上一篇文章《电量优化 - 电量的统计原理与监控》已经讲到了 Android App 电量的计算方式，也分析了系统源码 Android 是怎么统计电量的。那么现在我们可以开始给自己的 App 开发电量异常检测功能了，实现的方案就是用系统源码类似的计算方案，在 App 内部进行电量统计，主要也就两个部分：线程监控与系统服务调用监控。如果大家觉得麻烦的话可以尝试一下我们的开源方案 matrix-battery-canary ，这套方案在我们的项目项目中全量运行了一年多，期间发现了很多电量问题。如果大家感兴趣，这期文章我先带大家来实现系统服务调用监控，后面再带大家实现线程监控。
如何 Hook 系统服务的调用？主流上一般有三种方案：字节码插桩，动态代理，Native Hook。这三种方案我们都有讲过也有用过，这里我们用动态代理来实现，大家可以自己先去试着实现下。套路印象中至少应该讲了十次，第一步肯定首先是要看源码流程了，第二步找单例和接口切入点，第三步就是设计实现类。源码我就简单贴了，因为在 《Framework 源码分析》中都讲到过了：
// 调用一般都是通过 context 获取系统服务
WifiManager mWifi = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
mWifi.startScan();
对应找到 /frameworks/base/core/java/android/app/ContextImpl.java 中的 getSystemService 方法
@Override
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}
再找到 /frameworks/base/core/java/android/app/SystemServiceRegistry.java 中的 getSystemService 方法
private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();
static {
registerService(Context.WIFI_SERVICE, WifiManager.class,
new CachedServiceFetcher<WifiManager>() {
@Override
public WifiManager createService(ContextImpl ctx) throws ServiceNotFoundException {
IBinder b = ServiceManager.getServiceOrThrow(Context.WIFI_SERVICE);
IWifiManager service = IWifiManager.Stub.asInterface(b);
return new WifiManager(ctx.getOuterContext(), service,
ConnectivityThread.getInstanceLooper());
}});
}
/**
* Gets a system service from a given context.
*/
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
private static <T> void registerService(String serviceName, Class<T> serviceClass,
ServiceFetcher<T> serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
看到这里第二步的方案已经出来了，单例就是 WifiManager 而接口对象就是 WifiManager 中的 mService 对象，只要 Hook 住 mService 就可以了，在 《Android 源码分析实战 - 授权时拦截 QQ 用户名和密码》一文中就是用的这种方案。这里我们再分析一个切入点，我们接着往 ServiceManager.getServiceOrThrow 中看：
private static HashMap<String, IBinder> sCache = new HashMap<String, IBinder>();
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;
}
public static IBinder getServiceOrThrow(String name) throws ServiceNotFoundException {
final IBinder binder = getService(name);
if (binder != null) {
return binder;
} else {
throw new ServiceNotFoundException(name);
}
}
再往 IWifiManager.Stub.asInterface 中看：
public static IWifiManager asInterface(android.os.IBinder obj)
{
if ((obj==null)) {
return null;
}
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin!=null)&&(iin instanceof IWifiManager))) {
return ((IWifiManager)iin);
}
return new IWifiManager.Stub.Proxy(obj);
}
看到这里我们就有了第二种方案了，Hook 住 Binder 对象的 queryLocalInterface 方法返回一个代理对象即可。最后一步就是设计实现类了：
public class SystemServiceBinderHooker {
public interface HookCallback {
void onServiceMethodInvoke(Method method, Object[] args);
Object onServiceMethodIntercept(Object receiver, Method method, Object[] args) throws Throwable;
}
private String mServiceName;
private String mServiceClassName;
private HookCallback mHookCallback;
public SystemServiceBinderHooker(String serviceName, String serviceClassName, HookCallback hookCallback){
this.mServiceName = serviceName;
this.mServiceClassName = serviceClassName;
this.mHookCallback = hookCallback;
}
public boolean hook(){
try {
// 1. 先获取 origin 的 IBinder 对象
Class<?> serviceManagerClass = Class.forName("android.os.ServiceManager");
Method getServiceMethod = serviceManagerClass.getDeclaredMethod("getService",String.class);
getServiceMethod.setAccessible(true);
final IBinder serviceBinder = (IBinder) getServiceMethod.invoke(null,mServiceName);
// 2. hook 住 serviceBinder 创建代理对象
IBinder proxyServiceBinder = (IBinder) Proxy.newProxyInstance(serviceManagerClass.getClassLoader(), new Class<?>[]{IBinder.class}, new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (TextUtils.equals(method.getName(), "queryLocalInterface")) {
return createServiceProxy(serviceBinder);
}
return method.invoke(serviceBinder, args);
}
});
// 3. 把代理对象塞到 ServiceManager 中的 sCache
Field sCacheField = serviceManagerClass.getDeclaredField("sCache");
sCacheField.setAccessible(true);
Map<String, IBinder> sCache = (Map<String, IBinder>) sCacheField.get(null);
sCache.put(mServiceName, proxyServiceBinder);
return true;
} catch (Exception e) {
e.printStackTrace();
return false;
}
}
private Object createServiceProxy(IBinder serviceBinder) {
try {
// new IWifiManager.Stub.Proxy
Class<?> serviceProxyClass = Class.forName(mServiceClassName + "$Stub$Proxy");
Constructor<?> serviceProxyConstructor = serviceProxyClass.getDeclaredConstructor(IBinder.class);
serviceProxyConstructor.setAccessible(true);
final Object originServiceProxy = serviceProxyConstructor.newInstance(serviceBinder);
// hook serviceProxy
Object serviceProxyHooker = Proxy.newProxyInstance(serviceProxyClass.getClassLoader(), new Class<?>[]{IBinder.class, IInterface.class, Class.forName(mServiceClassName)}, new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (mHookCallback != null) {
mHookCallback.onServiceMethodInvoke(method, args);
Object result = mHookCallback.onServiceMethodIntercept(originServiceProxy, method, args);
if (result != null) {
return result;
}
}
return method.invoke(originServiceProxy, args);
}
});
return serviceProxyHooker;
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
}
视频链接：https://pan.baidu.com/s/164aJyOYlXm-JCOC0_HVBtQ
视频密码：vsfu