上一节我们详细的看了下Android应用进程的启动过程分析，知道了应用进程是由Zygote进程调用Linux的系统函数fork复制出来的，那么Zygote进程是怎么启动起来的？这节我们就来看一下Zygote进程的启动过程。
当我们的Android手机开机时，Linux的init进程会去加载init.rc配置文件，老罗博客上讲的是Android 2.3的系统，当前应该还没有64位的虚拟机，所以Zygote进程的启动都是配置在init.rc文件中，而8.0的Android源码中已经支持了32位的虚拟机和64位的虚拟机，而且有主次之分，所以就会有四个配置文件，截图如下：
该配置文件的目录路径为：system\core\rootdir\，init.zygote32.rc文件表示当前的手机只配置有32位的Zygote，init.zygote32_64.rc文件表示当前的Zygote同时配置有32位和64位，而且以32位为主Zygote，64位为次Zygote，主次是怎么区分的呢？就是从配置文件中的--socket-name属性来区分的。另外两个init.zygote64.rc、init.zygote64_32.rc分别表示只支持64位的Zygote和同时两个支持，但是以64位为主。init.zygote32_64.rc配置文件的源码如下：
service zygote /system/bin/app_process32 -Xzygote /system/bin --zygote --start-system-server --socket-name=zygote
class main
priority -20
user root
group root readproc
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart audioserver
onrestart restart cameraserver
onrestart restart media
onrestart restart netd
onrestart restart wificond
writepid /dev/cpuset/foreground/tasks
service zygote_secondary /system/bin/app_process64 -Xzygote /system/bin --zygote --socket-name=zygote_secondary
class main
priority -20
user root
group root readproc
socket zygote_secondary stream 660 root system
onrestart restart zygote
writepid /dev/cpuset/foreground/tasks
大家可以看到，两个Zygote的name属性就表示出它们的主次了。init.rc文件加载完成后，两个Zygote虚拟机的name属性也用来在AMS请求创建新的应用进程时，进行匹配，这个过程我们上节已经看到了。Zygote是以service配置的，所以当解析该完成时，init进程就会调用fork去创建Zygote进程，并且执行app_main.cpp文件中的main函数，作为Zygote进行的启动入口。app_main.cpp文件的目录路径为：frameworks\base\cmds\app_process\app_main.cpp，它的main函数的源码如下：
int main(int argc, char* const argv[])
{
if (!LOG_NDEBUG) {
String8 argv_String;
for (int i = 0; i < argc; ++i) {
argv_String.append("\"");
argv_String.append(argv[i]);
argv_String.append("\" ");
}
ALOGV("app_process main with argv: %s", argv_String.string());
}
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// Process command line arguments
// ignore argv[0]
argc--;
argv++;
// Everything up to '--' or first non '-' arg goes to the vm.
//
// The first argument after the VM args is the "parent dir", which
// is currently unused.
//
// After the parent dir, we expect one or more the following internal
// arguments :
//
// --zygote : Start in zygote mode
// --start-system-server : Start the system server.
// --application : Start in application (stand alone, non zygote) mode.
// --nice-name : The nice name for this process.
//
// For non zygote starts, these arguments will be followed by
// the main class name. All remaining arguments are passed to
// the main method of this class.
//
// For zygote starts, all remaining arguments are passed to the zygote.
// main function.
//
// Note that we must copy argument string values since we will rewrite the
// entire argument block when we apply the nice name to argv0.
//
// As an exception to the above rule, anything in "spaced commands"
// goes to the vm even though it has a space in it.
const char* spaced_commands[] = { "-cp", "-classpath" };
// Allow "spaced commands" to be succeeded by exactly 1 argument (regardless of -s).
bool known_command = false;
int i;
for (i = 0; i < argc; i++) {
if (known_command == true) {
runtime.addOption(strdup(argv[i]));
ALOGV("app_process main add known option '%s'", argv[i]);
known_command = false;
continue;
}
for (int j = 0;
j < static_cast<int>(sizeof(spaced_commands) / sizeof(spaced_commands[0]));
++j) {
if (strcmp(argv[i], spaced_commands[j]) == 0) {
known_command = true;
ALOGV("app_process main found known command '%s'", argv[i]);
}
}
if (argv[i][0] != '-') {
break;
}
if (argv[i][1] == '-' && argv[i][2] == 0) {
++i; // Skip --.
break;
}
runtime.addOption(strdup(argv[i]));
ALOGV("app_process main add option '%s'", argv[i]);
}
// Parse runtime arguments. Stop at first unrecognized option.
bool zygote = false;
bool startSystemServer = false;
bool application = false;
String8 niceName;
String8 className;
++i; // Skip unused "parent dir" argument.
while (i < argc) {
const char* arg = argv[i++];
if (strcmp(arg, "--zygote") == 0) {
zygote = true;
niceName = ZYGOTE_NICE_NAME;
} else if (strcmp(arg, "--start-system-server") == 0) {
startSystemServer = true;
} else if (strcmp(arg, "--application") == 0) {
application = true;
} else if (strncmp(arg, "--nice-name=", 12) == 0) {
niceName.setTo(arg + 12);
} else if (strncmp(arg, "--", 2) != 0) {
className.setTo(arg);
break;
} else {
--i;
break;
}
}
Vector<String8> args;
if (!className.isEmpty()) {
// We're not in zygote mode, the only argument we need to pass
// to RuntimeInit is the application argument.
//
// The Remainder of args get passed to startup class main(). Make
// copies of them before we overwrite them with the process name.
args.add(application ? String8("application") : String8("tool"));
runtime.setClassNameAndArgs(className, argc - i, argv + i);
if (!LOG_NDEBUG) {
String8 restOfArgs;
char* const* argv_new = argv + i;
int argc_new = argc - i;
for (int k = 0; k < argc_new; ++k) {
restOfArgs.append("\"");
restOfArgs.append(argv_new[k]);
restOfArgs.append("\" ");
}
ALOGV("Class name = %s, args = %s", className.string(), restOfArgs.string());
}
} else {
// We're in zygote mode.
maybeCreateDalvikCache();
if (startSystemServer) {
args.add(String8("start-system-server"));
}
char prop[PROP_VALUE_MAX];
if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {
LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
ABI_LIST_PROPERTY);
return 11;
}
String8 abiFlag("--abi-list=");
abiFlag.append(prop);
args.add(abiFlag);
// In zygote mode, pass all remaining arguments to the zygote
// main() method.
for (; i < argc; ++i) {
args.add(String8(argv[i]));
}
}
if (!niceName.isEmpty()) {
runtime.setArgv0(niceName.string(), true /* setProcName */);
}
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
} else if (className) {
runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
} else {
fprintf(stderr, "Error: no class name or --zygote supplied.\n");
app_usage();
LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
}
}
该函数首先创建一个AppRuntime对象，然后对init.rc配置文件中的启动参数argc、argv[]进行解析，while循环中我们可以看到，如果配置的启动参数为--zygote，表示要启动Zygote进程，如果为--start-system-server表示要启动SystemServer进程，如果为--application就表示是普通的应用进程。我们当前的场景中就是第一个，参数解析完成后，此时的局部变量zygote的值为true，最后的if/else分支就会执行runtime.start("com.android.internal.os.ZygoteInit", args, zygote)来继续完成Zygote进行的启动。
接下来我们就看看AppRuntime的创建过程，AppRuntime类的定义也是在app_main文件中，该类的源码如下：
class AppRuntime : public AndroidRuntime
{
public:
AppRuntime(char* argBlockStart, const size_t argBlockLength)
: AndroidRuntime(argBlockStart, argBlockLength)
, mClass(NULL)
{
}
void setClassNameAndArgs(const String8& className, int argc, char * const *argv) {
mClassName = className;
for (int i = 0; i < argc; ++i) {
mArgs.add(String8(argv[i]));
}
}
virtual void onVmCreated(JNIEnv* env)
{
if (mClassName.isEmpty()) {
return; // Zygote. Nothing to do here.
}
/*
* This is a little awkward because the JNI FindClass call uses the
* class loader associated with the native method we're executing in.
* If called in onStarted (from RuntimeInit.finishInit because we're
* launching "am", for example), FindClass would see that we're calling
* from a boot class' native method, and so wouldn't look for the class
* we're trying to look up in CLASSPATH. Unfortunately it needs to,
* because the "am" classes are not boot classes.
*
* The easiest fix is to call FindClass here, early on before we start
* executing boot class Java code and thereby deny ourselves access to
* non-boot classes.
*/
char* slashClassName = toSlashClassName(mClassName.string());
mClass = env->FindClass(slashClassName);
if (mClass == NULL) {
ALOGE("ERROR: could not find class '%s'\n", mClassName.string());
}
free(slashClassName);
mClass = reinterpret_cast<jclass>(env->NewGlobalRef(mClass));
}
virtual void onStarted()
{
sp<ProcessState> proc = ProcessState::self();
ALOGV("App process: starting thread pool.\n");
proc->startThreadPool();
AndroidRuntime* ar = AndroidRuntime::getRuntime();
ar->callMain(mClassName, mClass, mArgs);
IPCThreadState::self()->stopProcess();
hardware::IPCThreadState::self()->stopProcess();
}
virtual void onZygoteInit()
{
sp<ProcessState> proc = ProcessState::self();
ALOGV("App process: starting thread pool.\n");
proc->startThreadPool();
}
virtual void onExit(int code)
{
if (mClassName.isEmpty()) {
// if zygote
IPCThreadState::self()->stopProcess();
hardware::IPCThreadState::self()->stopProcess();
}
AndroidRuntime::onExit(code);
}
String8 mClassName;
Vector<String8> mArgs;
jclass mClass;
};
它是AndroidRuntime类的子类，它的构造方法中没有任何逻辑。那我们接着来看看AndroidRuntime类的构造方法，AndroidRuntime类的定义在AndroidRuntime.cpp文件中，目录路径为：frameworks\base\core\jni\AndroidRuntime.cpp，它的构造方法的源码如下：
AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
mExitWithoutCleanup(false),
mArgBlockStart(argBlockStart),
mArgBlockLength(argBlockLength)
{
SkGraphics::Init();
// There is also a global font cache, but its budget is specified by
// SK_DEFAULT_FONT_CACHE_COUNT_LIMIT and SK_DEFAULT_FONT_CACHE_LIMIT.
// Pre-allocate enough space to hold a fair number of options.
mOptions.setCapacity(20);
assert(gCurRuntime == NULL); // one per process
gCurRuntime = this;
}
这里就是给成员变量gCurRuntime赋值，后面的注释也写的非常清楚了，一个进程只会执行一次，如果gCurRuntime不为空，那么assert方法进行参数检查时就会抛出异常。好了，AppRuntime类的构造方法我们就看到这里，接下来我们继续分析app_main文件中的main函数的最后一句runtime.start("com.android.internal.os.ZygoteInit", args, zygote)来看看Zygote进行是如何启动起来的。runtime对象就是AppRuntime，它没有重写start方法，所以会执行父类AndroidRuntime类的start方法，AndroidRuntime类的start方法源码如下：
/*
* Start the Android runtime. This involves starting the virtual machine
* and calling the "static void main(String[] args)" method in the class
* named by "className".
*
* Passes the main function two arguments, the class name and the specified
* options string.
*/
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
ALOGD(">>>>>> START %s uid %d <<<<<<\n",
className != NULL ? className : "(unknown)", getuid());
static const String8 startSystemServer("start-system-server");
/*
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
for (size_t i = 0; i < options.size(); ++i) {
if (options[i] == startSystemServer) {
/* track our progress through the boot sequence */
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}
}
const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /android does not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}
//const char* kernelHack = getenv("LD_ASSUME_KERNEL");
//ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env, zygote) != 0) {
return;
}
onVmCreated(env);
/*
* Register android functions.
*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
/*
* We want to call main() with a String array with arguments in it.
* At present we have two arguments, the class name and an option string.
* Create an array to hold them.
*/
jclass stringClass;
jobjectArray strArray;
jstring classNameStr;
stringClass = env->FindClass("java/lang/String");
assert(stringClass != NULL);
strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);
assert(classNameStr != NULL);
env->SetObjectArrayElement(strArray, 0, classNameStr);
for (size_t i = 0; i < options.size(); ++i) {
jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
assert(optionsStr != NULL);
env->SetObjectArrayElement(strArray, i + 1, optionsStr);
}
/*
* Start VM. This thread becomes the main thread of the VM, and will
* not return until the VM exits.
*/
char* slashClassName = toSlashClassName(className);
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
/* keep going */
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);
#if 0
if (env->ExceptionCheck())
threadExitUncaughtException(env);
#endif
}
}
free(slashClassName);
ALOGD("Shutting down VM\n");
if (mJavaVM->DetachCurrentThread() != JNI_OK)
ALOGW("Warning: unable to detach main thread\n");
if (mJavaVM->DestroyJavaVM() != 0)
ALOGW("Warning: VM did not shut down cleanly\n");
}
该方法一共三个参数，第一个是启动类的类路径全名，也就是com.android.internal.os.ZygoteInit，第二个参数options就是前面解析好的启动参数，第三个zygote表示当前是否要启动Zygote进程，当前场景下该值为true，接下来就调用startVm启动虚拟机，我们来看一下该方法的实现。startVm方法的源码如下：
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{
JavaVMInitArgs initArgs;
char propBuf[PROPERTY_VALUE_MAX];
char stackTraceFileBuf[sizeof("-Xstacktracefile:")-1 + PROPERTY_VALUE_MAX];
char jniOptsBuf[sizeof("-Xjniopts:")-1 + PROPERTY_VALUE_MAX];
char heapstartsizeOptsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];
char heapsizeOptsBuf[sizeof("-Xmx")-1 + PROPERTY_VALUE_MAX];
char heapgrowthlimitOptsBuf[sizeof("-XX:HeapGrowthLimit=")-1 + PROPERTY_VALUE_MAX];
char heapminfreeOptsBuf[sizeof("-XX:HeapMinFree=")-1 + PROPERTY_VALUE_MAX];
char heapmaxfreeOptsBuf[sizeof("-XX:HeapMaxFree=")-1 + PROPERTY_VALUE_MAX];
char usejitOptsBuf[sizeof("-Xusejit:")-1 + PROPERTY_VALUE_MAX];
char jitmaxsizeOptsBuf[sizeof("-Xjitmaxsize:")-1 + PROPERTY_VALUE_MAX];
char jitinitialsizeOptsBuf[sizeof("-Xjitinitialsize:")-1 + PROPERTY_VALUE_MAX];
char jitthresholdOptsBuf[sizeof("-Xjitthreshold:")-1 + PROPERTY_VALUE_MAX];
char useJitProfilesOptsBuf[sizeof("-Xjitsaveprofilinginfo:")-1 + PROPERTY_VALUE_MAX];
char jitprithreadweightOptBuf[sizeof("-Xjitprithreadweight:")-1 + PROPERTY_VALUE_MAX];
char jittransitionweightOptBuf[sizeof("-Xjittransitionweight:")-1 + PROPERTY_VALUE_MAX];
char gctypeOptsBuf[sizeof("-Xgc:")-1 + PROPERTY_VALUE_MAX];
char backgroundgcOptsBuf[sizeof("-XX:BackgroundGC=")-1 + PROPERTY_VALUE_MAX];
char heaptargetutilizationOptsBuf[sizeof("-XX:HeapTargetUtilization=")-1 + PROPERTY_VALUE_MAX];
char cachePruneBuf[sizeof("-Xzygote-max-boot-retry=")-1 + PROPERTY_VALUE_MAX];
char dex2oatXmsImageFlagsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];
char dex2oatXmxImageFlagsBuf[sizeof("-Xmx")-1 + PROPERTY_VALUE_MAX];
char dex2oatXmsFlagsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];
char dex2oatXmxFlagsBuf[sizeof("-Xmx")-1 + PROPERTY_VALUE_MAX];
char dex2oatCompilerFilterBuf[sizeof("--compiler-filter=")-1 + PROPERTY_VALUE_MAX];
char dex2oatImageCompilerFilterBuf[sizeof("--compiler-filter=")-1 + PROPERTY_VALUE_MAX];
char dex2oatThreadsBuf[sizeof("-j")-1 + PROPERTY_VALUE_MAX];
char dex2oatThreadsImageBuf[sizeof("-j")-1 + PROPERTY_VALUE_MAX];
char dex2oat_isa_variant_key[PROPERTY_KEY_MAX];
char dex2oat_isa_variant[sizeof("--instruction-set-variant=") -1 + PROPERTY_VALUE_MAX];
char dex2oat_isa_features_key[PROPERTY_KEY_MAX];
char dex2oat_isa_features[sizeof("--instruction-set-features=") -1 + PROPERTY_VALUE_MAX];
char dex2oatFlagsBuf[PROPERTY_VALUE_MAX];
char dex2oatImageFlagsBuf[PROPERTY_VALUE_MAX];
char extraOptsBuf[PROPERTY_VALUE_MAX];
char voldDecryptBuf[PROPERTY_VALUE_MAX];
enum {
kEMDefault,
kEMIntPortable,
kEMIntFast,
kEMJitCompiler,
} executionMode = kEMDefault;
char localeOption[sizeof("-Duser.locale=") + PROPERTY_VALUE_MAX];
char lockProfThresholdBuf[sizeof("-Xlockprofthreshold:")-1 + PROPERTY_VALUE_MAX];
char nativeBridgeLibrary[sizeof("-XX:NativeBridge=") + PROPERTY_VALUE_MAX];
char cpuAbiListBuf[sizeof("--cpu-abilist=") + PROPERTY_VALUE_MAX];
char methodTraceFileBuf[sizeof("-Xmethod-trace-file:") + PROPERTY_VALUE_MAX];
char methodTraceFileSizeBuf[sizeof("-Xmethod-trace-file-size:") + PROPERTY_VALUE_MAX];
char fingerprintBuf[sizeof("-Xfingerprint:") + PROPERTY_VALUE_MAX];
bool checkJni = false;
property_get("dalvik.vm.checkjni", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
checkJni = true;
} else if (strcmp(propBuf, "false") != 0) {
/* property is neither true nor false; fall back on kernel parameter */
property_get("ro.kernel.android.checkjni", propBuf, "");
if (propBuf[0] == '1') {
checkJni = true;
}
}
ALOGV("CheckJNI is %s\n", checkJni ? "ON" : "OFF");
if (checkJni) {
/* extended JNI checking */
addOption("-Xcheck:jni");
/* with -Xcheck:jni, this provides a JNI function call trace */
//addOption("-verbose:jni");
}
property_get("dalvik.vm.execution-mode", propBuf, "");
if (strcmp(propBuf, "int:portable") == 0) {
executionMode = kEMIntPortable;
} else if (strcmp(propBuf, "int:fast") == 0) {
executionMode = kEMIntFast;
} else if (strcmp(propBuf, "int:jit") == 0) {
executionMode = kEMJitCompiler;
}
parseRuntimeOption("dalvik.vm.stack-trace-file", stackTraceFileBuf, "-Xstacktracefile:");
strcpy(jniOptsBuf, "-Xjniopts:");
if (parseRuntimeOption("dalvik.vm.jniopts", jniOptsBuf, "-Xjniopts:")) {
ALOGI("JNI options: '%s'\n", jniOptsBuf);
}
/* route exit() to our handler */
addOption("exit", (void*) runtime_exit);
/* route fprintf() to our handler */
addOption("vfprintf", (void*) runtime_vfprintf);
/* register the framework-specific "is sensitive thread" hook */
addOption("sensitiveThread", (void*) runtime_isSensitiveThread);
/* enable verbose; standard options are { jni, gc, class } */
//addOption("-verbose:jni");
addOption("-verbose:gc");
//addOption("-verbose:class");
/*
* The default starting and maximum size of the heap. Larger
* values should be specified in a product property override.
*/
parseRuntimeOption("dalvik.vm.heapstartsize", heapstartsizeOptsBuf, "-Xms", "4m");
parseRuntimeOption("dalvik.vm.heapsize", heapsizeOptsBuf, "-Xmx", "16m");
parseRuntimeOption("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");
parseRuntimeOption("dalvik.vm.heapminfree", heapminfreeOptsBuf, "-XX:HeapMinFree=");
parseRuntimeOption("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");
parseRuntimeOption("dalvik.vm.heaptargetutilization",
heaptargetutilizationOptsBuf,
"-XX:HeapTargetUtilization=");
/*
* JIT related options.
*/
parseRuntimeOption("dalvik.vm.usejit", usejitOptsBuf, "-Xusejit:");
parseRuntimeOption("dalvik.vm.jitmaxsize", jitmaxsizeOptsBuf, "-Xjitmaxsize:");
parseRuntimeOption("dalvik.vm.jitinitialsize", jitinitialsizeOptsBuf, "-Xjitinitialsize:");
parseRuntimeOption("dalvik.vm.jitthreshold", jitthresholdOptsBuf, "-Xjitthreshold:");
property_get("dalvik.vm.usejitprofiles", useJitProfilesOptsBuf, "");
if (strcmp(useJitProfilesOptsBuf, "true") == 0) {
addOption("-Xjitsaveprofilinginfo");
}
parseRuntimeOption("dalvik.vm.jitprithreadweight",
jitprithreadweightOptBuf,
"-Xjitprithreadweight:");
parseRuntimeOption("dalvik.vm.jittransitionweight",
jittransitionweightOptBuf,
"-Xjittransitionweight:");
property_get("ro.config.low_ram", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
addOption("-XX:LowMemoryMode");
}
parseRuntimeOption("dalvik.vm.gctype", gctypeOptsBuf, "-Xgc:");
parseRuntimeOption("dalvik.vm.backgroundgctype", backgroundgcOptsBuf, "-XX:BackgroundGC=");
/*
* Enable debugging only for apps forked from zygote.
* Set suspend=y to pause during VM init and use android ADB transport.
*/
if (zygote) {
addOption("-agentlib:jdwp=transport=dt_android_adb,suspend=n,server=y");
}
parseRuntimeOption("dalvik.vm.lockprof.threshold",
lockProfThresholdBuf,
"-Xlockprofthreshold:");
if (executionMode == kEMIntPortable) {
addOption("-Xint:portable");
} else if (executionMode == kEMIntFast) {
addOption("-Xint:fast");
} else if (executionMode == kEMJitCompiler) {
addOption("-Xint:jit");
}
// If we are booting without the real /data, don't spend time compiling.
property_get("vold.decrypt", voldDecryptBuf, "");
bool skip_compilation = ((strcmp(voldDecryptBuf, "trigger_restart_min_framework") == 0) ||
(strcmp(voldDecryptBuf, "1") == 0));
// Extra options for boot.art/boot.oat image generation.
parseCompilerRuntimeOption("dalvik.vm.image-dex2oat-Xms", dex2oatXmsImageFlagsBuf,
"-Xms", "-Ximage-compiler-option");
parseCompilerRuntimeOption("dalvik.vm.image-dex2oat-Xmx", dex2oatXmxImageFlagsBuf,
"-Xmx", "-Ximage-compiler-option");
if (skip_compilation) {
addOption("-Ximage-compiler-option");
addOption("--compiler-filter=assume-verified");
} else {
parseCompilerOption("dalvik.vm.image-dex2oat-filter", dex2oatImageCompilerFilterBuf,
"--compiler-filter=", "-Ximage-compiler-option");
}
// Make sure there is a preloaded-classes file.
if (!hasFile("/system/etc/preloaded-classes")) {
ALOGE("Missing preloaded-classes file, /system/etc/preloaded-classes not found: %s\n",
strerror(errno));
return -1;
}
addOption("-Ximage-compiler-option");
addOption("--image-classes=/system/etc/preloaded-classes");
// If there is a compiled-classes file, push it.
if (hasFile("/system/etc/compiled-classes")) {
addOption("-Ximage-compiler-option");
addOption("--compiled-classes=/system/etc/compiled-classes");
}
property_get("dalvik.vm.image-dex2oat-flags", dex2oatImageFlagsBuf, "");
parseExtraOpts(dex2oatImageFlagsBuf, "-Ximage-compiler-option");
// Extra options for DexClassLoader.
parseCompilerRuntimeOption("dalvik.vm.dex2oat-Xms", dex2oatXmsFlagsBuf,
"-Xms", "-Xcompiler-option");
parseCompilerRuntimeOption("dalvik.vm.dex2oat-Xmx", dex2oatXmxFlagsBuf,
"-Xmx", "-Xcompiler-option");
if (skip_compilation) {
addOption("-Xcompiler-option");
addOption("--compiler-filter=assume-verified");
// We skip compilation when a minimal runtime is brought up for decryption. In that case
// /data is temporarily backed by a tmpfs, which is usually small.
// If the system image contains prebuilts, they will be relocated into the tmpfs. In this
// specific situation it is acceptable to *not* relocate and run out of the prebuilts
// directly instead.
addOption("--runtime-arg");
addOption("-Xnorelocate");
} else {
parseCompilerOption("dalvik.vm.dex2oat-filter", dex2oatCompilerFilterBuf,
"--compiler-filter=", "-Xcompiler-option");
}
parseCompilerOption("dalvik.vm.dex2oat-threads", dex2oatThreadsBuf, "-j", "-Xcompiler-option");
parseCompilerOption("dalvik.vm.image-dex2oat-threads", dex2oatThreadsImageBuf, "-j",
"-Ximage-compiler-option");
// The runtime will compile a boot image, when necessary, not using installd. Thus, we need to
// pass the instruction-set-features/variant as an image-compiler-option.
// TODO: Find a better way for the instruction-set.
#if defined(__arm__)
constexpr const char* instruction_set = "arm";
#elif defined(__aarch64__)
constexpr const char* instruction_set = "arm64";
#elif defined(__mips__) && !defined(__LP64__)
constexpr const char* instruction_set = "mips";
#elif defined(__mips__) && defined(__LP64__)
constexpr const char* instruction_set = "mips64";
#elif defined(__i386__)
constexpr const char* instruction_set = "x86";
#elif defined(__x86_64__)
constexpr const char* instruction_set = "x86_64";
#else
constexpr const char* instruction_set = "unknown";
#endif
// Note: it is OK to reuse the buffer, as the values are exactly the same between
// * compiler-option, used for runtime compilation (DexClassLoader)
// * image-compiler-option, used for boot-image compilation on device
// Copy the variant.
sprintf(dex2oat_isa_variant_key, "dalvik.vm.isa.%s.variant", instruction_set);
parseCompilerOption(dex2oat_isa_variant_key, dex2oat_isa_variant,
"--instruction-set-variant=", "-Ximage-compiler-option");
parseCompilerOption(dex2oat_isa_variant_key, dex2oat_isa_variant,
"--instruction-set-variant=", "-Xcompiler-option");
// Copy the features.
sprintf(dex2oat_isa_features_key, "dalvik.vm.isa.%s.features", instruction_set);
parseCompilerOption(dex2oat_isa_features_key, dex2oat_isa_features,
"--instruction-set-features=", "-Ximage-compiler-option");
parseCompilerOption(dex2oat_isa_features_key, dex2oat_isa_features,
"--instruction-set-features=", "-Xcompiler-option");
property_get("dalvik.vm.dex2oat-flags", dex2oatFlagsBuf, "");
parseExtraOpts(dex2oatFlagsBuf, "-Xcompiler-option");
/* extra options; parse this late so it overrides others */
property_get("dalvik.vm.extra-opts", extraOptsBuf, "");
parseExtraOpts(extraOptsBuf, NULL);
/* Set the properties for locale */
{
strcpy(localeOption, "-Duser.locale=");
const std::string locale = readLocale();
strncat(localeOption, locale.c_str(), PROPERTY_VALUE_MAX);
addOption(localeOption);
}
// Trace files are stored in /data/misc/trace which is writable only in debug mode.
property_get("ro.debuggable", propBuf, "0");
if (strcmp(propBuf, "1") == 0) {
property_get("dalvik.vm.method-trace", propBuf, "false");
if (strcmp(propBuf, "true") == 0) {
addOption("-Xmethod-trace");
parseRuntimeOption("dalvik.vm.method-trace-file",
methodTraceFileBuf,
"-Xmethod-trace-file:");
parseRuntimeOption("dalvik.vm.method-trace-file-siz",
methodTraceFileSizeBuf,
"-Xmethod-trace-file-size:");
property_get("dalvik.vm.method-trace-stream", propBuf, "false");
if (strcmp(propBuf, "true") == 0) {
addOption("-Xmethod-trace-stream");
}
}
}
// Native bridge library. "0" means that native bridge is disabled.
property_get("ro.dalvik.vm.native.bridge", propBuf, "");
if (propBuf[0] == '\0') {
ALOGW("ro.dalvik.vm.native.bridge is not expected to be empty");
} else if (strcmp(propBuf, "0") != 0) {
snprintf(nativeBridgeLibrary, sizeof("-XX:NativeBridge=") + PROPERTY_VALUE_MAX,
"-XX:NativeBridge=%s", propBuf);
addOption(nativeBridgeLibrary);
}
#if defined(__LP64__)
const char* cpu_abilist_property_name = "ro.product.cpu.abilist64";
#else
const char* cpu_abilist_property_name = "ro.product.cpu.abilist32";
#endif // defined(__LP64__)
property_get(cpu_abilist_property_name, propBuf, "");
if (propBuf[0] == '\0') {
ALOGE("%s is not expected to be empty", cpu_abilist_property_name);
return -1;
}
snprintf(cpuAbiListBuf, sizeof(cpuAbiListBuf), "--cpu-abilist=%s", propBuf);
addOption(cpuAbiListBuf);
// Dalvik-cache pruning counter.
parseRuntimeOption("dalvik.vm.zygote.max-boot-retry", cachePruneBuf,
"-Xzygote-max-boot-retry=");
/*
* When running with debug.generate-debug-info, add --generate-debug-info to
* the compiler options so that the boot image, if it is compiled on device,
* will include native debugging information.
*/
property_get("debug.generate-debug-info", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
addOption("-Xcompiler-option");
addOption("--generate-debug-info");
addOption("-Ximage-compiler-option");
addOption("--generate-debug-info");
}
/*
* Retrieve the build fingerprint and provide it to the runtime. That way, ANR dumps will
* contain the fingerprint and can be parsed.
*/
parseRuntimeOption("ro.build.fingerprint", fingerprintBuf, "-Xfingerprint:");
initArgs.version = JNI_VERSION_1_4;
initArgs.options = mOptions.editArray();
initArgs.nOptions = mOptions.size();
initArgs.ignoreUnrecognized = JNI_FALSE;
/*
* Initialize the VM.
*
* The JavaVM* is essentially per-process, and the JNIEnv* is per-thread.
* If this call succeeds, the VM is ready, and we can start issuing
* JNI calls.
*/
if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
ALOGE("JNI_CreateJavaVM failed\n");
return -1;
}
return 0;
}
可以看到，该方法的代码非常多。前面的一大堆char数组都是用来指定虚拟机的启动属性的，这里我们也可以看到checkJni的默认值为false，如果我们指定了dalvik.vm.checkjni配置项的话，那么它就会为true，在执行JNI调用时就会进行合法性检查。dalvik.vm.heapsize用来指定虚拟机启动时的堆内存大小，可以看到8.0默认的虚拟机的堆内存配置为16M，再往下还有上节我们讲的ro.product.cpu.abilist64、ro.product.cpu.abilist32配置项，一大堆参数封装完成后，最后就直接调用JNI_CreateJavaVM方法来创建虚拟机。第一个方法参数pJavaVM是一个类型为JavaVM的双重指针，它的定义在libnativehelper\include\nativehelper\jni.h文件中，源码如下：
typedef _JavaVM JavaVM;startVm方法执行完，AndroidRuntime类的成员变量mJavaVM就会被赋值，接下来执行onVmCreated方法，该方法在AndroidRuntime类中的实现为空，它是由AppRuntime类重写的，来执行一些扩展的工作。接下来继续执行startReg方法，进行native层的方法注册，它的源码如下：
struct _JavaVM {
const struct JNIInvokeInterface* functions;
#if defined(__cplusplus)
jint DestroyJavaVM()
{ return functions->DestroyJavaVM(this); }
jint AttachCurrentThread(JNIEnv** p_env, void* thr_args)
{ return functions->AttachCurrentThread(this, p_env, thr_args); }
jint DetachCurrentThread()
{ return functions->DetachCurrentThread(this); }
jint GetEnv(void** env, jint version)
{ return functions->GetEnv(this, env, version); }
jint AttachCurrentThreadAsDaemon(JNIEnv** p_env, void* thr_args)
{ return functions->AttachCurrentThreadAsDaemon(this, p_env, thr_args); }
#endif /*__cplusplus*/
};
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)该方法中最重要的就是register_jni_procs逻辑了，该方法会将AndroidRuntime.cpp文件中定义的RegJNIRec gRegJNI[]数组中的所有方法注册进来，这样我们才可以从native层回调到Java层， gRegJNI数组中声明的都是方法指针，在Java层的实现方法的修饰符都会被定义为private，不允许其他地方调用，这样也是一种非常好的编码习惯。接下来最后的一段逻辑就是找到我们传入的class的Java类，然后调用该类的main方法，也就是frameworks\base\core\java\com\android\internal\os\ZygoteInit.java类的main方法了，它的main方法中会进入无限循环，直到虚拟机退出。ZygoteInit.java类的main方法的源码如下：
{
ATRACE_NAME("RegisterAndroidNatives");
/*
* This hook causes all future threads created in this process to be
* attached to the JavaVM. (This needs to go away in favor of JNI
* Attach calls.)
*/
androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);
ALOGV("--- registering native functions ---\n");
/*
* Every "register" function calls one or more things that return
* a local reference (e.g. FindClass). Because we haven't really
* started the VM yet, they're all getting stored in the base frame
* and never released. Use Push/Pop to manage the storage.
*/
env->PushLocalFrame(200);
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
env->PopLocalFrame(NULL);
//createJavaThread("fubar", quickTest, (void*) "hello");
return 0;
}
public static void main(String argv[]) {
ZygoteServer zygoteServer = new ZygoteServer();
// Mark zygote start. This ensures that thread creation will throw
// an error.
ZygoteHooks.startZygoteNoThreadCreation();
// Zygote goes into its own process group.
try {
Os.setpgid(0, 0);
} catch (ErrnoException ex) {
throw new RuntimeException("Failed to setpgid(0,0)", ex);
}
try {
// Report Zygote start time to tron unless it is a runtime restart
if (!"1".equals(SystemProperties.get("sys.boot_completed"))) {
MetricsLogger.histogram(null, "boot_zygote_init",
(int) SystemClock.elapsedRealtime());
}
String bootTimeTag = Process.is64Bit() ? "Zygote64Timing" : "Zygote32Timing";
BootTimingsTraceLog bootTimingsTraceLog = new BootTimingsTraceLog(bootTimeTag,
Trace.TRACE_TAG_DALVIK);
bootTimingsTraceLog.traceBegin("ZygoteInit");
RuntimeInit.enableDdms();
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
boolean startSystemServer = false;
String socketName = "zygote";
String abiList = null;
boolean enableLazyPreload = false;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true;
} else if ("--enable-lazy-preload".equals(argv[i])) {
enableLazyPreload = true;
} else if (argv[i].startsWith(ABI_LIST_ARG)) {
abiList = argv[i].substring(ABI_LIST_ARG.length());
} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
socketName = argv[i].substring(SOCKET_NAME_ARG.length());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
zygoteServer.registerServerSocket(socketName);
// In some configurations, we avoid preloading resources and classes eagerly.
// In such cases, we will preload things prior to our first fork.
if (!enableLazyPreload) {
bootTimingsTraceLog.traceBegin("ZygotePreload");
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload(bootTimingsTraceLog);
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
bootTimingsTraceLog.traceEnd(); // ZygotePreload
} else {
Zygote.resetNicePriority();
}
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
bootTimingsTraceLog.traceBegin("PostZygoteInitGC");
gcAndFinalize();
bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC
bootTimingsTraceLog.traceEnd(); // ZygoteInit
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
// Zygote process unmounts root storage spaces.
Zygote.nativeUnmountStorageOnInit();
// Set seccomp policy
Seccomp.setPolicy();
ZygoteHooks.stopZygoteNoThreadCreation();
if (startSystemServer) {
startSystemServer(abiList, socketName, zygoteServer);
}
Log.i(TAG, "Accepting command socket connections");
zygoteServer.runSelectLoop(abiList);
zygoteServer.closeServerSocket();
} catch (Zygote.MethodAndArgsCaller caller) {
caller.run();
} catch (Throwable ex) {
Log.e(TAG, "System zygote died with exception", ex);
zygoteServer.closeServerSocket();
throw ex;
}
}
首先构造一个ZygoteServer对象，名字取的非常形象，就像是服务器一样等待客户端来请求。接下来对我们传入的argv参数进行解析，几个if/else分支的判断就可以很明显的看出来意图。然后调用zygoteServer.registerServerSocket(socketName)进行socket注册，参数就是我们在init.rc中写入的socket名称。当然在ZygoteServer中注册时，还会加上private static final String ANDROID_SOCKET_PREFIX = "ANDROID_SOCKET_"前缀，和zygoteName拼接起来。大家可以自己去看一下该方法的实现。注册完成后继续调用zygoteServer.runSelectLoop(abiList)进入无限循环，如果无限循环退出了，那说明虚拟机也要退出了，退出前执行zygoteServer.closeServerSocket()来将socket关闭。我们继续来看一下
runSelectLoop方法，该方法实现在frameworks\base\core\java\com\android\internal\os\ZygoteServer.java文件中，源码如下：
void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
fds.add(mServerSocket.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(this);
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}
该方法中首先调用mServerSocket.getFileDescriptor()将刚才注册的socket的文件描述符添加到局部变量fds中，然后在while循环中新建局部变量pollFds，用来监控该文件描述符上的事件，Os.poll(pollFds, -1)就是调用Linux的epoll机制，传入的第二个参数为-1表示该文件描述符上如果没有事件则一直睡眠，如果有事件的话，该函数就会立刻返回去处理事件。我们启动新的进程时，也就是在else分支中通过peers.get(i).runOnce(this)将当前的连接事件取出来执行，新进程创建成员后肯定需要调用peers.remove(i)将对应的ZygoteConnection连接移除，以免重复创建。
那么到这里，我们的Zygote进程就启动起来了，后续就会在这个while无限循环中等待AMS来请求创建新的进程了。