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]