Zygote进程是Android世界中的第一个进程,其父进程为init进程。Zygote是系统启动时吗,通过解析zygote.rc脚本来启动的,Zygote启动时会创建一个虚拟机,并注册了系统的JNI方法,考虑如下问题:
- Zygote是由app_main.cpp负责启动的,很明显,这是在native层的逻辑,在Zygote的真正实现是在ZygoteInit这个类中,这个类是Java代码实现的,这就意味着ZygoteInit无法通过fork和exec的方式启动,ZygoteInit该如何启动呢?
- Android中每一个进程都运行在一个虚拟机中,是不是说每次启动一个应用时都要创建一个新的虚拟机?
- 每个应用在运行时都需要加载Java类和Android的核心类和资源,而这些过程又是非常耗时的,Android是如何解决这个问题的?
在分析完Zyogte的启动过程后,再回答上面的三个问题。
一. 解析启动参数
Zygote启动参数如下:
service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
class main
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
writepid /dev/cpuset/foreground/tasks
Zygote启动执行的文件为app_main.cpp,入口为main方法,在main中,主要分为两步:
解析启动参数
根据上文中的启动参数可知,可确定:启动的进程为Zygote,进程名为“zygote”,启动完毕后要启动SystemServer进程构造AppRuntime对象,根据解析出的启动参数,执行AppRuntime的start方法。AppRuntime继承AndroidRuntime,重写了启动生命过程中的几个函数,start方法还是在AndroidRuntime中定义的
int main(int argc, char* const argv[]) { if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) { // Older kernels don't understand PR_SET_NO_NEW_PRIVS and return // EINVAL. Don't die on such kernels. if (errno != EINVAL) { LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno)); return 12; } } AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv)); // Process command line arguments // ignore argv[0] argc--; argv++; int i; for (i = 0; i < argc; i++) { if (argv[i][0] != '-') { break; } if (argv[i][1] == '-' && argv[i][2] == 0) { ++i; // Skip --. break; } runtime.addOption(strdup(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()) { args.add(application ? String8("application") : String8("tool")); runtime.setClassNameAndArgs(className, argc - i, argv + i); } 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()); set_process_name(niceName.string()); } 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."); return 10; } }
AndroidRuntime的start方法如下,在start中,主要做了四件事:
1. 启动虚拟机
2. 注册JNI
3. 载入ZygoteInit的main方法
4. Zygote退出时,清理资源
```java
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);
/* 启动虚拟机 */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env, zygote) != 0) {
return;
}
onVmCreated(env);
/*
*注册Android中的JNI方法
*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
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);
}
/*
* 载入ZygoteInit的main方法
*/
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);
//zygote退出,清理资源
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");
}
二. 启动虚拟机
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote){
...
if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
ALOGE("JNI_CreateJavaVM failed\n");
return -1;
}
return 0;
}
三. 注册JNI
int AndroidRuntime::startReg(JNIEnv* env)
{
/* * 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;
}
四. 启动ZygoteInit
在app_main.cpp的main方法中,启动AndroidRuntime的参数如下:
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
可知在Zygote所在的虚拟机创建完毕后,将通过JNIEnv载入com.android.internal.os.ZygoteInit这个类:
- 根据传入的类名构造ZygoteInit的jclass对象
- 通过JNIEnv的GetStaticMethodID的方法,获取ZygoteInit的main方法
- 通过JNI的CallStaticVoidMethod的的方法,执行ZygoteInit的main方法 “`java 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
}
}
#### 五. 退出清理
在系统运行过程中,是不应该走到这一步的。Zygote启动后,会通过while(true)的方式循环监听系统创建进程的请求。
Zygote退出时,要执行两个清理函数:
* DetachCurrentThread:将当前进程中主线程的JNIEnv取消与虚拟机的关联
* DestroyJavaVM:销毁虚拟机
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”);
**`现在来回答本文开头的三个问题:`**
`1. Zygote创建过程中会创建虚拟机,并创建虚拟机中主线程的JNIEnv对象,ZygoteInit是通过JNIEnv中的方法载入的`
`2. 每次启动应用时会创建新的虚拟机,但是是通过copy on wirte的技术实现的。每次创建应用时,zygote会通过fork自身进程,来 创建出一个新的进程,fork只是创建了一个zygote进程的副本,并没有完全从头的创建一个新的进程`
`3. zygote启动过程中提前加载的核心类和Android资源,存放在虚拟机中的zygote heap中,子进程会直接复用zygote进程的这个zygote heap,这样会大大加快应用的启动速度`