Add ClientHttpRequestInitializer support

Add `ClientHttpRequestInitializer` interface that can be used with any
`HttpAccessor` to initialize each `ClientHttpRequest` before it's used.

This provides a useful alternative to `ClientHttpRequestInterceptor`
when users need to configure things like `HttpHeaders`.

Closes spring-projectsgh-22002

spring-web/src/main/java/org/springframework/http/client/ClientHttpRequestInitializer.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2002-2019 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.http.client;
+
+import org.springframework.http.client.support.HttpAccessor;
+
+/**
+ * Callback interface for initializing a {@link ClientHttpRequest} prior to it
+ * being used.
+ *
+ * <p> Typically used with {@link HttpAccessor} and subclasses such as
+ * {@link org.springframework.web.client.RestTemplate RestTemplate} to apply
+ * consistent settings or headers to each request.
+ *
+ * <p>Unlike {@link ClientHttpRequestInterceptor}, this interface can apply
+ * customizations without needing to read the entire request body into memory.
+ *
+ * @author Phillip Webb
+ * @since 5.2
+ * @see HttpAccessor#getClientHttpRequestInitializers()
+ */
+@FunctionalInterface
+public interface ClientHttpRequestInitializer {
+
+	/**
+	 * Initialize the given client HTTP request.
+	 * @param request the request to configure
+	 */
+	void initialize(ClientHttpRequest request);
+
+}

spring-web/src/main/java/org/springframework/http/client/support/HttpAccessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -18,13 +18,17 @@
 
 import java.io.IOException;
 import java.net.URI;
+import java.util.ArrayList;
+import java.util.List;
 
 import org.apache.commons.logging.Log;
 
+import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.http.HttpLogging;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpRequestFactory;
+import org.springframework.http.client.ClientHttpRequestInitializer;
 import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.util.Assert;
 
@@ -49,6 +53,8 @@ public abstract class HttpAccessor {
 
 	private ClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
 
+	private List<ClientHttpRequestInitializer> clientHttpRequestInitializers = new ArrayList<>();
+
 
 	/**
 	 * Set the request factory that this accessor uses for obtaining client request handles.
@@ -74,6 +80,28 @@ public ClientHttpRequestFactory getRequestFactory() {
 	}
 
 
+	/**
+	 * Set the request initializers the this accessor should use.
+	 * <p>The initializers will get sorted according to their order
+	 * before the {@link ClientHttpRequest} is initialized.
+	 */
+	public void setClientHttpRequestInitializers(
+			List<ClientHttpRequestInitializer> clientHttpRequestInitializers) {
+		if (this.clientHttpRequestInitializers != clientHttpRequestInitializers) {
+			this.clientHttpRequestInitializers.clear();
+			this.clientHttpRequestInitializers.addAll(clientHttpRequestInitializers);
+			AnnotationAwareOrderComparator.sort(this.clientHttpRequestInitializers);
+		}
+	}
+
+	/**
+	 * Return the request initializers that this accessor uses.
+	 * <p>The returned {@link List} is active and may get appended to.
+	 */
+	public List<ClientHttpRequestInitializer> getClientHttpRequestInitializers() {
+		return this.clientHttpRequestInitializers;
+	}
+
 	/**
 	 * Create a new {@link ClientHttpRequest} via this template's {@link ClientHttpRequestFactory}.
 	 * @param url the URL to connect to
@@ -85,10 +113,16 @@ public ClientHttpRequestFactory getRequestFactory() {
 	 */
 	protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
 		ClientHttpRequest request = getRequestFactory().createRequest(url, method);
+		initialize(request);
 		if (logger.isDebugEnabled()) {
 			logger.debug("HTTP " + method.name() + " " + url);
 		}
 		return request;
 	}
 
+	private void initialize(ClientHttpRequest request) {
+		this.clientHttpRequestInitializers.forEach(
+				initializer -> initializer.initialize(request));
+	}
+
 }

spring-web/src/test/java/org/springframework/web/client/RestTemplateTests.java

@@ -40,6 +40,7 @@
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpRequestFactory;
+import org.springframework.http.client.ClientHttpRequestInitializer;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.http.converter.GenericHttpMessageConverter;
@@ -679,6 +680,32 @@ public void requestInterceptorCanAddExistingHeaderValueWithBody() throws Excepti
 		verify(response).close();
 	}
 
+	@Test
+	public void clientHttpRequestInitializerAndRequestInterceptorAreBothApplied() throws Exception {
+		ClientHttpRequestInitializer initializer = request ->
+			request.getHeaders().add("MyHeader", "MyInitializerValue");
+		ClientHttpRequestInterceptor interceptor = (request, body, execution) -> {
+			request.getHeaders().add("MyHeader", "MyInterceptorValue");
+			return execution.execute(request, body);
+		};
+		template.setClientHttpRequestInitializers(Collections.singletonList(initializer));
+		template.setInterceptors(Collections.singletonList(interceptor));
+
+		MediaType contentType = MediaType.TEXT_PLAIN;
+		given(converter.canWrite(String.class, contentType)).willReturn(true);
+		HttpHeaders requestHeaders = new HttpHeaders();
+		mockSentRequest(POST, "https://example.com", requestHeaders);
+		mockResponseStatus(HttpStatus.OK);
+
+		HttpHeaders entityHeaders = new HttpHeaders();
+		entityHeaders.setContentType(contentType);
+		HttpEntity<String> entity = new HttpEntity<>("Hello World", entityHeaders);
+		template.exchange("https://example.com", POST, entity, Void.class);
+		assertThat(requestHeaders.get("MyHeader")).contains("MyInterceptorValue", "MyInitializerValue");
+
+		verify(response).close();
+	}
+
 	private void mockSentRequest(HttpMethod method, String uri) throws Exception {
 		mockSentRequest(method, uri, new HttpHeaders());
 	}