fix generic type constraint issues and try to avoid issues covered by SonarQube squid:S1452 rule

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionMessage.java

@@ -420,7 +420,7 @@ protected String applyToItem(Object item) {
 			}
 		};
 
-		public Collection<?> applyTo(Collection<?> items) {
+		public Collection applyTo(Collection items) {
 			List<Object> result = new ArrayList<>();
 			for (Object item : items) {
 				result.add(applyToItem(item));

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceProperties.java

@@ -182,7 +182,7 @@ public void afterPropertiesSet() throws Exception {
 	 * @return a {@link DataSourceBuilder} initialized with the customizations defined on
 	 * this instance
 	 */
-	public DataSourceBuilder<?> initializeDataSourceBuilder() {
+	public DataSourceBuilder<? extends DataSource> initializeDataSourceBuilder() {
 		return DataSourceBuilder.create(getClassLoader()).type(getType())
 				.driverClassName(determineDriverClassName()).url(determineUrl())
 				.username(determineUsername()).password(determinePassword());

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/SessionRepositoryFilterConfiguration.java

@@ -26,6 +26,7 @@
 import org.springframework.boot.web.servlet.FilterRegistrationBean;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.session.Session;
 import org.springframework.session.web.http.SessionRepositoryFilter;
 
 /**
@@ -39,7 +40,7 @@
 class SessionRepositoryFilterConfiguration {
 
 	@Bean
-	public FilterRegistrationBean<SessionRepositoryFilter<?>> sessionRepositoryFilterRegistration(
+	public FilterRegistrationBean<SessionRepositoryFilter<? extends Session>> sessionRepositoryFilterRegistration(
 			SessionProperties sessionProperties, SessionRepositoryFilter<?> filter) {
 		FilterRegistrationBean<SessionRepositoryFilter<?>> registration = new FilterRegistrationBean<>(
 				filter);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -45,6 +45,7 @@
 import org.springframework.boot.context.properties.source.ConfigurationPropertySources;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.AnnotatedBeanDefinitionReader;
@@ -225,9 +226,9 @@ public class SpringApplication {
 
 	private boolean registerShutdownHook = true;
 
-	private List<ApplicationContextInitializer<?>> initializers;
+	private List<ApplicationContextInitializer<? extends ConfigurableApplicationContext>> initializers;
 
-	private List<ApplicationListener<?>> listeners;
+	private List<ApplicationListener<? extends ApplicationEvent>> listeners;
 
 	private Map<String, Object> defaultProperties;
 
@@ -1170,7 +1171,7 @@ private boolean isWebApplicationContext(Class<?> applicationContextClass) {
 	 * @param initializers the initializers to set
 	 */
 	public void setInitializers(
-			Collection<? extends ApplicationContextInitializer<?>> initializers) {
+			Collection<? extends ApplicationContextInitializer<? extends ConfigurableApplicationContext>> initializers) {
 		this.initializers = new ArrayList<>();
 		this.initializers.addAll(initializers);
 	}
@@ -1180,7 +1181,7 @@ public void setInitializers(
 	 * {@link ApplicationContext}.
 	 * @param initializers the initializers to add
 	 */
-	public void addInitializers(ApplicationContextInitializer<?>... initializers) {
+	public void addInitializers(ApplicationContextInitializer<? extends ConfigurableApplicationContext>... initializers) {
 		this.initializers.addAll(Arrays.asList(initializers));
 	}
 
@@ -1189,7 +1190,7 @@ public void addInitializers(ApplicationContextInitializer<?>... initializers) {
 	 * will be applied to the Spring {@link ApplicationContext}.
 	 * @return the initializers
 	 */
-	public Set<ApplicationContextInitializer<?>> getInitializers() {
+	public Set<ApplicationContextInitializer<? extends ConfigurableApplicationContext>> getInitializers() {
 		return asUnmodifiableOrderedSet(this.initializers);
 	}
 
@@ -1198,7 +1199,7 @@ public Set<ApplicationContextInitializer<?>> getInitializers() {
 	 * and registered with the {@link ApplicationContext}.
 	 * @param listeners the listeners to set
 	 */
-	public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
+	public void setListeners(Collection<? extends ApplicationListener<? extends ApplicationEvent>> listeners) {
 		this.listeners = new ArrayList<>();
 		this.listeners.addAll(listeners);
 	}
@@ -1208,7 +1209,7 @@ public void setListeners(Collection<? extends ApplicationListener<?>> listeners)
 	 * registered with the {@link ApplicationContext}.
 	 * @param listeners the listeners to add
 	 */
-	public void addListeners(ApplicationListener<?>... listeners) {
+	public void addListeners(ApplicationListener<? extends ApplicationEvent>... listeners) {
 		this.listeners.addAll(Arrays.asList(listeners));
 	}
 
@@ -1218,7 +1219,7 @@ public void addListeners(ApplicationListener<?>... listeners) {
 	 * .
 	 * @return the listeners
 	 */
-	public Set<ApplicationListener<?>> getListeners() {
+	public Set<ApplicationListener<? extends ApplicationEvent>> getListeners() {
 		return asUnmodifiableOrderedSet(this.listeners);
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java

@@ -33,6 +33,7 @@
 import org.springframework.boot.WebApplicationType;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -509,7 +510,7 @@ public SpringApplicationBuilder resourceLoader(ResourceLoader resourceLoader) {
 	 * @return the current builder
 	 */
 	public SpringApplicationBuilder initializers(
-			ApplicationContextInitializer<?>... initializers) {
+			ApplicationContextInitializer<? extends ConfigurableApplicationContext>... initializers) {
 		this.application.addInitializers(initializers);
 		return this;
 	}
@@ -522,7 +523,7 @@ public SpringApplicationBuilder initializers(
 	 * @param listeners some listeners to add
 	 * @return the current builder
 	 */
-	public SpringApplicationBuilder listeners(ApplicationListener<?>... listeners) {
+	public SpringApplicationBuilder listeners(ApplicationListener<? extends ApplicationEvent>... listeners) {
 		this.application.addListeners(listeners);
 		return this;
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jdbc/DataSourceBuilder.java

@@ -57,11 +57,11 @@ public final class DataSourceBuilder<T extends DataSource> {
 
 	private Map<String, String> properties = new HashMap<>();
 
-	public static DataSourceBuilder<?> create() {
+	public static DataSourceBuilder<? extends DataSource> create() {
 		return new DataSourceBuilder<>(null);
 	}
 
-	public static DataSourceBuilder<?> create(ClassLoader classLoader) {
+	public static DataSourceBuilder<? extends DataSource> create(ClassLoader classLoader) {
 		return new DataSourceBuilder<>(classLoader);
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/AbstractFilterRegistrationBean.java

@@ -27,6 +27,7 @@
 import javax.servlet.Filter;
 import javax.servlet.FilterRegistration;
 import javax.servlet.FilterRegistration.Dynamic;
+import javax.servlet.Servlet;
 import javax.servlet.ServletContext;
 
 import org.apache.commons.logging.Log;
@@ -93,7 +94,7 @@ public void setServletRegistrationBeans(
 	 * @see #setServletNames
 	 * @see #setUrlPatterns
 	 */
-	public Collection<ServletRegistrationBean<?>> getServletRegistrationBeans() {
+	public Collection<ServletRegistrationBean<? extends Servlet>> getServletRegistrationBeans() {
 		return this.servletRegistrationBeans;
 	}