Android内存泄漏：谨慎使用getSystemService-CFANZ编程社区

Android中有很多服务，比如PowerManager,AlarmManager，NotificationManager等，通常使用起来也很方便，就是使用Context.getSystemService方法来获得。

一次在公司开发项目开发中，突然LeakCanary弹出了一个内存泄漏的通知栏，不好，内存泄漏发生了。原因竟是和getSystemService有关。

为了排除干扰因素，我们使用一个简单的示例代码

public class MainActivity extends AppCompatActivity { private static PowerManager powerManager; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); powerManager = (PowerManager)getSystemService(Context.POWER_SERVICE); } }

public class MainActivity extends AppCompatActivity {
    private static PowerManager powerManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        powerManager = (PowerManager)getSystemService(Context.POWER_SERVICE);
    }
}

当退出MainActivity时，得到了LeakCanary的内存泄漏报告。如下图。

Android内存泄漏：谨慎使用getSystemService_ide

奇怪了，为什么PowerManager会持有Activity的实例呢，按照理解，PowerManager应该是持有Application的Context对象的。

因此，我们有必要对PowerManager的源码分析一下

1.PowerManager会持有一个Context实例，具体使用Activity还是Application的Context取决于调用者。

final Context mContext; final IPowerManager mService; final Handler mHandler; /** * {@hide} */ public PowerManager(Context context, IPowerManager service, Handler handler) { mContext = context; mService = service; mHandler = handler; }

final Context mContext;
    final IPowerManager mService;
    final Handler mHandler;

    /**
     * {@hide}
     */
    public PowerManager(Context context, IPowerManager service, Handler handler) {
        mContext = context;
        mService = service;
        mHandler = handler;
    }

2.负责缓存服务的实现在ContextImpl.java文件中

// The system service cache for the system services that are cached per-ContextImpl. final Object[] mServiceCache = SystemServiceRegistry.createServiceCache();

而Activity通过ContextImpl提供的setOuterContext方法设置mOuterContext

final void setOuterContext(Context context) { mOuterContext = context; }

因此Activity与ContextImpl的关系如下图

Android内存泄漏：谨慎使用getSystemService_生命周期_02

SystemServiceRegistry.java中获取PowerManager的实现。

registerService(Context.POWER_SERVICE, PowerManager.class, new CachedServiceFetcher<PowerManager>() { @Override public PowerManager createService(ContextImpl ctx) { IBinder b = ServiceManager.getService(Context.POWER_SERVICE); IPowerManager service = IPowerManager.Stub.asInterface(b); if (service == null) { Log.wtf(TAG, "Failed to get power manager service."); } return new PowerManager(ctx.getOuterContext(), service, ctx.mMainThread.getHandler()); }});

registerService(Context.POWER_SERVICE, PowerManager.class,
                new CachedServiceFetcher<PowerManager>() {
            @Override
            public PowerManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.POWER_SERVICE);
                IPowerManager service = IPowerManager.Stub.asInterface(b);
                if (service == null) {
                    Log.wtf(TAG, "Failed to get power manager service.");
                }
                return new PowerManager(ctx.getOuterContext(),
                        service, ctx.mMainThread.getHandler());
            }});

创建具体的服务的实现为core/java/android/app/SystemServiceRegistry.java

如何解决

不使用静态持有PowerManager

因为static是一个很容易和内存泄漏产生关联的因素

static变量与类的生命周期相同
类的生命周期等同于类加载器
类加载器通常和进程的生命周期一致

所以通过去除static可以保证变量周期和Activity实例相同。这样就不会产生内存泄漏问题。

使用ApplicationContext

除了上面的方法之外，传入Application的Context而不是Activity Context也可以解决问题。

PowerManager powerManager = (PowerManager)getApplicationContext().getSystemService(Context.POWER_SERVICE);

是不是都要使用Application Context？

然而并非如此

以Activity为例，一些和UI相关的服务已经优先进行了处理

@Override public Object getSystemService(@ServiceName @NonNull String name) { if (getBaseContext() == null) { throw new IllegalStateException( "System services not available to Activities before onCreate()"); } if (WINDOW_SERVICE.equals(name)) { return mWindowManager; } else if (SEARCH_SERVICE.equals(name)) { ensureSearchManager(); return mSearchManager; } return super.getSystemService(name); }

@Override
public Object getSystemService(@ServiceName @NonNull String name) {
    if (getBaseContext() == null) {
        throw new IllegalStateException(
                "System services not available to Activities before onCreate()");
    }

    if (WINDOW_SERVICE.equals(name)) {
        return mWindowManager;
    } else if (SEARCH_SERVICE.equals(name)) {
        ensureSearchManager();
        return mSearchManager;
    }
    return super.getSystemService(name);
}

ContextThemeWrapper也优先处理了LayoutManager服务

@Override public Object getSystemService(String name) { if (LAYOUT_INFLATER_SERVICE.equals(name)) { if (mInflater == null) { mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this); } return mInflater; } return getBaseContext().getSystemService(name); }

@Override
public Object getSystemService(String name) {
    if (LAYOUT_INFLATER_SERVICE.equals(name)) {
        if (mInflater == null) {
            mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);
        }
        return mInflater;
    }
    return getBaseContext().getSystemService(name);
}