一、基础知识
1.Android 进程优先级
1.1 进程优先级等级一般分法：
- Activte process
- Visible Process
- Service process
- Background process
- Empty process
1.2 进程优先级号
// Adjustment used in certain places where we don't know it yet. // (Generally this is something that is going to be cached, but we // don't know the exact value in the cached range to assign yet.) static final int UNKNOWN_ADJ = 16; // This is a process only hosting activities that are not visible, // so it can be killed without any disruption. static final int CACHED_APP_MAX_ADJ = 15; static final int CACHED_APP_MIN_ADJ = 9; // The B list of SERVICE_ADJ -- these are the old and decrepit // services that aren't as shiny and interesting as the ones in the A list. static final int SERVICE_B_ADJ = 8; // This is the process of the previous application that the user was in. // This process is kept above other things, because it is very common to // switch back to the previous app. This is important both for recent // task switch (toggling between the two top recent apps) as well as normal // UI flow such as clicking on a URI in the e-mail app to view in the browser, // and then pressing back to return to e-mail. static final int PREVIOUS_APP_ADJ = 7; // This is a process holding the home application -- we want to try // avoiding killing it, even if it would normally be in the background, // because the user interacts with it so much. static final int HOME_APP_ADJ = 6; // This is a process holding an application service -- killing it will not // have much of an impact as far as the user is concerned. static final int SERVICE_ADJ = 5; // This is a process with a heavy-weight application. It is in the // background, but we want to try to avoid killing it. Value set in // system/rootdir/init.rc on startup. static final int HEAVY_WEIGHT_APP_ADJ = 4; // This is a process currently hosting a backup operation. Killing it // is not entirely fatal but is generally a bad idea. static final int BACKUP_APP_ADJ = 3; // This is a process only hosting components that are perceptible to the // user, and we really want to avoid killing them, but they are not // immediately visible. An example is background music playback. static final int PERCEPTIBLE_APP_ADJ = 2; // This is a process only hosting activities that are visible to the // user, so we'd prefer they don't disappear. static final int VISIBLE_APP_ADJ = 1; // This is the process running the current foreground app. We'd really // rather not kill it! static final int FOREGROUND_APP_ADJ = 0; // This is a process that the system or a persistent process has bound to, // and indicated it is important. static final int PERSISTENT_SERVICE_ADJ = -11; // This is a system persistent process, such as telephony. Definitely // don't want to kill it, but doing so is not completely fatal. static final int PERSISTENT_PROC_ADJ = -12; // The system process runs at the default adjustment. static final int SYSTEM_ADJ = -16; // Special code for native processes that are not being managed by the system (so // don't have an oom adj assigned by the system). static final int NATIVE_ADJ = -17;
2. Android Low Memory Killer
Android系统内存不足时，系统会杀掉一部分进程以释放空间，谁生谁死的这个生死大权就是由LMK所决定的，这就是Android系统中的Low Memory Killer，其基于Linux的OOM机制,其阈值定义如下面所示的lowmemorykiller文件中，当然也可以通过系统的init.rc实现自定义。
lowmemorykiller.c
static uint32_t lowmem_debug_level = 1; static int lowmem_adj[6] = { 0, 1, 6, 12, }; static int lowmem_adj_size = 4; static int lowmem_minfree[6] = { 3 * 512, /* 6MB */ 2 * 1024, /* 8MB */ 4 * 1024, /* 16MB */ 16 * 1024, /* 64MB */ }; static int lowmem_minfree_size = 4;
① 在Low Memory Killer中通过进程的oom_adj与占用内存的大小决定要杀死的进程，oom_adj值越小越不容易被杀死。其中，lowmem_minfree是杀进程的时机，谁被杀，则取决于lowmem_adj，具体值得含义参考上面 Android进程优先级 所述.
② 在init.rc中定义了init进程（系统进程）的oom_adj为-16，其不可能会被杀死(init的PID是1)，而前台进程是0（这里的前台进程是指用户正在使用的Activity所在的进程），用户按Home键回到桌面时的优先级是6，普通的Service的进程是8.
init.rc
1 # Set init and its forked children's oom_adj. 2 write /proc/1/oom_adj -16
关于Low Memory Killer的具体实现原理可参考Ref-2.
3. 查看某个App的进程
步骤（手机与PC连接）
1. adb shell
2. ps | grep 进程名
3. cat /proc/pid/oom_adj //其中pid是上述grep得到的进程号
4. Android账号和同步机制
属于Android中较偏冷的知识，具体参考 Ref 3/4/5
二、现有方法
1. 网络连接保活方法
a. GCM
b. 公共的第三方push通道(信鸽等)
c. 自身跟服务器通过轮询，或者长连接
具体实现请参考 微信架构师杨干荣的"微信Android客户端后台保活经验分享" （Ref-1）.
2. 双service 提高进程优先级
思路：（API level > 18 ）
① 应用启动时启动一个假的Service（FakeService）, startForeground()，传一个空的Notification
② 启动真正的Service（AlwaysLiveService），startForeground()，注意必须相同Notification ID
③ FakeService stopForeground()
效果：通过adb查看,运行在后台的服务其进程号变成了1（优先级仅次于前台进程）
风险：Android系统前台service的一个漏洞，可能在6.0以上系统中修复
实现：核心代码如下
AlwaysLiveService 常驻内存服务
@Override public int onStartCommand(Intent intent, int flags, int startId) { startForeground(R.id.notify, new Notification()); startService(new Intent(this, FakeService.class)); return super.onStartCommand(intent, flags, startId); }
FakeService 临时服务
public class FakeService extends Service { @Nullable @Override public IBinder onBind(Intent intent) { return null; } @Override public int onStartCommand(Intent intent, int flags, int startId) { startForeground(R.id.notify, new Notification()); stopSelf(); return super.onStartCommand(intent, flags, startId); } @Override public void onDestroy() { stopForeground(true); super.onDestroy(); } }
3. 守护进程及时拉起
AlarmReceiver， ConnectReceiver，BootReceiver等
三、新方法(AccountSyncAdapter)
1. 思路：
利用Android系统提供的账号和同步机制实现
2. 效果：
① 通过adb查看,运行在后台的服务其进程号变成了1（优先级仅次于前台进程），能提高进程优先级，对比如下图
正常情况
采用AccountSyncAdapter方法后
② 进程被系统kill后，可以由syn拉起
3. 风险：
① SyncAdapter时间进度不高，往往会因为手机处于休眠状态，而时间往后调整，同步间隔最低为1分钟
② 用户可以单独停止或者删除，有些手机账号默认是不同步的，需要手动开启
4. 实现：核心代码如下
① 建立数据同步系统（ContentProvider）
通过一个ContentProvider用来作数据同步，由于并没有实际数据同步，所以此处就直接建立一个空的ContentProvider即可
public class XXAccountProvider extends ContentProvider { public static final String AUTHORITY = "包名.provider"; public static final String CONTENT_URI_BASE = "content://" + AUTHORITY; public static final String TABLE_NAME = "data"; public static final Uri CONTENT_URI = Uri.parse(CONTENT_URI_BASE + "/" + TABLE_NAME); @Override public boolean onCreate() { return true; } @Nullable @Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { return null; } @Nullable @Override public String getType(Uri uri) { return new String(); } @Nullable @Override public Uri insert(Uri uri, ContentValues values) { return null; } @Override public int delete(Uri uri, String selection, String[] selectionArgs) { return 0; } @Override public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) { return 0; } }
然后再Manifest中声明
<provider android:name="**.XXAccountProvider" android:authorities="@string/account_auth_provider" android:exported="false" android:syncable="true"/>
② 建立Sync系统 (SyncAdapter)
通过实现SyncAdapter这个系统服务后, 利用系统的定时器对程序数据ContentProvider进行更新，具体步骤为：
- 创建Sync服务
public class XXSyncService extends Service { private static final Object sSyncAdapterLock = new Object(); private static XXSyncAdapter sSyncAdapter = null; @Override public void onCreate() { synchronized (sSyncAdapterLock) { if (sSyncAdapter == null) { sSyncAdapter = new XXSyncAdapter(getApplicationContext(), true); } } } @Override public IBinder onBind(Intent intent) { return sSyncAdapter.getSyncAdapterBinder(); } static class XXSyncAdapter extends AbstractThreadedSyncAdapter { public XXSyncAdapter(Context context, boolean autoInitialize) { super(context, autoInitialize); } @Override public void onPerformSync(Account account, Bundle extras, String authority, ContentProviderClient provider, SyncResult syncResult) { getContext().getContentResolver().notifyChange(XXAccountProvider.CONTENT_URI, null, false); } } }
- 声明Sync服务
<service android:name="**.XXSyncService" android:exported="true" android:process=":core"> <intent-filter> <action android:name="android.content.SyncAdapter"/> </intent-filter> <meta-data android:name="android.content.SyncAdapter" android:resource="@xml/sync_adapter"/> </service>
其中sync_adapter为：
<sync-adapter xmlns:android="http://schemas.android.com/apk/res/android" android:accountType="@string/account_auth_type" android:allowParallelSyncs="false" android:contentAuthority="@string/account_auth_provide" android:isAlwaysSyncable="true" android:supportsUploading="false" android:userVisible="true"/>
参数说明：
android:contentAuthority 指定要同步的ContentProvider在其AndroidManifest.xml文件中有个android:authorities属性。
android:accountType 表示进行同步的账号的类型。
android:userVisible 设置是否在“设置”中显示
android:supportsUploading 设置是否必须notifyChange通知才能同步
android:allowParallelSyncs 是否支持多账号同时同步
android:isAlwaysSyncable 设置所有账号的isSyncable为1
android:syncAdapterSettingsAction 指定一个可以设置同步的activity的Action。
- 账户调用Sync服务
首先配置好Account（第三步），然后再通过ContentProvider实现
手动更新
public void triggerRefresh() { Bundle b = new Bundle(); b.putBoolean(ContentResolver.SYNC_EXTRAS_MANUAL, true); b.putBoolean(ContentResolver.SYNC_EXTRAS_EXPEDITED, true); ContentResolver.requestSync( account, CONTENT_AUTHORITY, b); }
添加账号
Account account = AccountService.GetAccount(); AccountManager accountManager = (AccountManager) context.getSystemService(Context.ACCOUNT_SERVICE); accountManager.addAccountExplicitly(...)
同步周期设置
ContentResolver.setIsSyncable(account, CONTENT_AUTHORITY, 1); ContentResolver.setSyncAutomatically(account, CONTENT_AUTHORITY, true); ContentResolver.addPeriodicSync(account, CONTENT_AUTHORITY, new Bundle(), SYNC_FREQUENCY);
③ 建立账号系统 (Account Authenticator)
通过建立Account账号，并关联SyncAdapter服务实现同步
- 创建Account服务
public class XXAuthService extends Service { private XXAuthenticator mAuthenticator; @Override public void onCreate() { mAuthenticator = new XXAuthenticator(this); } private XXAuthenticator getAuthenticator() { if (mAuthenticator == null) mAuthenticator = new XXAuthenticator(this); return mAuthenticator; } @Override public IBinder onBind(Intent intent) { return getAuthenticator().getIBinder(); } class XXAuthenticator extends AbstractAccountAuthenticator { private final Context context; private AccountManager accountManager; public XXAuthenticator(Context context) { super(context); this.context = context; accountManager = AccountManager.get(context); } @Override public Bundle addAccount(AccountAuthenticatorResponse response, String accountType, String authTokenType, String[] requiredFeatures, Bundle options) throws NetworkErrorException { // 添加账号 示例代码 final Bundle bundle = new Bundle(); final Intent intent = new Intent(context, AuthActivity.class); intent.putExtra(AccountManager.KEY_ACCOUNT_AUTHENTICATOR_RESPONSE, response); bundle.putParcelable(AccountManager.KEY_INTENT, intent); return bundle; } @Override public Bundle getAuthToken(AccountAuthenticatorResponse response, Account account, String authTokenType, Bundle options) throws NetworkErrorException { // 认证 示例代码 String authToken = accountManager.peekAuthToken(account, getString(R.string.account_token_type)); //if not, might be expired, register again if (TextUtils.isEmpty(authToken)) { final String password = accountManager.getPassword(account); if (password != null) { //get new token authToken = account.name + password; } } //without password, need to sign again final Bundle bundle = new Bundle(); if (!TextUtils.isEmpty(authToken)) { bundle.putString(AccountManager.KEY_ACCOUNT_NAME, account.name); bundle.putString(AccountManager.KEY_ACCOUNT_TYPE, account.type); bundle.putString(AccountManager.KEY_AUTHTOKEN, authToken); return bundle; } //no account data at all, need to do a sign final Intent intent = new Intent(context, AuthActivity.class); intent.putExtra(AccountManager.KEY_ACCOUNT_AUTHENTICATOR_RESPONSE, response); intent.putExtra(AuthActivity.ARG_ACCOUNT_NAME, account.name); bundle.putParcelable(AccountManager.KEY_INTENT, intent); return bundle; } @Override public String getAuthTokenLabel(String authTokenType) { // throw new UnsupportedOperationException(); return null; } @Override public Bundle editProperties(AccountAuthenticatorResponse response, String accountType) { return null; } @Override public Bundle confirmCredentials(AccountAuthenticatorResponse response, Account account, Bundle options) throws NetworkErrorException { return null; } @Override public Bundle updateCredentials(AccountAuthenticatorResponse response, Account account, String authTokenType, Bundle options) throws NetworkErrorException { return null; } @Override public Bundle hasFeatures(AccountAuthenticatorResponse response, Account account, String[] features) throws NetworkErrorException { return null; } } }
- 声明Account服务
<service android:name="**.XXAuthService" android:exported="true" android:process=":core"> <intent-filter> <action android:name="android.accounts.AccountAuthenticator"/> </intent-filter> <meta-data android:name="android.accounts.AccountAuthenticator" android:resource="@xml/authenticator"/> </service>
其中authenticator为：
<?xml version="1.0" encoding="utf-8"?> <account-authenticator xmlns:android="http://schemas.android.com/apk/res/android" android:accountType="@string/account_auth_type" android:icon="@drawable/icon" android:smallIcon="@drawable/icon" android:label="@string/app_name" />
- 使用Account服务
同SyncAdapter，通过AccountManager使用
- 申请Token主要是通过 [AccountManager.getAuthToken]系列方法
- 添加账号则通过 [AccountManager.addAccount]
- 查看是否存在账号通过 [AccountManager.getAccountsByType]
Refs
1. [微信Android客户端后台保活经验分享]
2. [Android Low Memory Killer原理]
3. [stackOverflow 上介绍的双Service方法]
4. [Write your own Android Sync Adapter]
5. [Write your own Android Authenticator]
6. Android developer
- [android.accounts]
- [AccountManager]
- [AbstractAccountAuthenticator]
- [AccountAuthenticatorActivity]
- [Creating a Sync Adapter]