Android内存泄漏:谨慎使用getSystemService
joker67
7年前
<p>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。</p> <p>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。</p> <p>为了排除干扰因素,我们使用一个简单的示例代码</p> <pre> <code class="language-java">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); } } </code></pre> <p style="text-align: center;">当退出MainActivity时,得到了LeakCanary的内存泄漏报告。如下图。 <img src="https://simg.open-open.com/show/648fd4bb949c0051cf7a23053340910d.png"></p> <p>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。</p> <p>因此,我们有必要对PowerManager的源码分析一下</p> <p>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。</p> <pre> <code class="language-java">final Context mContext; final IPowerManager mService; final Handler mHandler; /** * {@hide} */ public PowerManager(Context context, IPowerManager service, Handler handler) { mContext = context; mService = service; mHandler = handler; } </code></pre> <p>2.负责缓存服务的实现在ContextImpl.java文件中</p> <pre> <code class="language-java">// The system service cache for the system services that are cached per-ContextImpl. final Object[] mServiceCache = SystemServiceRegistry.createServiceCache(); </code></pre> <p>而Activity通过ContextImpl提供的setOuterContext方法设置mOuterContext</p> <pre> <code class="language-java">final void setOuterContext(Context context) { mOuterContext = context; } </code></pre> <p>因此Activity与ContextImpl的关系如下图 <img src="https://simg.open-open.com/show/33d90606195a425bfd527b4dadded029.png"></p> <p>SystemServiceRegistry.java中获取PowerManager的实现。</p> <pre> <code class="language-java">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()); }}); </code></pre> <p>创建具体的服务的实现为core/java/android/app/SystemServiceRegistry.java</p> <h2><strong>如何解决</strong></h2> <h3><strong>不使用静态持有PowerManager</strong></h3> <p>因为static是一个很容易和内存泄漏产生关联的因素</p> <ul> <li>static变量与类的生命周期相同</li> <li>类的生命周期等同于类加载器</li> <li>类加载器通常和进程的生命周期一致</li> </ul> <p>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。</p> <h3><strong>使用ApplicationContext</strong></h3> <p>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。</p> <pre> <code class="language-java">PowerManager powerManager = (PowerManager)getApplicationContext().getSystemService(Context.POWER_SERVICE); </code></pre> <h2><strong>是不是都要使用Application Context?</strong></h2> <p>然而并非如此</p> <p>以Activity为例,一些和UI相关的服务已经优先进行了处理</p> <pre> <code class="language-java">@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); } </code></pre> <p>ContextThemeWrapper也优先处理了LayoutManager服务</p> <pre> <code class="language-java">@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); } </code></pre> <h2><strong>那到底改用哪个Context</strong></h2> <ul> <li>如果服务和UI相关,则用Activity</li> <li>如果是类似ALARM_SERVICE,CONNECTIVITY_SERVICE建议有限选用Application Context</li> <li>如果出现出现了内存泄漏,排除问题,可以考虑使用Application Context</li> </ul> <p>所以,当我们再次使用getSystemService时要慎重考虑这样的问题。</p> <p> </p> <p>来自:http://droidyue.com/blog/2016/11/14/be-careful-using-getsystemservice/</p> <p> </p>