Clean UnknownNullness issues for webkit

Explicitly adding a lint rule to break on UnknownNullness. Fix all
reported lint errors.

Fixes a few other reported code quality issues from Android Studio, such
as fields that could be final and using try-with-resource.

Fixes: 139769993
Fixes: 139774819
Test: ./gradlew webkit:webkit:lint
Test: ./gradlew webkit:webkit:connectedAndroid
Change-Id: I64159337850bee45d7f6fe8f24908c8051933e4e

17 files changed, 147 insertions, 86 deletions

diff --git a/webkit/webkit/lint.xml b/webkit/webkit/lint.xml
index 6262c9b67f6..dd56e16974e 100644
--- a/webkit/webkit/lint.xml
+++ b/webkit/webkit/lint.xml
@@ -22,7 +22,18 @@
 
 
     <!-- Webkit-specific lint rules: -->
+
+    <!-- We cannot cause ClassVerificationFailure in embedding apps -->
     <issue id="ClassVerificationFailure" severity="fatal" />
+    <!-- Developers need to call our code from Kotlin code, so nullness is important.-->
+    <issue id="UnknownNullness" severity="fatal" >
+        <!-- The boundary interfaces are not annotated, and they represent external chromium code,
+             so skip them.
+             The catch-all ignore below does not disable this check,
+             so it needs to be done here too. -->
+        <ignore path="**/org/chromium/support_lib_boundary/**" />
+    </issue>
+
     <!-- Ignore all warnings for webview-support-interfaces, since it's external chromium code. -->
     <issue id="all" severity="ignore">
         <ignore path="**/org/chromium/support_lib_boundary/**" />
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/PollingCheck.java b/webkit/webkit/src/androidTest/java/androidx/webkit/PollingCheck.java
index d660b997df3..f7098fb4fd0 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/PollingCheck.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/PollingCheck.java
@@ -16,6 +16,8 @@
 
 package androidx.webkit;
 
+import androidx.annotation.NonNull;
+
 import org.junit.Assert;
 
 import java.util.concurrent.Callable;
@@ -61,7 +63,8 @@ public abstract class PollingCheck {
         Assert.fail("unexpected timeout");
     }
 
-    public static void check(CharSequence message, long timeout, Callable<Boolean> condition)
+    public static void check(@NonNull CharSequence message, long timeout,
+            @NonNull Callable<Boolean> condition)
             throws Exception {
         while (timeout > 0) {
             if (condition.call()) {
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/ServiceWorkerClientCompatTest.java b/webkit/webkit/src/androidTest/java/androidx/webkit/ServiceWorkerClientCompatTest.java
index e40b4d52d60..180fea344a2 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/ServiceWorkerClientCompatTest.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/ServiceWorkerClientCompatTest.java
@@ -26,6 +26,7 @@ import android.webkit.WebResourceResponse;
 import android.webkit.WebView;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import androidx.annotation.RequiresApi;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import androidx.test.filters.MediumTest;
@@ -116,8 +117,9 @@ public class ServiceWorkerClientCompatTest {
     }
 
     public static class InterceptServiceWorkerClient extends ServiceWorkerClientCompat {
-        private List<WebResourceRequest> mInterceptedRequests = new ArrayList<>();
+        private final List<WebResourceRequest> mInterceptedRequests = new ArrayList<>();
 
+        @Nullable
         @Override
         public WebResourceResponse shouldInterceptRequest(@NonNull WebResourceRequest request) {
             // Records intercepted requests and only return content for SW_URL.
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/TargetSdkActivityTestRule.java b/webkit/webkit/src/androidTest/java/androidx/webkit/TargetSdkActivityTestRule.java
index 1b70b672462..205980214de 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/TargetSdkActivityTestRule.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/TargetSdkActivityTestRule.java
@@ -23,6 +23,7 @@ import android.app.Activity;
 import android.content.Context;
 import android.content.pm.ApplicationInfo;
 
+import androidx.annotation.NonNull;
 import androidx.test.platform.app.InstrumentationRegistry;
 
 /**
@@ -34,7 +35,7 @@ public class TargetSdkActivityTestRule<T extends Activity> extends
     private int mTargetSdk;
     private Context mAppContext;
 
-    public TargetSdkActivityTestRule(Class<T> activityClass, int targetSdk) {
+    public TargetSdkActivityTestRule(@NonNull Class<T> activityClass, int targetSdk) {
         super(activityClass);
         mTargetSdk = targetSdk;
     }
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebSettingsCompatDarkModeTestBase.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebSettingsCompatDarkModeTestBase.java
index 50c432cac0a..568cf608916 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebSettingsCompatDarkModeTestBase.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebSettingsCompatDarkModeTestBase.java
@@ -68,7 +68,7 @@ public class WebSettingsCompatDarkModeTestBase<T extends Activity> {
     @Rule
     public final TargetSdkActivityTestRule<T> mActivityRule;
 
-    public WebSettingsCompatDarkModeTestBase(Class<T> activityClass, int targetSdk) {
+    public WebSettingsCompatDarkModeTestBase(@NonNull Class<T> activityClass, int targetSdk) {
         mActivityRule = new TargetSdkActivityTestRule<T>(activityClass,
                 targetSdk);
     }
@@ -154,6 +154,7 @@ public class WebSettingsCompatDarkModeTestBase<T extends Activity> {
     /**
      * Returns a matcher to check if a color int is mostly green.
      */
+    @NonNull
     public static Matcher<Integer> isGreen() {
         return new TypeSafeMatcher<Integer>() {
             private int mPageColor;
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewDocumentStartJavaScriptTest.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewDocumentStartJavaScriptTest.java
index 619ef7a13d4..b8ae25dbf19 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewDocumentStartJavaScriptTest.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewDocumentStartJavaScriptTest.java
@@ -59,11 +59,11 @@ public class WebViewDocumentStartJavaScriptTest {
     private static class TestWebMessageListener implements WebViewCompat.WebMessageListener {
         private BlockingQueue<Data> mQueue = new LinkedBlockingQueue<>();
 
-        public static class Data {
-            public WebMessageCompat mMessage;
-            public Uri mSourceOrigin;
-            public boolean mIsMainFrame;
-            public JavaScriptReplyProxy mReplyProxy;
+        static class Data {
+            WebMessageCompat mMessage;
+            Uri mSourceOrigin;
+            boolean mIsMainFrame;
+            JavaScriptReplyProxy mReplyProxy;
 
             Data(WebMessageCompat message, Uri sourceOrigin, boolean isMainFrame,
                     JavaScriptReplyProxy replyProxy) {
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewOnUiThread.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewOnUiThread.java
index 215696a36d3..f28c63c4c82 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewOnUiThread.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewOnUiThread.java
@@ -16,6 +16,7 @@
 
 package androidx.webkit;
 
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -98,7 +99,7 @@ public class WebViewOnUiThread implements AutoCloseable{
      *
      * The caller is responsible for destroying the WebView instance.
      */
-    public WebViewOnUiThread(final WebView webView) {
+    public WebViewOnUiThread(final @NonNull WebView webView) {
         this(webView, false);
     }
 
@@ -120,6 +121,7 @@ public class WebViewOnUiThread implements AutoCloseable{
         volatile WebView mView;
     }
 
+    @NonNull
     public static WebView createWebView() {
         final Holder h = new Holder();
         final Context ctx = ApplicationProvider.getApplicationContext();
@@ -157,7 +159,7 @@ public class WebViewOnUiThread implements AutoCloseable{
      *
      * The task will be executed on the same thread that executes the cleanup.
      */
-    public void setCleanupTask(Runnable cleanupTask) {
+    public void setCleanupTask(@Nullable Runnable cleanupTask) {
         mCleanupTask = cleanupTask;
     }
 
@@ -186,89 +188,97 @@ public class WebViewOnUiThread implements AutoCloseable{
         this.notifyAll();
     }
 
-    public static void destroy(final WebView webView) {
+    public static void destroy(final @NonNull WebView webView) {
         WebkitUtils.onMainThreadSync(webView::destroy);
     }
 
-    public void setWebViewClient(final WebViewClient webviewClient) {
+    public void setWebViewClient(final @NonNull WebViewClient webviewClient) {
         setWebViewClient(mWebView, webviewClient);
     }
 
     public static void setWebViewClient(
-            final WebView webView, final WebViewClient webviewClient) {
+            final @NonNull WebView webView, final @NonNull WebViewClient webviewClient) {
         WebkitUtils.onMainThreadSync(() -> webView.setWebViewClient(webviewClient));
     }
 
-    public void setWebChromeClient(final WebChromeClient webChromeClient) {
+    public void setWebChromeClient(final @Nullable WebChromeClient webChromeClient) {
         setWebChromeClient(mWebView, webChromeClient);
     }
 
     public static void setWebChromeClient(
-            final WebView webView, final WebChromeClient webChromeClient) {
+            final @NonNull WebView webView, final @Nullable WebChromeClient webChromeClient) {
         WebkitUtils.onMainThreadSync(() -> webView.setWebChromeClient(webChromeClient));
     }
 
     public void setWebViewRenderProcessClient(
-            final WebViewRenderProcessClient webViewRenderProcessClient) {
+            final @NonNull WebViewRenderProcessClient webViewRenderProcessClient) {
         setWebViewRenderProcessClient(mWebView, webViewRenderProcessClient);
     }
 
     public static void setWebViewRenderProcessClient(
-            final WebView webView, final WebViewRenderProcessClient webViewRenderProcessClient) {
+            final @NonNull WebView webView,
+            final @NonNull WebViewRenderProcessClient webViewRenderProcessClient) {
         WebkitUtils.onMainThreadSync(() -> WebViewCompat.setWebViewRenderProcessClient(
                 webView, webViewRenderProcessClient));
     }
 
     public void setWebViewRenderProcessClient(
-            final Executor executor, final WebViewRenderProcessClient webViewRenderProcessClient) {
+            final @NonNull Executor executor,
+            final @NonNull WebViewRenderProcessClient webViewRenderProcessClient) {
         setWebViewRenderProcessClient(mWebView, executor, webViewRenderProcessClient);
     }
 
     public static void setWebViewRenderProcessClient(
-            final WebView webView,
-            final Executor executor,
-            final WebViewRenderProcessClient webViewRenderProcessClient) {
+            final @NonNull WebView webView,
+            final @NonNull Executor executor,
+            final @NonNull WebViewRenderProcessClient webViewRenderProcessClient) {
         WebkitUtils.onMainThreadSync(() -> WebViewCompat.setWebViewRenderProcessClient(
                 webView, executor, webViewRenderProcessClient));
     }
 
+    @Nullable
     public WebViewRenderProcessClient getWebViewRenderProcessClient() {
         return getWebViewRenderProcessClient(mWebView);
     }
 
+    @Nullable
     public static WebViewRenderProcessClient getWebViewRenderProcessClient(
-            final WebView webView) {
+            final @NonNull WebView webView) {
         return WebkitUtils.onMainThreadSync(
                 () -> WebViewCompat.getWebViewRenderProcessClient(webView));
     }
 
+    @NonNull
     public WebMessagePortCompat[] createWebMessageChannelCompat() {
         return WebkitUtils.onMainThreadSync(() -> WebViewCompat.createWebMessageChannel(mWebView));
     }
 
-    public void postWebMessageCompat(final WebMessageCompat message, final Uri targetOrigin) {
+    public void postWebMessageCompat(final @NonNull WebMessageCompat message,
+            final @NonNull Uri targetOrigin) {
         WebkitUtils.onMainThreadSync(
                 () -> WebViewCompat.postWebMessage(mWebView, message, targetOrigin));
     }
 
-    public void addWebMessageListener(String jsObjectName, Set<String> allowedOriginRules,
-            final WebViewCompat.WebMessageListener listener) {
+    public void addWebMessageListener(@NonNull String jsObjectName,
+            @NonNull Set<String> allowedOriginRules,
+            final @NonNull WebViewCompat.WebMessageListener listener) {
         WebkitUtils.onMainThreadSync(() -> WebViewCompat.addWebMessageListener(
                 mWebView, jsObjectName, allowedOriginRules, listener));
     }
 
-    public void removeWebMessageListener(final String jsObjectName) {
+    public void removeWebMessageListener(final @NonNull String jsObjectName) {
         WebkitUtils.onMainThreadSync(
                 () -> WebViewCompat.removeWebMessageListener(mWebView, jsObjectName));
     }
 
+    @NonNull
     public ScriptHandler addDocumentStartJavaScript(
-            String script, Set<String> allowedOriginRules) {
+            @NonNull String script, @NonNull Set<String> allowedOriginRules) {
         return WebkitUtils.onMainThreadSync(() -> WebViewCompat.addDocumentStartJavaScript(
                 mWebView, script, allowedOriginRules));
     }
 
-    public void addJavascriptInterface(final Object object, final String name) {
+    public void addJavascriptInterface(final @NonNull Object object, final @NonNull String name) {
         WebkitUtils.onMainThreadSync(() -> mWebView.addJavascriptInterface(object, name));
     }
 
@@ -282,7 +292,7 @@ public class WebViewOnUiThread implements AutoCloseable{
         callAndWait(() -> mWebView.loadUrl(url));
     }
 
-    public void loadUrl(final String url) {
+    public void loadUrl(final @NonNull String url) {
         WebkitUtils.onMainThreadSync(() -> mWebView.loadUrl(url));
     }
 
@@ -299,9 +309,10 @@ public class WebViewOnUiThread implements AutoCloseable{
         callAndWait(() -> mWebView.loadData(data, mimeType, encoding));
     }
 
-    public void loadDataWithBaseURLAndWaitForCompletion(final String baseUrl,
-            final String data, final String mimeType, final String encoding,
-            final String historyUrl) {
+    public void loadDataWithBaseURLAndWaitForCompletion(final @Nullable String baseUrl,
+            final @NonNull String data, final @Nullable String mimeType,
+            final @Nullable String encoding,
+            final @Nullable String historyUrl) {
         callAndWait(() -> mWebView.loadDataWithBaseURL(
                 baseUrl, data, mimeType, encoding, historyUrl));
     }
@@ -324,20 +335,23 @@ public class WebViewOnUiThread implements AutoCloseable{
         }
     }
 
+    @Nullable
     public String getTitle() {
         return WebkitUtils.onMainThreadSync(() -> mWebView.getTitle());
     }
 
+    @NonNull
     public WebSettings getSettings() {
         return WebkitUtils.onMainThreadSync(() -> mWebView.getSettings());
     }
 
+    @Nullable
     public String getUrl() {
         return WebkitUtils.onMainThreadSync(() -> mWebView.getUrl());
     }
 
     public void postVisualStateCallbackCompat(final long requestId,
-            final WebViewCompat.VisualStateCallback callback) {
+            final @NonNull WebViewCompat.VisualStateCallback callback) {
         WebkitUtils.onMainThreadSync(() -> WebViewCompat.postVisualStateCallback(
                 mWebView, requestId, callback));
     }
@@ -345,29 +359,35 @@ public class WebViewOnUiThread implements AutoCloseable{
     /**
      * Execute javascript synchronously, returning the result.
      */
-    public String evaluateJavascriptSync(final String script) {
+    @Nullable
+    public String evaluateJavascriptSync(final @NonNull String script) {
         final ResolvableFuture<String> future = ResolvableFuture.create();
         evaluateJavascript(script, future::set);
         return WebkitUtils.waitForFuture(future);
     }
 
-    public void evaluateJavascript(final String script, final ValueCallback<String> result) {
+    public void evaluateJavascript(final @NonNull String script,
+            final @Nullable ValueCallback<String> result) {
         WebkitUtils.onMainThread(() -> mWebView.evaluateJavascript(script, result));
     }
 
+    @NonNull
     public WebViewClient getWebViewClient() {
         return getWebViewClient(mWebView);
     }
 
-    public static WebViewClient getWebViewClient(final WebView webView) {
+    @NonNull
+    public static WebViewClient getWebViewClient(final @NonNull WebView webView) {
         return WebkitUtils.onMainThreadSync(() -> WebViewCompat.getWebViewClient(webView));
     }
 
+    @Nullable
     public WebChromeClient getWebChromeClient() {
         return getWebChromeClient(mWebView);
     }
 
-    public static WebChromeClient getWebChromeClient(final WebView webView) {
+    @Nullable
+    public static WebChromeClient getWebChromeClient(final @NonNull WebView webView) {
         return WebkitUtils.onMainThreadSync(() -> WebViewCompat.getWebChromeClient(webView));
     }
 
@@ -402,6 +422,7 @@ public class WebViewOnUiThread implements AutoCloseable{
      * This synchronises so that the bitmap contents reflects the current DOM state, rather than
      * potentially capturing a previously generated frame.
      */
+    @NonNull
     public Bitmap captureBitmap() {
         WebSettingsCompat.setOffscreenPreRaster(getSettings(), true);
         waitForDOMReadyToRender();
@@ -435,10 +456,9 @@ public class WebViewOnUiThread implements AutoCloseable{
      * @param call The call to make on the UI thread prior to waiting.
      */
     private void callAndWait(Runnable call) {
-        assertTrue("WebViewOnUiThread.load*AndWaitForCompletion calls "
-                        + "may not be mixed with load* calls directly on WebView "
-                        + "without calling waitForLoadCompletion after the load",
-                !isLoaded());
+        assertFalse("WebViewOnUiThread.load*AndWaitForCompletion calls "
+                + "may not be mixed with load* calls directly on WebView "
+                + "without calling waitForLoadCompletion after the load", isLoaded());
         clearLoad(); // clear any extraneous signals from a previous load.
         if (Looper.myLooper() == Looper.getMainLooper()) {
             call.run();
@@ -522,7 +542,7 @@ public class WebViewOnUiThread implements AutoCloseable{
      * directly.
      */
     public static class WaitForProgressClient extends WebChromeClient {
-        private WebViewOnUiThread mOnUiThread;
+        private final WebViewOnUiThread mOnUiThread;
 
         WaitForProgressClient(WebViewOnUiThread onUiThread) {
             mOnUiThread = onUiThread;
@@ -544,7 +564,7 @@ public class WebViewOnUiThread implements AutoCloseable{
      * WaitForLoadedClient or call WebViewOnUiThread.onPageFinished.
      */
     public static class WaitForLoadedClient extends WebViewClientCompat {
-        private WebViewOnUiThread mOnUiThread;
+        private final WebViewOnUiThread mOnUiThread;
 
         WaitForLoadedClient(WebViewOnUiThread onUiThread) {
             mOnUiThread = onUiThread;
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewTestActivity.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewTestActivity.java
index 940153722ba..0cd87a3e0e0 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewTestActivity.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewTestActivity.java
@@ -21,15 +21,19 @@ import android.os.Bundle;
 import android.view.ViewGroup;
 import android.webkit.WebView;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 public class WebViewTestActivity extends Activity {
     private WebView mWebView;
 
+    @NonNull
     public WebView getWebView() {
         return mWebView;
     }
 
     @Override
-    protected void onCreate(Bundle savedInstanceState) {
+    protected void onCreate(@Nullable Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
         mWebView = new WebView(this);
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewWebMessageListenerTest.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewWebMessageListenerTest.java
index 10a9bdfeaea..f53207166d6 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewWebMessageListenerTest.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebViewWebMessageListenerTest.java
@@ -65,16 +65,16 @@ public class WebViewWebMessageListenerTest {
     private static final Set<String> MATCH_EXAMPLE_COM = new HashSet<>(Arrays.asList(BASE_URI));
 
     private WebViewOnUiThread mWebViewOnUiThread;
-    private TestWebMessageListener mListener = new TestWebMessageListener();
+    private final TestWebMessageListener mListener = new TestWebMessageListener();
 
     private static class TestWebMessageListener implements WebViewCompat.WebMessageListener {
-        private BlockingQueue<Data> mQueue = new LinkedBlockingQueue<>();
+        private final BlockingQueue<Data> mQueue = new LinkedBlockingQueue<>();
 
-        public static class Data {
-            public WebMessageCompat mMessage;
-            public Uri mSourceOrigin;
-            public boolean mIsMainFrame;
-            public JavaScriptReplyProxy mReplyProxy;
+        static class Data {
+            WebMessageCompat mMessage;
+            Uri mSourceOrigin;
+            boolean mIsMainFrame;
+            JavaScriptReplyProxy mReplyProxy;
 
             Data(WebMessageCompat message, Uri sourceOrigin, boolean isMainFrame,
                     JavaScriptReplyProxy replyProxy) {
diff --git a/webkit/webkit/src/androidTest/java/androidx/webkit/WebkitUtils.java b/webkit/webkit/src/androidTest/java/androidx/webkit/WebkitUtils.java
index 75a8387fd4c..62bd52e8882 100644
--- a/webkit/webkit/src/androidTest/java/androidx/webkit/WebkitUtils.java
+++ b/webkit/webkit/src/androidTest/java/androidx/webkit/WebkitUtils.java
@@ -66,7 +66,8 @@ public final class WebkitUtils {
      * @param callable the {@link Callable} to execute.
      * @return a {@link ListenableFuture} representing the result of {@code callable}.
      */
-    public static <T> ListenableFuture<T> onMainThread(final Callable<T> callable)  {
+    @NonNull
+    public static <T> ListenableFuture<T> onMainThread(final @NonNull Callable<T> callable)  {
         return onMainThreadDelayed(0, callable);
     }
 
@@ -75,7 +76,7 @@ public final class WebkitUtils {
      *
      * @param runnable the {@link Runnable} to execute.
      */
-    public static void onMainThread(final Runnable runnable)  {
+    public static void onMainThread(final @NonNull Runnable runnable)  {
         onMainThreadDelayed(0, runnable);
     }
 
@@ -86,8 +87,9 @@ public final class WebkitUtils {
      * @param callable the {@link Callable} to execute.
      * @return a {@link ListenableFuture} representing the result of {@code callable}.
      */
+    @NonNull
     public static <T> ListenableFuture<T> onMainThreadDelayed(
-            long delayMs, final Callable<T> callable)  {
+            long delayMs, final @NonNull Callable<T> callable)  {
         final ResolvableFuture<T> future = ResolvableFuture.create();
         sMainHandler.postDelayed(() -> {
             try {
@@ -105,7 +107,7 @@ public final class WebkitUtils {
      * @param delayMs the delay in milliseconds
      * @param runnable the {@link Runnable} to execute.
      */
-    public static void onMainThreadDelayed(long delayMs, final Runnable runnable) {
+    public static void onMainThreadDelayed(long delayMs, final @NonNull Runnable runnable) {
         sMainHandler.postDelayed(runnable, delayMs);
     }
 
@@ -120,7 +122,7 @@ public final class WebkitUtils {
      * @param callable the {@link Callable} to execute.
      * @return the result of the {@link Callable}.
      */
-    public static <T> T onMainThreadSync(final Callable<T> callable) {
+    public static <T> T onMainThreadSync(final @NonNull Callable<T> callable) {
         if (Looper.myLooper() == Looper.getMainLooper()) {
             throw new IllegalStateException("This cannot be called from the UI thread.");
         }
@@ -135,9 +137,9 @@ public final class WebkitUtils {
      *
      * <p class="note"><b>Note:</b> this should not be called from the UI thread.
      *
-     * @param Runnable the {@link Runnable} to execute.
+     * @param runnable the {@link Runnable} to execute.
      */
-    public static void onMainThreadSync(final Runnable runnable) {
+    public static void onMainThreadSync(final @NonNull Runnable runnable) {
         if (Looper.myLooper() == Looper.getMainLooper()) {
             throw new IllegalStateException("This cannot be called from the UI thread.");
         }
@@ -172,7 +174,7 @@ public final class WebkitUtils {
      * @param featureName the feature to be checked
      */
     @RequiresApi(21)
-    public static void checkFeature(String featureName) {
+    public static void checkFeature(@NonNull String featureName) {
         final String msg = "This device does not have the feature '" +  featureName + "'";
         final boolean hasFeature = WebViewFeature.isFeatureSupported(featureName);
         Assume.assumeTrue(msg, hasFeature);
@@ -187,7 +189,7 @@ public final class WebkitUtils {
      * @param future the {@link Future} representing a value of interest.
      * @return the value {@code future} represents.
      */
-    public static <T> T waitForFuture(Future<T> future) {
+    public static <T> T waitForFuture(@NonNull Future<T> future) {
         try {
             return future.get(TEST_TIMEOUT_MS, TimeUnit.MILLISECONDS);
         } catch (ExecutionException e) {
@@ -219,7 +221,7 @@ public final class WebkitUtils {
     /**
      * Takes an element out of the {@link BlockingQueue} (or times out).
      */
-    public static <T> T waitForNextQueueElement(BlockingQueue<T> queue) {
+    public static <T> T waitForNextQueueElement(@NonNull BlockingQueue<T> queue) {
         try {
             T value = queue.poll(TEST_TIMEOUT_MS, TimeUnit.MILLISECONDS);
             if (value == null) {
@@ -242,14 +244,11 @@ public final class WebkitUtils {
     /**
      * Write a string to a file, and create the whole parent directories if they don't exist.
      */
-    public static void writeToFile(File file, String content)
+    public static void writeToFile(@NonNull File file, @NonNull String content)
                   throws IOException {
         file.getParentFile().mkdirs();
-        FileOutputStream fos = new FileOutputStream(file);
-        try {
+        try (FileOutputStream fos = new FileOutputStream(file)) {
             fos.write(content.getBytes("utf-8"));
-        } finally {
-            fos.close();
         }
     }
 
@@ -258,7 +257,7 @@ public final class WebkitUtils {
      * @param currentFile The file or directory to delete. Does not need to exist.
      * @return Whether currentFile does not exist afterwards.
      */
-    public static boolean recursivelyDeleteFile(File currentFile) {
+    public static boolean recursivelyDeleteFile(@NonNull File currentFile) {
         if (!currentFile.exists()) {
             return true;
         }
@@ -271,8 +270,7 @@ public final class WebkitUtils {
             }
         }
 
-        boolean ret = currentFile.delete();
-        return ret;
+        return currentFile.delete();
     }
 
     /**
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/VisualStateCallbackAdapter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/VisualStateCallbackAdapter.java
index 4e95e228b1c..29529c5d1da 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/VisualStateCallbackAdapter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/VisualStateCallbackAdapter.java
@@ -16,6 +16,7 @@
 
 package androidx.webkit.internal;
 
+import androidx.annotation.NonNull;
 import androidx.webkit.WebViewCompat;
 
 import org.chromium.support_lib_boundary.VisualStateCallbackBoundaryInterface;
@@ -25,9 +26,10 @@ import org.chromium.support_lib_boundary.VisualStateCallbackBoundaryInterface;
  * corresponding interface shared with the support library glue in the WebView APK).
  */
 public class VisualStateCallbackAdapter implements VisualStateCallbackBoundaryInterface {
-    private WebViewCompat.VisualStateCallback mVisualStateCallback;
+    private final WebViewCompat.VisualStateCallback mVisualStateCallback;
 
-    public VisualStateCallbackAdapter(WebViewCompat.VisualStateCallback visualStateCallback) {
+    public VisualStateCallbackAdapter(
+            @NonNull WebViewCompat.VisualStateCallback visualStateCallback) {
         mVisualStateCallback = visualStateCallback;
     }
 
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebMessageCallbackAdapter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebMessageCallbackAdapter.java
index 9fe9b211eb4..f68ba94ac57 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebMessageCallbackAdapter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebMessageCallbackAdapter.java
@@ -30,14 +30,14 @@ import java.lang.reflect.InvocationHandler;
  * Adapter between {@link WebMessageCallbackCompat} and {@link WebMessageCallbackBoundaryInterface}.
  */
 public class WebMessageCallbackAdapter implements WebMessageCallbackBoundaryInterface {
-    WebMessageCallbackCompat mImpl;
+    private final WebMessageCallbackCompat mImpl;
 
     public WebMessageCallbackAdapter(@NonNull WebMessageCallbackCompat impl) {
         mImpl = impl;
     }
 
     @Override
-    public void onMessage(InvocationHandler port, InvocationHandler message) {
+    public void onMessage(@NonNull InvocationHandler port, @NonNull InvocationHandler message) {
         mImpl.onMessage(new WebMessagePortImpl(port),
                 WebMessageAdapter.webMessageCompatFromBoundaryInterface(
                         BoundaryInterfaceReflectionUtil.castToSuppLibClass(
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebResourceRequestAdapter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebResourceRequestAdapter.java
index 0d6a05d3348..5fbd02e55e3 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebResourceRequestAdapter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebResourceRequestAdapter.java
@@ -18,6 +18,8 @@ package androidx.webkit.internal;
 
 import android.webkit.WebResourceRequest;
 
+import androidx.annotation.NonNull;
+
 import org.chromium.support_lib_boundary.WebResourceRequestBoundaryInterface;
 
 /**
@@ -27,7 +29,8 @@ import org.chromium.support_lib_boundary.WebResourceRequestBoundaryInterface;
 public class WebResourceRequestAdapter {
     private final WebResourceRequestBoundaryInterface mBoundaryInterface;
 
-    public WebResourceRequestAdapter(WebResourceRequestBoundaryInterface boundaryInterface) {
+    public WebResourceRequestAdapter(
+            @NonNull WebResourceRequestBoundaryInterface boundaryInterface) {
         mBoundaryInterface = boundaryInterface;
     }
 
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebSettingsAdapter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebSettingsAdapter.java
index 1b7e3343a00..963fb991a02 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebSettingsAdapter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebSettingsAdapter.java
@@ -16,6 +16,8 @@
 
 package androidx.webkit.internal;
 
+import androidx.annotation.NonNull;
+
 import org.chromium.support_lib_boundary.WebSettingsBoundaryInterface;
 
 /**
@@ -24,9 +26,9 @@ import org.chromium.support_lib_boundary.WebSettingsBoundaryInterface;
  * corresponding interface shared with the support library glue in the WebView APK).
  */
 public class WebSettingsAdapter {
-    private WebSettingsBoundaryInterface mBoundaryInterface;
+    private final WebSettingsBoundaryInterface mBoundaryInterface;
 
-    public WebSettingsAdapter(WebSettingsBoundaryInterface boundaryInterface) {
+    public WebSettingsAdapter(@NonNull WebSettingsBoundaryInterface boundaryInterface) {
         mBoundaryInterface = boundaryInterface;
     }
 
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactory.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactory.java
index 080233e8a59..9eb736b395f 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactory.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactory.java
@@ -16,6 +16,7 @@
 
 package androidx.webkit.internal;
 
+import android.webkit.TracingController;
 import android.webkit.WebView;
 
 import androidx.annotation.NonNull;
@@ -39,23 +40,27 @@ public interface WebViewProviderFactory {
     /**
      * Create a support library version of {@link android.webkit.WebViewProvider}.
      */
-    WebViewProviderBoundaryInterface createWebView(WebView webview);
+    @NonNull
+    WebViewProviderBoundaryInterface createWebView(@NonNull WebView webview);
 
     /**
-     * Create the boundary interface for {@link androidx.webkit.internal.WebkitToCompatConverter}
+     * Create the boundary interface for {@link WebkitToCompatConverter}
      * which converts android.webkit classes into their corresponding support library classes.
      */
+    @NonNull
     WebkitToCompatConverterBoundaryInterface getWebkitToCompatConverter();
 
     /**
      * Fetch the boundary interface representing
      * {@link android.webkit.WebViewFactoryProvider#Statics}.
      */
+    @NonNull
     StaticsBoundaryInterface getStatics();
 
     /**
      * Fetch the features supported by the current WebView APK.
      */
+    @NonNull
     String[] getWebViewFeatures();
 
     /**
@@ -65,12 +70,14 @@ public interface WebViewProviderFactory {
     ServiceWorkerControllerBoundaryInterface getServiceWorkerController();
 
     /**
-     * Fetch the boundary interface representing {@link android.webkit.TracingController}.
+     * Fetch the boundary interface representing {@link TracingController}.
      */
+    @NonNull
     TracingControllerBoundaryInterface getTracingController();
 
     /**
      * Fetch the boundary interface representing {@link android.webkit.ProxyController}.
      */
+    @NonNull
     ProxyControllerBoundaryInterface getProxyController();
 }
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactoryAdapter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactoryAdapter.java
index 7f0a6b21e10..b30a66b651a 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactoryAdapter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebViewProviderFactoryAdapter.java
@@ -35,9 +35,9 @@ import org.chromium.support_lib_boundary.util.BoundaryInterfaceReflectionUtil;
  */
 @SuppressWarnings("JavadocReference") // WebViewFactoryProvider and WebViewProvider are hidden.
 public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
-    WebViewProviderFactoryBoundaryInterface mImpl;
+    final WebViewProviderFactoryBoundaryInterface mImpl;
 
-    public WebViewProviderFactoryAdapter(WebViewProviderFactoryBoundaryInterface impl) {
+    public WebViewProviderFactoryAdapter(@NonNull WebViewProviderFactoryBoundaryInterface impl) {
         mImpl = impl;
     }
 
@@ -46,8 +46,9 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
      * {@link android.webkit.WebViewProvider} - the class used to implement
      * {@link androidx.webkit.WebViewCompat}.
      */
+    @NonNull
     @Override
-    public WebViewProviderBoundaryInterface createWebView(WebView webview) {
+    public WebViewProviderBoundaryInterface createWebView(@NonNull WebView webview) {
         return BoundaryInterfaceReflectionUtil.castToSuppLibClass(
                 WebViewProviderBoundaryInterface.class, mImpl.createWebView(webview));
     }
@@ -57,6 +58,7 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
      * {@link androidx.webkit.internal.WebkitToCompatConverter}, which converts android.webkit
      * classes into their corresponding support library classes.
      */
+    @NonNull
     @Override
     public WebkitToCompatConverterBoundaryInterface getWebkitToCompatConverter() {
         return BoundaryInterfaceReflectionUtil.castToSuppLibClass(
@@ -67,6 +69,7 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
      * Adapter method for fetching the support library class representing
      * {@link android.webkit.WebViewFactoryProvider#Statics}.
      */
+    @NonNull
     @Override
     public StaticsBoundaryInterface getStatics() {
         return BoundaryInterfaceReflectionUtil.castToSuppLibClass(
@@ -76,6 +79,7 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
     /**
      * Adapter method for fetching the features supported by the current WebView APK.
      */
+    @NonNull
     @Override
     public String[] getWebViewFeatures() {
         return mImpl.getSupportedFeatures();
@@ -96,6 +100,7 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
      * Adapter method for fetching the support library class representing
      * {@link android.webkit.TracingController}.
      */
+    @NonNull
     @Override
     public TracingControllerBoundaryInterface getTracingController() {
         return BoundaryInterfaceReflectionUtil.castToSuppLibClass(
@@ -106,6 +111,7 @@ public class WebViewProviderFactoryAdapter implements WebViewProviderFactory {
      * Adapter method for fetching the support library class representing
      * {@link android.webkit.ProxyController}.
      */
+    @NonNull
     @Override
     public ProxyControllerBoundaryInterface getProxyController() {
         return BoundaryInterfaceReflectionUtil.castToSuppLibClass(
diff --git a/webkit/webkit/src/main/java/androidx/webkit/internal/WebkitToCompatConverter.java b/webkit/webkit/src/main/java/androidx/webkit/internal/WebkitToCompatConverter.java
index c7c51d2064f..ef80a917540 100644
--- a/webkit/webkit/src/main/java/androidx/webkit/internal/WebkitToCompatConverter.java
+++ b/webkit/webkit/src/main/java/androidx/webkit/internal/WebkitToCompatConverter.java
@@ -43,7 +43,7 @@ import java.lang.reflect.InvocationHandler;
 public class WebkitToCompatConverter {
     private final WebkitToCompatConverterBoundaryInterface mImpl;
 
-    public WebkitToCompatConverter(WebkitToCompatConverterBoundaryInterface impl) {
+    public WebkitToCompatConverter(@NonNull WebkitToCompatConverterBoundaryInterface impl) {
         mImpl = impl;
     }
 
@@ -53,7 +53,7 @@ public class WebkitToCompatConverter {
      * {@link androidx.webkit.WebSettingsCompat}.
      */
     @NonNull
-    public WebSettingsAdapter convertSettings(WebSettings webSettings) {
+    public WebSettingsAdapter convertSettings(@NonNull WebSettings webSettings) {
         return new WebSettingsAdapter(BoundaryInterfaceReflectionUtil.castToSuppLibClass(
                 WebSettingsBoundaryInterface.class, mImpl.convertSettings(webSettings)));
     }
@@ -63,7 +63,8 @@ public class WebkitToCompatConverter {
      * that calls on either of those objects affect the other object.
      */
     @NonNull
-    public WebResourceRequestAdapter convertWebResourceRequest(WebResourceRequest request) {
+    public WebResourceRequestAdapter convertWebResourceRequest(
+            @NonNull WebResourceRequest request) {
         return new WebResourceRequestAdapter(BoundaryInterfaceReflectionUtil.castToSuppLibClass(
                 WebResourceRequestBoundaryInterface.class,
                 mImpl.convertWebResourceRequest(request)));