前言
如果把Android系统比喻成一个人类生命体，那么Zygote 和 SystemServer 就如同支撑生命体站立行走的两根大腿。无论哪根大腿倒了，整个系统都需要重新启动，重新运行。
SystemServer和 Zygote一样是Android系统的独立进程。用于支持应用功能的系统服务诸如WindowManagerServer（WMS）、ActivityManagerSystemService（AMS）、 PackageManagerServer（PMS） 都是通过此进程来创建和管理的。
在进入正题之前，先思考几个问题：
1、systemServer是如何启动的？
2、SystemServer启动之后干了什么？
3、SystemServer 启动了那些系统服务，是如何解决不同服务的依赖关系的呢？
正文
1、systemServer是怎么启动的？
SystemServer 虽然与Zygote 并称为Android系统的两大基石，但与zygote不同的是，SystemServer并不是定义在 init rc文件中，通过init进程启动的。由上文zygote进程解析 可知zygote进程启动之后会进入ZygoteInit 的main方法。SystemServer是在zygote进程初始化阶段进行启动的。
1.1 ZygoteInit.java -> main()
ZygoteInit.java 的 main() 方法是zygote进程启动的第一个入口函数。在main方法的后半段调用了forkSystemServer方法开始了启动SystemServer进程的旅程。
frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
@UnsupportedAppUsage
public static void main(String argv[]) {
........省略其他逻辑..............
try {
........省略其他逻辑..............
// 启动systemServer
if (startSystemServer) {
Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
// {@code r == null} in the parent (zygote) process, and {@code r != null} in the
// child (system_server) process.
if (r != null) {
r.run();
return;
}
}
..............
} catch (Throwable ex) {
Log.e(TAG, "System zygote died with exception", ex);
throw ex;
} finally {
if (zygoteServer != null) {
zygoteServer.closeServerSocket();
}
}
// We're in the child process and have exited the select loop. Proceed to execute the
// command.
if (caller != null) {
caller.run();
}
}1.2 ZygoteInit.java -> forkSystemServer()
ZygoteInit 的 forkSystemServer 方法主要是准备好fork 子进程的启动参数，为真正启动进程做准备。
frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/**
* Prepare the arguments and forks for the system server process.
*
* @return A {@code Runnable} that provides an entrypoint into system_server code in the child
* process; {@code null} in the parent.
*/
private static Runnable forkSystemServer(String abiList, String socketName,
ZygoteServer zygoteServer) {
long capabilities = posixCapabilitiesAsBits(
OsConstants.CAP_IPC_LOCK,
OsConstants.CAP_KILL,
OsConstants.CAP_NET_ADMIN,
OsConstants.CAP_NET_BIND_SERVICE,
OsConstants.CAP_NET_BROADCAST,
OsConstants.CAP_NET_RAW,
OsConstants.CAP_SYS_MODULE,
OsConstants.CAP_SYS_NICE,
OsConstants.CAP_SYS_PTRACE,
OsConstants.CAP_SYS_TIME,
OsConstants.CAP_SYS_TTY_CONFIG,
OsConstants.CAP_WAKE_ALARM,
OsConstants.CAP_BLOCK_SUSPEND
);
/* Containers run without some capabilities, so drop any caps that are not available. */
StructCapUserHeader header = new StructCapUserHeader(
OsConstants._LINUX_CAPABILITY_VERSION_3, 0);
StructCapUserData[] data;
try {
data = Os.capget(header);
} catch (ErrnoException ex) {
throw new RuntimeException("Failed to capget()", ex);
}
capabilities &= ((long) data[0].effective) | (((long) data[1].effective) << 32);
/* Hardcoded command line to start the system server */
//SystemServer 启动参数
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,"
+ "1024,1032,1065,3001,3002,3003,3006,3007,3009,3010,3011",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server",
"--runtime-args",
"--target-sdk-version=" + VMRuntime.SDK_VERSION_CUR_DEVELOPMENT,
"com.android.server.SystemServer",
};
ZygoteArguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteArguments(args);
Zygote.applyDebuggerSystemProperty(parsedArgs);
Zygote.applyInvokeWithSystemProperty(parsedArgs);
if (Zygote.nativeSupportsTaggedPointers()) {
/* Enable pointer tagging in the system server. Hardware support for this is present
* in all ARMv8 CPUs. */
parsedArgs.mRuntimeFlags |= Zygote.MEMORY_TAG_LEVEL_TBI;
}
/* Enable gwp-asan on the system server with a small probability. This is the same
* policy as applied to native processes and system apps. */
parsedArgs.mRuntimeFlags |= Zygote.GWP_ASAN_LEVEL_LOTTERY;
if (shouldProfileSystemServer()) {
parsedArgs.mRuntimeFlags |= Zygote.PROFILE_SYSTEM_SERVER;
}
/* Request to fork the system server process 用fork子进程的方式，启动SystemServer进程 */
pid = Zygote.forkSystemServer(
parsedArgs.mUid, parsedArgs.mGid,
parsedArgs.mGids,
parsedArgs.mRuntimeFlags,
null,
parsedArgs.mPermittedCapabilities,
parsedArgs.mEffectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process 返回pid为0则为fork子进程返回值*/
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
zygoteServer.closeServerSocket();
return handleSystemServerProcess(parsedArgs);
}
return null;
}
1.3 Zygote.java -> forkSystemServer()
这个方法主要是调用native的fork子进程方法。
frameworks/base/core/java/com/android/internal/os/Zygote.java
/**
* Special method to start the system server process. In addition to the
* common actions performed in forkAndSpecialize, the pid of the child
* process is recorded such that the death of the child process will cause
* zygote to exit.
*
* @param uid the UNIX uid that the new process should setuid() to after
* fork()ing and and before spawning any threads.
* @param gid the UNIX gid that the new process should setgid() to after
* fork()ing and and before spawning any threads.
* @param gids null-ok; a list of UNIX gids that the new process should
* setgroups() to after fork and before spawning any threads.
* @param runtimeFlags bit flags that enable ART features.
* @param rlimits null-ok an array of rlimit tuples, with the second
* dimension having a length of 3 and representing
* (resource, rlim_cur, rlim_max). These are set via the posix
* setrlimit(2) call.
* @param permittedCapabilities argument for setcap()
* @param effectiveCapabilities argument for setcap()
*
* @return 0 if this is the child, pid of the child
* if this is the parent, or -1 on error.
*/
static int forkSystemServer(int uid, int gid, int[] gids, int runtimeFlags,
int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
ZygoteHooks.preFork();
//执行fork子进程的native逻辑
int pid = nativeForkSystemServer(
uid, gid, gids, runtimeFlags, rlimits,
permittedCapabilities, effectiveCapabilities);
// Set the Java Language thread priority to the default value for new apps.
Thread.currentThread().setPriority(Thread.NORM_PRIORITY);
ZygoteHooks.postForkCommon();
return pid;
}1.4 android_internal_os_Zygote_nativeForkSystemServer.cpp -> nativeForkSystemServer()
static jint com_android_internal_os_Zygote_nativeForkSystemServer(
JNIEnv* env, jclass, uid_t uid, gid_t gid, jintArray gids,
jint runtime_flags, jobjectArray rlimits, jlong permitted_capabilities,
jlong effective_capabilities) {
std::vector<int> fds_to_close(MakeUsapPipeReadFDVector()),
fds_to_ignore(fds_to_close);
fds_to_close.push_back(gUsapPoolSocketFD);
if (gUsapPoolEventFD != -1) {
fds_to_close.push_back(gUsapPoolEventFD);
fds_to_ignore.push_back(gUsapPoolEventFD);
}
if (gSystemServerSocketFd != -1) {
fds_to_close.push_back(gSystemServerSocketFd);
fds_to_ignore.push_back(gSystemServerSocketFd);
}
pid_t pid = ForkCommon(env, true,
fds_to_close,
fds_to_ignore,
true);
if (pid == 0) {
// System server prcoess does not need data isolation so no need to
// know pkg_data_info_list.
SpecializeCommon(env, uid, gid, gids, runtime_flags, rlimits,
permitted_capabilities, effective_capabilities,
MOUNT_EXTERNAL_DEFAULT, nullptr, nullptr, true,
false, nullptr, nullptr, /* is_top_app= */ false,
/* pkg_data_info_list */ nullptr,
/* whitelisted_data_info_list */ nullptr, false, false);
} else if (pid > 0) {
// The zygote process checks whether the child process has died or not.
ALOGI("System server process %d has been created", pid);
gSystemServerPid = pid;
// There is a slight window that the system server process has crashed
// but it went unnoticed because we haven't published its pid yet. So
// we recheck here just to make sure that all is well.
int status;
if (waitpid(pid, &status, WNOHANG) == pid) {
ALOGE("System server process %d has died. Restarting Zygote!", pid);
RuntimeAbort(env, __LINE__, "System server process has died. Restarting Zygote!");
}
if (UsePerAppMemcg()) {
// Assign system_server to the correct memory cgroup.
// Not all devices mount memcg so check if it is mounted first
// to avoid unnecessarily printing errors and denials in the logs.
if (!SetTaskProfiles(pid, std::vector<std::string>{"SystemMemoryProcess"})) {
ALOGE("couldn't add process %d into system memcg group", pid);
}
}
}
return pid;
}1.5 android_internal_os_Zygote_nativeForkSystemServer.cpp -> ForkCommon()
fork流程的真正实现的方法，调用系统的fork来生成子进程。
系统的fork方法会返回两次
① 返回值为0，表示fork子进程成功，当前执行在子进程
② 返回值大于 0，fork子进程成功当前执行在父进程，返回新创建子进程的进程ID
③ 返回值为-1，fork子进程失败
fork()完以后，父进程和子进程由于有着同样的数据段和代码段，栈，程序计数器。子进程诞生后，是直接运行return返回的，然后接着执行后面的程序，这里注意：子进程并不会执行前面父进程已经执行过的程序，因为进程管理模块中记录了当前进程运行到哪里，而子进程又是完全拷贝过来的，所以子进程程序计数器也是和父进程相同的，所以是从fork()后面的程序继续执行。
// Utility routine to fork a process from the zygote.
static pid_t ForkCommon(JNIEnv* env, bool is_system_server,
const std::vector<int>& fds_to_close,
const std::vector<int>& fds_to_ignore,
bool is_priority_fork) {
SetSignalHandlers();
// Curry a failure function.
auto fail_fn = std::bind(ZygoteFailure, env, is_system_server ? "system_server" : "zygote",
nullptr, _1);
// Temporarily block SIGCHLD during forks. The SIGCHLD handler might
// log, which would result in the logging FDs we close being reopened.
// This would cause failures because the FDs are not whitelisted.
//
// Note that the zygote process is single threaded at this point.
BlockSignal(SIGCHLD, fail_fn);
// Close any logging related FDs before we start evaluating the list of
// file descriptors.
__android_log_close();
AStatsSocket_close();
// If this is the first fork for this zygote, create the open FD table. If
// it isn't, we just need to check whether the list of open files has changed
// (and it shouldn't in the normal case).
if (gOpenFdTable == nullptr) {
gOpenFdTable = FileDescriptorTable::Create(fds_to_ignore, fail_fn);
} else {
gOpenFdTable->Restat(fds_to_ignore, fail_fn);
}
android_fdsan_error_level fdsan_error_level = android_fdsan_get_error_level();
// Purge unused native memory in an attempt to reduce the amount of false
// sharing with the child process. By reducing the size of the libc_malloc
// region shared with the child process we reduce the number of pages that
// transition to the private-dirty state when malloc adjusts the meta-data
// on each of the pages it is managing after the fork.
mallopt(M_PURGE, 0);
//调用fork孵化子进程
pid_t pid = fork();
if (pid == 0) {
if (is_priority_fork) {
setpriority(PRIO_PROCESS, 0, PROCESS_PRIORITY_MAX);
} else {
setpriority(PRIO_PROCESS, 0, PROCESS_PRIORITY_MIN);
}
// The child process.
PreApplicationInit();
// Clean up any descriptors which must be closed immediately
DetachDescriptors(env, fds_to_close, fail_fn);
// Invalidate the entries in the USAP table.
ClearUsapTable();
// Re-open all remaining open file descriptors so that they aren't shared
// with the zygote across a fork.
gOpenFdTable->ReopenOrDetach(fail_fn);
// Turn fdsan back on.
android_fdsan_set_error_level(fdsan_error_level);
// Reset the fd to the unsolicited zygote socket
gSystemServerSocketFd = -1;
} else {
ALOGD("Forked child process %d", pid);
}
// We blocked SIGCHLD prior to a fork, we unblock it here.
UnblockSignal(SIGCHLD, fail_fn);
return pid;
}1.6 ZygoteInit.java -> ⑦ handleSystemServerProcess()
完成forkSystemServer 的剩余工作
/**
* Finish remaining work for the newly forked system server process.
*/
private static Runnable handleSystemServerProcess(ZygoteArguments parsedArgs) {
// set umask to 0077 so new files and directories will default to owner-only permissions.
Os.umask(S_IRWXG | S_IRWXO);
//给当前进程设置一个niceName，也就是 system_server
if (parsedArgs.mNiceName != null) {
Process.setArgV0(parsedArgs.mNiceName);
}
//设置环境变量
final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
if (systemServerClasspath != null) {
//根据这个路径进行dex优化
performSystemServerDexOpt(systemServerClasspath);
// Capturing profiles is only supported for debug or eng builds since selinux normally
// prevents it.
if (shouldProfileSystemServer() && (Build.IS_USERDEBUG || Build.IS_ENG)) {
try {
Log.d(TAG, "Preparing system server profile");
prepareSystemServerProfile(systemServerClasspath);
} catch (Exception e) {
Log.wtf(TAG, "Failed to set up system server profile", e);
}
}
}
if (parsedArgs.mInvokeWith != null) {
String[] args = parsedArgs.mRemainingArgs;
// If we have a non-null system server class path, we'll have to duplicate the
// existing arguments and append the classpath to it. ART will handle the classpath
// correctly when we exec a new process.
if (systemServerClasspath != null) {
String[] amendedArgs = new String[args.length + 2];
amendedArgs[0] = "-cp";
amendedArgs[1] = systemServerClasspath;
System.arraycopy(args, 0, amendedArgs, 2, args.length);
args = amendedArgs;
}
//初始化应用进程
WrapperInit.execApplication(parsedArgs.mInvokeWith,
parsedArgs.mNiceName, parsedArgs.mTargetSdkVersion,
VMRuntime.getCurrentInstructionSet(), null, args);
throw new IllegalStateException("Unexpected return from WrapperInit.execApplication");
} else {
ClassLoader cl = null;
if (systemServerClasspath != null) {
//为当前线程设置classloader
cl = createPathClassLoader(systemServerClasspath, parsedArgs.mTargetSdkVersion);
Thread.currentThread().setContextClassLoader(cl);
}
/*
* Pass the remaining arguments to SystemServer. 进入SystemServer的Main函数
*/
return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mDisabledCompatChanges,
parsedArgs.mRemainingArgs, cl);
}
/* should never reach here */
}
1.7 ZygoteInit.java -> zygoteInit()
zygotinit主要是干了两件事
① 调用nativeZygoteInit方法启动Binder线程池，使得SystemServer可以与其他进程进行Binder通信。
② 通过反射进入SystemServer 的main方法
至此，SystemServer的的启动流程到此结束
public static final Runnable zygoteInit(int targetSdkVersion, long[] disabledCompatChanges,
String[] argv, ClassLoader classLoader) {
if (RuntimeInit.DEBUG) {
Slog.d(RuntimeInit.TAG, "RuntimeInit: Starting application from zygote");
}
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ZygoteInit");
RuntimeInit.redirectLogStreams();
RuntimeInit.commonInit();
//启动Binder 线程池，如此SystemServer 进程就可以使用 Binder与其他进程进行通信。
ZygoteInit.nativeZygoteInit();
//通过反射进入SystemServer的main 方法
return RuntimeInit.applicationInit(targetSdkVersion, disabledCompatChanges, argv,
classLoader);
}
protected static Runnable applicationInit(int targetSdkVersion, long[] disabledCompatChanges,
String[] argv, ClassLoader classLoader) {
// If the application calls System.exit(), terminate the process
// immediately without running any shutdown hooks. It is not possible to
// shutdown an Android application gracefully. Among other things, the
// Android runtime shutdown hooks close the Binder driver, which can cause
// leftover running threads to crash before the process actually exits.
nativeSetExitWithoutCleanup(true);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
VMRuntime.getRuntime().setDisabledCompatChanges(disabledCompatChanges);
final Arguments args = new Arguments(argv);
// The end of of the RuntimeInit event (see #zygoteInit).
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
// Remaining arguments are passed to the start class's static main
return findStaticMain(args.startClass, args.startArgs, classLoader);
} 2 、SystemServer启动之后干了什么？
进入SystemServer 的main方法之后主要是初始化已SystemServer的对象，并调用其run方法
在run方法中比较主要工作是
1、 启动主线程的looper
2、加载共享库（SystemServer的native代码）
3、创建系统上下文
4 、启动系统服务
5、进入loop循环监听消息
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}
private void run() {
TimingsTraceAndSlog t = new TimingsTraceAndSlog();
try {
t.traceBegin("InitBeforeStartServices");
// Record the process start information in sys props.
SystemProperties.set(SYSPROP_START_COUNT, String.valueOf(mStartCount));
SystemProperties.set(SYSPROP_START_ELAPSED, String.valueOf(mRuntimeStartElapsedTime));
SystemProperties.set(SYSPROP_START_UPTIME, String.valueOf(mRuntimeStartUptime));
EventLog.writeEvent(EventLogTags.SYSTEM_SERVER_START,
mStartCount, mRuntimeStartUptime, mRuntimeStartElapsedTime);
//
// Default the timezone property to GMT if not set.
//
String timezoneProperty = SystemProperties.get("persist.sys.timezone");
if (timezoneProperty == null || timezoneProperty.isEmpty()) {
Slog.w(TAG, "Timezone not set; setting to GMT.");
SystemProperties.set("persist.sys.timezone", "GMT");
}
// If the system has "persist.sys.language" and friends set, replace them with
// "persist.sys.locale". Note that the default locale at this point is calculated
// using the "-Duser.locale" command line flag. That flag is usually populated by
// AndroidRuntime using the same set of system properties, but only the system_server
// and system apps are allowed to set them.
//
// NOTE: Most changes made here will need an equivalent change to
// core/jni/AndroidRuntime.cpp
if (!SystemProperties.get("persist.sys.language").isEmpty()) {
final String languageTag = Locale.getDefault().toLanguageTag();
SystemProperties.set("persist.sys.locale", languageTag);
SystemProperties.set("persist.sys.language", "");
SystemProperties.set("persist.sys.country", "");
SystemProperties.set("persist.sys.localevar", "");
}
// The system server should never make non-oneway calls
Binder.setWarnOnBlocking(true);
// The system server should always load safe labels
PackageItemInfo.forceSafeLabels();
// Default to FULL within the system server.
SQLiteGlobal.sDefaultSyncMode = SQLiteGlobal.SYNC_MODE_FULL;
// Deactivate SQLiteCompatibilityWalFlags until settings provider is initialized
SQLiteCompatibilityWalFlags.init(null);
// Here we go!
Slog.i(TAG, "Entered the Android system server!");
final long uptimeMillis = SystemClock.elapsedRealtime();
EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);
if (!mRuntimeRestart) {
FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
FrameworkStatsLog
.BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__SYSTEM_SERVER_INIT_START,
uptimeMillis);
}
// In case the runtime switched since last boot (such as when
// the old runtime was removed in an OTA), set the system
// property so that it is in sync. We can't do this in
// libnativehelper's JniInvocation::Init code where we already
// had to fallback to a different runtime because it is
// running as root and we need to be the system user to set
// the property. http://b/11463182
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
// Mmmmmm... more memory!
VMRuntime.getRuntime().clearGrowthLimit();
// Some devices rely on runtime fingerprint generation, so make sure
// we've defined it before booting further.
Build.ensureFingerprintProperty();
// Within the system server, it is an error to access Environment paths without
// explicitly specifying a user.
Environment.setUserRequired(true);
// Within the system server, any incoming Bundles should be defused
// to avoid throwing BadParcelableException.
BaseBundle.setShouldDefuse(true);
// Within the system server, when parceling exceptions, include the stack trace
Parcel.setStackTraceParceling(true);
// Ensure binder calls into the system always run at foreground priority.
BinderInternal.disableBackgroundScheduling(true);
// Increase the number of binder threads in system_server
BinderInternal.setMaxThreads(sMaxBinderThreads);
// Prepare the main looper thread (this thread). 1、 启动主线程的looper
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
Looper.getMainLooper().setSlowLogThresholdMs(
SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);
SystemServiceRegistry.sEnableServiceNotFoundWtf = true;
// Initialize native services. 2、 加载共享库（SystemServer的native代码）
System.loadLibrary("android_servers");
// Allow heap / perf profiling.
initZygoteChildHeapProfiling();
// Debug builds - spawn a thread to monitor for fd leaks.
if (Build.IS_DEBUGGABLE) {
spawnFdLeakCheckThread();
}
// Check whether we failed to shut down last time we tried.
// This call may not return.
performPendingShutdown();
// Initialize the system context. 3、创建系统上下文
createSystemContext();
// Call per-process mainline module initialization.
ActivityThread.initializeMainlineModules();
// Create the system service manager.
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setStartInfo(mRuntimeRestart,
mRuntimeStartElapsedTime, mRuntimeStartUptime);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Prepare the thread pool for init tasks that can be parallelized
SystemServerInitThreadPool.start();
// Attach JVMTI agent if this is a debuggable build and the system property is set.
if (Build.IS_DEBUGGABLE) {
// Property is of the form "library_path=parameters".
String jvmtiAgent = SystemProperties.get("persist.sys.dalvik.jvmtiagent");
if (!jvmtiAgent.isEmpty()) {
int equalIndex = jvmtiAgent.indexOf('=');
String libraryPath = jvmtiAgent.substring(0, equalIndex);
String parameterList =
jvmtiAgent.substring(equalIndex + 1, jvmtiAgent.length());
// Attach the agent.
try {
Debug.attachJvmtiAgent(libraryPath, parameterList, null);
} catch (Exception e) {
Slog.e("System", "*************************************************");
Slog.e("System", "********** Failed to load jvmti plugin: " + jvmtiAgent);
}
}
}
} finally {
t.traceEnd(); // InitBeforeStartServices
}
// Setup the default WTF handler
RuntimeInit.setDefaultApplicationWtfHandler(SystemServer::handleEarlySystemWtf);
// Start services. 4 启动系统服务
try {
t.traceBegin("StartServices");
startBootstrapServices(t);
startCoreServices(t);
startOtherServices(t);
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
t.traceEnd(); // StartServices
}
StrictMode.initVmDefaults(null);
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
final long uptimeMillis = SystemClock.elapsedRealtime();
FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__SYSTEM_SERVER_READY,
uptimeMillis);
final long maxUptimeMillis = 60 * 1000;
if (uptimeMillis > maxUptimeMillis) {
Slog.wtf(SYSTEM_SERVER_TIMING_TAG,
"SystemServer init took too long. uptimeMillis=" + uptimeMillis);
}
}
// Diagnostic to ensure that the system is in a base healthy state. Done here as a common
// non-zygote process.
if (!VMRuntime.hasBootImageSpaces()) {
Slog.wtf(TAG, "Runtime is not running with a boot image!");
}
// Loop forever. 5、进入loop循环监听消息
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在启动系统服务阶段，SystemServer将系统服务分为三大类 BootstrapServices （引导服务）、 CoreService（核心服务）、otherService（其他服务）一共有90多个系统服务。
这么多服务，许多存在着依赖关系，系统是如何控制启动顺序，以及处理依赖关系的呢？
1、是分批启动
类似 AMS PMS PKMS 之类的基础服务就可以优先启动，上层一些的服务需要依赖到这些基础服务的就放在后边。
2 分阶段启动
每完成一个阶段就去通知已经启动service 已经完成什么阶段，哪些资源可以用，可以去干什么事了。
在SystemServer中每启动到一个阶段都会调用 SystemServiceManager.startBootPhase 方法。这个方法会通知所有已经启动的方法当前启动到什么阶段。
这个方法会回调Service的 onBootPhase 方法。不同的Service可以根据回调的参数在合适的时间做合适的事情。
/**
* Starts the specified boot phase for all system services that have been started up to
* this point.
*
* @param t trace logger
* @param phase The boot phase to start.
*/
public void startBootPhase(@NonNull TimingsTraceAndSlog t, int phase) {
if (phase <= mCurrentPhase) {
throw new IllegalArgumentException("Next phase must be larger than previous");
}
mCurrentPhase = phase;
Slog.i(TAG, "Starting phase " + mCurrentPhase);
try {
t.traceBegin("OnBootPhase_" + phase);
final int serviceLen = mServices.size();
for (int i = 0; i < serviceLen; i++) {
final SystemService service = mServices.get(i);
long time = SystemClock.elapsedRealtime();
t.traceBegin("OnBootPhase_" + phase + "_" + service.getClass().getName());
try {
service.onBootPhase(mCurrentPhase);
} catch (Exception ex) {
throw new RuntimeException("Failed to boot service "
+ service.getClass().getName()
+ ": onBootPhase threw an exception during phase "
+ mCurrentPhase, ex);
}
warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onBootPhase");
t.traceEnd();
}
} finally {
t.traceEnd();
}
if (phase == SystemService.PHASE_BOOT_COMPLETED) {
final long totalBootTime = SystemClock.uptimeMillis() - mRuntimeStartUptime;
t.logDuration("TotalBootTime", totalBootTime);
SystemServerInitThreadPool.shutdown();
}
}按照SystemService中常量值的定义主要分为以下阶段
总结
1、systemServer是如何启动的？
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