diff --git a/parent/pom.xml b/parent/pom.xml
index 968711c2..03a55fb4 100644
--- a/parent/pom.xml
+++ b/parent/pom.xml
@@ -179,17 +179,13 @@ application while protecting against XSS.
                 <value>bar</value>
               </property>
             </javaApiLinks>
-            <source>6</source>
+            <source>${maven.compiler.source}</source>
           </configuration>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
-          <version>3.3</version>
-          <configuration>
-            <source>9</source>
-            <target>9</target>
-          </configuration>
+          <version>3.12.1</version>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
diff --git a/src/main/java/org/owasp/html/CollectionsHelper.java b/src/main/java/org/owasp/html/CollectionsHelper.java
new file mode 100644
index 00000000..bb8fa100
--- /dev/null
+++ b/src/main/java/org/owasp/html/CollectionsHelper.java
@@ -0,0 +1,29 @@
+package org.owasp.html;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Internal helper for common Collection creation/copy methods
+ */
+final class CollectionsHelper {
+  public static <E> List<E> copyToUnmodifiableList(Collection<? extends E> list) {
+    final ArrayList<E> newList = new ArrayList<>(list.size());
+    newList.addAll(list);
+    return Collections.unmodifiableList(newList);
+  }
+
+  public static <E> Set<E> copyToUnmodifiableSet(Collection<? extends E> set) {
+    return Collections.unmodifiableSet(new HashSet<E>(set));
+  }
+
+  public static <K, V> Map<K, V> copyToUnmodifiableMap(Map<? extends K, ? extends V> map) {
+    return Collections.unmodifiableMap(new HashMap<>(map));
+  }
+}
diff --git a/src/main/java/org/owasp/html/CssSchema.java b/src/main/java/org/owasp/html/CssSchema.java
index 342dd1ee..8ce2f661 100644
--- a/src/main/java/org/owasp/html/CssSchema.java
+++ b/src/main/java/org/owasp/html/CssSchema.java
@@ -69,8 +69,8 @@ public Property(
         int bits, Set<String> literals,
         Map<String, String> fnKeys) {
       this.bits = bits;
-      this.literals = Set.copyOf(literals);
-      this.fnKeys = Map.copyOf(fnKeys);
+      this.literals = CollectionsHelper.copyToUnmodifiableSet(literals);
+      this.fnKeys = CollectionsHelper.copyToUnmodifiableMap(fnKeys);
     }
 
     @Override
diff --git a/src/main/java/org/owasp/html/ElementAndAttributePolicies.java b/src/main/java/org/owasp/html/ElementAndAttributePolicies.java
index da4a0f2d..fb010440 100644
--- a/src/main/java/org/owasp/html/ElementAndAttributePolicies.java
+++ b/src/main/java/org/owasp/html/ElementAndAttributePolicies.java
@@ -54,7 +54,7 @@ final class ElementAndAttributePolicies {
       HtmlTagSkipType htmlTagSkipType) {
     this.elementName = elementName;
     this.elPolicy = elPolicy;
-    this.attrPolicies = Map.copyOf(attrPolicies);
+    this.attrPolicies = CollectionsHelper.copyToUnmodifiableMap(attrPolicies);
     this.htmlTagSkipType = htmlTagSkipType;
   }
 
diff --git a/src/main/java/org/owasp/html/ElementAndAttributePolicyBasedSanitizerPolicy.java b/src/main/java/org/owasp/html/ElementAndAttributePolicyBasedSanitizerPolicy.java
index 4f78e2c8..df1cfc84 100644
--- a/src/main/java/org/owasp/html/ElementAndAttributePolicyBasedSanitizerPolicy.java
+++ b/src/main/java/org/owasp/html/ElementAndAttributePolicyBasedSanitizerPolicy.java
@@ -63,8 +63,8 @@ class ElementAndAttributePolicyBasedSanitizerPolicy
       Map<String, ElementAndAttributePolicies> elAndAttrPolicies,
       Set<String> allowedTextContainers) {
     this.out = out;
-    this.elAndAttrPolicies = Map.copyOf(elAndAttrPolicies);
-    this.allowedTextContainers = Set.copyOf(allowedTextContainers);
+    this.elAndAttrPolicies = CollectionsHelper.copyToUnmodifiableMap(elAndAttrPolicies);
+    this.allowedTextContainers = CollectionsHelper.copyToUnmodifiableSet(allowedTextContainers);
   }
 
   static final Set<String> SKIPPABLE_ELEMENT_CONTENT
diff --git a/src/main/java/org/owasp/html/HtmlElementTables.java b/src/main/java/org/owasp/html/HtmlElementTables.java
index e230b870..09b0fbbb 100644
--- a/src/main/java/org/owasp/html/HtmlElementTables.java
+++ b/src/main/java/org/owasp/html/HtmlElementTables.java
@@ -401,7 +401,7 @@ public static final class HtmlElementNames {
 
     /** */
     public HtmlElementNames(List<String> canonNames) {
-      this.canonNames = List.copyOf(canonNames);
+      this.canonNames = CollectionsHelper.copyToUnmodifiableList(canonNames);
     }
 
     /** */
diff --git a/src/main/java/org/owasp/html/HtmlPolicyBuilder.java b/src/main/java/org/owasp/html/HtmlPolicyBuilder.java
index 54c0305f..043e874d 100644
--- a/src/main/java/org/owasp/html/HtmlPolicyBuilder.java
+++ b/src/main/java/org/owasp/html/HtmlPolicyBuilder.java
@@ -698,7 +698,7 @@ public PolicyFactory toFactory() {
 
     return new PolicyFactory(
         compiled.compiledPolicies, Collections.unmodifiableSet(textContainerSetBuilder),
-        Map.copyOf(compiled.globalAttrPolicies),
+        CollectionsHelper.copyToUnmodifiableMap(compiled.globalAttrPolicies),
         preprocessor, postprocessor);
   }
 
@@ -740,7 +740,7 @@ private CompiledState compilePolicies() {
     Map<String, AttributePolicy> globalAttrPolicies
         = new LinkedHashMap<>(this.globalAttrPolicies);
     @SuppressWarnings("hiding")
-    Set<String> allowedProtocols = Set.copyOf(this.allowedProtocols);
+    Set<String> allowedProtocols = CollectionsHelper.copyToUnmodifiableSet(this.allowedProtocols);
 
     // Implement requireRelsOnLinks & skip...
     {
@@ -874,7 +874,7 @@ public final class AttributeBuilder {
     private AttributePolicy policy = AttributePolicy.IDENTITY_ATTRIBUTE_POLICY;
 
     AttributeBuilder(List<? extends String> attributeNames) {
-      this.attributeNames = List.copyOf(attributeNames);
+      this.attributeNames = CollectionsHelper.copyToUnmodifiableList(attributeNames);
     }
 
     /**
@@ -939,7 +939,7 @@ public AttributeBuilder matching(
      */
     public AttributeBuilder matching(
         final boolean ignoreCase, Set<? extends String> allowedValues) {
-      final Set<String> allowed = Set.copyOf(allowedValues);
+      final Set<String> allowed = CollectionsHelper.copyToUnmodifiableSet(allowedValues);
       return matching(new AttributePolicy() {
         public @Nullable String apply(
             String elementName, String attributeName, String uncanonValue) {
@@ -1007,13 +1007,13 @@ static RelsOnLinksPolicy create(
     RelsOnLinksPolicy(
         Set<? extends String> extra,
         Set<? extends String> skip) {
-      this.extra = Set.copyOf(extra);
-      this.skip = Set.copyOf(skip);
+      this.extra = CollectionsHelper.copyToUnmodifiableSet(extra);
+      this.skip = CollectionsHelper.copyToUnmodifiableSet(skip);
       Set<String> targetOnly = new HashSet<>();
       targetOnly.addAll(DEFAULT_RELS_ON_TARGETTED_LINKS);
       targetOnly.removeAll(extra);
       targetOnly.removeAll(skip);
-      this.whenTargetPresent = List.copyOf(targetOnly);
+      this.whenTargetPresent = CollectionsHelper.copyToUnmodifiableList(targetOnly);
     }
 
     private static int indexOfAttributeValue(
diff --git a/src/test/java/org/owasp/html/Benchmark.java b/src/test/java/org/owasp/html/Benchmark.java
index ea172b7d..b69c1af1 100644
--- a/src/test/java/org/owasp/html/Benchmark.java
+++ b/src/test/java/org/owasp/html/Benchmark.java
@@ -28,10 +28,10 @@
 
 package org.owasp.html;
 
-import java.io.File;
 import java.io.StringReader;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
+import java.nio.file.Paths;
 import java.util.List;
 import java.util.ListIterator;
 
@@ -58,7 +58,7 @@ public class Benchmark {
    * specifies a benchmark to run and unspecified ones are not run.
    */
   public static void main(String[] args) throws Exception {
-    String html = new String(Files.readAllBytes(new File(args[0]).toPath()), StandardCharsets.UTF_8);
+    String html = new String(Files.readAllBytes(Paths.get(args[0])), StandardCharsets.UTF_8);
 
     boolean timeLibhtmlparser = true;
     boolean timeSanitize = true;