_StriveG Blog

android应用进程是如何启动的

1.前言

我们在了解了四大组件之后,有必要去了解下进程是如何启动的,毕竟,进程是一个很重要的感念。我们知道,我们可以在配置文件中,通过process属性指定进程。在ams中,如果组件需要运行在一个新的进程中,这时候就会去新建进程。让我们看下代码。

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if (entryPoint == null) entryPoint = "android.app.ActivityThread";
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
app.processName);
checkTime(startTime, "startProcess: asking zygote to start proc");
Process.ProcessStartResult startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
app.info.dataDir, entryPointArgs);
  • 其中entryPoint是进程的运行入口

2.Process#start

在start方法中,会调用startViaZygote方法。

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private static ProcessStartResult startViaZygote(final String processClass,
final String niceName,
final int uid, final int gid,
final int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String[] extraArgs)
throws ZygoteStartFailedEx {
synchronized(Process.class) {
ArrayList<String> argsForZygote = new ArrayList<String>();
// --runtime-args, --setuid=, --setgid=,
// and --setgroups= must go first
argsForZygote.add("--runtime-args");
argsForZygote.add("--setuid=" + uid);
argsForZygote.add("--setgid=" + gid);
if ((debugFlags & Zygote.DEBUG_ENABLE_JNI_LOGGING) != 0) {
argsForZygote.add("--enable-jni-logging");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_SAFEMODE) != 0) {
argsForZygote.add("--enable-safemode");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_DEBUGGER) != 0) {
argsForZygote.add("--enable-debugger");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_CHECKJNI) != 0) {
argsForZygote.add("--enable-checkjni");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_JIT) != 0) {
argsForZygote.add("--enable-jit");
}
if ((debugFlags & Zygote.DEBUG_GENERATE_DEBUG_INFO) != 0) {
argsForZygote.add("--generate-debug-info");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_ASSERT) != 0) {
argsForZygote.add("--enable-assert");
}
if (mountExternal == Zygote.MOUNT_EXTERNAL_DEFAULT) {
argsForZygote.add("--mount-external-default");
} else if (mountExternal == Zygote.MOUNT_EXTERNAL_READ) {
argsForZygote.add("--mount-external-read");
} else if (mountExternal == Zygote.MOUNT_EXTERNAL_WRITE) {
argsForZygote.add("--mount-external-write");
}
argsForZygote.add("--target-sdk-version=" + targetSdkVersion);
//TODO optionally enable debuger
//argsForZygote.add("--enable-debugger");
// --setgroups is a comma-separated list
if (gids != null && gids.length > 0) {
StringBuilder sb = new StringBuilder();
sb.append("--setgroups=");
int sz = gids.length;
for (int i = 0; i < sz; i++) {
if (i != 0) {
sb.append(',');
}
sb.append(gids[i]);
}
argsForZygote.add(sb.toString());
}
if (niceName != null) {
argsForZygote.add("--nice-name=" + niceName);
}
if (seInfo != null) {
argsForZygote.add("--seinfo=" + seInfo);
}
if (instructionSet != null) {
argsForZygote.add("--instruction-set=" + instructionSet);
}
if (appDataDir != null) {
argsForZygote.add("--app-data-dir=" + appDataDir);
}
argsForZygote.add(processClass);
if (extraArgs != null) {
for (String arg : extraArgs) {
argsForZygote.add(arg);
}
}
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
}

在经过一系列参数设置之后,会调用zygoteSendArgsAndGetResult方法,这里需要两个参数,一个是ZygoteState,通过openZygoteSocketIfNeeded函数返回,另一个就是启动配置。接下来就看下openZygoteSocketIfNeeded干了什么?

3.Process#openZygoteSocketIfNeeded

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private static ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
try {
primaryZygoteState = ZygoteState.connect(ZYGOTE_SOCKET);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);
}
}
if (primaryZygoteState.matches(abi)) {
return primaryZygoteState;
}
// The primary zygote didn't match. Try the secondary.
if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {
try {
secondaryZygoteState = ZygoteState.connect(SECONDARY_ZYGOTE_SOCKET);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);
}
}
if (secondaryZygoteState.matches(abi)) {
return secondaryZygoteState;
}
throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}

这个方法会根据需要是否开启和zygote进程的socket通道,去做操作。在这里能看到两种不同的,这里是因为android5.0开始,支持64位编译,上面分别对应32和64,这里就不说多了。这里通过ZygoteState的connect方法,去链接到在zygote进程中的server端。

4. Process#zygoteSendArgsAndGetResult

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final BufferedWriter writer = zygoteState.writer;
final DataInputStream inputStream = zygoteState.inputStream;
writer.write(Integer.toString(args.size()));
writer.newLine();
int sz = args.size();
for (int i = 0; i < sz; i++) {
String arg = args.get(i);
if (arg.indexOf('\n') >= 0) {
throw new ZygoteStartFailedEx(
"embedded newlines not allowed");
}
writer.write(arg);
writer.newLine();
}
writer.flush();
// Should there be a timeout on this?
ProcessStartResult result = new ProcessStartResult();
result.pid = inputStream.readInt();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
result.usingWrapper = inputStream.readBoolean();
return result;

在这个方法中,向socke通道写入进程启动参数,等待socket server相应并返回,读取返回结果。

那么,现在我们就需要这里socket服务端的处理。因为这里没有分析zygote进程的启动,所以讲起来比较麻烦,直接告诉大家,其socket服务端实现在ZygoteInit中,在mian方法中,会调用registerZygoteSocket方法去启动socket server。在然后会调用runSelectLoop方法,去等待socket客户端的连接。

5. ZygoteInit#runSelectLoop

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private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
fds.add(sServerSocket.getFileDescriptor());
peers.add(null);
while (true) {
StructPollfd[] pollFds = new StructPollfd[fds.size()];
for (int i = 0; i < pollFds.length; ++i) {
pollFds[i] = new StructPollfd();
pollFds[i].fd = fds.get(i);
pollFds[i].events = (short) POLLIN;
}
try {
Os.poll(pollFds, -1);
} catch (ErrnoException ex) {
throw new RuntimeException("poll failed", ex);
}
for (int i = pollFds.length - 1; i >= 0; --i) {
if ((pollFds[i].revents & POLLIN) == 0) {
continue;
}
if (i == 0) {
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
boolean done = peers.get(i).runOnce();
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}

首先会通过Os.poll等待事件的到来,这里应该是用的poll模型,然后处理,当i=0的时候,为socket请求连接的事件,这时会调用acceptCommandPeer与客户端建立一个连接,然后加入监听数组,等待参数的到来,一旦i!=0,则为参数到来,那么,就调用runOnce去处理参数。完成之后,移除连接、移除监听。

6.ZygoteConnection#runOnce

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boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {
String args[];
Arguments parsedArgs = null;
FileDescriptor[] descriptors;
try {
args = readArgumentList();
descriptors = mSocket.getAncillaryFileDescriptors();
} catch (IOException ex) {
Log.w(TAG, "IOException on command socket " + ex.getMessage());
closeSocket();
return true;
}
if (args == null) {
// EOF reached.
closeSocket();
return true;
}
/** the stderr of the most recent request, if avail */
PrintStream newStderr = null;
if (descriptors != null && descriptors.length >= 3) {
newStderr = new PrintStream(
new FileOutputStream(descriptors[2]));
}
int pid = -1;
FileDescriptor childPipeFd = null;
FileDescriptor serverPipeFd = null;
try {
parsedArgs = new Arguments(args);
if (parsedArgs.abiListQuery) {
return handleAbiListQuery();
}
if (parsedArgs.permittedCapabilities != 0 || parsedArgs.effectiveCapabilities != 0) {
throw new ZygoteSecurityException("Client may not specify capabilities: " +
"permitted=0x" + Long.toHexString(parsedArgs.permittedCapabilities) +
", effective=0x" + Long.toHexString(parsedArgs.effectiveCapabilities));
}
applyUidSecurityPolicy(parsedArgs, peer);
applyInvokeWithSecurityPolicy(parsedArgs, peer);
applyDebuggerSystemProperty(parsedArgs);
applyInvokeWithSystemProperty(parsedArgs);
int[][] rlimits = null;
if (parsedArgs.rlimits != null) {
rlimits = parsedArgs.rlimits.toArray(intArray2d);
}
if (parsedArgs.invokeWith != null) {
FileDescriptor[] pipeFds = Os.pipe2(O_CLOEXEC);
childPipeFd = pipeFds[1];
serverPipeFd = pipeFds[0];
Os.fcntlInt(childPipeFd, F_SETFD, 0);
}
/**
* In order to avoid leaking descriptors to the Zygote child,
* the native code must close the two Zygote socket descriptors
* in the child process before it switches from Zygote-root to
* the UID and privileges of the application being launched.
*
* In order to avoid "bad file descriptor" errors when the
* two LocalSocket objects are closed, the Posix file
* descriptors are released via a dup2() call which closes
* the socket and substitutes an open descriptor to /dev/null.
*/
int [] fdsToClose = { -1, -1 };
FileDescriptor fd = mSocket.getFileDescriptor();
if (fd != null) {
fdsToClose[0] = fd.getInt$();
}
fd = ZygoteInit.getServerSocketFileDescriptor();
if (fd != null) {
fdsToClose[1] = fd.getInt$();
}
fd = null;
pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,
parsedArgs.appDataDir);
} catch (ErrnoException ex) {
logAndPrintError(newStderr, "Exception creating pipe", ex);
} catch (IllegalArgumentException ex) {
logAndPrintError(newStderr, "Invalid zygote arguments", ex);
} catch (ZygoteSecurityException ex) {
logAndPrintError(newStderr,
"Zygote security policy prevents request: ", ex);
}
try {
if (pid == 0) {
// in child
IoUtils.closeQuietly(serverPipeFd);
serverPipeFd = null;
handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
// should never get here, the child is expected to either
// throw ZygoteInit.MethodAndArgsCaller or exec().
return true;
} else {
// in parent...pid of < 0 means failure
IoUtils.closeQuietly(childPipeFd);
childPipeFd = null;
return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
}
} finally {
IoUtils.closeQuietly(childPipeFd);
IoUtils.closeQuietly(serverPipeFd);
}
}
  • readArgumentList读区启动参数
  • 构造Arguments,在这个的构造函数中,会调用parseArgs去解析参数
  • 随后进行参数检查和配置
  • 调用Zygote.forkAndSpecialize进行fork进程,返回进程id

7.Zygote#forkAndSpecialize

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public static int forkAndSpecialize(int uid, int gid, int[] gids, int debugFlags,
int[][] rlimits, int mountExternal, String seInfo, String niceName, int[] fdsToClose,
String instructionSet, String appDataDir) {
VM_HOOKS.preFork();
int pid = nativeForkAndSpecialize(
uid, gid, gids, debugFlags, rlimits, mountExternal, seInfo, niceName, fdsToClose,
instructionSet, appDataDir);
// Enable tracing as soon as possible for the child process.
if (pid == 0) {
Trace.setTracingEnabled(true);
// Note that this event ends at the end of handleChildProc,
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "PostFork");
}
VM_HOOKS.postForkCommon();
return pid;
}
  • VM_HOOKS是ZygoteHooks
  • 在preFork中,会中断HeapTaskDaemon、ReferenceQueueDaemon、FinalizerDaemon、FinalizerWatchdogDaemon,这四个守护线程。并调用nativePreFork在native层做一些fork之前的操作。其对应实现在daivik_system_ZygoteHocks.cc文件中,函数对应表如下

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    static JNINativeMethod gMethods[] = {
    NATIVE_METHOD(ZygoteHooks, nativePreFork, "()J"),
    NATIVE_METHOD(ZygoteHooks, nativePostForkChild, "(JILjava/lang/String;)V"),
    };
  • 然后调用nativeForkAndSpecialize去fork进程,对应实现在com_android_internal_os_Zygote.cpp中。

  • 调用VM_HOOKS的postForkCommon,去启动先前中断的几个线程。

8. nativePreFork

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static jlong ZygoteHooks_nativePreFork(JNIEnv* env, jclass) {
Runtime* runtime = Runtime::Current();
CHECK(runtime->IsZygote()) << "runtime instance not started with -Xzygote";
runtime->PreZygoteFork();
if (Trace::GetMethodTracingMode() != TracingMode::kTracingInactive) {
// Tracing active, pause it.
Trace::Pause();
}
// Grab thread before fork potentially makes Thread::pthread_key_self_ unusable.
return reinterpret_cast<jlong>(ThreadForEnv(env));
}

这里会调用runtime、runtime中调用heap,最终调用heap的PreZygoteFork方法。去做一些初始化操作,本人太渣,看不太懂。略

9. nativeForkAndSpecialize

在com_android_internal_os_Zygote_nativeForkAndSpecialize方法中,会调用ForkAndSpecializeCommon。

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static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArray javaGids,
jint debug_flags, jobjectArray javaRlimits,
jlong permittedCapabilities, jlong effectiveCapabilities,
jint mount_external,
jstring java_se_info, jstring java_se_name,
bool is_system_server, jintArray fdsToClose,
jstring instructionSet, jstring dataDir) {
SetSigChldHandler();
pid_t pid = fork();
if (pid == 0) {
// The child process.
gMallocLeakZygoteChild = 1;
// Clean up any descriptors which must be closed immediately
DetachDescriptors(env, fdsToClose);
// Keep capabilities across UID change, unless we're staying root.
if (uid != 0) {
EnableKeepCapabilities(env);
}
DropCapabilitiesBoundingSet(env);
bool use_native_bridge = !is_system_server && (instructionSet != NULL)
&& android::NativeBridgeAvailable();
if (use_native_bridge) {
ScopedUtfChars isa_string(env, instructionSet);
use_native_bridge = android::NeedsNativeBridge(isa_string.c_str());
}
if (use_native_bridge && dataDir == NULL) {
// dataDir should never be null if we need to use a native bridge.
// In general, dataDir will never be null for normal applications. It can only happen in
// special cases (for isolated processes which are not associated with any app). These are
// launched by the framework and should not be emulated anyway.
use_native_bridge = false;
ALOGW("Native bridge will not be used because dataDir == NULL.");
}
if (!MountEmulatedStorage(uid, mount_external, use_native_bridge)) {
ALOGW("Failed to mount emulated storage: %s", strerror(errno));
if (errno == ENOTCONN || errno == EROFS) {
// When device is actively encrypting, we get ENOTCONN here
// since FUSE was mounted before the framework restarted.
// When encrypted device is booting, we get EROFS since
// FUSE hasn't been created yet by init.
// In either case, continue without external storage.
} else {
ALOGE("Cannot continue without emulated storage");
RuntimeAbort(env);
}
}
if (!is_system_server) {
int rc = createProcessGroup(uid, getpid());
if (rc != 0) {
if (rc == -EROFS) {
ALOGW("createProcessGroup failed, kernel missing CONFIG_CGROUP_CPUACCT?");
} else {
ALOGE("createProcessGroup(%d, %d) failed: %s", uid, pid, strerror(-rc));
}
}
}
SetGids(env, javaGids);
SetRLimits(env, javaRlimits);
if (use_native_bridge) {
ScopedUtfChars isa_string(env, instructionSet);
ScopedUtfChars data_dir(env, dataDir);
android::PreInitializeNativeBridge(data_dir.c_str(), isa_string.c_str());
}
int rc = setresgid(gid, gid, gid);
if (rc == -1) {
ALOGE("setresgid(%d) failed: %s", gid, strerror(errno));
RuntimeAbort(env);
}
rc = setresuid(uid, uid, uid);
if (rc == -1) {
ALOGE("setresuid(%d) failed: %s", uid, strerror(errno));
RuntimeAbort(env);
}
if (NeedsNoRandomizeWorkaround()) {
// Work around ARM kernel ASLR lossage (http://b/5817320).
int old_personality = personality(0xffffffff);
int new_personality = personality(old_personality | ADDR_NO_RANDOMIZE);
if (new_personality == -1) {
ALOGW("personality(%d) failed: %s", new_personality, strerror(errno));
}
}
SetCapabilities(env, permittedCapabilities, effectiveCapabilities);
SetSchedulerPolicy(env);
const char* se_info_c_str = NULL;
ScopedUtfChars* se_info = NULL;
if (java_se_info != NULL) {
se_info = new ScopedUtfChars(env, java_se_info);
se_info_c_str = se_info->c_str();
if (se_info_c_str == NULL) {
ALOGE("se_info_c_str == NULL");
RuntimeAbort(env);
}
}
const char* se_name_c_str = NULL;
ScopedUtfChars* se_name = NULL;
if (java_se_name != NULL) {
se_name = new ScopedUtfChars(env, java_se_name);
se_name_c_str = se_name->c_str();
if (se_name_c_str == NULL) {
ALOGE("se_name_c_str == NULL");
RuntimeAbort(env);
}
}
rc = selinux_android_setcontext(uid, is_system_server, se_info_c_str, se_name_c_str);
if (rc == -1) {
ALOGE("selinux_android_setcontext(%d, %d, \"%s\", \"%s\") failed", uid,
is_system_server, se_info_c_str, se_name_c_str);
RuntimeAbort(env);
}
// Make it easier to debug audit logs by setting the main thread's name to the
// nice name rather than "app_process".
if (se_info_c_str == NULL && is_system_server) {
se_name_c_str = "system_server";
}
if (se_info_c_str != NULL) {
SetThreadName(se_name_c_str);
}
delete se_info;
delete se_name;
UnsetSigChldHandler();
env->CallStaticVoidMethod(gZygoteClass, gCallPostForkChildHooks, debug_flags,
is_system_server ? NULL : instructionSet);
if (env->ExceptionCheck()) {
ALOGE("Error calling post fork hooks.");
RuntimeAbort(env);
}
} else if (pid > 0) {
// the parent process
}
return pid;
}
  • 设置子进程的signal信号处理函数 SetSigChldHandler函数
  • fork进程,fork函数
  • pid为0,进入子进程
    • DetachDescriptors 关闭清理文件描述符
    • SetGids 设置group
    • SetRLimits 设置资源限制
    • 进行其他的初始化设置
    • CallStaticVoidMethod,调用ZygotecallPostForkChildHooks方法。这里又会调用nativePostForkChild。
  • 父进程分支,啥也不做
  • 返回pid
    当这些都执行完之后,回到ZygoteConnection的runonce方法,进行后续操作
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try {
if (pid == 0) {
// in child
IoUtils.closeQuietly(serverPipeFd);
serverPipeFd = null;
handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
// should never get here, the child is expected to either
// throw ZygoteInit.MethodAndArgsCaller or exec().
return true;
} else {
// in parent...pid of < 0 means failure
IoUtils.closeQuietly(childPipeFd);
childPipeFd = null;
return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
}
} finally {
IoUtils.closeQuietly(childPipeFd);
IoUtils.closeQuietly(serverPipeFd);
}

我们重点看handleChildProc。

10.ZygoteConnection#handleChildProc

在这个方法中,有如下代码。

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if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
VMRuntime.getCurrentInstructionSet(),
pipeFd, parsedArgs.remainingArgs);
} else {
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion,
parsedArgs.remainingArgs, null /* classLoader */);
}

大部分情况下,invokeWith为null,所以我们看下面的分支。

11.RuntimeInit.zygoteInit

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public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");
redirectLogStreams();
commonInit();
nativeZygoteInit();
applicationInit(targetSdkVersion, argv, classLoader);
}
  • 重定向log输出
  • commonInit,进行通用的一些设置如时区。
  • zygote初始化
  • 应用初始化

12.nativeZygoteInit

该函数的实现在AndroidRuntime.cpp中,

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static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
gCurRuntime->onZygoteInit();
}

这里onZygoteInit在app_main.cpp中,这里就不多说了。

13.RuntimeInit.applicationInit

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private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
// 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);
// We want to be fairly aggressive about heap utilization, to avoid
// holding on to a lot of memory that isn't needed.
VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
final Arguments args;
try {
args = new Arguments(argv);
} catch (IllegalArgumentException ex) {
Slog.e(TAG, ex.getMessage());
// let the process exit
return;
}
// 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
invokeStaticMain(args.startClass, args.startArgs, classLoader);
}

这里设置一些参数,并且调用invokeStaticMain,从名字上来看,就知道是调用静态main方法,也就是我们指定的进程入口ActivityThread的main方法。

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private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
Class<?> cl;
try {
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
throw new ZygoteInit.MethodAndArgsCaller(m, argv);
}

注意看最后一行代码的注释,因为我们之前经过了复杂的调用,堆栈信息比较多了,这里通过抛异常处理来清理调用栈。最后调用如下代码。

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public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}

就这样我们的应用进程就启动起来了。当然,启动应用程序也是这个流程,简单说下吧:

在点击luncher上的图标,会通过startactivity启动我们的程序,但是,这时候没有进程,通过上面这些繁琐的流程启动之后,在启动activity,这样,应用程序也启动起来了。


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