Index: oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/GroupPrincipal.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/GroupPrincipal.java	(revision 1862145)
+++ oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/GroupPrincipal.java	(date 1565965076000)
@@ -19,6 +19,7 @@
 import java.security.Principal;
 import java.util.Enumeration;
 
+import org.jetbrains.annotations.NotNull;
 import org.osgi.annotation.versioning.ProviderType;
 
 /**
@@ -37,7 +38,7 @@
      * @return true if the principal is a member of this group,
      * false otherwise.
      */
-    public boolean isMember(Principal member);
+    boolean isMember(@NotNull Principal member);
 
     /**
      * Returns an enumeration of the members in the group. This includes both
@@ -47,6 +48,7 @@
      *
      * @return an enumeration of the group members.
      */
-    public Enumeration<? extends Principal> members();
+    @NotNull
+    Enumeration<? extends Principal> members();
 
 }
Index: oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/ItemBasedPrincipal.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/ItemBasedPrincipal.java	(revision 1862145)
+++ oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/ItemBasedPrincipal.java	(date 1561560298000)
@@ -16,6 +16,8 @@
  */
 package org.apache.jackrabbit.api.security.principal;
 
+import org.jetbrains.annotations.NotNull;
+
 import javax.jcr.RepositoryException;
 
 /**
@@ -35,5 +37,6 @@
      * @throws RepositoryException If an error occurs while retrieving the
      * <code>Item</code> path.
      */
+    @NotNull
     String getPath() throws RepositoryException;
 }
Index: oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalIterator.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalIterator.java	(revision 1862145)
+++ oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalIterator.java	(date 1561560298000)
@@ -16,6 +16,8 @@
  */
 package org.apache.jackrabbit.api.security.principal;
 
+import org.jetbrains.annotations.NotNull;
+
 import javax.jcr.RangeIterator;
 import java.security.Principal;
 
@@ -29,6 +31,7 @@
      * 
      * @return the next principal
      */
+    @NotNull
     Principal nextPrincipal();
 
 }
Index: oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalManager.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalManager.java	(revision 1862145)
+++ oak-jackrabbit-api/src/main/java/org/apache/jackrabbit/api/security/principal/PrincipalManager.java	(date 1565964611000)
@@ -16,6 +16,9 @@
  */
 package org.apache.jackrabbit.api.security.principal;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
 import java.security.Principal;
 
 /**
@@ -75,7 +78,7 @@
      * @return return <code>true</code> if the principal with this name is known
      *         to this manager; <code>false</code> otherwise.
      */
-    boolean hasPrincipal(String principalName);
+    boolean hasPrincipal(@NotNull String principalName);
 
     /**
      * Returns the principal with the given name if is known to this manager
@@ -89,7 +92,8 @@
      * principal with the given name does not exist or is not accessible
      * for the editing session.
      */
-    Principal getPrincipal(String principalName);
+    @Nullable
+    Principal getPrincipal(@NotNull String principalName);
 
     /**
      * Gets the principals matching a simple filter expression applied against
@@ -102,7 +106,8 @@
      * @return a <code>PrincipalIterator</code> over the <code>Principal</code>s
      * matching the given filter.
      */
-    PrincipalIterator findPrincipals(String simpleFilter);
+    @NotNull
+    PrincipalIterator findPrincipals(@Nullable String simpleFilter);
 
     /**
      * Gets the principals matching a simple filter expression applied against
@@ -122,7 +127,8 @@
      * @return a <code>PrincipalIterator</code> over the <code>Principal</code>s
      * matching the given filter and search type.
      */
-    PrincipalIterator findPrincipals(String simpleFilter, int searchType);
+    @NotNull
+    PrincipalIterator findPrincipals(@Nullable String simpleFilter, int searchType);
 
     /**
      * Returns all <code>Principal</code>s matching the specified search type.
@@ -136,6 +142,7 @@
      * @return a <code>PrincipalIterator</code> over all the <code>Principal</code>s
      * matching the given search type.
      */
+    @NotNull
     PrincipalIterator getPrincipals(int searchType);
 
     /**
@@ -149,7 +156,8 @@
      * @param principal the principal to return it's membership from.
      * @return an iterator returning all groups the given principal is member of.
      */
-    PrincipalIterator getGroupMembership(Principal principal);
+    @NotNull
+    PrincipalIterator getGroupMembership(@NotNull Principal principal);
 
     /**
      * Returns the <code>Principal</code> which is implicitly applied to
@@ -157,5 +165,6 @@
      *
      * @return the 'everyone' principal
      */
+    @NotNull
     Principal getEveryone();
 }