安卓获取APP对应的Android id的原理分析

android_id 的生成原理是由系统生成的随机数，并与应用 app 签名，经过 HmacSHA256 算法生成的；
从 android 8 以后开始就是随机的了，每个应用获取到的简要步骤；
获取的方式如下所示：

androidId = android.provider.Settings.Secure.getString(context.getContentResolver(), android.provider.Settings.Secure.ANDROID_ID);

 public String getStringForUser(ContentResolver cr, String name, final int userHandle) {final boolean isSelf = (userHandle == UserHandle.myUserId());if (isSelf) {synchronized (NameValueCache.this) {if (mGenerationTracker != null) {if (mGenerationTracker.isGenerationChanged()) {if (true) {Log.i(TAG, "Generation changed for type:"+ mUri.getPath() + " in package:"+ cr.getPackageName() +" and user:" + userHandle);}mValues.clear();} else if (mValues.containsKey(name)) {//如果APP已经安装第二次读取位置Log.v(TAG, "getStringForUser clear name= " + name  + " key: " + mValues.get(name));mValues.remove(name);// return mValues.get(name);}}}final long token = Binder.clearCallingIdentity();try {Log.w(TAG, "call here ------" );b = cp.call(cr.getPackageName(), mCallGetCommand, name, args);} finally {Binder.restoreCallingIdentity(token);}} else {b = cp.call(cr.getPackageName(), mCallGetCommand, name, args);}if (b != null) {//这里 call getSecureSetting获取 valueString value = b.getString(Settings.NameValueTable.VALUE);Log.w(TAG, "NameValueCache b.getString(Settings " + value + " isSelf " + isSelf + " needsGenerationTracker " + needsGenerationTracker);Log.w(TAG, "NameValueCache value " + value);// Don't update our cache for reads of other users' dataif (isSelf) {synchronized (NameValueCache.this) {if (needsGenerationTracker) {MemoryIntArray array = b.getParcelable(CALL_METHOD_TRACK_GENERATION_KEY);final int index = b.getInt(CALL_METHOD_GENERATION_INDEX_KEY, -1);if (array != null && index >= 0) {final int generation = b.getInt(CALL_METHOD_GENERATION_KEY, 0);if (true) {Log.i(TAG, "Received generation tracker for type:"+ mUri.getPath() + " in package:"+ cr.getPackageName() + " and user:"+ userHandle + " with index:" + index);}if (mGenerationTracker != null) {mGenerationTracker.destroy();}mGenerationTracker = new GenerationTracker(array, index,generation, () -> {synchronized (NameValueCache.this) {Log.e(TAG, "Error accessing generation"+ " tracker - removing");if (mGenerationTracker != null) {GenerationTracker generationTracker =mGenerationTracker;mGenerationTracker = null;generationTracker.destroy();mValues.clear();}}});}}//第一次APP安装执行到这里，保存生成的Android idmValues.put(name, value);}

首先从缓存mValues变量中去找，如果没有查询到，就调用SettingsProvider的call()接口，如果call()接口也没有查询到，再调用query()接口。这里用的是call()接口，下文就以call()接口往下分析。cp.call()会调用到SettingsProvider的call()方法
APP安装后第一次尝试读取Android id ，mValues 一定为NULL，所有就会调用SettingsProvider 相关接口读取Android id。读取成功会保存到mValues缓存中。
frameworks/base/packages/SettingsProvider/src/com/android/providers/settings/SettingsProvider.java
接着会调用到SettingsProvider.java内部函数getSecureSetting

private Setting getSecureSetting(String name, int requestingUserId) {执行到这里说明此App在缓存中不存在。在这里获取 if (isNewSsaidSetting(name)) {PackageInfo callingPkg = getCallingPackageInfo(owningUserId);synchronized (mLock) {//getStringForUser -> getSecureSetting ,进入下一步生成android idSlog.e(LOG_TAG, "getSecureSetting to getSsaidSettingLocked " +  callingPkg.packageName + " owningUserId " +   owningUserId);return getSsaidSettingLocked(callingPkg, owningUserId);}}
}

 private Setting getSsaidSettingLocked(PackageInfo callingPkg, int owningUserId) {//首先获取APP的 userid，因为Android id 是根据 key = userid 保存在文件和settings中String name = Integer.toString(UserHandle.getUid(owningUserId, UserHandle.getAppId(Binder.getCallingUid())));//获取msettings 保存的 app 的 android id 对应的 settingfinal Setting ssaid = mSettingsRegistry.getSettingLocked(SETTINGS_TYPE_SSAID, owningUserId,name);//获取 setting对应的SettingsStatefinal SettingsState ssaidSettings = mSettingsRegistry.getSettingsLocked(SETTINGS_TYPE_SSAID, owningUserId);//如果 app 对应的setting 不存在，就重新生成 走到这里生成userkeyif (ssaid == null || ssaid.isNull() || ssaid.getValue() == null) {Slog.e(LOG_TAG, "ssaid is null . go generate====");Setting setting = mSettingsRegistry.generateSsaidLocked(callingPkg, owningUserId);return mascaradeSsaidSetting(ssaidSettings, setting);}return mascaradeSsaidSetting(ssaidSettings, ssaid);}

上段代码主要是获取相关的app 对应的setting ，获取不到就重新生成一个。
下面就进入到真正生成Android id的相关逻辑流程

        public Setting generateSsaidLocked(PackageInfo callingPkg, int userId) {// Read the user's key from the ssaid table.//这里获取生成Android id所用的系统随机数Setting userKeySetting = getSettingLocked(SETTINGS_TYPE_SSAID, userId, SSAID_USER_KEY);if (userKeySetting == null || userKeySetting.isNull()|| userKeySetting.getValue() == null) {// Lazy initialize and store the user key.//如果默认的系统key不存在，这里生成默认Android userkeySlog.e(LOG_TAG, "generateSsaidLocked -------");generateUserKeyLocked(userId);userKeySetting = getSettingLocked(SETTINGS_TYPE_SSAID, userId, SSAID_USER_KEY);if (userKeySetting == null || userKeySetting.isNull()|| userKeySetting.getValue() == null) {throw new IllegalStateException("User key not accessible");}}final String userKey = userKeySetting.getValue();// Convert the user's key back to a byte array.final byte[] keyBytes = ByteStringUtils.fromHexToByteArray(userKey);// Validate that the key is of expected length.// Keys are currently 32 bytes, but were once 16 bytes during Android O development.if (keyBytes == null || (keyBytes.length != 16 && keyBytes.length != 32)) {throw new IllegalStateException("User key invalid");}//下面就是对APP做相关的算法生成Android idfinal Mac m;try {m = Mac.getInstance("HmacSHA256");m.init(new SecretKeySpec(keyBytes, m.getAlgorithm()));} catch (NoSuchAlgorithmException e) {throw new IllegalStateException("HmacSHA256 is not available", e);} catch (InvalidKeyException e) {throw new IllegalStateException("Key is corrupted", e);}Slog.e(LOG_TAG, "generateSsaidLocked -------Mac " +  m);// Mac each of the developer signatures.for (int i = 0; i < callingPkg.signatures.length; i++) {byte[] sig = callingPkg.signatures[i].toByteArray();m.update(getLengthPrefix(sig), 0, 4);m.update(sig);}// Convert result to a string for storage in settings table. Only want first 64 bits.final String ssaid = ByteStringUtils.toHexString(m.doFinal()).substring(0, 16).toLowerCase(Locale.US);// Save the ssaid in the ssaid table.final String uid = Integer.toString(callingPkg.applicationInfo.uid);生成新的Android id 插入到msetting表中和写入到ssid.xml中final SettingsState ssaidSettings = getSettingsLocked(SETTINGS_TYPE_SSAID, userId);final boolean success = ssaidSettings.insertSettingLocked(uid, ssaid, null, true,callingPkg.packageName);if (!success) {throw new IllegalStateException("Ssaid settings not accessible");}return getSettingLocked(SETTINGS_TYPE_SSAID, userId, uid);}

Android生成 userkey 所用的生成随机数的代码如下所示。

 final byte[] keyBytes1 = new byte[32];final SecureRandom rand1 = new SecureRandom();rand1.nextBytes(keyBytes1);final String userKey = ByteStringUtils.toHexString(keyBytes1);

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