Properly documenting null behavior of User-related methods.

core/src/main/java/hudson/model/User.java

@@ -75,6 +75,9 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import javax.annotation.CheckForNull;
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 /**
  * Represents a user.
@@ -279,7 +282,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
      * <p>
      * This is used to avoid null {@link User} instance.
      */
-    public static User getUnknown() {
+    public static @Nonnull User getUnknown() {
         return get("unknown");
     }
 
@@ -292,7 +295,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
      *      If false, this method will return null if {@link User} object
      *      with the given name doesn't exist.
      */
-    public static User get(String idOrFullName, boolean create) {
+    public static @Nullable User get(String idOrFullName, boolean create) {
         if(idOrFullName==null)
             return null;
         String id = idOrFullName.replace('\\', '_').replace('/', '_').replace('<','_')
@@ -314,7 +317,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
     /**
      * Gets the {@link User} object by its id or full name.
      */
-    public static User get(String idOrFullName) {
+    public static @Nonnull User get(String idOrFullName) {
         return get(idOrFullName,true);
     }
 
@@ -323,7 +326,7 @@ public class User extends AbstractModelObject implements AccessControlled, Descr
      * if the current user is anonymous.
      * @since 1.172
      */
-    public static User current() {
+    public static @CheckForNull User current() {
         Authentication a = Jenkins.getAuthentication();
         if(a instanceof AnonymousAuthenticationToken)
             return null;

core/src/main/java/jenkins/model/Jenkins.java

@@ -298,6 +298,7 @@ import static java.util.logging.Level.SEVERE;
 import java.util.logging.LogRecord;
 import java.util.logging.Logger;
 import java.util.regex.Pattern;
+import javax.annotation.CheckForNull;
 
 /**
  * Root object of the system.
@@ -2290,10 +2291,10 @@ public class Jenkins extends AbstractCIBase implements ModifiableItemGroup<TopLe
     /**
      * Gets the user of the given name.
      *
-     * @return
-     *      This method returns a non-null object for any user name, without validation.
+     * @return the user of the given name, if that person exists or the invoker {@link #hasPermission} on {@link #ADMINISTER}; else null
+     * @see User#get(String,boolean)
      */
-    public User getUser(String name) {
+    public @CheckForNull User getUser(String name) {
         return User.get(name,hasPermission(ADMINISTER));
     }