diff --git a/testng-core/src/main/java/org/testng/internal/ClassImpl.java b/testng-core/src/main/java/org/testng/internal/ClassImpl.java
index cc682033a7..9bc821ef59 100644
--- a/testng-core/src/main/java/org/testng/internal/ClassImpl.java
+++ b/testng-core/src/main/java/org/testng/internal/ClassImpl.java
@@ -173,13 +173,12 @@ private static int computeHashCode(IdentifiableObject identifiable) {
   }
 
   private DetailedAttributes newDetailedAttributes(boolean create, String errMsgPrefix) {
-    DetailedAttributes ea = new DetailedAttributes();
-    ea.setXmlTest(m_testContext.getCurrentXmlTest());
-    ea.setClasses(m_classes);
-    ea.setFinder(m_annotationFinder);
-    ea.setDeclaringClass(m_class);
-    ea.setErrorMsgPrefix(errMsgPrefix);
-    ea.setCreate(create);
-    return ea;
+    return new DetailedAttributes(
+        m_class,
+        m_classes,
+        m_testContext.getCurrentXmlTest(),
+        m_annotationFinder,
+        create,
+        errMsgPrefix);
   }
 }
diff --git a/testng-core/src/main/java/org/testng/internal/collections/Pair.java b/testng-core/src/main/java/org/testng/internal/collections/Pair.java
index a7072b3f8d..256cc21e7a 100644
--- a/testng-core/src/main/java/org/testng/internal/collections/Pair.java
+++ b/testng-core/src/main/java/org/testng/internal/collections/Pair.java
@@ -1,5 +1,6 @@
 package org.testng.internal.collections;
 
+import org.jspecify.annotations.Nullable;
 import org.testng.collections.Objects;
 
 public class Pair<A, B> {
@@ -28,7 +29,7 @@ public int hashCode() {
   }
 
   @Override
-  public boolean equals(Object obj) {
+  public boolean equals(@Nullable Object obj) {
     if (this == obj) {
       return true;
     }
diff --git a/testng-core/src/main/java/org/testng/internal/collections/ResourceAwareIterator.java b/testng-core/src/main/java/org/testng/internal/collections/ResourceAwareIterator.java
index a080ebc18a..ce41dfb4a4 100644
--- a/testng-core/src/main/java/org/testng/internal/collections/ResourceAwareIterator.java
+++ b/testng-core/src/main/java/org/testng/internal/collections/ResourceAwareIterator.java
@@ -4,6 +4,7 @@
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.Iterator;
+import org.jspecify.annotations.Nullable;
 import org.testng.internal.Utils;
 
 /**
@@ -18,7 +19,7 @@
 public class ResourceAwareIterator<T> implements CloseableIterator<T> {
 
   private final Iterator<T> delegate;
-  private final AutoCloseable resource;
+  private final @Nullable AutoCloseable resource;
   private boolean closed;
 
   /**
@@ -26,7 +27,7 @@ public class ResourceAwareIterator<T> implements CloseableIterator<T> {
    * @param resource the resource to release on {@link #close()}, or {@code null} if there is
    *     nothing to release.
    */
-  public ResourceAwareIterator(Iterator<T> delegate, AutoCloseable resource) {
+  public ResourceAwareIterator(Iterator<T> delegate, @Nullable AutoCloseable resource) {
     this.delegate = delegate;
     this.resource = resource;
   }
@@ -43,7 +44,7 @@ public ResourceAwareIterator(Iterator<T> delegate, AutoCloseable resource) {
    *     iterator was derived from), or {@code null} if there is nothing to release.
    */
   public static CloseableIterator<Object[]> forDataProvider(
-      Iterator<Object> iterator, Type returnType, AutoCloseable resource) {
+      Iterator<Object> iterator, Type returnType, @Nullable AutoCloseable resource) {
     return new ResourceAwareIterator<>(toObjectArrayIterator(iterator, returnType), resource);
   }
 
diff --git a/testng-core/src/main/java/org/testng/internal/collections/package-info.java b/testng-core/src/main/java/org/testng/internal/collections/package-info.java
new file mode 100644
index 0000000000..9b6a9cd341
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/internal/collections/package-info.java
@@ -0,0 +1,5 @@
+/** Iterators and small containers used to shape data provider rows and internal state. */
+@NullMarked
+package org.testng.internal.collections;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/internal/invokers/objects/package-info.java b/testng-core/src/main/java/org/testng/internal/invokers/objects/package-info.java
new file mode 100644
index 0000000000..64a3b344a1
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/internal/invokers/objects/package-info.java
@@ -0,0 +1,5 @@
+/** Suite-level state the invokers carry forward, detached from the XML model it came from. */
+@NullMarked
+package org.testng.internal.invokers.objects;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/internal/objects/pojo/BasicAttributes.java b/testng-core/src/main/java/org/testng/internal/objects/pojo/BasicAttributes.java
index 7583fb2ae4..bdbdf814ca 100644
--- a/testng-core/src/main/java/org/testng/internal/objects/pojo/BasicAttributes.java
+++ b/testng-core/src/main/java/org/testng/internal/objects/pojo/BasicAttributes.java
@@ -1,25 +1,26 @@
 package org.testng.internal.objects.pojo;
 
+import org.jspecify.annotations.Nullable;
 import org.testng.IClass;
 
 /** Represents the basic attributes associated with object creation. */
 public class BasicAttributes {
 
-  private final IClass iClass;
-  private final Class<?> clazz;
+  private final @Nullable IClass iClass;
+  private final @Nullable Class<?> clazz;
 
-  public BasicAttributes(IClass iClass, Class<?> clazz) {
+  public BasicAttributes(@Nullable IClass iClass, @Nullable Class<?> clazz) {
     this.iClass = iClass;
     this.clazz = clazz;
   }
 
   /** @return - The actual {@link Class} */
-  public Class<?> getRawClass() {
+  public @Nullable Class<?> getRawClass() {
     return clazz;
   }
 
   /** @return - The wrapped {@link IClass} that represents a TestNG test class. */
-  public IClass getTestClass() {
+  public @Nullable IClass getTestClass() {
     return iClass;
   }
 }
diff --git a/testng-core/src/main/java/org/testng/internal/objects/pojo/CreationAttributes.java b/testng-core/src/main/java/org/testng/internal/objects/pojo/CreationAttributes.java
index 68e89483da..38e778b98d 100644
--- a/testng-core/src/main/java/org/testng/internal/objects/pojo/CreationAttributes.java
+++ b/testng-core/src/main/java/org/testng/internal/objects/pojo/CreationAttributes.java
@@ -1,5 +1,6 @@
 package org.testng.internal.objects.pojo;
 
+import org.jspecify.annotations.Nullable;
 import org.testng.ITestContext;
 import org.testng.internal.invokers.objects.GuiceContext;
 
@@ -7,11 +8,12 @@
 public class CreationAttributes {
 
   private final BasicAttributes basic;
-  private final DetailedAttributes detailed;
-  private final ITestContext context;
-  private final GuiceContext suiteContext;
+  private final @Nullable DetailedAttributes detailed;
+  private final @Nullable ITestContext context;
+  private final @Nullable GuiceContext suiteContext;
 
-  public CreationAttributes(ITestContext ctx, BasicAttributes basic, DetailedAttributes detailed) {
+  public CreationAttributes(
+      ITestContext ctx, BasicAttributes basic, @Nullable DetailedAttributes detailed) {
     this.basic = basic;
     this.detailed = detailed;
     this.context = ctx;
@@ -25,7 +27,7 @@ public CreationAttributes(BasicAttributes basic, GuiceContext suiteContext) {
     this.suiteContext = suiteContext;
   }
 
-  public DetailedAttributes getDetailedAttributes() {
+  public @Nullable DetailedAttributes getDetailedAttributes() {
     return detailed;
   }
 
@@ -33,11 +35,11 @@ public BasicAttributes getBasicAttributes() {
     return basic;
   }
 
-  public ITestContext getContext() {
+  public @Nullable ITestContext getContext() {
     return context;
   }
 
-  public GuiceContext getSuiteContext() {
+  public @Nullable GuiceContext getSuiteContext() {
     return suiteContext;
   }
 }
diff --git a/testng-core/src/main/java/org/testng/internal/objects/pojo/DetailedAttributes.java b/testng-core/src/main/java/org/testng/internal/objects/pojo/DetailedAttributes.java
index 2e646ec7af..2f99079972 100644
--- a/testng-core/src/main/java/org/testng/internal/objects/pojo/DetailedAttributes.java
+++ b/testng-core/src/main/java/org/testng/internal/objects/pojo/DetailedAttributes.java
@@ -8,58 +8,49 @@
 /** Represents the elaborate set of attributes required for object creation. */
 public class DetailedAttributes {
 
-  private Class<?> declaringClass;
-  private Map<Class<?>, IClass> classes;
-  private XmlTest xmlTest;
-  private IAnnotationFinder finder;
-  private boolean create;
-  private String errorMsgPrefix;
+  private final Class<?> declaringClass;
+  private final Map<Class<?>, IClass> classes;
+  private final XmlTest xmlTest;
+  private final IAnnotationFinder finder;
+  private final boolean create;
+  private final String errorMsgPrefix;
+
+  public DetailedAttributes(
+      Class<?> declaringClass,
+      Map<Class<?>, IClass> classes,
+      XmlTest xmlTest,
+      IAnnotationFinder finder,
+      boolean create,
+      String errorMsgPrefix) {
+    this.declaringClass = declaringClass;
+    this.classes = classes;
+    this.xmlTest = xmlTest;
+    this.finder = finder;
+    this.create = create;
+    this.errorMsgPrefix = errorMsgPrefix;
+  }
 
   public Class<?> getDeclaringClass() {
     return declaringClass;
   }
 
-  public void setDeclaringClass(Class<?> declaringClass) {
-    this.declaringClass = declaringClass;
-  }
-
   public Map<Class<?>, IClass> getClasses() {
     return classes;
   }
 
-  public void setClasses(Map<Class<?>, IClass> classes) {
-    this.classes = classes;
-  }
-
   public XmlTest getXmlTest() {
     return xmlTest;
   }
 
-  public void setXmlTest(XmlTest xmlTest) {
-    this.xmlTest = xmlTest;
-  }
-
   public IAnnotationFinder getFinder() {
     return finder;
   }
 
-  public void setFinder(IAnnotationFinder finder) {
-    this.finder = finder;
-  }
-
   public boolean isCreate() {
     return create;
   }
 
-  public void setCreate(boolean create) {
-    this.create = create;
-  }
-
   public String getErrorMsgPrefix() {
     return errorMsgPrefix;
   }
-
-  public void setErrorMsgPrefix(String errorMsgPrefix) {
-    this.errorMsgPrefix = errorMsgPrefix;
-  }
 }
diff --git a/testng-core/src/main/java/org/testng/internal/objects/pojo/package-info.java b/testng-core/src/main/java/org/testng/internal/objects/pojo/package-info.java
new file mode 100644
index 0000000000..65dc0e0961
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/internal/objects/pojo/package-info.java
@@ -0,0 +1,5 @@
+/** The attribute bundles the object dispensers are handed to create a test instance. */
+@NullMarked
+package org.testng.internal.objects.pojo;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/internal/thread/graph/GraphOrchestrator.java b/testng-core/src/main/java/org/testng/internal/thread/graph/GraphOrchestrator.java
index f1a6bdac27..25a6277a0f 100644
--- a/testng-core/src/main/java/org/testng/internal/thread/graph/GraphOrchestrator.java
+++ b/testng-core/src/main/java/org/testng/internal/thread/graph/GraphOrchestrator.java
@@ -8,6 +8,7 @@
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import org.jspecify.annotations.Nullable;
 import org.testng.IDynamicGraph;
 import org.testng.internal.AutoCloseableLock;
 import org.testng.internal.RuntimeBehavior;
@@ -24,7 +25,7 @@ public class GraphOrchestrator<T> {
   private final IDynamicGraph<T> graph;
   private final Map<T, IWorker<T>> mapping = new ConcurrentHashMap<>();
   private final Map<T, T> upstream = new ConcurrentHashMap<>();
-  private final Comparator<T> comparator;
+  private final @Nullable Comparator<T> comparator;
   private final IThreadWorkerFactory<T> factory;
   private final boolean shutdownExecutorOnFinish;
   private final CountDownLatch completed = new CountDownLatch(1);
@@ -35,7 +36,7 @@ public GraphOrchestrator(
       ExecutorService service,
       IThreadWorkerFactory<T> factory,
       IDynamicGraph<T> graph,
-      Comparator<T> comparator) {
+      @Nullable Comparator<T> comparator) {
     this(service, factory, graph, comparator, true);
   }
 
@@ -49,7 +50,7 @@ public GraphOrchestrator(
       ExecutorService service,
       IThreadWorkerFactory<T> factory,
       IDynamicGraph<T> graph,
-      Comparator<T> comparator,
+      @Nullable Comparator<T> comparator,
       boolean shutdownExecutorOnFinish) {
     this.service = service;
     this.graph = graph;
diff --git a/testng-core/src/main/java/org/testng/internal/thread/graph/PhoneyWorker.java b/testng-core/src/main/java/org/testng/internal/thread/graph/PhoneyWorker.java
index c8d9f3ad51..4631fc03a6 100644
--- a/testng-core/src/main/java/org/testng/internal/thread/graph/PhoneyWorker.java
+++ b/testng-core/src/main/java/org/testng/internal/thread/graph/PhoneyWorker.java
@@ -1,7 +1,6 @@
 package org.testng.internal.thread.graph;
 
 import java.util.List;
-import javax.annotation.Nonnull;
 import org.testng.thread.IWorker;
 
 class PhoneyWorker<T> implements IWorker<T> {
@@ -13,7 +12,8 @@ public PhoneyWorker(long threadId) {
 
   @Override
   public List<T> getTasks() {
-    return null;
+    // A PhoneyWorker stands in for a thread id, never for work: it has no tasks to report.
+    return List.of();
   }
 
   @Override
@@ -27,7 +27,7 @@ public int getPriority() {
   }
 
   @Override
-  public int compareTo(@Nonnull IWorker<T> o) {
+  public int compareTo(IWorker<T> o) {
     return 0;
   }
 
diff --git a/testng-core/src/main/java/org/testng/internal/thread/graph/package-info.java b/testng-core/src/main/java/org/testng/internal/thread/graph/package-info.java
new file mode 100644
index 0000000000..2ef9a42fd7
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/internal/thread/graph/package-info.java
@@ -0,0 +1,5 @@
+/** Drives a dynamic graph's ready nodes through an executor, worker by worker. */
+@NullMarked
+package org.testng.internal.thread.graph;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/log/package-info.java b/testng-core/src/main/java/org/testng/log/package-info.java
new file mode 100644
index 0000000000..4cec819143
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/log/package-info.java
@@ -0,0 +1,5 @@
+/** The java.util.logging formatter TestNG installs on its own handlers. */
+@NullMarked
+package org.testng.log;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/reporters/util/StackTraceTools.java b/testng-core/src/main/java/org/testng/reporters/util/StackTraceTools.java
index f425bef6ac..f400ff8f85 100644
--- a/testng-core/src/main/java/org/testng/reporters/util/StackTraceTools.java
+++ b/testng-core/src/main/java/org/testng/reporters/util/StackTraceTools.java
@@ -1,5 +1,6 @@
 package org.testng.reporters.util;
 
+import org.jspecify.annotations.Nullable;
 import org.testng.ITestNGMethod;
 
 /**
@@ -20,7 +21,8 @@ private StackTraceTools() {
    * @return topmost position of the test method in the stack, or top of stack if <code>method
    *     </code> is not in it.
    */
-  public static int getTestRoot(StackTraceElement[] stack, ITestNGMethod method) {
+  public static int getTestRoot(
+      StackTraceElement @Nullable [] stack, @Nullable ITestNGMethod method) {
     if (stack == null || method == null) {
       return -1;
     }
@@ -41,7 +43,7 @@ public static int getTestRoot(StackTraceElement[] stack, ITestNGMethod method) {
    *     </code> is not in it.
    */
   public static StackTraceElement[] getTestNGInfrastructure(
-      StackTraceElement[] stack, ITestNGMethod method) {
+      StackTraceElement @Nullable [] stack, @Nullable ITestNGMethod method) {
     if (method == null || stack == null) {
       return new StackTraceElement[] {};
     }
diff --git a/testng-core/src/main/java/org/testng/reporters/util/package-info.java b/testng-core/src/main/java/org/testng/reporters/util/package-info.java
new file mode 100644
index 0000000000..1a51c09acf
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/reporters/util/package-info.java
@@ -0,0 +1,5 @@
+/** Stack trace slicing for reporters. */
+@NullMarked
+package org.testng.reporters.util;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/thread/package-info.java b/testng-core/src/main/java/org/testng/thread/package-info.java
new file mode 100644
index 0000000000..8fb7b357aa
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/thread/package-info.java
@@ -0,0 +1,5 @@
+/** The worker and thread-pool contracts the graph executors are written against. */
+@NullMarked
+package org.testng.thread;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/testng-core/src/main/java/org/testng/xml/internal/Parser.java b/testng-core/src/main/java/org/testng/xml/internal/Parser.java
index 5548d3841e..d07875a7b6 100644
--- a/testng-core/src/main/java/org/testng/xml/internal/Parser.java
+++ b/testng-core/src/main/java/org/testng/xml/internal/Parser.java
@@ -13,6 +13,7 @@
 import java.util.Map;
 import java.util.Queue;
 import java.util.ServiceLoader;
+import org.jspecify.annotations.Nullable;
 import org.testng.xml.IFileParser;
 import org.testng.xml.IPostProcessor;
 import org.testng.xml.ISuiteParser;
@@ -50,13 +51,13 @@ public class Parser {
   }
 
   /**
-   * The file name of the xml suite being parsed. This may be null if the Parser has not been
-   * initialized with a file name. TODO CQ This member is never used.
+   * The file name of the xml suite being parsed, or {@link #DEFAULT_FILENAME} when the Parser was
+   * not given one.
    */
   private String m_fileName;
 
-  private InputStream m_inputStream;
-  private IPostProcessor m_postProcessor;
+  private @Nullable InputStream m_inputStream;
+  private @Nullable IPostProcessor m_postProcessor;
 
   private boolean m_loadClasses = true;
 
@@ -66,7 +67,7 @@ public class Parser {
    *
    * @param fileName the filename corresponding to the inputStream or null if unknown.
    */
-  public Parser(String fileName) {
+  public Parser(@Nullable String fileName) {
     init(fileName, null);
   }
 
@@ -79,7 +80,7 @@ public Parser(InputStream is) {
     init(null, is);
   }
 
-  private void init(String fileName, InputStream is) {
+  private void init(@Nullable String fileName, @Nullable InputStream is) {
     m_fileName = fileName != null ? fileName : DEFAULT_FILENAME;
     m_inputStream = is;
   }
@@ -122,17 +123,15 @@ public Collection<XmlSuite> parse() throws IOException {
     List<String> toBeAdded = new ArrayList<>();
     List<String> toBeRemoved = new ArrayList<>();
 
-    if (m_fileName != null) {
-      URI uri = constructURI(m_fileName);
-      if (uri == null || uri.getScheme() == null) {
-        uri = new File(m_fileName).toURI();
-      }
-      if ("file".equalsIgnoreCase(uri.getScheme())) {
-        File mainFile = new File(uri);
-        toBeParsed.add(mainFile.getCanonicalPath());
-      } else {
-        toBeParsed.add(uri.toString());
-      }
+    URI uri = constructURI(m_fileName);
+    if (uri == null || uri.getScheme() == null) {
+      uri = new File(m_fileName).toURI();
+    }
+    if ("file".equalsIgnoreCase(uri.getScheme())) {
+      File mainFile = new File(uri);
+      toBeParsed.add(mainFile.getCanonicalPath());
+    } else {
+      toBeParsed.add(uri.toString());
     }
 
     /*
@@ -268,7 +267,7 @@ private static Parser newParser(InputStream is, IPostProcessor processor) {
     return result;
   }
 
-  private static URI constructURI(String text) {
+  private static @Nullable URI constructURI(String text) {
     try {
       return URI.create(text);
     } catch (Exception e) {
diff --git a/testng-core/src/main/java/org/testng/xml/internal/TestNamesMatcher.java b/testng-core/src/main/java/org/testng/xml/internal/TestNamesMatcher.java
index 82eb162b6f..1a37cbafa2 100644
--- a/testng-core/src/main/java/org/testng/xml/internal/TestNamesMatcher.java
+++ b/testng-core/src/main/java/org/testng/xml/internal/TestNamesMatcher.java
@@ -4,6 +4,7 @@
 import java.util.LinkedList;
 import java.util.List;
 import java.util.regex.Pattern;
+import org.jspecify.annotations.Nullable;
 import org.testng.TestNGException;
 import org.testng.log4testng.Logger;
 import org.testng.util.Strings;
@@ -110,13 +111,13 @@ public List<XmlTest> getMatchedTests() {
     return matchedTests;
   }
 
-  private void addIfNotNull(XmlSuite xmlSuite) {
+  private void addIfNotNull(@Nullable XmlSuite xmlSuite) {
     if (xmlSuite != null) {
       cloneSuites.add(xmlSuite);
     }
   }
 
-  private XmlSuite cloneIfSuiteContainTestsWithNamesMatchingAny(XmlSuite suite) {
+  private @Nullable XmlSuite cloneIfSuiteContainTestsWithNamesMatchingAny(XmlSuite suite) {
     List<XmlTest> tests = new LinkedList<>();
     for (XmlTest xt : suite.getTests()) {
       if (xt.nameMatchesAny(testNames)) {
diff --git a/testng-core/src/main/java/org/testng/xml/internal/package-info.java b/testng-core/src/main/java/org/testng/xml/internal/package-info.java
new file mode 100644
index 0000000000..dfd9430d17
--- /dev/null
+++ b/testng-core/src/main/java/org/testng/xml/internal/package-info.java
@@ -0,0 +1,5 @@
+/** Reads suite files into XmlSuite, and the checks that run over the result. */
+@NullMarked
+package org.testng.xml.internal;
+
+import org.jspecify.annotations.NullMarked;