轉(zhuǎn)自android handler的警告Handler Class Should be Static or Leaks Occur

在使用Handler更新UI的時(shí)候，我是這樣寫的：

public class SampleActivity extends Activity {private final Handler mLeakyHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {// TODO}} }

看起來(lái)很正常的，但是 Android Lint 卻給出了警告：

This Handler class should be static or leaks might occur

意思是說(shuō)：這個(gè)Handler必須是static的，否則就會(huì)引發(fā)內(nèi)存泄露。

其實(shí)，對(duì)于這個(gè)問(wèn)題，Android Framework 的工程師 Romain Guy 早已經(jīng)在Google論壇上做出過(guò)解釋，并且給出了他的建議寫法：

I wrote that debugging code because of a couple of memory leaks I
found in the Android codebase. Like you said, a Message has a
reference to the Handler which, when it’s inner and non-static, has a
reference to the outer this (an Activity for instance.) If the Message
lives in the queue for a long time, which happens fairly easily when
posting a delayed message for instance, you keep a reference to the
Activity and “l(fā)eak” all the views and resources. It gets even worse
when you obtain a Message and don’t post it right away but keep it
somewhere (for instance in a static structure) for later use.

他的建議寫法是：

class OuterClass {class InnerClass {private final WeakReference<OuterClass> mTarget;InnerClass(OuterClass target) {mTarget = new WeakReference<OuterClass>(target);}void doSomething() {OuterClass target = mTarget.get();if (target != null) {target.do(); }} }

下面，我們進(jìn)一步解釋一下：

• Android App啟動(dòng)的時(shí)候，Android Framework 為主線程創(chuàng)建一個(gè)Looper對(duì)象，這個(gè)Looper對(duì)象將貫穿這個(gè)App的整個(gè)生命周期，它實(shí)現(xiàn)了一個(gè)消息隊(duì)列（Message Queue），并且開(kāi)啟一個(gè)循環(huán)來(lái)處理Message對(duì)象。而Framework的主要事件都包含著內(nèi)部Message對(duì)象，當(dāng)這些事件被觸發(fā)的時(shí)候，Message對(duì)象會(huì)被加到消息隊(duì)列中執(zhí)行。
• 當(dāng)一個(gè)Handler被實(shí)例化時(shí)（如上面那樣），它將和主線程Looper對(duì)象的消息隊(duì)列相關(guān)聯(lián)，被推到消息隊(duì)列中的Message對(duì)象將持有一個(gè)Handler的引用以便于當(dāng)Looper處理到這個(gè)Message的時(shí)候，Framework執(zhí)行Handler的handleMessage(Message)方法。
• 在 Java 語(yǔ)言中，非靜態(tài)匿名內(nèi)部類將持有一個(gè)對(duì)外部類的隱式引用，而靜態(tài)內(nèi)部類則不會(huì)。

到底內(nèi)存泄露是在哪里發(fā)生的呢？以下面代碼為例：

public class SampleActivity extends Activity { private final Handler mLeakyHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {// ...}}@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);// Post a message and delay its execution for 10 minutes.mLeakyHandler.postDelayed(new Runnable() {@Overridepublic void run() { }}, 60 * 10 * 1000);// Go back to the previous Activity.finish();} }

當(dāng)Activity被finish()掉，Message將存在于消息隊(duì)列中長(zhǎng)達(dá)10分鐘的時(shí)間才會(huì)被執(zhí)行到。這個(gè)Message持有一個(gè)對(duì)Handler的引用，Handler也會(huì)持有一個(gè)對(duì)于外部類（SampleActivity）的隱式引用，這些引用在Message被執(zhí)行前將一直保持，這樣會(huì)保證Activity的上下文不被垃圾回收機(jī)制回收，同時(shí)也會(huì)泄露應(yīng)用程序的資源（views and resources）。

為解決這個(gè)問(wèn)題，下面這段代碼中的Handler則是一個(gè)靜態(tài)匿名內(nèi)部類。靜態(tài)匿名內(nèi)部類不會(huì)持有一個(gè)對(duì)外部類的隱式引用，因此Activity將不會(huì)被泄露。如果你需要在Handler中調(diào)用外部Activity的方法，就讓Handler持有一個(gè)對(duì)Activity的WeakReference，這樣就不會(huì)泄露Activity的上下文了，如下所示：

public class SampleActivity extends Activity { /*** Instances of static inner classes do not hold an implicit* reference to their outer class.*/private static class MyHandler extends Handler {private final WeakReference<SampleActivity> mActivity;public MyHandler(SampleActivity activity) {mActivity = new WeakReference<SampleActivity>(activity);}@Overridepublic void handleMessage(Message msg) {SampleActivity activity = mActivity.get();if (activity != null) {// ...}} }private final MyHandler mHandler = new MyHandler(this); /*** Instances of anonymous classes do not hold an implicit* reference to their outer class when they are "static".*/private static final Runnable sRunnable = new Runnable() {@Overridepublic void run() { }};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);// Post a message and delay its execution for 10 minutes.mHandler.postDelayed(sRunnable, 60 * 10 * 1000);// Go back to the previous Activity.finish();} }

總結(jié)：
在實(shí)際開(kāi)發(fā)中，如果內(nèi)部類的生命周期和Activity的生命周期不一致（比如上面那種，Activity finish()之后要等10分鐘，內(nèi)部類的實(shí)例才會(huì)執(zhí)行），則在Activity中要避免使用非靜態(tài)的內(nèi)部類，這種情況，就使用一個(gè)靜態(tài)內(nèi)部類，同時(shí)持有一個(gè)對(duì)Activity的WeakReference。

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