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Apex的由来，我们都知道普通的apk我们可以通过应用商店playstore等进行更新，apex的引入是google希望也能通过playstore更新bin文件.so etc配置文件等类型文件. 这些文件的安装实际通过apexd来进行，现在我们来解析一下apexd， apexd的启动分为两个阶段，bootstrap和普通apexd启动，下面分析apexd bootstrap

1. rc文件启动
/system/core/rootdir/init.rc#80

 本阶段启动三个bootstrap apex，分别为com.android.i18n，com.android.runtime com.android.tzdata，在bootstrap 阶段主要就是提供 critical shared libraries

77     # Run apexd-bootstrap so that APEXes that provide critical libraries
78     # become available. Note that this is executed as exec_start to ensure that
79     # the libraries are available to the processes started after this statement.
80     exec_start apexd-bootstrap    

2. main函数
/system/apex/apexd/apexd_main.cpp#118

int main(int /*argc*/, char** argv) {
...
118    android::apex::SetConfig(android::apex::kDefaultConfig); → 全局config
...
151    if (has_subcommand) {
152      return HandleSubcommand(argv);    →  这里走 apexd --bootstrap
153    }

全局config 就是这些常量:

61  static const ApexdConfig kDefaultConfig = {
62      kApexStatusSysprop,       →    kApexStatusSysprop = "apexd.status" apex的状态
63      kApexPackageBuiltinDirs,   →  所有apex文件所在的目录data、product、system、system_ext、vendor
64      kActiveApexPackagesDataDir,   →  kActiveApexPackagesDataDir = "/data/apex/active"
65      kApexDecompressedDir,         →    kApexDecompressedDir = "/data/apex/decompressed";
66      kOtaReservedDir,             →        kOtaReservedDir = "/data/apex/ota_reserved";
67      kApexHashTreeDir,           →   kApexHashTreeDir = "/data/apex/hashtree";
68      kStagedSessionsDir,        →   StagedSessionsDir = "/data/app-staging"
69      kMetadataSepolicyStagedDir,  →  kMetadataSepolicyStagedDir = "/metadata/sepolicy/staged";
70      kVmPayloadMetadataPartitionProp,  →  kVmPayloadMetadataPartitionProp = "apexd.payload_metadata.path"
71      "u:object_r:staging_data_file",  → staging_data_file的 file contexts
72  };

3. OnBootstrap()
 

/system/apex/apexd/apexd_main.cpp#38
36  int HandleSubcommand(char** argv) {
37    if (strcmp("--bootstrap", argv[1]) == 0) {
38      SetDefaultTag("apexd-bootstrap");
39      LOG(INFO) << "Bootstrap subcommand detected";
40      return android::apex::OnBootstrap();
41    }/system/apex/apexd/apexd.cpp#2566
2566  int OnBootstrap() {
2567    ATRACE_NAME("OnBootstrap");
2568    auto time_started = boot_clock::now();
2569  
2570    ApexFileRepository& instance = ApexFileRepository::GetInstance(); → 创建个实例 啥也没做
2571    Result<void> status =
2572        instance.AddPreInstalledApex(gConfig->apex_built_in_dirs);  →  3.1 scan kApexPackageBuiltinDirs下的所有apex, 详见3.1.1
2573    if (!status.ok()) {
2574      LOG(ERROR) << "Failed to collect APEX keys : " << status.error();
2575      return 1;
2576    }
2577  
2578    const auto& pre_installed_apexes = instance.GetPreInstalledApexFiles(); → 从全局变量中得到 pre_installed_store_
2579    int loop_device_cnt = pre_installed_apexes.size();
2580    // Find all bootstrap apexes
2581    std::vector<ApexFileRef> bootstrap_apexes;
2582    for (const auto& apex : pre_installed_apexes) {      →  遍历 所有apexfile
2583      if (IsBootstrapApex(apex.get())) {      →    判断是否是bootstrap apex，有三个com.android.i18n com.android.runtime com.android.tzdata
2584        LOG(INFO) << "Found bootstrap APEX " << apex.get().GetPath();
2585        bootstrap_apexes.push_back(apex);
2586        loop_device_cnt++;
2587      }
2588      if (apex.get().GetManifest().providesharedapexlibs()) {
2589        LOG(INFO) << "Found sharedlibs APEX " << apex.get().GetPath();
2590        // Sharedlis APEX might be mounted 2 times:
2591        //   * Pre-installed sharedlibs APEX will be mounted in OnStart
2592        //   * Updated sharedlibs APEX (if it exists) will be mounted in OnStart
2593        //
2594        // We already counted a loop device for one of these 2 mounts, need to add
2595        // 1 more.
2596        loop_device_cnt++;
2597      }
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