[WebView] initial webview-side implementation of safebrowsing

android_webview/java/src/org/chromium/android_webview/PlatformServiceBridge.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.android_webview;
 
 import android.content.Context;
 import android.webkit.ValueCallback;
 
 import org.chromium.base.Log;
 import org.chromium.base.ThreadUtils;
 
 import java.lang.reflect.InvocationTargetException;
 
 /**
  * This class manages platform-specific services. (i.e. Google Services) The platform
  * should extend this class and use this base class to fetch their specialized version.
  */
 public class PlatformServiceBridge {
     private static final String TAG = "PlatformServiceBrid-";
     private static final String PLATFORM_SERVICE_BRIDGE =
             "com.android.webview.chromium.PlatformServiceBridgeGoogle";
 
+    private static final Object sInstanceLock = new Object();
     private static PlatformServiceBridge sInstance;
 
     protected PlatformServiceBridge() {}
 
     public static PlatformServiceBridge getInstance(Context appContext) {
-        ThreadUtils.assertOnUiThread(); // Avoid race conditions on sInstance.
+        // TODO(timvolodine): consider removing the lock, crbug.com/681805.
+        synchronized (sInstanceLock) {
+            if (sInstance != null) {
+                return sInstance;
+            }
 
-        if (sInstance != null) {
-            return sInstance;
+            // Try to get a specialized service bridge.
+            try {
+                Class<?> cls = Class.forName(PLATFORM_SERVICE_BRIDGE);
+                sInstance = (PlatformServiceBridge) cls.getDeclaredConstructor(Context.class)
+                                    .newInstance(appContext);
+                return sInstance;
+            } catch (ClassNotFoundException e) {
+                // This is not an error; it just means this device doesn't have specialized
+                // services.
+            } catch (IllegalAccessException | IllegalArgumentException | InstantiationException
+                    | NoSuchMethodException e) {
+                Log.e(TAG, "Failed to get " + PLATFORM_SERVICE_BRIDGE + ": " + e);
+            } catch (InvocationTargetException e) {
+                Log.e(TAG, "Failed invocation to get " + PLATFORM_SERVICE_BRIDGE + ":",
+                        e.getCause());
+            }
+
+            // Otherwise, get the generic service bridge.
+            sInstance = new PlatformServiceBridge();
         }
-
-        // Try to get a specialized service bridge.
-        try {
-            Class<?> cls = Class.forName(PLATFORM_SERVICE_BRIDGE);
-            sInstance = (PlatformServiceBridge) cls.getDeclaredConstructor(Context.class)
-                    .newInstance(appContext);
-            return sInstance;
-        } catch (ClassNotFoundException e) {
-            // This is not an error; it just means this device doesn't have specialized services.
-        } catch (IllegalAccessException | IllegalArgumentException | InstantiationException
-                | NoSuchMethodException e) {
-            Log.e(TAG, "Failed to get " + PLATFORM_SERVICE_BRIDGE + ": " + e);
-        } catch (InvocationTargetException e) {
-            Log.e(TAG, "Failed invocation to get " + PLATFORM_SERVICE_BRIDGE + ":", e.getCause());
-        }
-
-        // Otherwise, get the generic service bridge.
-        sInstance = new PlatformServiceBridge();
         return sInstance;
     }
 
     // TODO(paulmiller): remove; replaced by canUseGms
     public boolean tryEnableGms() {
         return false;
     }
 
     // Can WebView use Google Play Services (a.k.a. GMS)?
     public boolean canUseGms() {
         return false;
     }
 
     // Overriding implementations may call "callback" asynchronously. For simplicity (and not
     // because of any technical limitation) we require that "queryMetricsSetting" and "callback"
     // both get called on WebView's UI thread.
     public void queryMetricsSetting(ValueCallback<Boolean> callback) {
         ThreadUtils.assertOnUiThread();
         callback.onReceiveValue(false);
     }
 }