diff --git a/pom.xml b/pom.xml
index f326abb..6900d3e 100644
--- a/pom.xml
+++ b/pom.xml
@@ -40,7 +40,7 @@
     <gpg.plugin.version>3.1.0</gpg.plugin.version>
     <staging.plugin.version>1.6.13</staging.plugin.version>
     <checkstyle.plugin.version>3.3.1</checkstyle.plugin.version>
-    <spotbugs.plugin.version>4.7.3.6</spotbugs.plugin.version>
+    <spotbugs.plugin.version>4.8.1.0</spotbugs.plugin.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
   </properties>
 
diff --git a/src/main/software/amazon/event/ruler/Event.java b/src/main/software/amazon/event/ruler/Event.java
index 3fe8af9..5893593 100644
--- a/src/main/software/amazon/event/ruler/Event.java
+++ b/src/main/software/amazon/event/ruler/Event.java
@@ -46,7 +46,7 @@
 // TODO: Improve unit-test coverage, there are surprising gaps
 @Immutable
 @ThreadSafe
-class Event {
+final class Event {
 
     private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
     private static final JsonFactory JSON_FACTORY = new JsonFactory();
diff --git a/src/main/software/amazon/event/ruler/IntIntMap.java b/src/main/software/amazon/event/ruler/IntIntMap.java
index 46b054a..8803d5c 100644
--- a/src/main/software/amazon/event/ruler/IntIntMap.java
+++ b/src/main/software/amazon/event/ruler/IntIntMap.java
@@ -7,7 +7,7 @@
 /**
  * A fast primitive int-int map implementation. Keys and values may only be positive.
  */
-class IntIntMap implements Cloneable {
+final class IntIntMap implements Cloneable {
 
     // taken from FastUtil
     private static final int INT_PHI = 0x9E3779B9;
diff --git a/src/main/software/amazon/event/ruler/Range.java b/src/main/software/amazon/event/ruler/Range.java
index 9a79a86..39454a0 100644
--- a/src/main/software/amazon/event/ruler/Range.java
+++ b/src/main/software/amazon/event/ruler/Range.java
@@ -8,7 +8,7 @@
  * "Numeric" means that the character repertoire is "digits"; initially, either 0-9 or 0-9a-f. In the current
  *  implementation, the number of digits in the top and bottom of the range is the same.
  */
-public class Range extends Patterns {
+public final class Range extends Patterns {
     /**
      * Bottom and top of the range. openBottom true means we're looking for > bottom, false means >=
      *  Similarly, openTop true means we're looking for < top, false means <= top.
diff --git a/src/main/software/amazon/event/ruler/input/MultiByte.java b/src/main/software/amazon/event/ruler/input/MultiByte.java
index 7b3c32e..c7ea117 100644
--- a/src/main/software/amazon/event/ruler/input/MultiByte.java
+++ b/src/main/software/amazon/event/ruler/input/MultiByte.java
@@ -9,7 +9,7 @@
  * A grouping of multiple bytes. This can be used to represent a character that has a UTF-8 representation requiring
  * multiple bytes.
  */
-public class MultiByte {
+public final class MultiByte {
 
     public static final byte MIN_FIRST_BYTE_FOR_ONE_BYTE_CHAR = (byte) 0x00;
     public static final byte MAX_FIRST_BYTE_FOR_ONE_BYTE_CHAR = (byte) 0x7F;