Consistent use of @Nullable across the codebase (even for internals)

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

spring-test/src/main/java/org/springframework/mock/http/MockHttpInputMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -37,8 +37,9 @@ public class MockHttpInputMessage implements HttpInputMessage {
 	private final InputStream body;
 
 
-	public MockHttpInputMessage(byte[] contents) {
-		this.body = (contents != null ? new ByteArrayInputStream(contents) : null);
+	public MockHttpInputMessage(byte[] content) {
+		Assert.notNull(content, "Byte array must not be null");
+		this.body = new ByteArrayInputStream(content);
 	}
 
 	public MockHttpInputMessage(InputStream body) {

spring-test/src/main/java/org/springframework/mock/http/MockHttpOutputMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -19,7 +19,6 @@ package org.springframework.mock.http;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
-import java.io.UnsupportedEncodingException;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 
@@ -77,13 +76,7 @@ public class MockHttpOutputMessage implements HttpOutputMessage {
 	 */
 	public String getBodyAsString(Charset charset) {
 		byte[] bytes = getBodyAsBytes();
-		try {
-			return new String(bytes, charset.name());
-		}
-		catch (UnsupportedEncodingException ex) {
-			// should not occur
-			throw new IllegalStateException(ex);
-		}
+		return new String(bytes, charset);
 	}
 
 }

spring-test/src/main/java/org/springframework/test/web/client/DefaultRequestExpectation.java

@@ -22,6 +22,7 @@ import java.util.List;
 
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -61,6 +62,7 @@ public class DefaultRequestExpectation implements RequestExpectation {
 		return this.requestMatchers;
 	}
 
+	@Nullable
 	protected ResponseCreator getResponseCreator() {
 		return this.responseCreator;
 	}
@@ -86,7 +88,7 @@ public class DefaultRequestExpectation implements RequestExpectation {
 	}
 
 	@Override
-	public ClientHttpResponse createResponse(ClientHttpRequest request) throws IOException {
+	public ClientHttpResponse createResponse(@Nullable ClientHttpRequest request) throws IOException {
 		ResponseCreator responseCreator = getResponseCreator();
 		Assert.state(responseCreator != null, "createResponse() called before ResponseCreator was set");
 		getRequestCount().incrementAndValidate();

spring-test/src/main/java/org/springframework/test/web/client/ResponseCreator.java

@@ -20,6 +20,7 @@ import java.io.IOException;
 
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.test.web.client.response.MockRestResponseCreators;
 
 /**
@@ -36,6 +37,6 @@ public interface ResponseCreator {
 	 * Create a response for the given request.
 	 * @param request the request
 	 */
-	ClientHttpResponse createResponse(ClientHttpRequest request) throws IOException;
+	ClientHttpResponse createResponse(@Nullable ClientHttpRequest request) throws IOException;
 
 }

spring-test/src/main/java/org/springframework/test/web/client/response/DefaultResponseCreator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -27,6 +27,7 @@ import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.mock.http.client.MockClientHttpResponse;
 import org.springframework.test.web.client.ResponseCreator;
 import org.springframework.util.Assert;
@@ -41,7 +42,7 @@ public class DefaultResponseCreator implements ResponseCreator {
 
 	private HttpStatus statusCode;
 
-	private byte[] content;
+	private byte[] content = new byte[0];
 
 	private Resource contentResource;
 
@@ -86,9 +87,7 @@ public class DefaultResponseCreator implements ResponseCreator {
 	 * Set the {@code Content-Type} header.
 	 */
 	public DefaultResponseCreator contentType(MediaType mediaType) {
-		if (mediaType != null) {
-			this.headers.setContentType(mediaType);
-		}
+		this.headers.setContentType(mediaType);
 		return this;
 	}
 
@@ -104,17 +103,13 @@ public class DefaultResponseCreator implements ResponseCreator {
 	 * Copy all given headers.
 	 */
 	public DefaultResponseCreator headers(HttpHeaders headers) {
-		for (String headerName : headers.keySet()) {
-			for (String headerValue : headers.get(headerName)) {
-				this.headers.add(headerName, headerValue);
-			}
-		}
+		this.headers.putAll(headers);
 		return this;
 	}
 
 
 	@Override
-	public ClientHttpResponse createResponse(ClientHttpRequest request) throws IOException {
+	public ClientHttpResponse createResponse(@Nullable ClientHttpRequest request) throws IOException {
 		MockClientHttpResponse response;
 		if (this.contentResource != null) {
 			InputStream stream = this.contentResource.getInputStream();

spring-test/src/main/java/org/springframework/test/web/client/response/MockRestResponseCreators.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.test.web.client.response;
 
 import java.net.URI;
@@ -44,28 +45,31 @@ public abstract class MockRestResponseCreators {
 	/**
 	 * {@code ResponseCreator} for a 200 response (OK) with String body.
 	 * @param body the response body, a "UTF-8" string
-	 * @param mediaType the type of the content, may be {@code null}
+	 * @param contentType the type of the content (may be {@code null})
 	 */
-	public static DefaultResponseCreator withSuccess(String body, @Nullable MediaType mediaType) {
-		return new DefaultResponseCreator(HttpStatus.OK).body(body).contentType(mediaType);
+	public static DefaultResponseCreator withSuccess(String body, @Nullable MediaType contentType) {
+		DefaultResponseCreator creator = new DefaultResponseCreator(HttpStatus.OK).body(body);
+		return (contentType != null ? creator.contentType(contentType) : creator);
 	}
 
 	/**
 	 * {@code ResponseCreator} for a 200 response (OK) with byte[] body.
 	 * @param body the response body
-	 * @param contentType the type of the content, may be {@code null}
+	 * @param contentType the type of the content (may be {@code null})
 	 */
 	public static DefaultResponseCreator withSuccess(byte[] body, @Nullable MediaType contentType) {
-		return new DefaultResponseCreator(HttpStatus.OK).body(body).contentType(contentType);
+		DefaultResponseCreator creator = new DefaultResponseCreator(HttpStatus.OK).body(body);
+		return (contentType != null ? creator.contentType(contentType) : creator);
 	}
 
 	/**
 	 * {@code ResponseCreator} for a 200 response (OK) content with {@link Resource}-based body.
 	 * @param body the response body
-	 * @param contentType the type of the content, may be {@code null}
+	 * @param contentType the type of the content (may be {@code null})
 	 */
 	public static DefaultResponseCreator withSuccess(Resource body, @Nullable MediaType contentType) {
-		return new DefaultResponseCreator(HttpStatus.OK).body(body).contentType(contentType);
+		DefaultResponseCreator creator = new DefaultResponseCreator(HttpStatus.OK).body(body);
+		return (contentType != null ? creator.contentType(contentType) : creator);
 	}
 
 	/**

spring-test/src/main/java/org/springframework/test/web/servlet/setup/StandaloneMockMvcBuilder.java

@@ -32,7 +32,6 @@ import org.springframework.context.ApplicationContextAware;
 import org.springframework.format.support.DefaultFormattingConversionService;
 import org.springframework.format.support.FormattingConversionService;
 import org.springframework.http.converter.HttpMessageConverter;
-import org.springframework.lang.Nullable;
 import org.springframework.mock.web.MockServletContext;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
@@ -496,16 +495,11 @@ public class StandaloneMockMvcBuilder extends AbstractMockMvcBuilder<StandaloneM
 
 		public StaticStringValueResolver(final Map<String, String> values) {
 			this.helper = new PropertyPlaceholderHelper("${", "}", ":", false);
-			this.resolver = new PlaceholderResolver() {
-				@Override
-				public String resolvePlaceholder(String placeholderName) {
-					return values.get(placeholderName);
-				}
-			};
+			this.resolver = values::get;
 		}
 
 		@Override
-		public String resolveStringValue(@Nullable String strVal) throws BeansException {
+		public String resolveStringValue(String strVal) throws BeansException {
 			return this.helper.replacePlaceholders(strVal, this.resolver);
 		}
 	}

spring-test/src/main/java/org/springframework/test/web/servlet/setup/StubWebApplicationContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -141,13 +141,12 @@ class StubWebApplicationContext implements WebApplicationContext {
 		this.beanFactory.addBean(name, bean);
 	}
 
-	public void addBeans(List<?> beans) {
-		if (beans == null) {
-			return;
-		}
-		for (Object bean : beans) {
-			String name = bean.getClass().getName() + "#" +  ObjectUtils.getIdentityHexString(bean);
-			this.beanFactory.addBean(name, bean);
+	public void addBeans(@Nullable List<?> beans) {
+		if (beans != null) {
+			for (Object bean : beans) {
+				String name = bean.getClass().getName() + "#" + ObjectUtils.getIdentityHexString(bean);
+				this.beanFactory.addBean(name, bean);
+			}
 		}
 	}
 
@@ -167,13 +166,13 @@ class StubWebApplicationContext implements WebApplicationContext {
 	}
 
 	@Override
-	public <T> T getBean(Class<T> requiredType) throws BeansException {
-		return this.beanFactory.getBean(requiredType);
+	public Object getBean(String name, Object... args) throws BeansException {
+		return this.beanFactory.getBean(name, args);
 	}
 
 	@Override
-	public Object getBean(String name, Object... args) throws BeansException {
-		return this.beanFactory.getBean(name, args);
+	public <T> T getBean(Class<T> requiredType) throws BeansException {
+		return this.beanFactory.getBean(requiredType);
 	}
 
 	@Override
@@ -202,7 +201,7 @@ class StubWebApplicationContext implements WebApplicationContext {
 	}
 
 	@Override
-	public boolean isTypeMatch(String name, Class<?> typeToMatch) throws NoSuchBeanDefinitionException {
+	public boolean isTypeMatch(String name, @Nullable Class<?> typeToMatch) throws NoSuchBeanDefinitionException {
 		return this.beanFactory.isTypeMatch(name, typeToMatch);
 	}
 
@@ -405,7 +404,7 @@ class StubWebApplicationContext implements WebApplicationContext {
 		}
 
 		@Override
-		public Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
+		public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
 				@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) {
 			throw new UnsupportedOperationException("Dependency resolution not supported");
 		}