Add methods to RestTemplateBuilder for configuring interceptors

Closes gh-6701

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

@@ -72,6 +72,9 @@ public class RestTemplateBuilderTests {
 	@Mock
 	private HttpMessageConverter<Object> messageConverter;
 
+	@Mock
+	private ClientHttpRequestInterceptor interceptor;
+
 	@Before
 	public void setup() {
 		MockitoAnnotations.initMocks(this);
@@ -203,6 +206,62 @@ public class RestTemplateBuilderTests {
 				.hasSameSizeAs(new RestTemplate().getMessageConverters());
 	}
 
+	@Test
+	public void interceptorsWhenInterceptorsAreNullShouldThrowException()
+			throws Exception {
+		this.thrown.expect(IllegalArgumentException.class);
+		this.thrown.expectMessage("interceptors must not be null");
+		this.builder.interceptors((ClientHttpRequestInterceptor[]) null);
+	}
+
+	@Test
+	public void interceptorsCollectionWhenInterceptorsAreNullShouldThrowException()
+			throws Exception {
+		this.thrown.expect(IllegalArgumentException.class);
+		this.thrown.expectMessage("interceptors must not be null");
+		this.builder.interceptors((Set<ClientHttpRequestInterceptor>) null);
+	}
+
+	@Test
+	public void interceptorsShouldApply() throws Exception {
+		RestTemplate template = this.builder.interceptors(this.interceptor).build();
+		assertThat(template.getInterceptors()).containsOnly(this.interceptor);
+	}
+
+	@Test
+	public void interceptorsShouldReplaceExisting() throws Exception {
+		RestTemplate template = this.builder
+				.interceptors(mock(ClientHttpRequestInterceptor.class))
+				.interceptors(Collections.singleton(this.interceptor)).build();
+		assertThat(template.getInterceptors()).containsOnly(this.interceptor);
+	}
+
+	@Test
+	public void additionalInterceptorsWhenInterceptorsAreNullShouldThrowException()
+			throws Exception {
+		this.thrown.expect(IllegalArgumentException.class);
+		this.thrown.expectMessage("interceptors must not be null");
+		this.builder.additionalInterceptors((ClientHttpRequestInterceptor[]) null);
+	}
+
+	@Test
+	public void additionalInterceptorsCollectionWhenInterceptorsAreNullShouldThrowException()
+			throws Exception {
+		this.thrown.expect(IllegalArgumentException.class);
+		this.thrown.expectMessage("interceptors must not be null");
+		this.builder.additionalInterceptors((Set<ClientHttpRequestInterceptor>) null);
+	}
+
+	@Test
+	public void additionalInterceptorsShouldAddToExisting() throws Exception {
+		ClientHttpRequestInterceptor interceptor = mock(
+				ClientHttpRequestInterceptor.class);
+		RestTemplate template = this.builder.interceptors(interceptor)
+				.additionalInterceptors(this.interceptor).build();
+		assertThat(template.getInterceptors()).containsOnly(interceptor,
+				this.interceptor);
+	}
+
 	@Test
 	public void requestFactoryClassWhenFactoryIsNullShouldThrowException()
 			throws Exception {