Add nullability contract to PasswordEncoder#encode implementations

Signed-off-by: Stefano Cordio <stefano.cordio@gmail.com>

Author: scordio

config/src/main/java/org/springframework/security/config/annotation/authentication/configuration/AuthenticationConfiguration.java

@@ -36,6 +36,7 @@
 import org.springframework.context.annotation.Import;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.core.log.LogMessage;
+import org.springframework.lang.Contract;
 import org.springframework.security.authentication.AuthenticationEventPublisher;
 import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.DefaultAuthenticationEventPublisher;
@@ -301,6 +302,7 @@ static class LazyPasswordEncoder implements PasswordEncoder {
 			this.applicationContext = applicationContext;
 		}
 
+		@Contract("!null -> !null; null -> null")
 		@Override
 		public String encode(CharSequence rawPassword) {
 			return getPasswordEncoder().encode(rawPassword);

config/src/main/java/org/springframework/security/config/annotation/web/configuration/HttpSecurityConfiguration.java

@@ -30,6 +30,7 @@
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.support.SpringFactoriesLoader;
+import org.springframework.lang.Contract;
 import org.springframework.security.authentication.AuthenticationEventPublisher;
 import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.DefaultAuthenticationEventPublisher;
@@ -292,6 +293,7 @@ static class LazyPasswordEncoder implements PasswordEncoder {
 			this.applicationContext = applicationContext;
 		}
 
+		@Contract("!null -> !null; null -> null")
 		@Override
 		public String encode(CharSequence rawPassword) {
 			return getPasswordEncoder().encode(rawPassword);

crypto/src/main/java/org/springframework/security/crypto/password/AbstractValidatingPasswordEncoder.java

@@ -18,9 +18,12 @@
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.lang.Contract;
+
 public abstract class AbstractValidatingPasswordEncoder implements PasswordEncoder {
 
 	@Override
+	@Contract("!null -> !null; null -> null")
 	public final @Nullable String encode(@Nullable CharSequence rawPassword) {
 		if (rawPassword == null) {
 			return null;

crypto/src/main/java/org/springframework/security/crypto/password/PasswordEncoder.java

@@ -38,7 +38,7 @@ public interface PasswordEncoder {
 	 * @return A non-null encoded password, unless the rawPassword was null in which case
 	 * the result must be null.
 	 */
-	@Contract("null -> null; !null -> !null")
+	@Contract("!null -> !null; null -> null")
 	@Nullable String encode(@Nullable CharSequence rawPassword);
 
 	/**