什么是 StrictMode(嚴(yán)苛模式)

strictmode是android在 API9后引入的檢測(cè)影響app運(yùn)行流暢性的一種機(jī)制,例如我們都知道的主線程中不允許有網(wǎng)絡(luò)操作這條規(guī)則就是嚴(yán)苛模式規(guī)則的一種.

strictmode.java 這個(gè)類(lèi)中設(shè)定了許多detect標(biāo)志位例如 DETECT_NETWORK ，還有許多 penalty標(biāo)志位例如 PENALTY_NETWORK ， DETECT標(biāo)志位決定strictmode是否要對(duì)這項(xiàng)內(nèi)容進(jìn)行檢測(cè)，PENALTY標(biāo)志位決定了在這項(xiàng)內(nèi)容發(fā)生時(shí)是否要拋出異常（相當(dāng)于一種懲罰機(jī)制，PENALTY的意思就是懲罰）.

StrictMode 類(lèi)的作用之一就是對(duì)這些標(biāo)志位進(jìn)行管理，通過(guò) setThreadPolicy() 方法可以設(shè)定 Policy 變量中的mask值.

之后會(huì)將 POLICY 變量傳入 BlockGuard 中，BlockGuard 運(yùn)行在 Dalvik虛擬機(jī)中，對(duì)所有的異常操作進(jìn)行統(tǒng)一的管理.

Android官方文檔中對(duì)于strict mode 給出的解釋

strictmode 是一種開(kāi)發(fā)工具，引入它可以使你發(fā)現(xiàn)在開(kāi)發(fā)過(guò)程中產(chǎn)生的問(wèn)題，并修復(fù)它們.

在 application main thread中常有UI相關(guān)的操作和動(dòng)畫(huà)發(fā)生，strictmode可以在主線程中檢測(cè)硬盤(pán)和網(wǎng)絡(luò)相關(guān)的操作.將硬盤(pán)讀寫(xiě)操作和網(wǎng)絡(luò)相關(guān)操作挪出主線程可以使你的app更加流暢和具有響應(yīng)性.同時(shí)為了使app更加響應(yīng)性，你可以屏蔽ANR發(fā)生時(shí)彈出的dialog.

需要注意的是，盡管android設(shè)備的硬盤(pán)類(lèi)型大多為 flash memory,建立在這種存儲(chǔ)介質(zhì)上的文件系統(tǒng)的并發(fā)性仍然是非常有限的(速度上肯定是RAM比較快).

大部分情況下，硬盤(pán)的讀寫(xiě)操作都是非?？斓模谀承┣闆r下，后臺(tái)進(jìn)程中會(huì)運(yùn)行耗費(fèi)很大的I/O操作，在這種情況下，app的響應(yīng)速度會(huì)下降很多.

一.setThreadPolicy()流程

StrictMode類(lèi)的文檔中給出的strictmode啟動(dòng)方式

 * public void onCreate() {
 * if (DEVELOPER_MODE) {
 * StrictMode.setThreadPolicy(new {@link ThreadPolicy.Builder StrictMode.ThreadPolicy.Builder}()
 *  .detectDiskReads()
 *  .detectDiskWrites()
 *  .detectNetwork() // or .detectAll() for all detectable problems
 *  .penaltyLog()
 *  .build());
 * StrictMode.setVmPolicy(new {@link VmPolicy.Builder StrictMode.VmPolicy.Builder}()
 *  .detectLeakedSqlLiteObjects()
 *  .detectLeakedClosableObjects()
 *  .penaltyLog()
 *  .penaltyDeath()
 *  .build());
 * }
 * super.onCreate();

1.

在執(zhí)行了 setThreadPolicy()函數(shù)后會(huì)調(diào)用 setThreadPolicyMask()方法.

 public static void setThreadPolicy(final ThreadPolicy policy) {
 setThreadPolicyMask(policy.mask);
 }

2.

在 setThreadPolicyMask()方法中,除了在java層的threadLocal中設(shè)置外,還需要在Native層也進(jìn)行一個(gè)設(shè)置.  

private static void setThreadPolicyMask(final int policyMask) {
 // In addition to the Java-level thread-local in Dalvik's
 // BlockGuard, we also need to keep a native thread-local in
 // Binder in order to propagate the value across Binder calls,
 // even across native-only processes. The two are kept in
 // sync via the callback to onStrictModePolicyChange, below.
 setBlockGuardPolicy(policyMask);
 // And set the Android native version...
 Binder.setThreadStrictModePolicy(policyMask);
 }

3.

首先分析java層的 setBlockGuardPolicy()方法.

如果policyMask==0,會(huì)返回一個(gè)默認(rèn)policy,默認(rèn)policy不進(jìn)行任何設(shè)置和檢測(cè),policy對(duì)象存儲(chǔ)在threadLocal變量中(每個(gè)線程保存一個(gè)policy的對(duì)象),首次運(yùn)行該方法會(huì)生成一個(gè)默認(rèn)policy(mMask=0)保存在threadLocal中,這里的policy對(duì)象是AndroidBlockGuardPolicy類(lèi)型.   

// Sets the policy in Dalvik/libcore (BlockGuard)
 private static void setBlockGuardPolicy(final int policyMask) {
 if (policyMask == 0) {
 BlockGuard.setThreadPolicy(BlockGuard.LAX_POLICY);
 return;
 }
 final BlockGuard.Policy policy = BlockGuard.getThreadPolicy();
 final AndroidBlockGuardPolicy androidPolicy;
 if (policy instanceof AndroidBlockGuardPolicy) {
 androidPolicy = (AndroidBlockGuardPolicy) policy;
 } else {
 androidPolicy = threadAndroidPolicy.get();
 BlockGuard.setThreadPolicy(androidPolicy);
 }
 androidPolicy.setPolicyMask(policyMask);
 }

4.

再看Native層的代碼:

設(shè)置了policy

386void IPCThreadState::setStrictModePolicy(int32_t policy)
387{
388 mStrictModePolicy = policy;
389}

二.StrictMode如何檢測(cè)問(wèn)題.

1.

CloseGuard檢測(cè)游標(biāo)是否正常關(guān)閉:

當(dāng)使用ContentResolver來(lái)查詢數(shù)據(jù)庫(kù)的時(shí)候,會(huì)返回一個(gè)CursorWrapperInner類(lèi)型的Cursor對(duì)象.

mCursor = mResolver.query(mUri, null, null, null, null);

CloseGuard對(duì)CursorWrapperInner是否正常關(guān)閉的檢測(cè)的邏輯在finalize()函數(shù)中,finalize()會(huì)在gc執(zhí)行垃圾回收的時(shí)候被調(diào)用(垃圾回收使用了GcRoot算法)

如果沒(méi)有執(zhí)行CursorWrapperInner的close()函數(shù)，僅將CursorWrapperInner對(duì)象置為null,當(dāng)主動(dòng)觸發(fā)gc的時(shí)候( Systemgc()),finalize()函數(shù)被調(diào)用 ,"Cursor finalized without prior close()"這段log被打印.但如果沒(méi)有將CursorWrapperInner對(duì)象置為null,這時(shí)主動(dòng)觸發(fā)gc并不會(huì)引起 finalize()函數(shù)的執(zhí)行,因?yàn)镃ursorWrapperInner對(duì)象被強(qiáng)引用,垃圾回收器在回收時(shí)不會(huì)考慮回收強(qiáng)引用對(duì)象，即使最后內(nèi)存不足而崩潰.

經(jīng)過(guò)測(cè)試程序的測(cè)試，發(fā)現(xiàn)"Cursor finalized without prior close()"這段log在 CursorWrapperInner對(duì)象置空并執(zhí)行 System.gc()后是會(huì)打印出來(lái)的.

但是 CloseGuard中的 warnIfOpen()函數(shù)始終沒(méi)有執(zhí)行

在 CursorWrapperInner的構(gòu)造函數(shù)中,mCloseGuard執(zhí)行 open()函數(shù),在 open函數(shù)中allocationSite被賦值,而 ENABLED 變量是默認(rèn)為true的，唯一改變它的setEnabled()方法在源碼中也并沒(méi)有被調(diào)用,所以應(yīng)該是會(huì)在REPORTER中打印SystemLog的,但最后SystemLog并沒(méi)有打印,具體原因分析不出來(lái).       

@Override
 protected void finalize() throws Throwable {
 try {
 if (mCloseGuard != null) {
  mCloseGuard.warnIfOpen();
 }
 if (!mProviderReleased && mContentProvider != null) {
  // Even though we are using CloseGuard, log this anyway so that
  // application developers always see the message in the log.
  Log.w(TAG, "Cursor finalized without prior close()");
  ContentResolver.this.releaseProvider(mContentProvider);
 }
 } finally {
 super.finalize();
 }
 }
 }
 public void warnIfOpen() {
 if (allocationSite == null || !ENABLED) {
 return;
 }
 String message = 
 ("A resource was acquired at attached stack trace but never released. "
  + "See java.io.Closeable for information on avoiding resource leaks.");
 REPORTER.report(message, allocationSite);
 }
 @Override public void report (String message, Throwable allocationSite) {
 System.logW(message, allocationSite);
 }
 CursorWrapperInner(Cursor cursor, IContentProvider icp) {
 super(cursor);
 mContentProvider = icp;
 mCloseGuard.open("close");
 }

2.

onSqliteObjectsLeaked()也是用來(lái)檢測(cè)數(shù)據(jù)庫(kù)游標(biāo)有沒(méi)有正常關(guān)閉,但這個(gè)函數(shù)檢測(cè)的是通過(guò)SqliteDataBase. query()得到的SqliteCursor游標(biāo)對(duì)象.

檢測(cè)位置也是在 finalize()函數(shù)中.

 /**
 * Release the native resources, if they haven't been released yet.
 */
 @Override
 protected void finalize() {
 try {
 // if the cursor hasn't been closed yet, close it first
 if (mWindow != null) {
 if (mStackTrace != null) {
  String sql = mQuery.getSql();
  int len = sql.length();
  StrictMode.onSqliteObjectLeaked(
  "Finalizing a Cursor that has not been deactivated or closed. " +
  "database = " + mQuery.getDatabase().getLabel() +
  ", table = " + mEditTable +
  ", query = " + sql.substring(0, (len > 1000) ? 1000 : len),
  mStackTrace);
 }
 close();
 }
 } finally {
 super.finalize();
 }
 }

流程如下圖

三.StrictMode中使用到的橋接模式

橋接模式:所謂橋接模式就是將邏輯的抽象與實(shí)現(xiàn)分開(kāi)的一種模式

總結(jié)

以上所述是小編給大家介紹的Android StrictMode運(yùn)行流程，希望對(duì)大家有所幫助，如果大家有任何疑問(wèn)請(qǐng)給我留言，小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)腳本之家網(wǎng)站的支持！

最新評(píng)論