指纹服务是Android系统中一个较为简单的服务（相比于AMS，WMS等），也比较独立，功能上包括几点
- 指纹的录入与删除
- 指纹认证
- 指纹的安全策略（错误次数判定）
和其他的system service 一样，应用程序通过FingerprintManager实现与FingerprintService的通信，除了上面所说的功能之外，FingerprintManager提供了一些别的的接口，重要的接口都会要求系统级别的权限，并且也不是公开的api（指纹的录入，删除，重命名，重置错误计数等）
/**
* Obtain the list of enrolled fingerprints templates.
* @return list of current fingerprint items
*
* @hide
*/
@RequiresPermission(USE_FINGERPRINT)
public List<Fingerprint> getEnrolledFingerprints(int userId) {
if (mService != null) try {
return mService.getEnrolledFingerprints(userId, mContext.getOpPackageName());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
return null;
}
/**
* @hide
*/
@RequiresPermission(allOf = {
USE_FINGERPRINT,
INTERACT_ACROSS_USERS})
public boolean hasEnrolledFingerprints(int userId) {
if (mService != null) try {
return mService.hasEnrolledFingerprints(userId, mContext.getOpPackageName());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
return false;
}
/**
* Determine if fingerprint hardware is present and functional.
*
* @return true if hardware is present and functional, false otherwise.
*/
@RequiresPermission(USE_FINGERPRINT)
public boolean isHardwareDetected() {
if (mService != null) {
try {
long deviceId = 0; /* TODO: plumb hardware id to FPMS */
return mService.isHardwareDetected(deviceId, mContext.getOpPackageName());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
} else {
Log.w(TAG, "isFingerprintHardwareDetected(): Service not connected!");
}
return false;
}
FingerprintService的启动过程
FingerprintService在system server中创建并初始化，当检测到手机支持指纹功能的时候就会启动这个service
...
if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_FINGERPRINT)) {
mSystemServiceManager.startService(FingerprintService.class);
}
...
FingerprintService在初始化后会建立和HAL层的通信，即连接到fingerprintd，拿到用于通信的IFingerprintDaemon对象（binder）
public void onStart() {
publishBinderService(Context.FINGERPRINT_SERVICE, new FingerprintServiceWrapper());
IFingerprintDaemon daemon = getFingerprintDaemon();
listenForUserSwitches();
}
public IFingerprintDaemon getFingerprintDaemon() {
if (mDaemon == null) {
mDaemon = IFingerprintDaemon.Stub.asInterface(ServiceManager.getService(FINGERPRINTD));
if (mDaemon != null) {
try {
mDaemon.asBinder().linkToDeath(this, 0);
mDaemon.init(mDaemonCallback);
mHalDeviceId = mDaemon.openHal();
if (mHalDeviceId != 0) {
updateActiveGroup(ActivityManager.getCurrentUser(), null);
} else {
Slog.w(TAG, "Failed to open Fingerprint HAL!");
MetricsLogger.count(mContext, "fingerprintd_openhal_error", 1);
mDaemon = null;
}
} catch (RemoteException e) {
Slog.e(TAG, "Failed to open fingeprintd HAL", e);
mDaemon = null; // try again later!
}
} else {
Slog.w(TAG, "fingerprint service not available");
}
}
return mDaemon;
}
本质上来说，除去安全相关的策略外，指纹的功能是依赖硬件实现的，FingerprintService也只是充当了framework java层与native层的消息传递者罢了，所以指纹的识别，录入和监听都是向fingerprintd发送命令和获取相应的结果
指纹监听认证过程
以指纹认证为例，介绍这一过程，录入和删除的过程和认证类似，不重复描述
FingerprintManager
public void authenticate(@Nullable CryptoObject crypto, @Nullable CancellationSignal cancel,
int flags, @NonNull AuthenticationCallback callback, Handler handler, int userId) {
if (callback == null) {
throw new IllegalArgumentException("Must supply an authentication callback");
}
if (cancel != null) {
if (cancel.isCanceled()) {
Log.w(TAG, "authentication already canceled");
return;
} else {
cancel.setOnCancelListener(new OnAuthenticationCancelListener(crypto));
}
}
if (mService != null) try {
useHandler(handler);
mAuthenticationCallback = callback;
mCryptoObject = crypto;
long sessionId = crypto != null ? crypto.getOpId() : 0;
mService.authenticate(mToken, sessionId, userId, mServiceReceiver, flags,
mContext.getOpPackageName());
} catch (RemoteException e) {
Log.w(TAG, "Remote exception while authenticating: ", e);
if (callback != null) {
// Though this may not be a hardware issue, it will cause apps to give up or try
// again later.
callback.onAuthenticationError(FINGERPRINT_ERROR_HW_UNAVAILABLE,
getErrorString(FINGERPRINT_ERROR_HW_UNAVAILABLE));
}
}
}
可以看到，最终仍然是向FingerprintService发送消息，但是开启指纹认证的函数传入了两个比较重要的参数，一个是CancellationSignal对象，用于取消指纹认证，另一个是指纹认证的回调对象AuthenticationCallback
public static abstract class AuthenticationCallback {
public void onAuthenticationError(int errorCode, CharSequence errString) { }
public void onAuthenticationHelp(int helpCode, CharSequence helpString) { }
public void onAuthenticationSucceeded(AuthenticationResult result) { }
public void onAuthenticationFailed() { }
public void onAuthenticationAcquired(int acquireInfo) {}
};
看函数名称也能知道其功能，他们分别代表了指纹认证时的回调结果（成功，失败，检测到指纹，认证异常等），参数包含了具体的信息，这些信息在FingerprintManager中都有对应的常量定义，有兴趣可以查看代码
FingerprintService
public void authenticate(final IBinder token, final long opId, final int groupId,
final IFingerprintServiceReceiver receiver, final int flags,
final String opPackageName) {
final int callingUid = Binder.getCallingUid();
final int callingUserId = UserHandle.getCallingUserId();
final int pid = Binder.getCallingPid();
final boolean restricted = isRestricted();
mHandler.post(new Runnable() {
@Override
public void run() {
if (!canUseFingerprint(opPackageName, true /* foregroundOnly */,
callingUid, pid)) {
if (DEBUG) Slog.v(TAG, "authenticate(): reject " + opPackageName);
return;
}
MetricsLogger.histogram(mContext, "fingerprint_token", opId != 0L ? 1 : 0);
// Get performance stats object for this user.
HashMap<Integer, PerformanceStats> pmap
= (opId == 0) ? mPerformanceMap : mCryptoPerformanceMap;
PerformanceStats stats = pmap.get(mCurrentUserId);
if (stats == null) {
stats = new PerformanceStats();
pmap.put(mCurrentUserId, stats);
}
mPerformanceStats = stats;
startAuthentication(token, opId, callingUserId, groupId, receiver,
flags, restricted, opPackageName);
}
});
}
前面会有对包名，userid以及应用进程是否在在前台的检查，继续看
private void startAuthentication(IBinder token, long opId, int callingUserId, int groupId,
IFingerprintServiceReceiver receiver, int flags, boolean restricted,
String opPackageName) {
updateActiveGroup(groupId, opPackageName);
if (DEBUG) Slog.v(TAG, "startAuthentication(" + opPackageName + ")");
AuthenticationClient client = new AuthenticationClient(getContext(), mHalDeviceId, token,
receiver, mCurrentUserId, groupId, opId, restricted, opPackageName) {
@Override
public boolean handleFailedAttempt() {
mFailedAttempts++;
if (mFailedAttempts == MAX_FAILED_ATTEMPTS) {
mPerformanceStats.lockout++;
}
if (inLockoutMode()) {
// Failing multiple times will continue to push out the lockout time.
scheduleLockoutReset();
return true;
}
return false;
}
@Override
public void resetFailedAttempts() {
FingerprintService.this.resetFailedAttempts();
}
@Override
public void notifyUserActivity() {
FingerprintService.this.userActivity();
}
@Override
public IFingerprintDaemon getFingerprintDaemon() {
return FingerprintService.this.getFingerprintDaemon();
}
};
if (inLockoutMode()) {
Slog.v(TAG, "In lockout mode; disallowing authentication");
// Don't bother starting the client. Just send the error message.
if (!client.onError(FingerprintManager.FINGERPRINT_ERROR_LOCKOUT)) {
Slog.w(TAG, "Cannot send timeout message to client");
}
return;
}
startClient(client, true /* initiatedByClient */);
}
AuthenticationClient继承自ClientMonitor，用于处理指纹认证相关的功能事务，ClientMonitor的其他子类如RemovalMonior，EnrollMonitor也是如此，ClientMonitor会直接与fingerprintd通信，其核心是调用其start()或stop()方法，
对于AuthenticationClient而言
private void startClient(ClientMonitor newClient, boolean initiatedByClient) {
ClientMonitor currentClient = mCurrentClient;
if (currentClient != null) {
if (DEBUG) Slog.v(TAG, "request stop current client " + currentClient.getOwnerString());
currentClient.stop(initiatedByClient);
mPendingClient = newClient;
mHandler.removeCallbacks(mResetClientState);
mHandler.postDelayed(mResetClientState, CANCEL_TIMEOUT_LIMIT);
} else if (newClient != null) {
mCurrentClient = newClient;
if (DEBUG) Slog.v(TAG, "starting client "
+ newClient.getClass().getSuperclass().getSimpleName()
+ "(" + newClient.getOwnerString() + ")"
+ ", initiatedByClient = " + initiatedByClient + ")");
newClient.start();
}
}
public int start() {
IFingerprintDaemon daemon = getFingerprintDaemon();
if (daemon == null) {
Slog.w(TAG, "start authentication: no fingeprintd!");
return ERROR_ESRCH;
}
try {
final int result = daemon.authenticate(mOpId, getGroupId());
if (result != 0) {
Slog.w(TAG, "startAuthentication failed, result=" + result);
MetricsLogger.histogram(getContext(), "fingeprintd_auth_start_error", result);
onError(FingerprintManager.FINGERPRINT_ERROR_HW_UNAVAILABLE);
return result;
}
if (DEBUG) Slog.w(TAG, "client " + getOwnerString() + " is authenticating...");
} catch (RemoteException e) {
Slog.e(TAG, "startAuthentication failed", e);
return ERROR_ESRCH;
}
return 0; // success
}
向底层发送认证命令后就只需要等待认证结果就可以了，前面我们说到在初始化的时候会建立与fingerprintd的通信，其核心是下面这行代码
mDaemon.init(mDaemonCallback);
mDaemonCallback是一个binder对象，接受来自底层的结果，然后通过FingerprintService和FingerManager一层层把结果发送到应用程序中去。
8.0的一些变化
8.0上的fingerprintd变化很大，甚至都不叫fingerprintd了，当然这是native层的东西，这里不讨论，对于FingerprintService而言，一个显著的变化是安全策略的调整
- 8.0之前，指纹只能错误5次，达到5次时会禁止指纹认证，同时开启30秒倒计时，等待结束后重置错误计数，继续认证
- 8.0之后，依然是每错误5次就会倒计时30秒，然而30秒结束后错误计数并不会被清空，8.0上加入了最大20次的限制，累计错误20次之后就无法使用指纹认证功能了，只能用密码的方式才能重置错误计数
private static final int MAX_FAILED_ATTEMPTS_LOCKOUT_TIMED = 5;
private static final int MAX_FAILED_ATTEMPTS_LOCKOUT_PERMANENT = 20;
private int getLockoutMode() {
if (mFailedAttempts >= MAX_FAILED_ATTEMPTS_LOCKOUT_PERMANENT) {
return AuthenticationClient.LOCKOUT_PERMANENT;
} else if (mFailedAttempts > 0 && mTimedLockoutCleared == false &&
(mFailedAttempts % MAX_FAILED_ATTEMPTS_LOCKOUT_TIMED == 0)) {
return AuthenticationClient.LOCKOUT_TIMED;
}
return AuthenticationClient.LOCKOUT_NONE;
}
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