diff --git a/packages/WallpaperBackup/Android.bp b/packages/WallpaperBackup/Android.bp
index 56020cd..748eb40 100644
--- a/packages/WallpaperBackup/Android.bp
+++ b/packages/WallpaperBackup/Android.bp
@@ -24,3 +24,27 @@
     certificate: "platform",
     privileged: false,
 }
+
+android_test {
+    name: "WallpaperBackupAgentTests",
+    manifest: "test/AndroidManifest.xml",
+    test_config: "test/AndroidTest.xml",
+    srcs: [
+        // Include the app source code because the app runs as the system user on-device.
+        "src/**/*.java",
+        "test/src/**/*.java"
+    ],
+    libs: [
+        "android.test.base",
+        "android.test.runner",
+    ],
+    static_libs: [
+        "androidx.test.core",
+        "androidx.test.rules",
+        "mockito-target-minus-junit4",
+        "truth-prebuilt",
+    ],
+    certificate: "platform",
+    platform_apis: true,
+    test_suites: ["device-tests"]
+}
diff --git a/packages/WallpaperBackup/src/com/android/wallpaperbackup/WallpaperBackupAgent.java b/packages/WallpaperBackup/src/com/android/wallpaperbackup/WallpaperBackupAgent.java
index da90189..1c2c640 100644
--- a/packages/WallpaperBackup/src/com/android/wallpaperbackup/WallpaperBackupAgent.java
+++ b/packages/WallpaperBackup/src/com/android/wallpaperbackup/WallpaperBackupAgent.java
@@ -39,6 +39,8 @@
 import android.util.Slog;
 import android.util.Xml;
 
+import com.android.internal.annotations.VisibleForTesting;
+
 import libcore.io.IoUtils;
 
 import org.xmlpull.v1.XmlPullParser;
@@ -89,7 +91,7 @@
             Slog.v(TAG, "onCreate()");
         }
 
-        File wallpaperDir = Environment.getUserSystemDirectory(UserHandle.USER_SYSTEM);
+        File wallpaperDir = getWallpaperDir();
         mWallpaperInfo = new File(wallpaperDir, WALLPAPER_INFO);
         mWallpaperFile = new File(wallpaperDir, WALLPAPER);
         mLockWallpaperFile = new File(wallpaperDir, WALLPAPER_LOCK);
@@ -102,6 +104,11 @@
         }
     }
 
+    @VisibleForTesting
+    protected File getWallpaperDir() {
+        return Environment.getUserSystemDirectory(UserHandle.USER_SYSTEM);
+    }
+
     @Override
     public void onFullBackup(FullBackupDataOutput data) throws IOException {
         // To avoid data duplication and disk churn, use links as the stage.
@@ -119,7 +126,7 @@
                 FileOutputStream touch = new FileOutputStream(empty);
                 touch.close();
             }
-            fullBackupFile(empty, data);
+            backupFile(empty, data);
 
             SharedPreferences prefs = getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
             final int lastSysGeneration = prefs.getInt(SYSTEM_GENERATION, -1);
@@ -155,7 +162,7 @@
                     FileUtils.copyFileOrThrow(mWallpaperInfo, infoStage);
                 }
                 if (DEBUG) Slog.v(TAG, "Storing wallpaper metadata");
-                fullBackupFile(infoStage, data);
+                backupFile(infoStage, data);
             }
             if (sysEligible && mWallpaperFile.exists()) {
                 if (sysChanged || !imageStage.exists()) {
@@ -163,7 +170,7 @@
                     FileUtils.copyFileOrThrow(mWallpaperFile, imageStage);
                 }
                 if (DEBUG) Slog.v(TAG, "Storing system wallpaper image");
-                fullBackupFile(imageStage, data);
+                backupFile(imageStage, data);
                 prefs.edit().putInt(SYSTEM_GENERATION, sysGeneration).apply();
             }
 
@@ -174,7 +181,7 @@
                     FileUtils.copyFileOrThrow(mLockWallpaperFile, lockImageStage);
                 }
                 if (DEBUG) Slog.v(TAG, "Storing lock wallpaper image");
-                fullBackupFile(lockImageStage, data);
+                backupFile(lockImageStage, data);
                 prefs.edit().putInt(LOCK_GENERATION, lockGeneration).apply();
             }
         } catch (Exception e) {
@@ -189,6 +196,12 @@
         }
     }
 
+    @VisibleForTesting
+    // fullBackupFile is final, so we intercept backups here in tests.
+    protected void backupFile(File file, FullBackupDataOutput data) {
+        fullBackupFile(file, data);
+    }
+
     @Override
     public void onQuotaExceeded(long backupDataBytes, long quotaBytes) {
         if (DEBUG) {
diff --git a/packages/WallpaperBackup/test/AndroidManifest.xml b/packages/WallpaperBackup/test/AndroidManifest.xml
new file mode 100644
index 0000000..44ab1b6
--- /dev/null
+++ b/packages/WallpaperBackup/test/AndroidManifest.xml
@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+          package="com.android.wallpaperbackup.tests">
+
+    <application android:label="WallpaperBackup Tests">
+        <uses-library android:name="android.test.runner" />
+    </application>
+
+    <instrumentation android:name="androidx.test.runner.AndroidJUnitRunner"
+                     android:targetPackage="com.android.wallpaperbackup.tests"
+                     android:label="WallpaperBackup Tests"/>
+</manifest>
+
diff --git a/packages/WallpaperBackup/test/AndroidTest.xml b/packages/WallpaperBackup/test/AndroidTest.xml
new file mode 100644
index 0000000..f2e7782
--- /dev/null
+++ b/packages/WallpaperBackup/test/AndroidTest.xml
@@ -0,0 +1,27 @@
+<!-- Copyright (C) 2019 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+<configuration description="Runs sample instrumentation test.">
+    <target_preparer class="com.android.tradefed.targetprep.TestAppInstallSetup">
+        <option name="test-file-name" value="WallpaperBackupAgentTests.apk" />
+    </target_preparer>
+
+    <option name="test-suite-tag" value="apct" />
+    <option name="test-tag" value="WallpaperBackupAgentTests" />
+    <test class="com.android.tradefed.testtype.AndroidJUnitTest" >
+        <option name="package" value="com.android.wallpaperbackup.tests" />
+        <option name="runner" value="androidx.test.runner.AndroidJUnitRunner" />
+        <option name="hidden-api-checks" value="false"/>
+    </test>
+</configuration>
diff --git a/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/tests/WallpaperBackupAgentTest.java b/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/tests/WallpaperBackupAgentTest.java
new file mode 100644
index 0000000..46a7dfe
--- /dev/null
+++ b/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/tests/WallpaperBackupAgentTest.java
@@ -0,0 +1,143 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.wallpaperbackup.tests;
+
+import static android.app.WallpaperManager.FLAG_LOCK;
+import static android.app.WallpaperManager.FLAG_SYSTEM;
+
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import android.app.WallpaperManager;
+import android.app.backup.FullBackupDataOutput;
+import android.content.SharedPreferences;
+import android.os.UserHandle;
+
+import androidx.test.core.app.ApplicationProvider;
+import androidx.test.runner.AndroidJUnit4;
+
+import com.android.wallpaperbackup.WallpaperBackupAgent;
+import com.android.wallpaperbackup.utils.ContextWithServiceOverrides;
+
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+@RunWith(AndroidJUnit4.class)
+public class WallpaperBackupAgentTest {
+    private static final String SYSTEM_GENERATION = "system_gen";
+    private static final String LOCK_GENERATION = "lock_gen";
+
+    @Mock private FullBackupDataOutput mOutput;
+    @Mock private WallpaperManager mWallpaperManager;
+    @Mock private SharedPreferences mSharedPreferences;
+
+    @Rule public TemporaryFolder mTemporaryFolder = new TemporaryFolder();
+
+    private ContextWithServiceOverrides mContext;
+
+    @Before
+    public void setUp() {
+        MockitoAnnotations.initMocks(this);
+
+        mContext = new ContextWithServiceOverrides(ApplicationProvider.getApplicationContext());
+        mContext.injectSystemService(WallpaperManager.class, mWallpaperManager);
+        mContext.setSharedPreferencesOverride(mSharedPreferences);
+    }
+
+    @Test
+    public void testOnFullBackup_withNoChanges_onlyBacksUpEmptyFile() throws IOException {
+        WallpaperBackupAgent wallpaperBackupAgent = new WallpaperBackupAgent();
+        initialiseAgent(wallpaperBackupAgent);
+
+        when(mWallpaperManager.getWallpaperIdForUser(eq(FLAG_SYSTEM), eq(UserHandle.USER_SYSTEM)))
+                .thenReturn(1);
+        when(mWallpaperManager.getWallpaperIdForUser(eq(FLAG_LOCK), eq(UserHandle.USER_SYSTEM)))
+                .thenReturn(1);
+        when(mSharedPreferences.getInt(eq(SYSTEM_GENERATION), eq(-1))).thenReturn(1);
+        when(mSharedPreferences.getInt(eq(LOCK_GENERATION), eq(-1))).thenReturn(1);
+
+        wallpaperBackupAgent.onFullBackup(mOutput);
+
+        verify(mOutput); // Backup of empty file only
+    }
+
+    @Test
+    public void testOnFullBackup_withOnlyChangedSystem_updatesTheSharedPreferences()
+            throws IOException {
+        // Create a system wallpaper file
+        mTemporaryFolder.newFile("wallpaper_orig");
+        // Create stageing file to simulate he wallpaper being ready to back up
+        new File(mContext.getFilesDir(), "wallpaper-stage").createNewFile();
+
+        WallpaperBackupAgent wallpaperBackupAgent =
+                new IsolatedWallpaperBackupAgent(mTemporaryFolder.getRoot());
+        initialiseAgent(wallpaperBackupAgent);
+
+        SharedPreferences.Editor preferenceEditor = mock(SharedPreferences.Editor.class);
+
+        when(mWallpaperManager.getWallpaperIdForUser(eq(FLAG_SYSTEM), eq(UserHandle.USER_SYSTEM)))
+                .thenReturn(2);
+        when(mWallpaperManager.getWallpaperIdForUser(eq(FLAG_LOCK), eq(UserHandle.USER_SYSTEM)))
+                .thenReturn(1);
+        when(mWallpaperManager.isWallpaperBackupEligible(eq(FLAG_SYSTEM))).thenReturn(true);
+        when(mWallpaperManager.isWallpaperBackupEligible(eq(FLAG_LOCK))).thenReturn(true);
+        when(mSharedPreferences.getInt(eq(SYSTEM_GENERATION), eq(-1))).thenReturn(1);
+        when(mSharedPreferences.getInt(eq(LOCK_GENERATION), eq(-1))).thenReturn(1);
+        when(mSharedPreferences.edit()).thenReturn(preferenceEditor);
+        when(preferenceEditor.putInt(eq(SYSTEM_GENERATION), eq(2))).thenReturn(preferenceEditor);
+
+        wallpaperBackupAgent.onFullBackup(mOutput);
+
+        verify(preferenceEditor).putInt(eq(SYSTEM_GENERATION), eq(2));
+    }
+
+    private void initialiseAgent(WallpaperBackupAgent agent) {
+        agent.attach(mContext);
+        agent.onCreate();
+    }
+
+    private static class IsolatedWallpaperBackupAgent extends WallpaperBackupAgent {
+        File mWallpaperBaseDirectory;
+        List<File> mBackedUpFiles = new ArrayList();
+
+        IsolatedWallpaperBackupAgent(File wallpaperBaseDirectory) {
+            mWallpaperBaseDirectory = wallpaperBaseDirectory;
+        }
+
+        @Override
+        protected File getWallpaperDir() {
+            return mWallpaperBaseDirectory;
+        }
+
+        @Override
+        protected void backupFile(File file, FullBackupDataOutput data) {
+            mBackedUpFiles.add(file);
+        }
+    }
+}
diff --git a/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/utils/ContextWithServiceOverrides.java b/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/utils/ContextWithServiceOverrides.java
new file mode 100644
index 0000000..df5d74a
--- /dev/null
+++ b/packages/WallpaperBackup/test/src/com/android/wallpaperbackup/utils/ContextWithServiceOverrides.java
@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.wallpaperbackup.utils;
+
+import android.content.Context;
+import android.content.ContextWrapper;
+import android.content.SharedPreferences;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class ContextWithServiceOverrides extends ContextWrapper {
+    private static final String TAG = "ContextWithOverrides";
+
+    private Map<String, Object> mInjectedSystemServices = new HashMap<>();
+    private SharedPreferences mSharedPreferencesOverride;
+
+    public ContextWithServiceOverrides(Context base) {
+        super(base);
+    }
+
+    public <S> void injectSystemService(Class<S> cls, S service) {
+        final String name = getSystemServiceName(cls);
+        mInjectedSystemServices.put(name, service);
+    }
+
+    @Override
+    public Context getApplicationContext() {
+        return this;
+    }
+
+    @Override
+    public Object getSystemService(String name) {
+        if (mInjectedSystemServices.containsKey(name)) {
+            return mInjectedSystemServices.get(name);
+        }
+        return super.getSystemService(name);
+    }
+
+    public void setSharedPreferencesOverride(SharedPreferences override) {
+        mSharedPreferencesOverride = override;
+    }
+
+    @Override
+    public SharedPreferences getSharedPreferences(String name, int mode) {
+        return mSharedPreferencesOverride == null
+                ? super.getSharedPreferences(name, mode)
+                : mSharedPreferencesOverride;
+    }
+}