diff --git a/app/src/main/java/com/smsreceive/app/ui/MainActivity.java b/app/src/main/java/com/smsreceive/app/ui/MainActivity.java
index 12b5b9c..8d1f8e6 100644
--- a/app/src/main/java/com/smsreceive/app/ui/MainActivity.java
+++ b/app/src/main/java/com/smsreceive/app/ui/MainActivity.java
@@ -73,7 +73,6 @@ public final class MainActivity extends Activity {
     private EditText feishuSecretEdit;
     private EditText pollingIntervalEdit;
     private AlertDialog debugInfoDialog;
-    private View debugInfoView;
     private long lastInboxSmsId = -1L;
     private String googlePlayTextValue = "";
     private String keepAliveTextValue = "";
@@ -189,7 +188,7 @@ public final class MainActivity extends Activity {
 
     private void showDebugInfoDialog() {
         if (debugInfoDialog == null) {
-            debugInfoView = LayoutInflater.from(this).inflate(R.layout.dialog_debug_info, null);
+            View debugInfoView = LayoutInflater.from(this).inflate(R.layout.dialog_debug_info, null);
             googlePlayText = debugInfoView.findViewById(R.id.google_play_text);
             keepAliveText = debugInfoView.findViewById(R.id.keep_alive_text);
             databaseHeartbeatText = debugInfoView.findViewById(R.id.database_heartbeat_text);
@@ -503,13 +502,11 @@ public final class MainActivity extends Activity {
     }
 
     private String buildLatestSmsTestMarkdown(SmsInboxReader.InboxResult inboxResult) {
-        StringBuilder builder = new StringBuilder();
-        builder.append("**SmsReceive 最近短信测试推送**").append('\n');
-        builder.append("时间：").append(formatTime(inboxResult.dateMillis)).append('\n');
-        builder.append("发送方：").append(maskSender(inboxResult.sender)).append('\n');
-        builder.append("短信ID：").append(inboxResult.id).append('\n');
-        builder.append("正文：").append(emptyAsDash(inboxResult.body));
-        return builder.toString();
+        return "**SmsReceive 最近短信测试推送**" + '\n' +
+                "时间：" + formatTime(inboxResult.dateMillis) + '\n' +
+                "发送方：" + maskSender(inboxResult.sender) + '\n' +
+                "短信ID：" + inboxResult.id + '\n' +
+                "正文：" + emptyAsDash(inboxResult.body);
     }
 
     private boolean hasReceiveSmsPermission() {
@@ -693,9 +690,6 @@ public final class MainActivity extends Activity {
     }
 
     private boolean isIgnoringBatteryOptimizations() {
-        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
-            return true;
-        }
         PowerManager powerManager = (PowerManager) getSystemService(Context.POWER_SERVICE);
         return powerManager != null && powerManager.isIgnoringBatteryOptimizations(getPackageName());
     }
diff --git a/build.gradle b/build.gradle
index 8f07d52..56fe1bc 100644
--- a/build.gradle
+++ b/build.gradle
@@ -18,12 +18,12 @@ allprojects {
 }
 
 ext {
-    minSdkVersion = 21
+    minSdkVersion = 23
     compileSdkVersion = 30
     targetSdkVersion = 30
     buildToolsVersion = '30.0.3'
 }
 
-task clean(type: Delete) {
+tasks.register('clean', Delete) {
     delete rootProject.buildDir
 }