Android系统服务与服务注册

一、服务

　　

　　系统服务将服务注册在ServiceManager中，Client通过ServiceManager获取服务的Bindler实例，通过Binder与服务通信。

　　ServiceManager源码（framework/base/core/java/android/os/ServiceManager.java）：

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.annotation.NonNull;
import android.compat.annotation.UnsupportedAppUsage;
import android.util.ArrayMap;
import android.util.Log;

import com.android.internal.annotations.GuardedBy;
import com.android.internal.os.BinderInternal;
import com.android.internal.util.StatLogger;

import java.util.Map;

/** @hide */
public final class ServiceManager {
    private static final String TAG = "ServiceManager";
    private static final Object sLock = new Object();

    @UnsupportedAppUsage
    private static IServiceManager sServiceManager;

    /**
     * Cache for the "well known" services, such as WM and AM.
     */
    @UnsupportedAppUsage
    private static Map<String, IBinder> sCache = new ArrayMap<String, IBinder>();

    /**
     * We do the "slow log" at most once every this interval.
     */
    private static final int SLOW_LOG_INTERVAL_MS = 5000;

    /**
     * We do the "stats log" at most once every this interval.
     */
    private static final int STATS_LOG_INTERVAL_MS = 5000;

    /**
     * Threshold in uS for a "slow" call, used on core UIDs. We use a more relax value to
     * avoid logspam.
     */
    private static final long GET_SERVICE_SLOW_THRESHOLD_US_CORE =
            SystemProperties.getInt("debug.servicemanager.slow_call_core_ms", 10) * 1000;

    /**
     * Threshold in uS for a "slow" call, used on non-core UIDs. We use a more relax value to
     * avoid logspam.
     */
    private static final long GET_SERVICE_SLOW_THRESHOLD_US_NON_CORE =
            SystemProperties.getInt("debug.servicemanager.slow_call_ms", 50) * 1000;

    /**
     * We log stats logging ever this many getService() calls.
     */
    private static final int GET_SERVICE_LOG_EVERY_CALLS_CORE =
            SystemProperties.getInt("debug.servicemanager.log_calls_core", 100);

    /**
     * We log stats logging ever this many getService() calls.
     */
    private static final int GET_SERVICE_LOG_EVERY_CALLS_NON_CORE =
            SystemProperties.getInt("debug.servicemanager.log_calls", 200);

    @GuardedBy("sLock")
    private static int sGetServiceAccumulatedUs;

    @GuardedBy("sLock")
    private static int sGetServiceAccumulatedCallCount;

    @GuardedBy("sLock")
    private static long sLastStatsLogUptime;

    @GuardedBy("sLock")
    private static long sLastSlowLogUptime;

    @GuardedBy("sLock")
    private static long sLastSlowLogActualTime;

    interface Stats {
        int GET_SERVICE = 0;

        int COUNT = GET_SERVICE + 1;
    }

    public static final StatLogger sStatLogger = new StatLogger(new String[] {
            "getService()",
    });

    @UnsupportedAppUsage
    public ServiceManager() {
    }

    @UnsupportedAppUsage
    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }

        // Find the service manager
        sServiceManager = ServiceManagerNative
                .asInterface(Binder.allowBlocking(BinderInternal.getContextObject()));
        return sServiceManager;
    }

    /**
     * Returns a reference to a service with the given name.
     *
     * @param name the name of the service to get
     * @return a reference to the service, or <code>null</code> if the service doesn't exist
     */
    @UnsupportedAppUsage
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return Binder.allowBlocking(rawGetService(name));
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }

    /**
     * Returns a reference to a service with the given name, or throws
     * {@link ServiceNotFoundException} if none is found.
     *
     * @hide
     */
    public static IBinder getServiceOrThrow(String name) throws ServiceNotFoundException {
        final IBinder binder = getService(name);
        if (binder != null) {
            return binder;
        } else {
            throw new ServiceNotFoundException(name);
        }
    }

    /**
     * Place a new @a service called @a name into the service
     * manager.
     *
     * @param name the name of the new service
     * @param service the service object
     */
    @UnsupportedAppUsage
    public static void addService(String name, IBinder service) {
        addService(name, service, false, IServiceManager.DUMP_FLAG_PRIORITY_DEFAULT);
    }

    /**
     * Place a new @a service called @a name into the service
     * manager.
     *
     * @param name the name of the new service
     * @param service the service object
     * @param allowIsolated set to true to allow isolated sandboxed processes
     * to access this service
     */
    @UnsupportedAppUsage
    public static void addService(String name, IBinder service, boolean allowIsolated) {
        addService(name, service, allowIsolated, IServiceManager.DUMP_FLAG_PRIORITY_DEFAULT);
    }

    /**
     * Place a new @a service called @a name into the service
     * manager.
     *
     * @param name the name of the new service
     * @param service the service object
     * @param allowIsolated set to true to allow isolated sandboxed processes
     * @param dumpPriority supported dump priority levels as a bitmask
     * to access this service
     */
    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    public static void addService(String name, IBinder service, boolean allowIsolated,
            int dumpPriority) {
        try {
            getIServiceManager().addService(name, service, allowIsolated, dumpPriority);
        } catch (RemoteException e) {
            Log.e(TAG, "error in addService", e);
        }
    }

    /**
     * Retrieve an existing service called @a name from the
     * service manager.  Non-blocking.
     */
    @UnsupportedAppUsage
    public static IBinder checkService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return Binder.allowBlocking(getIServiceManager().checkService(name));
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in checkService", e);
            return null;
        }
    }

    /**
     * Returns whether the specified service is declared.
     *
     * @return true if the service is declared somewhere (eg. VINTF manifest) and
     * waitForService should always be able to return the service.
     */
    public static boolean isDeclared(@NonNull String name) {
        try {
            return getIServiceManager().isDeclared(name);
        } catch (RemoteException e) {
            Log.e(TAG, "error in isDeclared", e);
            return false;
        }
    }

    /**
     * Returns the list of declared instances for an interface.
     *
     * @return true if the service is declared somewhere (eg. VINTF manifest) and
     * waitForService should always be able to return the service.
     */
    public static String[] getDeclaredInstances(@NonNull String iface) {
        try {
            return getIServiceManager().getDeclaredInstances(iface);
        } catch (RemoteException e) {
            Log.e(TAG, "error in getDeclaredInstances", e);
            return null;
        }
    }

    /**
     * Returns the specified service from the service manager.
     *
     * If the service is not running, servicemanager will attempt to start it, and this function
     * will wait for it to be ready.
     *
     * @return {@code null} only if there are permission problems or fatal errors.
     */
    public static IBinder waitForService(@NonNull String name) {
        return Binder.allowBlocking(waitForServiceNative(name));
    }

    private static native IBinder waitForServiceNative(@NonNull String name);


    /**
     * Returns the specified service from the service manager, if declared.
     *
     * If the service is not running, servicemanager will attempt to start it, and this function
     * will wait for it to be ready.
     *
     * @return {@code null} if the service is not declared in the manifest, or if there are
     * permission problems, or if there are fatal errors.
     */
    public static IBinder waitForDeclaredService(@NonNull String name) {
        return isDeclared(name) ? waitForService(name) : null;
    }

    /**
     * Return a list of all currently running services.
     * @return an array of all currently running services, or <code>null</code> in
     * case of an exception
     */
    @UnsupportedAppUsage
    public static String[] listServices() {
        try {
            return getIServiceManager().listServices(IServiceManager.DUMP_FLAG_PRIORITY_ALL);
        } catch (RemoteException e) {
            Log.e(TAG, "error in listServices", e);
            return null;
        }
    }

    /**
     * Get service debug info.
     * @return an array of information for each service (like listServices, but with PIDs)
     * @hide
     */
    public static ServiceDebugInfo[] getServiceDebugInfo() {
        try {
            return getIServiceManager().getServiceDebugInfo();
        } catch (RemoteException e) {
            Log.e(TAG, "error in getServiceDebugInfo", e);
            return null;
        }
    }

    /**
     * This is only intended to be called when the process is first being brought
     * up and bound by the activity manager. There is only one thread in the process
     * at that time, so no locking is done.
     *
     * @param cache the cache of service references
     * @hide
     */
    public static void initServiceCache(Map<String, IBinder> cache) {
        if (sCache.size() != 0) {
            throw new IllegalStateException("setServiceCache may only be called once");
        }
        sCache.putAll(cache);
    }

    /**
     * Exception thrown when no service published for given name. This might be
     * thrown early during boot before certain services have published
     * themselves.
     *
     * @hide
     */
    public static class ServiceNotFoundException extends Exception {
        public ServiceNotFoundException(String name) {
            super("No service published for: " + name);
        }
    }

    private static IBinder rawGetService(String name) throws RemoteException {
        final long start = sStatLogger.getTime();

        final IBinder binder = getIServiceManager().getService(name);

        final int time = (int) sStatLogger.logDurationStat(Stats.GET_SERVICE, start);

        final int myUid = Process.myUid();
        final boolean isCore = UserHandle.isCore(myUid);

        final long slowThreshold = isCore
                ? GET_SERVICE_SLOW_THRESHOLD_US_CORE
                : GET_SERVICE_SLOW_THRESHOLD_US_NON_CORE;

        synchronized (sLock) {
            sGetServiceAccumulatedUs += time;
            sGetServiceAccumulatedCallCount++;

            final long nowUptime = SystemClock.uptimeMillis();

            // Was a slow call?
            if (time >= slowThreshold) {
                // We do a slow log:
                // - At most once in every SLOW_LOG_INTERVAL_MS
                // - OR it was slower than the previously logged slow call.
                if ((nowUptime > (sLastSlowLogUptime + SLOW_LOG_INTERVAL_MS))
                        || (sLastSlowLogActualTime < time)) {
                    EventLogTags.writeServiceManagerSlow(time / 1000, name);

                    sLastSlowLogUptime = nowUptime;
                    sLastSlowLogActualTime = time;
                }
            }

            // Every GET_SERVICE_LOG_EVERY_CALLS calls, log the total time spent in getService().

            final int logInterval = isCore
                    ? GET_SERVICE_LOG_EVERY_CALLS_CORE
                    : GET_SERVICE_LOG_EVERY_CALLS_NON_CORE;

            if ((sGetServiceAccumulatedCallCount >= logInterval)
                    && (nowUptime >= (sLastStatsLogUptime + STATS_LOG_INTERVAL_MS))) {

                EventLogTags.writeServiceManagerStats(
                        sGetServiceAccumulatedCallCount, // Total # of getService() calls.
                        sGetServiceAccumulatedUs / 1000, // Total time spent in getService() calls.
                        (int) (nowUptime - sLastStatsLogUptime)); // Uptime duration since last log.
                sGetServiceAccumulatedCallCount = 0;
                sGetServiceAccumulatedUs = 0;
                sLastStatsLogUptime = nowUptime;
            }
        }
        return binder;
    }
}

二、如何获取使用系统服务

/**
   * Cache for the "well known" services, such as WM and AM.
   */
@UnsupportedAppUsage
private static Map<String, IBinder> sCache = new ArrayMap<String, IBinder>();

　　在ServiceManager中注册的Service都存储于sCache Map中，服务名作为key，value是Binder，通过Binder与Service通信。

　　获取Service，通过方法：

　　ContextImpl.java（frameworks/base/core/java/android/app/ContextImpl.java）：

@Override
    public Object getSystemService(String name) {
        if (vmIncorrectContextUseEnabled()) {
            // Check incorrect Context usage.
            if (isUiComponent(name) && !isSelfOrOuterUiContext()) {
                final String errorMessage = "Tried to access visual service "
                        + SystemServiceRegistry.getSystemServiceClassName(name)
                        + " from a non-visual Context:" + getOuterContext();
                final String message = "Visual services, such as WindowManager, WallpaperService "
                        + "or LayoutInflater should be accessed from Activity or other visual "
                        + "Context. Use an Activity or a Context created with "
                        + "Context#createWindowContext(int, Bundle), which are adjusted to "
                        + "the configuration and visual bounds of an area on screen.";
                final Exception exception = new IllegalAccessException(errorMessage);
                StrictMode.onIncorrectContextUsed(message, exception);
                Log.e(TAG, errorMessage + " " + message, exception);
            }
        }
        return SystemServiceRegistry.getSystemService(this, name);
    }

    @Override
    public String getSystemServiceName(Class<?> serviceClass) {
        return SystemServiceRegistry.getSystemServiceName(serviceClass);
    }

　　在启动SystemServer时，会将AMS、WMS、PMS等服务进行注册，注册的服务会存储在SystemServiceRegistry.SYSTEM_SERVICE_FETCHERS Map，存储的key是服务名。

　　源码：frameworks/base/core/java/android/app/SystemServiceRegistry.java

/**
    * Gets a system service from a given context.
    * @hide
    */
public static Object getSystemService(ContextImpl ctx, String name) {
    if (name == null) {
        return null;
    }
    final ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    if (fetcher == null) {
        if (sEnableServiceNotFoundWtf) {
            Slog.wtf(TAG, "Unknown manager requested: " + name);
        }
        return null;
    }

    final Object ret = fetcher.getService(ctx);
    if (sEnableServiceNotFoundWtf && ret == null) {
        // Some services do return null in certain situations, so don't do WTF for them.
        switch (name) {
            case Context.CONTENT_CAPTURE_MANAGER_SERVICE:
            case Context.APP_PREDICTION_SERVICE:
            case Context.INCREMENTAL_SERVICE:
                return null;
        }
        Slog.wtf(TAG, "Manager wrapper not available: " + name);
        return null;
    }
    return ret;
}

　　最会调用到ServiceManager类中，getService函数：

/**
     * Returns a reference to a service with the given name.
     *
     * @param name the name of the service to get
     * @return a reference to the service, or <code>null</code> if the service doesn't exist
     */
    @UnsupportedAppUsage
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return Binder.allowBlocking(rawGetService(name));
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }

三、Service注册

　　通过ServiceManager类中的addService函数注册：

/**
     * Place a new @a service called @a name into the service
     * manager.
     *
     * @param name the name of the new service
     * @param service the service object
     */
    @UnsupportedAppUsage
    public static void addService(String name, IBinder service) {
        addService(name, service, false, IServiceManager.DUMP_FLAG_PRIORITY_DEFAULT);
    }

四、系统服务什么时候注册 

　　先看下SystemServer启动都做了哪些事

 　图中启动各种服务，指的是启动系统服务，包括：AMS、PMS、WMS、PKMS等。