Support package private requestFactory classes

Update `RestTemplateBuilder` to support package private `requestFactory`
classes.

Fixes gh-7319

spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateBuilder.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.web.client;
 
+import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.util.ArrayList;
@@ -317,7 +318,19 @@ public class RestTemplateBuilder {
 	public RestTemplateBuilder requestFactory(
 			Class<? extends ClientHttpRequestFactory> requestFactory) {
 		Assert.notNull(requestFactory, "RequestFactory must not be null");
-		return requestFactory(BeanUtils.instantiate(requestFactory));
+		return requestFactory(createRequestFactory(requestFactory));
+	}
+
+	private ClientHttpRequestFactory createRequestFactory(
+			Class<? extends ClientHttpRequestFactory> requestFactory) {
+		try {
+			Constructor<?> constructor = requestFactory.getDeclaredConstructor();
+			constructor.setAccessible(true);
+			return (ClientHttpRequestFactory) constructor.newInstance();
+		}
+		catch (Exception ex) {
+			throw new IllegalStateException(ex);
+		}
 	}
 
 	/**

spring-boot/src/test/java/org/springframework/boot/web/client/RestTemplateBuilderTests.java

@@ -278,6 +278,14 @@ public class RestTemplateBuilderTests {
 				.isInstanceOf(SimpleClientHttpRequestFactory.class);
 	}
 
+	@Test
+	public void requestFactoryPackagePrivateClassShouldApply() throws Exception {
+		RestTemplate template = this.builder
+				.requestFactory(TestClientHttpRequestFactory.class).build();
+		assertThat(template.getRequestFactory())
+				.isInstanceOf(TestClientHttpRequestFactory.class);
+	}
+
 	@Test
 	public void requestFactoryWhenFactoryIsNullShouldThrowException() throws Exception {
 		this.thrown.expect(IllegalArgumentException.class);
@@ -547,4 +555,8 @@ public class RestTemplateBuilderTests {
 
 	}
 
+	static class TestClientHttpRequestFactory extends SimpleClientHttpRequestFactory {
+
+	}
+
 }