本文基于 AOSP android-10.0.0_r41 版别讲解
前面咱们说到,init 进程会解析 init.rc 文件,履行对应的指令,其中一个重要的任务便是发动 Zygote 进程。接下来咱们就来剖析 Zygote 进程的发动和履行进程。
上一节最终咱们剖析到调用 runtime.start()
发动 ZygoteInit
Java 类:
// AppRuntime 的初始化
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// 发动 ZygoteInit 类
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
这儿初始化一个 AppRuntime 目标,然后经过这个目标的 start 办法发动一个 ZygoteInit
Java 类。
AppRuntime 直译的话叫做使用运转时
,开发时,咱们写好 Java/Kotlin 代码,经过对应的编译器将代码编译为字节码,AppRuntime 的作用便是创建一个能够履行字节码的环境,这个环境主要由两部分内容构成:
- 一部分担任目标的创建与收回,比如类的加载器,垃圾收回器等
- 一部分担任程序逻辑的运转,比如即时编译体系,解说器等
咱们先看下 AppRuntime 类的完成:
class AppRuntime : public AndroidRuntime
{
public:
AppRuntime(char* argBlockStart, const size_t argBlockLength)
: AndroidRuntime(argBlockStart, argBlockLength)
, mClass(NULL)
{
}
//......
// 省掉一些代码
String8 mClassName;
Vector<String8> mArgs;
jclass mClass;
};
先看 AppRuntime 的结构函数,调用父类 AndroidRuntime 结构函数:
// AndroidRuntime 类在一个进程中只有一个实例目标,保存在全局变量 gCurRuntime 中。
static AndroidRuntime* gCurRuntime = NULL;
AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
mExitWithoutCleanup(false),
mArgBlockStart(argBlockStart),
mArgBlockLength(argBlockLength)
{
// 初始化 skia 图形体系
SkGraphics::Init();
// Pre-allocate enough space to hold a fair number of options.
mOptions.setCapacity(20);
// 只能被初始化一次
assert(gCurRuntime == NULL); // one per process
// 全局 static 变量
gCurRuntime = this;
}
结构函数中主要初始化 skia 图形体系,然后把 AndroidRuntime 保存到全局变量 gCurRuntime 中。
初始化完成后,就会调用 start 函数发动虚拟机:
// 发动 ZygoteInit 类
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
这个函数比较长,咱们逐步剖析:
//详细完成
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
// 打印发动 log
ALOGD(">>>>>> START %s uid %d <<<<<<n",
className != NULL ? className : "(unknown)", getuid());
当这个 Log 打印时,标志着 Java 的运转时开始发动,假如 Android 的体系 log 中反复呈现这段内容,而输出 ID都是Zygote,则说明体系或许呈现问题,Zygote进程在不断地重启.
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* 'startSystemServer == true' means runtime is obsolete and not run from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)));
}
}
这儿处理参数的反常,关于前期版别的 Runtime,才需要 startSystemServer == true
这样的参数,假如这儿呈现了这样的参数,打印一些 Log 告诉开发人员。
const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /system does not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}
这儿从环境变量 ANDROID_ROOT
中读取到根目录,假如没有读到,则默认设置为 /system
,假如 /system
目录不存在,直接退出。
const char* runtimeRootDir = getenv("ANDROID_RUNTIME_ROOT");
if (runtimeRootDir == NULL) {
LOG_FATAL("No runtime directory specified with ANDROID_RUNTIME_ROOT environment variable.");
return;
}
const char* tzdataRootDir = getenv("ANDROID_TZDATA_ROOT");
if (tzdataRootDir == NULL) {
LOG_FATAL("No tz data directory specified with ANDROID_TZDATA_ROOT environment variable.");
return;
}
这儿接着读取环境遍历。
//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;
}
这儿调用 startVm
发动虚拟机。
onVmCreated(env);
onVmCreated 是一个虚函数,实际上调用的是子类 AppRuntime 中的重载函数:
virtual void onVmCreated(JNIEnv* env)
{
if (mClassName.isEmpty()) {
return; // Zygote. Nothing to do here.
}
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));
}
假如是 Zygote 进程,变量 mClassName 的值会为 null,会立刻回来。假如是一个一般 Java 类的调用, mClassName 中会存放类的称号,toSlashClassName(mClassName.string())
的作用是将类名转换为类的全限定名,接着回去到类对应的 jclass 目标。
接着看 start 函数的后续部分:
/*
* Register android functions.
*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android nativesn");
return;
}
startReg 的详细完成如下:
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
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;
}
// 进一步调用 register_jni_procs
static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env)
{
for (size_t i = 0; i < count; i++) {
if (array[i].mProc(env) < 0) {
#ifndef NDEBUG
ALOGD("----------!!! %s failed to loadn", array[i].mName);
#endif
return -1;
}
}
return 0;
}
static const RegJNIRec gRegJNI[] = {
REG_JNI(register_com_android_internal_os_RuntimeInit),
REG_JNI(register_com_android_internal_os_ZygoteInit_nativeZygoteInit),
REG_JNI(register_android_os_SystemClock),
REG_JNI(register_android_util_EventLog),
REG_JNI(register_android_util_Log),
REG_JNI(register_android_util_MemoryIntArray),
REG_JNI(register_android_util_PathParser),
REG_JNI(register_android_util_StatsLog),
REG_JNI(register_android_util_StatsLogInternal),
REG_JNI(register_android_app_admin_SecurityLog),
REG_JNI(register_android_content_AssetManager),
REG_JNI(register_android_content_StringBlock),
REG_JNI(register_android_content_XmlBlock),
REG_JNI(register_android_content_res_ApkAssets),
REG_JNI(register_android_text_AndroidCharacter),
REG_JNI(register_android_text_Hyphenator),
REG_JNI(register_android_view_InputDevice),
REG_JNI(register_android_view_KeyCharacterMap),
REG_JNI(register_android_os_Process),
REG_JNI(register_android_os_SystemProperties),
REG_JNI(register_android_os_Binder),
REG_JNI(register_android_os_Parcel),
REG_JNI(register_android_os_HidlSupport),
REG_JNI(register_android_os_HwBinder),
REG_JNI(register_android_os_HwBlob),
REG_JNI(register_android_os_HwParcel),
REG_JNI(register_android_os_HwRemoteBinder),
REG_JNI(register_android_os_NativeHandle),
// ...... 省掉大部分
};
startReg 经过调用 register_jni_procs 函数将全局数组 gRegJNI 中的 JNI 本地函数注册到虚拟机中。咱们在 Java 层中调用的许多 Native 办法便是在这儿注册的。
接着看 start 函数的后续部分:
/*
* 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);
}
这儿主要是预备履行 Java 类的参数。这些参数保存在 strArray 中。
/*
* Start VM. This thread becomes the main thread of the VM, and will
* not return until the VM exits.
*/
char* slashClassName = toSlashClassName(className != NULL ? 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 VMn");
if (mJavaVM->DetachCurrentThread() != JNI_OK)
ALOGW("Warning: unable to detach main threadn");
if (mJavaVM->DestroyJavaVM() != 0)
ALOGW("Warning: VM did not shut down cleanlyn");
}
先找到 ZygoteInit 类对应的 jclass, 然后经过 GetStaticMethodID 获得ZygoteInit 类中 main 办法的 ID,接着调用 CallStaticVoidMethod
函数 main 函数。至此,整个 start 函数就剖析完了,Zygote进程的初始化进程将转到Java层了。