Android系統進行升級的時候，有兩種途徑，一種是通過接口傳遞升級包路逕自動升級（Android系統SD卡升級），升級完之後系統自動重啟；另一種是手動進入recovery模式下，選擇升級包進行升級，升級完成之後停留在recovery介面，需要手動選擇重啟。前者多用於手機廠商的客戶端在線升級，後者多用於開發和測試人員。但不管哪種，原理都是一樣的，都要在recovery模式下進行升級。
1、獲取升級包，可以從服務端下載，也可以直接拷貝到SD卡中
2、獲取升級包路徑，驗證簽名，通過installPackage接口升級
3、系統重啟進入Recovery模式
4、在install.cpp進行升級操作
5、try_update_binary執行升級腳本
6、finish_recovery，重啟
一、獲取升級包，可以從服務端下載，也可以直接拷貝到SD卡中
假設SD卡中已有升級包update.zip
二、獲取升級包路徑，驗證簽名，通過installPackage接口升級
1、調用RecoverySystem類提供的verifyPackage方法進行簽名驗證
public static void verifyPackage(File packageFile, ProgressListener listener, File deviceCertsZipFile) throws IOException, GeneralSecurityException 簽名驗證函數，實現過程就不貼出來了，參數，
packageFile--升級文件
listener--進度監督器
deviceCertsZipFile--簽名文件，如果為空，則使用系統默認的簽名
只有當簽名驗證正確才返回，否則將拋出異常。
在Recovery模式下進行升級時候也是會進行簽名驗證的，如果這裡先不進行驗證也不會有什麼問題。但是我們建議在重啟前，先驗證，以便及早發現問題。
如果簽名驗證沒有問題，就執行installPackage開始升級。
2、installPackage開始升級
如果簽名驗證沒有問題，就進行重啟升級，
public static void installPackage(Context context, File packageFile) throws IOException { String filename = packageFile.getCanonicalPath(); Log.w(TAG, "!!! REBOOTING TO INSTALL " + filename + " !!!"); final String filenameArg = "--update_package=" + filename; final String localeArg = "--locale=" + Locale.getDefault().toString(); bootCommand(context, filenameArg, localeArg); } 這裡定義了兩個參數，我們接著看，
private static void bootCommand(Context context, String... args) throws IOException { RECOVERY_DIR.mkdirs(); // In case we need it COMMAND_FILE.delete(); // In case it's not writable LOG_FILE.delete(); FileWriter command = new FileWriter(COMMAND_FILE); try { for (String arg : args) { if (!TextUtils.isEmpty(arg)) { command.write(arg); command.write("\n"); } } } finally { command.close(); } // Having written the command file, go ahead and reboot PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE); pm.reboot(PowerManager.REBOOT_RECOVERY); throw new IOException("Reboot failed (no permissions?)"); } 創建目錄/cache/recovery/，command文件保存在該目錄下；如果存在command文件，將其刪除；然後將上面一步生成的兩個參數寫入到command文件。
最後重啟設備，重啟過程就不再詳述了。
三、系統重啟進入Recovery模式
系統重啟時會判斷/cache/recovery目錄下是否有command文件，如果存在就進入recovery模式，否則就正常啟動。
進入到Recovery模式下，將執行recovery.cpp的main函數，下面貼出關鍵代碼片段，
int arg; while ((arg = getopt_long(argc, argv, "", OPTIONS, NULL)) != -1) { switch (arg) { case 's': send_intent = optarg; break; case 'u': update_package = optarg; break; case 'w': wipe_data = wipe_cache = 1; break; case 'c': wipe_cache = 1; break; case 't': show_text = 1; break; case 'x': just_exit = true; break; case 'l': locale = optarg; break; case 'g': { if (stage == NULL || *stage == '\0') { char buffer[20] = "1/"; strncat(buffer, optarg, sizeof(buffer)-3); stage = strdup(buffer); } break; } case 'p': shutdown_after = true; break; case 'r': reason = optarg; break; case '?': LOGE("Invalid command argument\n"); continue; } } 這是一個While 循環，用來讀取 recovery 的 command 參數， OPTIONS 的不同選項定義如下，
static const struct option OPTIONS[] = { { "send_intent", required_argument, NULL, 's' }, { "update_package", required_argument, NULL, 'u' }, { "wipe_data", no_argument, NULL, 'w' }, { "wipe_cache", no_argument, NULL, 'c' }, { "show_text", no_argument, NULL, 't' }, { "just_exit", no_argument, NULL, 'x' }, { "locale", required_argument, NULL, 'l' }, { "stages", required_argument, NULL, 'g' }, { "shutdown_after", no_argument, NULL, 'p' }, { "reason", required_argument, NULL, 'r' }, { NULL, 0, NULL, 0 },}; 顯然，根據第二步寫入的命令文件內容，將為update_package 賦值。
接著看，
if (update_package) { // For backwards compatibility on the cache partition only, if // we're given an old 'root' path "CACHE:foo", change it to // "/cache/foo". if (strncmp(update_package, "CACHE:", 6) == 0) { int len = strlen(update_package) + 10; char* modified_path = (char*)malloc(len); strlcpy(modified_path, "/cache/", len); strlcat(modified_path, update_package+6, len); printf("(replacing path \"%s\" with \"%s\")\n", update_package, modified_path); update_package = modified_path; } } 兼容性處理。
int status = INSTALL_SUCCESS; if (update_package != NULL) { status = install_package(update_package, &wipe_cache, TEMPORARY_INSTALL_FILE, true); if (status == INSTALL_SUCCESS && wipe_cache) { if (erase_volume("/cache")) { LOGE("Cache wipe (requested by package) failed."); } } if (status != INSTALL_SUCCESS) { ui->Print("Installation aborted.\n"); // If this is an eng or userdebug build, then automatically // turn the text display on if the script fails so the error // message is visible. char buffer[PROPERTY_VALUE_MAX+1]; property_get("ro.build.fingerprint", buffer, ""); if (strstr(buffer, ":userdebug/") || strstr(buffer, ":eng/")) { ui->ShowText(true); } } } else if (wipe_data) { if (device->WipeData()) status = INSTALL_ERROR; if (erase_volume("/data")) status = INSTALL_ERROR; if (wipe_cache && erase_volume("/cache")) status = INSTALL_ERROR; if (erase_persistent_partition() == -1 ) status = INSTALL_ERROR; if (status != INSTALL_SUCCESS) ui->Print("Data wipe failed.\n"); } else if (wipe_cache) { if (wipe_cache && erase_volume("/cache")) status = INSTALL_ERROR; if (status != INSTALL_SUCCESS) ui->Print("Cache wipe failed.\n"); } else if (!just_exit) { status = INSTALL_NONE; // No command specified ui->SetBackground(RecoveryUI::NO_COMMAND); } update_package不為空，執行install_package方法。
我們也可以看到擦除數據、緩存的實現也是在這個裡執行的，這裡就不展開了。
四、在install.cpp進行升級操作
具體的升級過程都是在install.cpp中執行的，先看install_package方法，
intinstall_package(const char* path, int* wipe_cache, const char* install_file, bool needs_mount){ FILE* install_log = fopen_path(install_file, "w"); if (install_log) { fputs(path, install_log); fputc('\n', install_log); } else { LOGE("failed to open last_install: %s\n", strerror(errno)); } int result; if (setup_install_mounts() != 0) { LOGE("failed to set up expected mounts for install; aborting\n"); result = INSTALL_ERROR; } else { result = really_install_package(path, wipe_cache, needs_mount); } if (install_log) { fputc(result == INSTALL_SUCCESS ? '1' : '0', install_log); fputc('\n', install_log); fclose(install_log); } return result;} 這個方法中首先創建了log文件，升級過程包括出錯的信息都會寫到這個文件中，便於後續的分析工作。繼續跟進， really_install_package，
static intreally_install_package(const char *path, int* wipe_cache, bool needs_mount){ ui->SetBackground(RecoveryUI::INSTALLING_UPDATE); ui->Print("Finding update package...\n"); // Give verification half the progress bar... ui->SetProgressType(RecoveryUI::DETERMINATE); ui->ShowProgress(VERIFICATION_PROGRESS_FRACTION, VERIFICATION_PROGRESS_TIME); LOGI("Update location: %s\n", path); // Map the update package into memory. ui->Print("Opening update package...\n"); if (path && needs_mount) { if (path[0] == '@') { ensure_path_mounted(path+1); } else { ensure_path_mounted(path); } } MemMapping map; if (sysMapFile(path, &map) != 0) { LOGE("failed to map file\n"); return INSTALL_CORRUPT; } // 裝入簽名文件 int numKeys; Certificate* loadedKeys = load_keys(PUBLIC_KEYS_FILE, &numKeys); if (loadedKeys == NULL) { LOGE("Failed to load keys\n"); return INSTALL_CORRUPT; } LOGI("%d key(s) loaded from %s\n", numKeys, PUBLIC_KEYS_FILE); ui->Print("Verifying update package...\n"); // 驗證簽名 int err; err = verify_file(map.addr, map.length, loadedKeys, numKeys); free(loadedKeys); LOGI("verify_file returned %d\n", err); // 簽名失敗的處理 if (err != VERIFY_SUCCESS) { LOGE("signature verification failed\n"); sysReleaseMap(&map); return INSTALL_CORRUPT; } /* Try to open the package. */ // 打開升級包 ZipArchive zip; err = mzOpenZipArchive(map.addr, map.length, &zip); if (err != 0) { LOGE("Can't open %s\n(%s)\n", path, err != -1 ? strerror(err) : "bad"); sysReleaseMap(&map); return INSTALL_CORRUPT; } /* Verify and install the contents of the package. */ ui->Print("Installing update...\n"); ui->SetEnableReboot(false); // 執行升級腳本文件，開始升級 int result = try_update_binary(path, &zip, wipe_cache); ui->SetEnableReboot(true); ui->Print("\n"); sysReleaseMap(&map); return result;} 該方法主要做了三件事
1、驗證簽名
int numKeys; Certificate* loadedKeys = load_keys(PUBLIC_KEYS_FILE, &numKeys); if (loadedKeys == NULL) { LOGE("Failed to load keys\n"); return INSTALL_CORRUPT; } 裝載簽名文件，如果為空 ，終止升級；
int err; err = verify_file(map.addr, map.length, loadedKeys, numKeys); free(loadedKeys); LOGI("verify_file returned %d\n", err); // 簽名失敗的處理 if (err != VERIFY_SUCCESS) { LOGE("signature verification failed\n"); sysReleaseMap(&map); return INSTALL_CORRUPT; } 調用verify_file進行簽名驗證，這個方法定義在verifier.cpp文件中，此處不展開，如果驗證失敗立即終止升級。
2、讀取升級包信息
ZipArchive zip; err = mzOpenZipArchive(map.addr, map.length, &zip); if (err != 0) { LOGE("Can't open %s\n(%s)\n", path, err != -1 ? strerror(err) : "bad"); sysReleaseMap(&map); return INSTALL_CORRUPT; } 執行mzOpenZipArchive方法，打開升級包並掃描，將包的內容拷貝到變量zip中，該變量將作為參數用來執行升級腳本。
3、執行升級腳本文件，開始升級
int result = try_update_binary(path, &zip, wipe_cache); try_update_binary方法用來處理升級包，執行製作升級包中的腳本文件 update_binary ，進行系統更新。
五、try_update_binary執行升級腳本
// If the package contains an update binary, extract it and run it.static inttry_update_binary(const char *path, ZipArchive *zip, int* wipe_cache) { // 檢查update-binary是否存在 const ZipEntry* binary_entry = mzFindZipEntry(zip, ASSUMED_UPDATE_BINARY_NAME); if (binary_entry == NULL) { mzCloseZipArchive(zip); return INSTALL_CORRUPT; } const char* binary = "/tmp/update_binary"; unlink(binary); int fd = creat(binary, 0755); if (fd < 0) { mzCloseZipArchive(zip); LOGE("Can't make %s\n", binary); return INSTALL_ERROR; } // update-binary拷貝到"/tmp/update_binary" bool ok = mzExtractZipEntryToFile(zip, binary_entry, fd); close(fd); mzCloseZipArchive(zip); if (!ok) { LOGE("Can't copy %s\n", ASSUMED_UPDATE_BINARY_NAME); return INSTALL_ERROR; } // 創建管道，用於下面的子進程和父進程之間的通信 int pipefd[2]; pipe(pipefd); // When executing the update binary contained in the package, the // arguments passed are: // // - the version number for this interface // // - an fd to which the program can write in order to update the // progress bar. The program can write single-line commands: // // progress <frac> <secs> // fill up the next <frac> part of of the progress bar // over <secs> seconds. If <secs> is zero, use // set_progress commands to manually control the // progress of this segment of the bar // // set_progress <frac> // <frac> should be between 0.0 and 1.0; sets the // progress bar within the segment defined by the most // recent progress command. // // firmware <"hboot"|"radio"> <filename> // arrange to install the contents of <filename> in the // given partition on reboot. // // (API v2: <filename> may start with "PACKAGE:" to // indicate taking a file from the OTA package.) // // (API v3: this command no longer exists.) // // ui_print <string> // display <string> on the screen. // // - the name of the package zip file. // const char** args = (const char**)malloc(sizeof(char*) * 5); args[0] = binary; args[1] = EXPAND(RECOVERY_API_VERSION); // defined in Android.mk char* temp = (char*)malloc(10); sprintf(temp, "%d", pipefd[1]); args[2] = temp; args[3] = (char*)path; args[4] = NULL; // 創建子進程。負責執行binary腳本 pid_t pid = fork(); if (pid == 0) { umask(022); close(pipefd[0]); execv(binary, (char* const*)args);// 執行binary腳本 fprintf(stdout, "E:Can't run %s (%s)\n", binary, strerror(errno)); _exit(-1); } close(pipefd[1]); *wipe_cache = 0; // 父進程負責接受子進程發送的命令去更新ui顯示 char buffer[1024]; FILE* from_child = fdopen(pipefd[0], "r"); while (fgets(buffer, sizeof(buffer), from_child) != NULL) { char* command = strtok(buffer, " \n"); if (command == NULL) { continue; } else if (strcmp(command, "progress") == 0) { char* fraction_s = strtok(NULL, " \n"); char* seconds_s = strtok(NULL, " \n"); float fraction = strtof(fraction_s, NULL); int seconds = strtol(seconds_s, NULL, 10); ui->ShowProgress(fraction * (1-VERIFICATION_PROGRESS_FRACTION), seconds); } else if (strcmp(command, "set_progress") == 0) { char* fraction_s = strtok(NULL, " \n"); float fraction = strtof(fraction_s, NULL); ui->SetProgress(fraction); } else if (strcmp(command, "ui_print") == 0) { char* str = strtok(NULL, "\n"); if (str) { ui->Print("%s", str); } else { ui->Print("\n"); } fflush(stdout); } else if (strcmp(command, "wipe_cache") == 0) { *wipe_cache = 1; } else if (strcmp(command, "clear_display") == 0) { ui->SetBackground(RecoveryUI::NONE); } else if (strcmp(command, "enable_reboot") == 0) { // packages can explicitly request that they want the user // to be able to reboot during installation (useful for // debugging packages that don't exit). ui->SetEnableReboot(true); } else { LOGE("unknown command [%s]\n", command); } } fclose(from_child); int status; waitpid(pid, &status, 0); if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) { LOGE("Error in %s\n(Status %d)\n", path, WEXITSTATUS(status)); return INSTALL_ERROR; } return INSTALL_SUCCESS;} try_update_binary函數，是真正實現讀取升級包中的腳本文件並執行相應的函數的地方。 在此函數中，通過調用fork函數創建出一個子進程，在子進程中開始讀取並執行升級腳本文件。在此需要注意的是函數fork的用法，fork被調用一次，將做兩次返回，在父進程中返回的是子進程的進程ID，為正數；而在子進程中，則返回0。子進程創建成功後，開始執行升級代碼，並通過管道與父進程交互，父進程則通過讀取子進程傳遞過來的信息更新UI。
六、finish_recovery，重啟
上一步完成之後，回到main函數，
// Save logs and clean up before rebooting or shutting down. finish_recovery(send_intent); 保存升級過程中的log，清除臨時文件，包括command文件（不清除的話，下次重啟還會進入recovery模式），最後重啟。
以上就是升級的一個流程。
補充：
手動升級的流程也基本差不多，通過power key + volume上鍵組合，進入recovery模式，進入prompt_and_wait函數等待用戶按鍵事件。
recovery.cpp的main函數，
Device::BuiltinAction after = shutdown_after ? Device::SHUTDOWN : Device::REBOOT; if (status != INSTALL_SUCCESS || ui->IsTextVisible()) { Device::BuiltinAction temp = prompt_and_wait(device, status); if (temp != Device::NO_ACTION) after = temp; } 根據用戶選擇進入到相應的分支進行處理，如下圖，
int chosen_item = get_menu_selection(headers, device->GetMenuItems(), 0, 0, device); // device-specific code may take some action here. It may // return one of the core actions handled in the switch // statement below. Device::BuiltinAction chosen_action = device->InvokeMenuItem(chosen_item); 
當我們選擇從外置 sdcard 升級，進入如下分支中，
case Device::APPLY_EXT: { ensure_path_mounted(SDCARD_ROOT); char* path = browse_directory(SDCARD_ROOT, device); if (path == NULL) { ui->Print("\n-- No package file selected.\n", path); break; } ui->Print("\n-- Install %s ...\n", path); set_sdcard_update_bootloader_message(); void* token = start_sdcard_fuse(path); int status = install_package(FUSE_SIDELOAD_HOST_PATHNAME, &wipe_cache, TEMPORARY_INSTALL_FILE, false); finish_sdcard_fuse(token); ensure_path_unmounted(SDCARD_ROOT); if (status == INSTALL_SUCCESS && wipe_cache) { ui->Print("\n-- Wiping cache (at package request)...\n"); if (erase_volume("/cache")) { ui->Print("Cache wipe failed.\n"); } else { ui->Print("Cache wipe complete.\n"); } } if (status >= 0) { if (status != INSTALL_SUCCESS) { ui->SetBackground(RecoveryUI::ERROR); ui->Print("Installation aborted.\n"); } else if (!ui->IsTextVisible()) { return Device::NO_ACTION; // reboot if logs aren't visible } else { ui->Print("\nInstall from sdcard complete.\n"); } } break; } char* path = browse_directory(SDCARD_ROOT, device); 這個函數瀏覽 SD card 下的文件 ， 並把路徑記錄下來 , 然後根據名稱排序 ， 並處理用戶按鍵。
·當用戶選擇第一個條目「../」,直接跳轉到上級目錄,並且繼續瀏覽文件
·當用戶選擇的條目以"/"開頭,直接進入子目錄
·其它情況表明,該條目就是zip包.寫入BCB,copy 更新包至臨時目錄，直接轉入install_package
選擇zip包後，同樣也是執行install_package函數，後面與自動升級的流程是一樣的。
int status = install_package(FUSE_SIDELOAD_HOST_PATHNAME, &wipe_cache, TEMPORARY_INSTALL_FILE, false); 
