Fix equals nullability annotations for jspecify compliance

In this commit, we added `@Nullable` to equals methods of classes that
support `jspecify` for consistency with other Spring projects and to
avoid bugs that caused other Spring projects to do this natively.

Closes: gh-18929, gh-18927

Signed-off-by: Andrey Litvitski <andrey1010102008@gmail.com>

acl/src/main/java/org/springframework/security/acls/domain/AbstractPermission.java

@@ -16,6 +16,8 @@
 
 package org.springframework.security.acls.domain;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.security.acls.model.Permission;
 
 /**
@@ -52,7 +54,7 @@ public abstract class AbstractPermission implements Permission {
 	}
 
 	@Override
-	public final boolean equals(Object obj) {
+	public final boolean equals(@Nullable Object obj) {
 		if (obj == null) {
 			return false;
 		}

acl/src/main/java/org/springframework/security/acls/domain/AccessControlEntryImpl.java

@@ -63,7 +63,7 @@ public class AccessControlEntryImpl implements AccessControlEntry, AuditableAcce
 	}
 
 	@Override
-	public boolean equals(Object arg0) {
+	public boolean equals(@Nullable Object arg0) {
 		if (!(arg0 instanceof AccessControlEntryImpl)) {
 			return false;
 		}

acl/src/main/java/org/springframework/security/acls/domain/AclImpl.java

@@ -278,7 +278,7 @@ public class AclImpl implements Acl, MutableAcl, AuditableAcl, OwnershipAcl {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (obj == this) {
 			return true;
 		}

acl/src/main/java/org/springframework/security/acls/domain/GrantedAuthoritySid.java

@@ -16,6 +16,8 @@
 
 package org.springframework.security.acls.domain;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.security.acls.model.Sid;
 import org.springframework.security.core.GrantedAuthority;
 import org.springframework.util.Assert;
@@ -47,7 +49,7 @@ public class GrantedAuthoritySid implements Sid {
 	}
 
 	@Override
-	public boolean equals(Object object) {
+	public boolean equals(@Nullable Object object) {
 		if ((object == null) || !(object instanceof GrantedAuthoritySid)) {
 			return false;
 		}

acl/src/main/java/org/springframework/security/acls/domain/ObjectIdentityImpl.java

@@ -19,6 +19,8 @@ package org.springframework.security.acls.domain;
 import java.io.Serializable;
 import java.lang.reflect.Method;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.security.acls.model.ObjectIdentity;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -97,7 +99,7 @@ public class ObjectIdentityImpl implements ObjectIdentity {
 	 * @return <code>true</code> if the presented object matches this object
 	 */
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (obj == null || !(obj instanceof ObjectIdentityImpl)) {
 			return false;
 		}

acl/src/main/java/org/springframework/security/acls/domain/PrincipalSid.java

@@ -16,6 +16,8 @@
 
 package org.springframework.security.acls.domain;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.security.acls.model.Sid;
 import org.springframework.security.core.Authentication;
 import org.springframework.util.Assert;
@@ -46,7 +48,7 @@ public class PrincipalSid implements Sid {
 	}
 
 	@Override
-	public boolean equals(Object object) {
+	public boolean equals(@Nullable Object object) {
 		if ((object == null) || !(object instanceof PrincipalSid)) {
 			return false;
 		}

acl/src/main/java/org/springframework/security/acls/model/ObjectIdentity.java

@@ -18,6 +18,8 @@ package org.springframework.security.acls.model;
 
 import java.io.Serializable;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Represents the identity of an individual domain object instance.
  *
@@ -40,7 +42,7 @@ public interface ObjectIdentity extends Serializable {
 	 * @see Object#equals(Object)
 	 */
 	@Override
-	boolean equals(Object obj);
+	boolean equals(@Nullable Object obj);
 
 	/**
 	 * Obtains the actual identifier. This identifier must not be reused to represent

acl/src/main/java/org/springframework/security/acls/model/Sid.java

@@ -18,6 +18,8 @@ package org.springframework.security.acls.model;
 
 import java.io.Serializable;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * A security identity recognised by the ACL system.
  *
@@ -40,7 +42,7 @@ public interface Sid extends Serializable {
 	 * @return <code>true</code> if the objects are equal, <code>false</code> otherwise
 	 */
 	@Override
-	boolean equals(Object obj);
+	boolean equals(@Nullable Object obj);
 
 	/**
 	 * Refer to the <code>java.lang.Object</code> documentation for the interface