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# 练习Sample跑起来 | 唯鹿同学的练习手记 第2辑
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你好，我是唯鹿。
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接着上篇[练习手记](https://time.geekbang.org/column/article/83742)，今天练习6～8、12、17、19这六期内容（主要针对有课后Sample练习的），相比1～5期轻松了很多。
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[**Chapter06**](https://github.com/AndroidAdvanceWithGeektime/Chapter06)
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> 该项目展示了使用PLT Hook技术来获取Atrace的日志，可以学习到systrace的一些底层机制。
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没有什么问题，项目直接可以运行起来。运行项目后点击开启Atrace日志，然后就可以在Logcat日志中查看到捕获的日志，如下：
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```
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11:40:07.031 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= install systrace hoook =========
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11:40:07.034 8537-8537/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|Record View#draw()
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11:40:07.034 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|DrawFrame
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11:40:07.035 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|syncFrameState
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========= B|8537|prepareTree
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========= E
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========= E
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========= B|8537|eglBeginFrame
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========= E
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========= B|8537|computeOrdering
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========= E
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========= B|8537|flush drawing commands
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========= E
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11:40:07.036 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|eglSwapBuffersWithDamageKHR
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========= B|8537|setSurfaceDamage
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========= E
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11:40:07.042 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|queueBuffer
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========= E
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11:40:07.043 8537-8552/com.dodola.atrace I/HOOOOOOOOK: ========= B|8537|dequeueBuffer
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========= E
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========= E
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========= E
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```
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通过B|事件和E|事件是成对出现的，这样就可以计算出应用执行每个事件使用的时间。那么上面的Log中View的draw()方法显示使用了9ms。
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这里实现方法是使用了[Profilo](https://github.com/facebookincubator/profilo)的PLT Hook来hook libc.so的`write`与`__write_chk`方法。libc是C的基础库函数，为什么要hook这些方法，需要我们补充C、Linux相关知识。
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同理[Chapter06-plus](https://github.com/AndroidAdvanceWithGeektime/Chapter06-plus)展示了如何使用 PLT Hook技术来获取线程创建的堆栈，README有详细的实现步骤介绍，我就不赘述了。
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[**Chapter07**](https://github.com/AndroidAdvanceWithGeektime/Chapter07)
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> 这个Sample是学习如何给代码加入Trace Tag，大家可以将这个代码运用到自己的项目中，然后利用systrace查看结果。这就是所谓的systrace + 函数插桩。
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操作步骤：
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* 使用Android Studio打开工程Chapter07。
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* 运行Gradle Task `:systrace-gradle-plugin:buildAndPublishToLocalMaven`编译plugin插件。
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* 使用Android Studio单独打开工程systrace-sample-android。
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* 编译运行App（插桩后的class文件在目录`Chapter07/systrace-sample-android/app/build/systrace_output/classes`中查看）。
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对比一下插桩效果，插桩前：
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插桩后:
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可以看到在方法执行前后插入了TraceTag，这样的话`beginSection`方法和`endSection`方法之间的代码就会被追踪。
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```
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public class TraceTag {
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@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
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public static void i(String name) {
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Trace.beginSection(name);
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}
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@TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
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public static void o() {
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Trace.endSection();
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}
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```
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其实Support-Compat库中也有类似的一个[TraceCompat](https://developer.android.google.cn/reference/android/support/v4/os/TraceCompat)，项目中可以直接使用。
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然后运行项目，打开systrace：
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```
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python $ANDROID_HOME/platform-tools/systrace/systrace.py gfx view wm am pm ss dalvik app sched -b 90960 -a com.sample.systrace -o test.log.html
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```
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最后打开生成的test.log.html文件就可以查看systrace记录：
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当然，这一步我们也可以使用SDK中的Monitor，效果是一样的。
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使用systrace + 函数插桩的方式，我们就可以很方便地观察每个方法的耗时，从而针对耗时的方法进行优化，尤其是Application的启动优化。
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[**Chapter08**](https://github.com/AndroidAdvanceWithGeektime/Chapter08)
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> 该项目展示了关闭掉虚拟机的class verify后对性能的影响。
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在加载类的过程有一个verify class的步骤，它需要校验方法的每一个指令，是一个比较耗时的操作。这个例子就是通过Hook去掉verify这个步骤。该例子尽量在Dalvik下执行，在ART下的效果并不明显。
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去除校验代码（可以参看阿里的[Atlas](https://github.com/alibaba/atlas)）：
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```
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AndroidRuntime runtime = AndroidRuntime.getInstance();
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runtime.init(this.getApplicationContext(), true);
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runtime.setVerificationEnabled(false);
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```
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具体运行效果这里我就不展示了，直接运行体验就可以了。
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[**Chapter12**](https://github.com/AndroidAdvanceWithGeektime/Chapter12)
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> 通过复写Application的`getSharedPreferences`替换系统`SharedPreferences`的实现，核心的优化在于修改了Apply的实现，将多个Apply方法在内存中合并，而不是多次提交。
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修改`SharedPreferencesImpl`的Apply部分如下：
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```
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public void apply() {
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// 先调用commitToMemory()
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final MemoryCommitResult mcr = commitToMemory();
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boolean hasDiskWritesInFlight = false;
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synchronized (SharedPreferencesImpl.this) {
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// mDiskWritesInFlight大于0说明之前已经有调用过commitToMemory()了
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hasDiskWritesInFlight = mDiskWritesInFlight > 0;
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}
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// 源码没有这层判断，直接提交。
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if (!hasDiskWritesInFlight) {
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final Runnable awaitCommit = new Runnable() {
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public void run() {
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try {
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mcr.writtenToDiskLatch.await();
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} catch (InterruptedException ignored) {
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}
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}
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};
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QueuedWork.add(awaitCommit);
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Runnable postWriteRunnable = new Runnable() {
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public void run() {
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awaitCommit.run();
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QueuedWork.remove(awaitCommit);
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}
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};
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SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);
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}
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// Okay to notify the listeners before it's hit disk
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// because the listeners should always get the same
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// SharedPreferences instance back, which has the
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// changes reflected in memory.
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notifyListeners(mcr);
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```
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[**Chapter14**](https://github.com/AndroidAdvanceWithGeektime/Chapter14)
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这个是全面解析SQLite的资料，有兴趣的可以下载看看。
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[**Chapter17**](https://github.com/AndroidAdvanceWithGeektime/Chapter17)
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> 该项目展示了如何使用PLT Hook技术来获取网络请求相关信息。
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通过PLT Hook，代理Socket相关的几个重要函数：
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```
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/**
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* 直接 hook 内存中的所有so，但是需要排除掉socket相关方法本身定义的libc(不然会出现循坏)
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* plt hook
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*/
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void hookLoadedLibs() {
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ALOG("hook_plt_method");
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hook_plt_method_all_lib("libc.so", "send", (hook_func) &socket_send_hook);
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hook_plt_method_all_lib("libc.so", "recv", (hook_func) &socket_recv_hook);
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hook_plt_method_all_lib("libc.so", "sendto", (hook_func) &socket_sendto_hook);
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hook_plt_method_all_lib("libc.so", "recvfrom", (hook_func) &socket_recvfrom_hook);
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hook_plt_method_all_lib("libc.so", "connect", (hook_func) &socket_connect_hook);
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}
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```
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```
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int hook_plt_method_all_lib(const char* exclueLibname, const char* name, hook_func hook) {
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if (refresh_shared_libs()) {
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// Could not properly refresh the cache of shared library data
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return -1;
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}
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int failures = 0;
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for (auto const& lib : allSharedLibs()) {
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if (strcmp(lib.first.c_str(), exclueLibname) != 0) {
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failures += hook_plt_method(lib.first.c_str(), name, hook);
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}
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}
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return failures;
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}
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```
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运行项目，访问百度的域名[https://www.baidu.com](https://www.baidu.com)，输出如下：
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```
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17:08:28.347 12145-12163/com.dodola.socket E/HOOOOOOOOK: socket_connect_hook sa_family: 10
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17:08:28.349 12145-12163/com.dodola.socket E/HOOOOOOOOK: stack:com.dodola.socket.SocketHook.getStack(SocketHook.java:13)
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java.net.PlainSocketImpl.socketConnect(Native Method)
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java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:334)
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java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:196)
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java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:178)
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java.net.SocksSocketImpl.connect(SocksSocketImpl.java:356)
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java.net.Socket.connect(Socket.java:586)
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com.android.okhttp.internal.Platform.connectSocket(Platform.java:113)
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com.android.okhttp.Connection.connectSocket(Connection.java:196)
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com.android.okhttp.Connection.connect(Connection.java:172)
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com.android.okhttp.Connection.connectAndSetOwner(Connection.java:367)
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com.android.okhttp.OkHttpClient$1.connectAndSetOwner(OkHttpClient.java:130)
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com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:329)
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com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:246)
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com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnection
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AF_INET6 ipv6 IP===>183.232.231.173:443
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socket_connect_hook sa_family: 1
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Ignore local socket connect
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02-07 17:08:28.637 12145-12163/com.dodola.socket E/HOOOOOOOOK: respond:<!DOCTYPE html>
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<html><!--STATUS OK--><head><meta charset="utf-8"><title>百度一下,你就知道</title>
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```
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可以看到我们获取到了网络请求的相关信息。
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最后，我们可以通过Connect函数的hook，实现很多需求，例如：
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* 禁用应用网络访问
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* 过滤广告IP
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* 禁用定位功能
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[**Chapter19**](https://github.com/AndroidAdvanceWithGeektime/Chapter19)
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> 使用Java Hook实现Alarm、WakeLock与GPS的耗电监控。
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实现原理
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根据老师提供的提示信息，动态代理对应的[PowerManager](http://androidxref.com/7.0.0_r1/xref/frameworks/base/core/java/android/os/PowerManager.java)、[AlarmManager](http://androidxref.com/7.0.0_r1/xref/frameworks/base/core/java/android/app/AlarmManager.java)、[LocationManager](http://androidxref.com/7.0.0_r1/xref/frameworks/base/location/java/android/location/LocationManager.java)的`mService`实现，要拦截的方法在[PowerManagerService](http://androidxref.com/7.0.0_r1/xref/frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java)、[AlarmManagerService](http://androidxref.com/7.0.0_r1/xref/frameworks/base/services/core/java/com/android/server/AlarmManagerService.java)、[LocationManagerService](http://androidxref.com/7.0.0_r1/xref/frameworks/base/services/core/java/com/android/server/LocationManagerService.java)中。
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实现核心代码：
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```
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Object oldObj = mHostContext.getSystemService(Context.XXX_SERVICE);
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Class<?> clazz = oldObj.getClass();
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Field field = clazz.getDeclaredField("mService");
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field.setAccessible(true);
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final Object mService = field.get(oldObj);
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setProxyObj(mService);
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Object newObj = Proxy.newProxyInstance(this.getClass().getClassLoader(), mService.getClass().getInterfaces(), this);
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field.set(oldObj, newObj)
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```
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写了几个调用方法去触发，通过判断对应的方法名来做堆栈信息的输出。
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输出的堆栈信息如下：
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当然，强大的Studio在3.2后也有了强大的耗电量分析器，同样可以监测到这些信息，如下图所示（我使用的Studio版本为3.3）。
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实现不足之处：
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* 可能兼容性上不是特别完善（期待老师的标准答案）。
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* 没有按照耗电监控的规则去做一些业务处理。
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心得体会：
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* 本身并不复杂，只是为了找到Hook点，看了对应的Service源码耗费了一些时间，对于它们的工作流程有了更深的认识。
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* 平时也很少使用动态代理，这回查漏补缺，一次用了个爽。
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这个作业前前后后用了一天时间，之前作业还有一些同学提供PR，所以相对轻松些，但这次没有参考，走了点弯路，不过收获也是巨大的。我就不细说了，感兴趣的话可以参考我的实现。完整代码参见[GitHub](https://github.com/simplezhli/Chapter19)，仅供参考。
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**参考**
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* [练习Sample跑起来 | 热点问题答疑第3期](https://time.geekbang.org/column/article/76413)
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* [练习Sample跑起来 | 热点问题答疑第4期](https://time.geekbang.org/column/article/79331)
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