Configure a RestClient.Builder with RestClientTest

This commit adds support for configuring a `RestClient.Builder` and
`MockRestServiceServer` support for the `RestClient` when using
`@RestClientTest` sliced tests.

Closes gh-37033

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/MockServerRestClientCustomizer.java

@@ -0,0 +1,139 @@
+/*
+ * Copyright 2012-2023 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.boot.test.web.client;
+
+import java.util.Collections;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Supplier;
+
+import org.springframework.beans.BeanUtils;
+import org.springframework.boot.web.client.RestClientCustomizer;
+import org.springframework.http.client.BufferingClientHttpRequestFactory;
+import org.springframework.test.web.client.MockRestServiceServer;
+import org.springframework.test.web.client.MockRestServiceServer.MockRestServiceServerBuilder;
+import org.springframework.test.web.client.RequestExpectationManager;
+import org.springframework.test.web.client.SimpleRequestExpectationManager;
+import org.springframework.util.Assert;
+import org.springframework.web.client.RestClient;
+
+/**
+ * {@link RestClientCustomizer} that can be applied to {@link RestClient.Builder}
+ * instances to add {@link MockRestServiceServer} support.
+ * <p>
+ * Typically applied to an existing builder before it is used, for example:
+ * <pre class="code">
+ * MockServerRestClientCustomizer customizer = new MockServerRestClientCustomizer();
+ * RestClient.Builder builder = RestClient.builder();
+ * customizer.customize(builder);
+ * MyBean bean = new MyBean(client.build());
+ * customizer.getServer().expect(requestTo("/hello")).andRespond(withSuccess());
+ * bean.makeRestCall();
+ * </pre>
+ * <p>
+ * If the customizer is only used once, the {@link #getServer()} method can be used to
+ * obtain the mock server. If the customizer has been used more than once the
+ * {@link #getServer(RestClient.Builder)} or {@link #getServers()} method must be used to
+ * access the related server.
+ *
+ * @author Scott Frederick
+ * @since 3.2.0
+ * @see #getServer()
+ * @see #getServer(RestClient.Builder)
+ */
+public class MockServerRestClientCustomizer implements RestClientCustomizer {
+
+	private final Map<RestClient.Builder, RequestExpectationManager> expectationManagers = new ConcurrentHashMap<>();
+
+	private final Map<RestClient.Builder, MockRestServiceServer> servers = new ConcurrentHashMap<>();
+
+	private final Supplier<? extends RequestExpectationManager> expectationManagerSupplier;
+
+	private boolean bufferContent = false;
+
+	public MockServerRestClientCustomizer() {
+		this(SimpleRequestExpectationManager::new);
+	}
+
+	/**
+	 * Crate a new {@link MockServerRestClientCustomizer} instance.
+	 * @param expectationManager the expectation manager class to use
+	 */
+	public MockServerRestClientCustomizer(Class<? extends RequestExpectationManager> expectationManager) {
+		this(() -> BeanUtils.instantiateClass(expectationManager));
+		Assert.notNull(expectationManager, "ExpectationManager must not be null");
+	}
+
+	/**
+	 * Crate a new {@link MockServerRestClientCustomizer} instance.
+	 * @param expectationManagerSupplier a supplier that provides the
+	 * {@link RequestExpectationManager} to use
+	 * @since 3.0.0
+	 */
+	public MockServerRestClientCustomizer(Supplier<? extends RequestExpectationManager> expectationManagerSupplier) {
+		Assert.notNull(expectationManagerSupplier, "ExpectationManagerSupplier must not be null");
+		this.expectationManagerSupplier = expectationManagerSupplier;
+	}
+
+	/**
+	 * Set if the {@link BufferingClientHttpRequestFactory} wrapper should be used to
+	 * buffer the input and output streams, and for example, allow multiple reads of the
+	 * response body.
+	 * @param bufferContent if request and response content should be buffered
+	 * @since 3.1.0
+	 */
+	public void setBufferContent(boolean bufferContent) {
+		this.bufferContent = bufferContent;
+	}
+
+	@Override
+	public void customize(RestClient.Builder restClientBuilder) {
+		RequestExpectationManager expectationManager = createExpectationManager();
+		MockRestServiceServerBuilder serverBuilder = MockRestServiceServer.bindTo(restClientBuilder);
+		if (this.bufferContent) {
+			serverBuilder.bufferContent();
+		}
+		MockRestServiceServer server = serverBuilder.build(expectationManager);
+		this.expectationManagers.put(restClientBuilder, expectationManager);
+		this.servers.put(restClientBuilder, server);
+	}
+
+	protected RequestExpectationManager createExpectationManager() {
+		return this.expectationManagerSupplier.get();
+	}
+
+	public MockRestServiceServer getServer() {
+		Assert.state(!this.servers.isEmpty(), "Unable to return a single MockRestServiceServer since "
+				+ "MockServerRestClientCustomizer has not been bound to a RestClient");
+		Assert.state(this.servers.size() == 1, "Unable to return a single MockRestServiceServer since "
+				+ "MockServerRestClientCustomizer has been bound to more than one RestClient");
+		return this.servers.values().iterator().next();
+	}
+
+	public Map<RestClient.Builder, RequestExpectationManager> getExpectationManagers() {
+		return this.expectationManagers;
+	}
+
+	public MockRestServiceServer getServer(RestClient.Builder restClientBuilder) {
+		return this.servers.get(restClientBuilder);
+	}
+
+	public Map<RestClient.Builder, MockRestServiceServer> getServers() {
+		return Collections.unmodifiableMap(this.servers);
+	}
+
+}