diff --git a/core/src/main/java/org/kohsuke/stapler/CaptureParameterNameTransformation.java b/core/src/main/java/org/kohsuke/stapler/CaptureParameterNameTransformation.java
index 6a456dc13b..0f784e56e1 100644
--- a/core/src/main/java/org/kohsuke/stapler/CaptureParameterNameTransformation.java
+++ b/core/src/main/java/org/kohsuke/stapler/CaptureParameterNameTransformation.java
@@ -36,7 +36,6 @@
 import org.codehaus.groovy.transform.GroovyASTTransformation;
 import org.kohsuke.MetaInfServices;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
@@ -60,8 +59,8 @@ private void handleClasses(List<ClassNode> classNodes) {
     }
 
     // set of annotation class names to capture
-    private static final Set<String> CONSTRUCTOR_ANN = Collections.singleton(DataBoundConstructor.class.getName());
-    private static final Set<String> INJECTED_PARAMETER_ANN = Collections.singleton(InjectedParameter.class.getName());
+    private static final Set<String> CONSTRUCTOR_ANN = Set.of(DataBoundConstructor.class.getName());
+    private static final Set<String> INJECTED_PARAMETER_ANN = Set.of(InjectedParameter.class.getName());
 
     private void handleMethods(List<MethodNode> methods) {
         for (MethodNode m : methods)
diff --git a/core/src/main/java/org/kohsuke/stapler/Stapler.java b/core/src/main/java/org/kohsuke/stapler/Stapler.java
index 1ba45ab2c6..9b36115088 100644
--- a/core/src/main/java/org/kohsuke/stapler/Stapler.java
+++ b/core/src/main/java/org/kohsuke/stapler/Stapler.java
@@ -52,7 +52,6 @@
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.io.PrintWriter;
-import java.io.UnsupportedEncodingException;
 import java.lang.reflect.InvocationTargetException;
 import java.net.JarURLConnection;
 import java.net.MalformedURLException;
@@ -60,6 +59,7 @@
 import java.net.URL;
 import java.net.URLConnection;
 import java.net.URLDecoder;
+import java.nio.charset.StandardCharsets;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
 import java.util.ArrayList;
@@ -671,11 +671,7 @@ private String getMimeType(String fileName) {
         //  File(String) does fs.normalize, which is really forgiving in fixing up
         // malformed stuff. I couldn't make the other URL.toURI() or File(URI) work
         // in all the cases that we test
-        try {
-            return new File(URLDecoder.decode(urlstr.substring(5),"UTF-8"));
-        } catch (UnsupportedEncodingException x) {
-            throw new AssertionError(x);
-        }
+        return new File(URLDecoder.decode(urlstr.substring(5), StandardCharsets.UTF_8));
     }
 
     /**
diff --git a/core/src/main/java/org/kohsuke/stapler/TokenList.java b/core/src/main/java/org/kohsuke/stapler/TokenList.java
index b86aea1d5a..0041347632 100644
--- a/core/src/main/java/org/kohsuke/stapler/TokenList.java
+++ b/core/src/main/java/org/kohsuke/stapler/TokenList.java
@@ -168,14 +168,14 @@ public static String decode(String s) {
                     }
                 } else {
                     if (baos.size()>0) {
-                        buf.append(new String(baos.toByteArray(),StandardCharsets.UTF_8));
+                        buf.append(baos.toString(StandardCharsets.UTF_8));
                         baos.reset();
                     }
                     buf.append(c);
                 }
             }
             if (baos.size()>0)
-                buf.append(new String(baos.toByteArray(),StandardCharsets.UTF_8));
+                buf.append(baos.toString(StandardCharsets.UTF_8));
             return buf.toString();
     }
 
diff --git a/core/src/main/java/org/kohsuke/stapler/bind/Bound.java b/core/src/main/java/org/kohsuke/stapler/bind/Bound.java
index 68b924e7e4..299328a223 100644
--- a/core/src/main/java/org/kohsuke/stapler/bind/Bound.java
+++ b/core/src/main/java/org/kohsuke/stapler/bind/Bound.java
@@ -30,7 +30,7 @@
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.Collections;
+import java.util.Set;
 
 /**
  * Handles to the object bound via {@link BoundObjectTable}.
@@ -71,13 +71,13 @@ public final String getProxyScript() {
         for (Method m : getTarget().getClass().getMethods()) {
             Collection<String> names;
             if (m.getName().startsWith("js")) {
-                names = Collections.singleton(camelize(m.getName().substring(2)));
+                names = Set.of(camelize(m.getName().substring(2)));
             } else {
                 JavaScriptMethod a = m.getAnnotation(JavaScriptMethod.class);
                 if (a!=null) {
                     names = Arrays.asList(a.name());
                     if (names.isEmpty())
-                        names = Collections.singleton(m.getName());
+                        names = Set.of(m.getName());
                 } else
                     continue;
             }
diff --git a/core/src/main/java/org/kohsuke/stapler/export/Model.java b/core/src/main/java/org/kohsuke/stapler/export/Model.java
index a9bc97402e..bb80497281 100644
--- a/core/src/main/java/org/kohsuke/stapler/export/Model.java
+++ b/core/src/main/java/org/kohsuke/stapler/export/Model.java
@@ -29,7 +29,6 @@
 import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Properties;
@@ -121,7 +120,7 @@ public class Model<T> {
      * Gets all the exported properties.
      */
     public List<Property> getProperties() {
-        return Collections.unmodifiableList(Arrays.asList(properties));
+        return List.of(properties);
     }
 
     /**
diff --git a/core/src/main/java/org/kohsuke/stapler/framework/io/WriterOutputStream.java b/core/src/main/java/org/kohsuke/stapler/framework/io/WriterOutputStream.java
index 09d4bedc18..66e2593906 100644
--- a/core/src/main/java/org/kohsuke/stapler/framework/io/WriterOutputStream.java
+++ b/core/src/main/java/org/kohsuke/stapler/framework/io/WriterOutputStream.java
@@ -87,8 +87,7 @@ public void flush() throws IOException {
 
     private void flushOutput() throws IOException {
         writer.write(out.array(),0,out.position());
-        // Downcasting to Buffer is needed to avoid NoSuchMethodError errors because Java 11 uses covariance in the ByteBuffer return type.
-        ((Buffer) out).clear();
+        out.clear();
     }
 
     @Override
@@ -97,8 +96,7 @@ public void close() throws IOException {
         flushOutput();
         writer.close();
 
-        // Downcasting to Buffer is needed to avoid NoSuchMethodError errors because Java 11 uses covariance in the ByteBuffer return type.
-        ((Buffer) buf).rewind();
+        buf.rewind();
     }
 
     /**
@@ -113,8 +111,7 @@ public void close() throws IOException {
      *      if true, tell the decoder that all the input bytes are ready.
      */
     private void decode(boolean last) throws IOException {
-        // Downcasting to Buffer is needed to avoid NoSuchMethodError errors because Java 11 uses covariance in the ByteBuffer return type.
-        ((Buffer) buf).flip();
+        buf.flip();
         while(true) {
             CoderResult r = decoder.decode(buf, out, last);
             if(r==CoderResult.OVERFLOW) {
diff --git a/core/src/main/java/org/kohsuke/stapler/jsr269/ConstructorProcessor.java b/core/src/main/java/org/kohsuke/stapler/jsr269/ConstructorProcessor.java
index 258e62508e..5eaf2089da 100644
--- a/core/src/main/java/org/kohsuke/stapler/jsr269/ConstructorProcessor.java
+++ b/core/src/main/java/org/kohsuke/stapler/jsr269/ConstructorProcessor.java
@@ -12,7 +12,7 @@
 import javax.lang.model.element.ExecutableElement;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.element.VariableElement;
-import javax.lang.model.util.ElementScanner6;
+import javax.lang.model.util.ElementScanner9;
 import java.io.IOException;
 import java.util.HashSet;
 import java.util.Properties;
@@ -31,7 +31,7 @@ public class ConstructorProcessor extends AbstractProcessorImpl {
     @Override
     public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
         try {
-            ElementScanner6<Void, Void> scanner = new ElementScanner6<Void, Void>() {
+            ElementScanner9<Void, Void> scanner = new ElementScanner9<Void, Void>() {
                 Set<Element> enclosingElementsWritten = new HashSet<>();
                 boolean messagePrinted;
 
diff --git a/core/src/test/java/org/kohsuke/stapler/RequestImplTest.java b/core/src/test/java/org/kohsuke/stapler/RequestImplTest.java
index da57a4bcb7..9af118ddb6 100644
--- a/core/src/test/java/org/kohsuke/stapler/RequestImplTest.java
+++ b/core/src/test/java/org/kohsuke/stapler/RequestImplTest.java
@@ -122,7 +122,7 @@ public int getContentLength() {
 
             @Override
             public Enumeration getParameterNames() {
-                return Collections.enumeration(Collections.singletonList("p1"));
+                return Collections.enumeration(List.of("p1"));
             }
 
             @Override
diff --git a/core/src/test/java/org/kohsuke/stapler/export/ModelTest.java b/core/src/test/java/org/kohsuke/stapler/export/ModelTest.java
index b26eaca5b0..cd8d250182 100644
--- a/core/src/test/java/org/kohsuke/stapler/export/ModelTest.java
+++ b/core/src/test/java/org/kohsuke/stapler/export/ModelTest.java
@@ -27,7 +27,6 @@
 import java.io.StringWriter;
 import java.lang.reflect.InvocationTargetException;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 
 import org.junit.Assert;
@@ -53,7 +52,7 @@ public static class Impl implements GenericInterface<Integer> {
         @Override
         @Exported
         public List<Integer> get() {
-            return Collections.singletonList(42);
+            return List.of(42);
         }
     }
 
diff --git a/core/src/test/java/org/kohsuke/stapler/export/XMLDataWriterTest.java b/core/src/test/java/org/kohsuke/stapler/export/XMLDataWriterTest.java
index 047e930ee6..e7fd11cfce 100644
--- a/core/src/test/java/org/kohsuke/stapler/export/XMLDataWriterTest.java
+++ b/core/src/test/java/org/kohsuke/stapler/export/XMLDataWriterTest.java
@@ -4,9 +4,9 @@
 import java.io.StringReader;
 import java.io.StringWriter;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Set;
 import javax.xml.parsers.SAXParser;
 import javax.xml.parsers.SAXParserFactory;
 
@@ -35,7 +35,7 @@ public String getName(){
 
         @Exported
         public Collection<Job> getJobs(){
-            return Collections.singleton(new Job());
+            return Set.of(new Job());
         }
     }
 
@@ -65,12 +65,12 @@ public String getName() {
         //        @Exported(visibility = 2)
         @Override
         public Iterator<ParameterValue> iterator() {
-            return Collections.singleton(new ParameterValue()).iterator();
+            return Set.of(new ParameterValue()).iterator();
         }
 
         @Exported(visibility = 2)
         public List<ParameterValue> getParameters(){
-            return Collections.singletonList(new ParameterValue());
+            return List.of(new ParameterValue());
         }
     }
 
diff --git a/core/src/test/java/org/kohsuke/stapler/lang/KlassTest.java b/core/src/test/java/org/kohsuke/stapler/lang/KlassTest.java
index 48206f641d..c0e91eb8dc 100644
--- a/core/src/test/java/org/kohsuke/stapler/lang/KlassTest.java
+++ b/core/src/test/java/org/kohsuke/stapler/lang/KlassTest.java
@@ -56,7 +56,7 @@ private static final class FooClass {
         
         public Object doDynamic(String token) {
             // Just return something potentially routable
-            return new Integer(0);
+            return Integer.valueOf(0);
         }
     }
 }
diff --git a/groovy/src/main/java/org/kohsuke/stapler/jelly/groovy/GroovyFacet.java b/groovy/src/main/java/org/kohsuke/stapler/jelly/groovy/GroovyFacet.java
index bcc0202223..75a768a728 100644
--- a/groovy/src/main/java/org/kohsuke/stapler/jelly/groovy/GroovyFacet.java
+++ b/groovy/src/main/java/org/kohsuke/stapler/jelly/groovy/GroovyFacet.java
@@ -40,7 +40,6 @@
 import javax.servlet.ServletException;
 import java.io.IOException;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 import java.util.logging.Level;
@@ -99,7 +98,7 @@ public boolean handleIndexRequest(RequestImpl req, ResponseImpl rsp, Object node
                 handleIndexRequest(nodeMetaClass.loadTearOff(GroovyServerPageTearOff.class), scriptExecutor, req, rsp, node);
     }
 
-    private static final Set<Class<GroovyClassTearOff>> TEAROFF_TYPES = Collections.singleton(GroovyClassTearOff.class);
+    private static final Set<Class<GroovyClassTearOff>> TEAROFF_TYPES = Set.of(GroovyClassTearOff.class);
 
-    private static final Set<String> EXTENSION = Collections.singleton(".groovy");
+    private static final Set<String> EXTENSION = Set.of(".groovy");
 }
diff --git a/groovy/src/test/java/org/kohsuke/stapler/jelly/groovy/GroovyClassLoaderTearOffTest.java b/groovy/src/test/java/org/kohsuke/stapler/jelly/groovy/GroovyClassLoaderTearOffTest.java
index d238dcd1b6..5fdeaa596d 100644
--- a/groovy/src/test/java/org/kohsuke/stapler/jelly/groovy/GroovyClassLoaderTearOffTest.java
+++ b/groovy/src/test/java/org/kohsuke/stapler/jelly/groovy/GroovyClassLoaderTearOffTest.java
@@ -54,7 +54,7 @@ public void testGettext() throws Exception {
             GroovyClassLoaderTearOff t = mcl.getTearOff(GroovyClassLoaderTearOff.class);
 
             Files.write(tmp, "output.write(_('localizable'))".getBytes(StandardCharsets.UTF_8));
-            Files.write(tmp.resolveSibling(tmp.getFileName().toString().replaceFirst("[.]groovy$", ".properties")), "localizable=Localizable".getBytes(StandardCharsets.ISO_8859_1));
+            Files.writeString(tmp.resolveSibling(tmp.getFileName().toString().replaceFirst("[.]groovy$", ".properties")), "localizable=Localizable", StandardCharsets.ISO_8859_1);
 
             JellyContext context = new JellyContext();
             ByteArrayOutputStream baos = new ByteArrayOutputStream();
diff --git a/jelly/src/main/java/org/kohsuke/stapler/jelly/AnnotationProcessorImpl.java b/jelly/src/main/java/org/kohsuke/stapler/jelly/AnnotationProcessorImpl.java
index 041da2a55c..7357df8959 100644
--- a/jelly/src/main/java/org/kohsuke/stapler/jelly/AnnotationProcessorImpl.java
+++ b/jelly/src/main/java/org/kohsuke/stapler/jelly/AnnotationProcessorImpl.java
@@ -33,7 +33,6 @@
 import javax.lang.model.util.ElementFilter;
 import javax.tools.Diagnostic.Kind;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -50,7 +49,7 @@ private static class MissingViews extends HashSet<String> {}
 
     @Override
     public Set<String> getSupportedAnnotationTypes() {
-        return Collections.singleton("*");
+        return Set.of("*");
     }
 
     @Override
diff --git a/jelly/src/main/java/org/kohsuke/stapler/jelly/InternationalizedStringExpression.java b/jelly/src/main/java/org/kohsuke/stapler/jelly/InternationalizedStringExpression.java
index 3e89704550..e2dc73b4ce 100644
--- a/jelly/src/main/java/org/kohsuke/stapler/jelly/InternationalizedStringExpression.java
+++ b/jelly/src/main/java/org/kohsuke/stapler/jelly/InternationalizedStringExpression.java
@@ -33,8 +33,6 @@
 
 import java.util.List;
 import java.util.ArrayList;
-import java.util.Collections;
-import java.util.Arrays;
 
 /**
  * Expression of the form "%messageName(arg1,arg2,...)" that represents
@@ -86,7 +84,7 @@ public InternationalizedStringExpression(ResourceBundle resourceBundle, String t
     }
     
     public List<Expression> getArguments() {
-        return Collections.unmodifiableList(Arrays.asList(arguments));
+        return List.of(arguments);
     }
 
     /**
diff --git a/jelly/src/main/java/org/kohsuke/stapler/jelly/JellyFacet.java b/jelly/src/main/java/org/kohsuke/stapler/jelly/JellyFacet.java
index 8e97303b4f..eac5ce8f76 100644
--- a/jelly/src/main/java/org/kohsuke/stapler/jelly/JellyFacet.java
+++ b/jelly/src/main/java/org/kohsuke/stapler/jelly/JellyFacet.java
@@ -41,7 +41,6 @@
 import javax.servlet.ServletException;
 import java.io.IOException;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 import java.util.logging.Level;
@@ -131,7 +130,7 @@ public static void setExpressionFactory( ServletContextEvent event, ExpressionFa
     @SuppressFBWarnings(value = "MS_SHOULD_BE_FINAL", justification = "Legacy switch.")
     public static boolean TRACE = Boolean.getBoolean("stapler.jelly.trace");
 
-    private static final Set<Class<JellyClassTearOff>> TEAROFF_TYPES = Collections.singleton(JellyClassTearOff.class);
+    private static final Set<Class<JellyClassTearOff>> TEAROFF_TYPES = Set.of(JellyClassTearOff.class);
 
-    private static final Set<String> EXTENSION = Collections.singleton(".jelly");
+    private static final Set<String> EXTENSION = Set.of(".jelly");
 }