Revise use of Objects.requireNonNull()

Historically, we have rarely intentionally thrown a
NullPointerException in the Spring Framework. Instead, we prefer to
throw either an IllegalArgumentException or IllegalStateException
instead of a NullPointerException.

However, changes to the code in recent times have introduced the use of
Objects.requireNonNull(Object) which throws a NullPointerException
without an explicit error message.

The latter ends up providing less context than a NullPointerException
thrown by the JVM (since Java 14) due to actually de-referencing a
null-pointer. See https://openjdk.org/jeps/358.

In light of that, this commit revises our current use of
Objects.requireNonNull(Object) by removing it or replacing it with
Assert.notNull().

However, we still use Objects.requireNonNull(T, String) in a few places
where we are required to throw a NullPointerException in order to
comply with a third-party contract such as Reactive Streams.

Closes gh-32430

spring-test/src/main/java/org/springframework/test/web/reactive/server/CookieAssertions.java

@@ -17,7 +17,6 @@
 package org.springframework.test.web.reactive.server;
 
 import java.time.Duration;
-import java.util.Objects;
 import java.util.function.Consumer;
 
 import org.hamcrest.Matcher;
@@ -213,10 +212,13 @@ public class CookieAssertions {
 
 	private ResponseCookie getCookie(String name) {
 		ResponseCookie cookie = this.exchangeResult.getResponseCookies().getFirst(name);
-		if (cookie == null) {
+		if (cookie != null) {
+			return cookie;
+		}
+		else {
 			this.exchangeResult.assertWithDiagnostics(() -> fail("No cookie with name '" + name + "'"));
 		}
-		return Objects.requireNonNull(cookie);
+		throw new IllegalStateException("This code path should not be reachable");
 	}
 
 	private static String getMessage(String cookie) {

spring-test/src/main/java/org/springframework/test/web/reactive/server/HeaderAssertions.java

@@ -19,7 +19,6 @@ package org.springframework.test.web.reactive.server;
 import java.net.URI;
 import java.util.Arrays;
 import java.util.List;
-import java.util.Objects;
 import java.util.function.Consumer;
 
 import org.hamcrest.Matcher;
@@ -200,10 +199,13 @@ public class HeaderAssertions {
 
 	private List<String> getRequiredValues(String name) {
 		List<String> values = getHeaders().get(name);
-		if (CollectionUtils.isEmpty(values)) {
+		if (!CollectionUtils.isEmpty(values)) {
+			return values;
+		}
+		else {
 			this.exchangeResult.assertWithDiagnostics(() -> fail(getMessage(name) + " not found"));
 		}
-		return Objects.requireNonNull(values);
+		throw new IllegalStateException("This code path should not be reachable");
 	}
 
 	/**

spring-test/src/test/java/org/springframework/test/context/bean/override/OverrideMetadataTests.java

@@ -18,7 +18,6 @@ package org.springframework.test.context.bean.override;
 
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Field;
-import java.util.Objects;
 
 import org.junit.jupiter.api.Test;
 
@@ -49,7 +48,7 @@ class OverrideMetadataTests {
 
 	private static OverrideMetadata exampleOverride() throws Exception {
 		Field field = OverrideMetadataTests.class.getDeclaredField("annotated");
-		return new ConcreteOverrideMetadata(Objects.requireNonNull(field), field.getAnnotation(NonNull.class),
+		return new ConcreteOverrideMetadata(field, field.getAnnotation(NonNull.class),
 				ResolvableType.forClass(String.class), BeanOverrideStrategy.REPLACE_DEFINITION);
 	}
 

spring-test/src/test/java/org/springframework/test/context/bean/override/convention/TestBeanOverrideProcessorTests.java

@@ -19,7 +19,6 @@ package org.springframework.test.context.bean.override.convention;
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.util.List;
-import java.util.Objects;
 
 import org.junit.jupiter.api.Test;
 
@@ -91,7 +90,8 @@ class TestBeanOverrideProcessorTests {
 		Class<?> clazz = ExplicitMethodNameConf.class;
 		Class<?> returnType = ExampleService.class;
 		Field field = clazz.getField("a");
-		TestBean overrideAnnotation = Objects.requireNonNull(field.getAnnotation(TestBean.class));
+		TestBean overrideAnnotation = field.getAnnotation(TestBean.class);
+		assertThat(overrideAnnotation).isNotNull();
 
 		TestBeanOverrideProcessor processor = new TestBeanOverrideProcessor();
 		assertThatIllegalStateException()
@@ -106,7 +106,8 @@ class TestBeanOverrideProcessorTests {
 	void createMetaDataForKnownExplicitMethod() throws Exception {
 		Class<?> returnType = ExampleService.class;
 		Field field = ExplicitMethodNameConf.class.getField("b");
-		TestBean overrideAnnotation = Objects.requireNonNull(field.getAnnotation(TestBean.class));
+		TestBean overrideAnnotation = field.getAnnotation(TestBean.class);
+		assertThat(overrideAnnotation).isNotNull();
 
 		TestBeanOverrideProcessor processor = new TestBeanOverrideProcessor();
 		assertThat(processor.createMetadata(field, overrideAnnotation, ResolvableType.forClass(returnType)))
@@ -117,7 +118,8 @@ class TestBeanOverrideProcessorTests {
 	void createMetaDataWithDeferredCheckForExistenceOfConventionBasedFactoryMethod() throws Exception {
 		Class<?> returnType = ExampleService.class;
 		Field field = MethodConventionConf.class.getField("field");
-		TestBean overrideAnnotation = Objects.requireNonNull(field.getAnnotation(TestBean.class));
+		TestBean overrideAnnotation = field.getAnnotation(TestBean.class);
+		assertThat(overrideAnnotation).isNotNull();
 
 		TestBeanOverrideProcessor processor = new TestBeanOverrideProcessor();
 		// When in convention-based mode, createMetadata() will not verify that