaringClass(TestData.class))
+				.withMessageContaining("Expecting actual not to be null");
+	}
+
+
+	private MethodAssert assertThat(@Nullable Method method) {
+		return new MethodAssert(method);
+	}
+
+
+	record TestData(String name, int counter) {}
+
+}