概述
在Android系统中，所有的应用程序进程，以及用来运行系统关键服务的System进程都是由zygote进程负责创建的。因此，我们将它称为进程孵化器。zygote进程是通过复制自身的方式来创建System进程和应用程序进程的。由于zygote进程在启动时会在内部创建一个虚拟机实例，因此，通过复制zygote进程而得到的System进程和应用程序进程可以快速地在内部获得一个虚拟机实例拷贝。
zygote进程在启动完成之后，会马上将System进程启动起来，以便它可以将系统的关键服务启动起来。下面我们将介绍zygote进程的启动脚本，然后分析它和System进程的启动过程。
zygote分析
zygote进程的启动脚本如下：
service zygote /system/bin/app_process -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
在我之前的一篇博客中已经分析了init进程是如何启动service服务了，需要了解的同学可以参考这篇文章：Android init进程——解析配置文件
通过zygote服务的启动脚本，我们可以知道，zygote进程的实际是二进制文件app_process的调用，我们就从这个应用程序的main函数入手去分析一下zygote进程的启动过程，源码如下（/frameworks/base/cmds/app_process/app_main.cpp）：
/**
* 将-Xzygote加入到JavaVMOption中，返回/system/bin参数指向的下标
*/
int AndroidRuntime::addVmArguments(int argc, const char* const argv[])
{
int i;
for (i = 0; i < argc; i ++) {
if (argv[i][0] != '-') {
return i;
}
if (argv[i][1] == '-' && argv[i][2] == 0) {
return i + 1;
}
JavaVMOption opt;
memset(&opt, 0, sizeof(opt));
opt.optionString = (char*)argv[i];
mOptions.add(opt);
}
return i;
}
int main(int argc, char* const argv[])
{
// zygote call parameters
// /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
// These are global variables in ProcessState.cpp
mArgC = argc;
mArgV = argv;
mArgLen = 0;
for (int i = 0; i < argc; i ++) {
mArgLen += strlen(argv[i]) + 1;
}
// 去除末尾的空格
mArgLen--;
AppRuntime runtime;
const char* argv0 = argv[0];
// Process command line arguments
// ignore argv[0]
argc --;
argv ++;
// Everything up tp '--' or first non '-' arg goes to the vm
int i = runtime.addVmArguments(argc, argv);
// Parse runtime arguments. Stop at first unrecognized option.
bool zygote = false;
bool startSystemServer = false;
bool application = false;
const char* parentDir = NULL;
const char* niceName = NULL;
const char* className = NULL;
while (i < argc) {
const char* arg = argv[i ++];
if (!parentDir) {
parentDir = arg;
} else if (strcmp(arg, "--zygote") == 0) {
zygote = true;
niceName = "zygote";
} 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)) {
niceName = arg + 12;
} else {
className = arg;
break;
}
}
if (niceName && *niceName) {
setArgv0(argv0, niceName);
set_process_name(niceName);
}
runtime.mParentDir = parentDir;
if (zygote) {
// 进入到AppRuntime的start函数
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer? "start-system-server" : "");
} else if (className) {
runtime.mClassName = className;
runtime.mArgc = argc - i;
runtime.mArgv = argv + i;
runtime.start("com.android.internal.os.RuntimeInit", application ? "application" : "tool");
} 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;
}
}
在zygote的main函数中，通过AppRuntime runtime代码创建了一个AppRuntime对象runtime，接下来Zygote进程就是通过它来进一步启动的。
init.rc中关于启动zygote命令中包含了–zygote参数，所以在if(strcmp(arg, “–zygote”) == 0)判断的时候，会将niceName赋值为”zygote”,然后通过set_process_name(niceName)函数将当前进程的名称设置为zygote。这也是为什么调用的脚本为/system/bin/app_process，而进程名为zygote的原因。set_process_name函数的源码如下(/system/core/libcutils/process_name.c)：
static const char* process_name = "unknown";
void set_process_name(const char* new_name)
{
if (new_name == NULL) {
return;
}
int len = strlen(new_name);
char* copy = (char*)malloc(len + 1);
strcpy(copy, new_name);
process_name = (const char*) copy;
}
从init.rc文件中关于zygote进程的配置参数可知，Zygote进程传递给应用程序app_process的启动参数arg还包含一个”–start-system-server”选项。因此，在调用AppRuntime对象runtime的成员函数start时，第二个参数为”start-system-server”，表示zygote进程启动完成之后，需要将system进程启动起来。
AppRuntime分析
AppRuntime类的成员函数start是从父类AndroidRuntime继承下来的，因此，接下来我们就继续分析AndroidRuntime类的成员函数start的实现，函数源码位置：/frameworks/base/core/jni/AndroidRuntime.cpp：
char* AndroidRuntime::toSlashClassName(const char* className)
{
char* result = strdup(className);
for (char* cp = result; *cp != '\0'; cp ++) {
if (*cp == '.') {
*cp = '/';
}
}
return result;
}
/**
* Start the Android runtime. This involves starting the virtual machine
* and calling the "static void main(String[] args)" method int the class
* named by "className".
*
* 这两个参数的值分别为:
* const char* className = "com.android.internal.os.ZygoteInit";
* const char* options = "start-system-server";
*/
void AndroidRuntime::start(const char* className, const char* options)
{
ALOGD("\n>>>>> AndroidRuntime START %s <<<<<<\n",
className != NULL ? className : "(unknown)");
/**
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
if (strcmp(options, "start-system-server") == 0) {
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}
// 设置ANDROID_ROOT环境变量
const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /android dose not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
// 1. 创建虚拟机
if (startVm(&mJavaVM, &env) != 0) {
return;
}
onVmCreated(env);
// 2. 注册JNI函数
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
jclass stringClass;
jobjectArray strArray;
jstring classNameStr;
jstring optionsStr;
stringClass = env->FindClass("java/lang/String");
assert(stringClass != NULL);
// 创建一个有两个元素的String数组，用Java代码表示为:String[] strArray = new String[2];
strArray = env->NewObjectArray(2, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);
assert(classNameStr != NULL);
// 设置第一个元素为"com.android.internal.os.ZygoteInit"
env->SetObjectArrayElement(strArray, 0, classNameStr);
optionsStr = env->NewStringUTF(options);
// 设置第二个元素为"start-system-server"
env->SetObjectArrayElement(strArray, 1, optionsStr);
// 将字符串"com.android.internal.os.ZygoteInit"转换为"com/android/internal/os/ZygoteInit"
char* slashClassName = toSlashClassName(className);
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V");
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s\n'", className);
} else {
// 3.
// 通过JNI调用java函数，注意调用的是main函数，所属的类是"com.android.internal.os.ZygoteInit".
// 传递的参数是"com.android.internal.os.ZygoteInit true"
env->CallStaticVoidMethod(startClass, startMeth, strArray);
}
}
free(slashClassName);
ALOGD("Shutting down VM\n");
if (mJavaVM->DetachCurrentThread() != JNI_OK) {
ALOGW("Warning: unable to detach main thread\n");
}
if (mJavaVM->DestoryJavaVM() != 0) {
ALOGW("Warning: VM did not shut down cleanly\n");
}
}
上述代码有几处关键点，分别是：
接下来，我们分别分析这三个关键点。
创建虚拟机——startVm
startVm并没有特别之处，就是调用JNI的虚拟机创建函数，但是创建虚拟机时的一些参数却是在startVm中确定的，其源码如下：
#define PROPERTY_VALUE_MAX 92
/**
* Start the Dalvik Virtual Machine.
*
* Various arguments, most determined by system properties, are passed in.
* The "mOptions" vector is updated.
*
* Returns 0 on success.
*/
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIENV** pEnv)
{
int result = -1;
JavaVMInitArgs initArgs;
JavaVMOption opt;
char propBuf[PROPERTY_VALUE_MAX];
char stackTraceFileBuf[PROPERTY_VALUE_MAX];
char dexoptFlagsBuf[PROPERTY_VALUE_MAX];
char enableAssertBuf[sizeof("-ea:")-1 + PROPERTY_VALUE_MAX];
char jniOptsBuf[sizeof("-Xjniopts:")-1 + PROPERTY_VALUE_MAX];
char heapstartsizeOptsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];
char heapsizeOptsBuf[sizeof("-Xms")-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 heaptargetutilizationOptsBuf[sizeof("-XX:HeapTargetUtilization=")-1 + PROPERTY_VALUE_MAX];
char jitcodecachesizeOptsBuf[sizeof("-Xjitcodecachesize:")-1 + PROPERTY_VALUE_MAX];
char extraOptsBuf[PROPERTY_VALUE_MAX];
char* stackTraceFile = NULL;
bool checkJni = false;
bool checkDexSum = false;
bool logStdio = false;
enum {
KEMDefault,
KEMIntPortable,
KEMIntFast,
KEMJitCompiler,
} executionMode = KEMDefault;
/**
* 这段代码是用了设置JNI_check选项的。JNI_check指的是Native层调用JNI函数时，系统所做的一些检查动作。
* 这个选项虽然能增加可靠性，但是还有一些副作用：
* 1. 因为检查工作比较耗时，所以会影响系统运行速度。
* 2. 有些检查工作比较耗时，一旦出错，整个进程会abort。
* 所以，JNI_check选项一般只在eng版本设置。
*/
property_get("dalvik.vm.checkjni", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
checkJni = true;
} else if (strcmp(propBuf, "false") != 0) {
property_get("ro.kernel.android.checkjni", propBuf, "");
if (propBuf[0] == '1') {
checkJni = true;
}
}
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;
}
// ... 省略大部分参数设置
/**
* 设置虚拟机的heapsize，默认为16m。绝大多数厂商都会在build.prop文件里修改这个属性，一般是256m。
* heapsize不能设置得过小，否则在操作大尺寸的图片时无法分配所需的内存。
*/
strcpy(heapsizeOptsBuf, "-Xmx");
property_get("dalvik.vm.heapsize", heapsizeOptsBuf+4, "16m");
opt.optionString = heapsizeOptsBuf;
mOptions.add(opt);
// ......
if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
ALOGE("JNI_CreateJavaVM failed\n");
goto bail;
}
result = 0;
bail:
free(stackTraceFile);
return result;
}
更多虚拟机参数的设置，我这里就不做特殊说明了，大家感兴趣可以自行google。(ps:因为我不太懂虚拟机这一块…)
注册JNI函数——startReg
上面讲了如何创建虚拟机，接下来需要给这个虚拟机注册一些JNI函数。正是因为后续的Java世界用到的一些函数是采用native方式实现的，所以才必须提前注册这些函数。
接下来，我们来看一下startReg函数的源码实现：
int AndroidRuntime::startReg(JNIEnv* env)
{
// 设置Thread类的线程创建函数为javaCreateThreadEtc
androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);
ALOGV("--- registering native functions ---\n");
env->PushLocalFrame(200);
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
env->PopLocalFrame(NULL);
return 0;
}
关键是需要注册JNI函数，具体实现是由register_jni_procs函数实现的，我们来看一下这个函数的具体实现(/frameworks/base/core/jni/AndroidRuntime.cpp)：
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 load\n", array[i].mName);
#endif
return -1;
}
}
return 0;
}
通过源码，我们可以看到，register_jni_procs只是对array数组的mProc函数的封装，而array数组指向的是gRegJNI数组，我们来看一下这个数组的实现：
static const RegJNIRec gRegJNI[] = {
REG_JNI(register_android_debug_JNITest),
REG_JNI(register_com_android_internal_os_RuntimeInit),
REG_JNI(register_android_os_SystemClock),
REG_JNI(register_android_util_EventLog),
REG_JNI(register_android_util_Log),
REG_JNI(register_android_util_FloatMath),
REG_JNI(register_android_text_format_Time),
REG_JNI(register_android_content_AssetManager),
REG_JNI(register_android_content_StringBlock),
REG_JNI(register_android_content_XmlBlock),
REG_JNI(register_android_emoji_EmojiFactory),
REG_JNI(register_android_text_AndroidCharacter),
REG_JNI(register_android_text_AndroidBidi),
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_view_DisplayEventReceiver),
REG_JNI(register_android_nio_utils),
REG_JNI(register_android_graphics_Graphics),
REG_JNI(register_android_view_GraphicBuffer),
REG_JNI(register_android_view_GLES20DisplayList),
REG_JNI(register_android_view_GLES20Canvas),
REG_JNI(register_android_view_HardwareRenderer),
REG_JNI(register_android_view_Surface),
REG_JNI(register_android_view_SurfaceControl),
REG_JNI(register_android_view_SurfaceSession),
REG_JNI(register_android_view_TextureView),
REG_JNI(register_com_google_android_gles_jni_EGLImpl),
REG_JNI(register_com_google_android_gles_jni_GLImpl),
REG_JNI(register_android_opengl_jni_EGL14),
REG_JNI(register_android_opengl_jni_EGLExt),
REG_JNI(register_android_opengl_jni_GLES10),
REG_JNI(register_android_opengl_jni_GLES10Ext),
REG_JNI(register_android_opengl_jni_GLES11),
REG_JNI(register_android_opengl_jni_GLES11Ext),
REG_JNI(register_android_opengl_jni_GLES20),
REG_JNI(register_android_opengl_jni_GLES30),
REG_JNI(register_android_graphics_Bitmap),
REG_JNI(register_android_graphics_BitmapFactory),
REG_JNI(register_android_graphics_BitmapRegionDecoder),
REG_JNI(register_android_graphics_Camera),
REG_JNI(register_android_graphics_CreateJavaOutputStreamAdaptor),
REG_JNI(register_android_graphics_Canvas),
REG_JNI(register_android_graphics_ColorFilter),
REG_JNI(register_android_graphics_DrawFilter),
REG_JNI(register_android_graphics_Interpolator),
REG_JNI(register_android_graphics_LayerRasterizer),
REG_JNI(register_android_graphics_MaskFilter),
REG_JNI(register_android_graphics_Matrix),
REG_JNI(register_android_graphics_Movie),
REG_JNI(register_android_graphics_NinePatch),
REG_JNI(register_android_graphics_Paint),
REG_JNI(register_android_graphics_Path),
REG_JNI(register_android_graphics_PathMeasure),
REG_JNI(register_android_graphics_PathEffect),
REG_JNI(register_android_graphics_Picture),
REG_JNI(register_android_graphics_PorterDuff),
REG_JNI(register_android_graphics_Rasterizer),
REG_JNI(register_android_graphics_Region),
REG_JNI(register_android_graphics_Shader),
REG_JNI(register_android_graphics_SurfaceTexture),
REG_JNI(register_android_graphics_Typeface),
REG_JNI(register_android_graphics_Xfermode),
REG_JNI(register_android_graphics_YuvImage),
REG_JNI(register_android_graphics_pdf_PdfDocument),
REG_JNI(register_android_database_CursorWindow),
REG_JNI(register_android_database_SQLiteConnection),
REG_JNI(register_android_database_SQLiteGlobal),
REG_JNI(register_android_database_SQLiteDebug),
REG_JNI(register_android_os_Debug),
REG_JNI(register_android_os_FileObserver),
REG_JNI(register_android_os_MessageQueue),
REG_JNI(register_android_os_SELinux),
REG_JNI(register_android_os_Trace),
REG_JNI(register_android_os_UEventObserver),
REG_JNI(register_android_net_LocalSocketImpl),
REG_JNI(register_android_net_NetworkUtils),
REG_JNI(register_android_net_TrafficStats),
REG_JNI(register_android_net_wifi_WifiNative),
REG_JNI(register_android_os_MemoryFile),
REG_JNI(register_com_android_internal_os_ZygoteInit),
REG_JNI(register_android_hardware_Camera),
REG_JNI(register_android_hardware_camera2_CameraMetadata),
REG_JNI(register_android_hardware_SensorManager),
REG_JNI(register_android_hardware_SerialPort),
REG_JNI(register_android_hardware_UsbDevice),
REG_JNI(register_android_hardware_UsbDeviceConnection),
REG_JNI(register_android_hardware_UsbRequest),
REG_JNI(register_android_media_AudioRecord),
REG_JNI(register_android_media_AudioSystem),
REG_JNI(register_android_media_AudioTrack),
REG_JNI(register_android_media_JetPlayer),
REG_JNI(register_android_media_RemoteDisplay),
REG_JNI(register_android_media_ToneGenerator),
REG_JNI(register_android_opengl_classes),
REG_JNI(register_android_server_NetworkManagementSocketTagger),
REG_JNI(register_android_server_Watchdog),
REG_JNI(register_android_ddm_DdmHandleNativeHeap),
REG_JNI(register_android_backup_BackupDataInput),
REG_JNI(register_android_backup_BackupDataOutput),
REG_JNI(register_android_backup_FileBackupHelperBase),
REG_JNI(register_android_backup_BackupHelperDispatcher),
REG_JNI(register_android_app_backup_FullBackup),
REG_JNI(register_android_app_ActivityThread),
REG_JNI(register_android_app_NativeActivity),
REG_JNI(register_android_view_InputChannel),
REG_JNI(register_android_view_InputEventReceiver),
REG_JNI(register_android_view_InputEventSender),
REG_JNI(register_android_view_InputQueue),
REG_JNI(register_android_view_KeyEvent),
REG_JNI(register_android_view_MotionEvent),
REG_JNI(register_android_view_PointerIcon),
REG_JNI(register_android_view_VelocityTracker),
REG_JNI(register_android_content_res_ObbScanner),
REG_JNI(register_android_content_res_Configuration),
REG_JNI(register_android_animation_PropertyValuesHolder),
REG_JNI(register_com_android_internal_content_NativeLibraryHelper),
REG_JNI(register_com_android_internal_net_NetworkStatsFactory),
};
#ifdef NDEBUG
#define REG_JNI(name) {name}
struct RegJNIRec {
int (*mProc)(JNIEnv*);
};
#else
#define REG_JNI(name) {name, #name}
struct RegJNIRec {
int (*mProc)(JNIEnv*);
const char* mName;
};
#endif
可以看到，REG_JNI是一个宏，宏里面包括的就是那个参数为JNIEnv*,返回值为int的函数指针mProc，我们以register_android_debug_JNITest为例，源码位置为/frameworks/base/core/jni/android_debug_JNITest.cpp：
#define NELEM(x) (sizeof(x)/sizeof(*(x)))
int register_android_debug_JNITest(JNIEnv* env)
{
return jniRegisterNativeMethods(env, "android/debug/JNITest", gMethods, NELEM(gMethods));
}
可以看到，mProc其实就是为Java类注册JNI函数。
进入JAVA世界
可以看到CallStaticVoidMethod最终将调用com.android.internal.os.ZygoteInit的main函数，下面就来看一下这个Java世界的入口函数。源码位置：/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java，源码如下：
public static void main(String argv[])
{
try {
SamplingProfilerIntegration.start();
// 1. 注册zygote用的socket
registerZygoteSocket();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis());
// 2. 预加载类和资源
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis());
SamplingProfilerIntegration.writeZygoteSnapshot();
// 强制执行一次垃圾收集
gc();
Trace.setTracingEnabled(false);
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("start-system-server")) {
// 3. 启动system-server
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
// 4. 进入请求应答模式
runSelectLoop();
closeServerSocket();
} catch(MethodAndArgsCaller caller) {
caller.run();
} catch(RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
上述代码中有5个重要的点，我已经通过标号标记出来了，接下来我们分别分析一下这5点函数的具体实现。
建立IPC通信服务端——registerZygoteSocket
zygote及系统中其他程序的通信没有使用Binder，而是采用了基于AF_UNIX类型的socket。registerZygoteSocket函数的使命正是建立这个Socket，实现代码如下：
private static void registerZygoteSocket()
{
if (sServerSocket == null) {
int fileDesc;
try {
String env = System.getenv(ANDROID_SOCKET_ENV);
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException(ANDROID_SOCKET_ENV + " unset or invalid", ex);
}
try {
sServerSocket = new LocalServerSocket(createFileDescriptor(fileDesc));
} catch(IOException ex) {
throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);
}
}
}
public class LocalServerSocket {
private final LocalSocketImpl impl;
private final LocalSocketAddress localAddress;
private static final int LISTEN_BACKLOG = 50;
/**
* Create a LocalServerSocket from a file descriptor that's already
* been created and bound. listen() will be called immediately on it.
* Used for cases where file descriptors are passed in via environment
* variables.
*/
public LocalServerSocket(FileDescriptor fd) throws IOException {
impl = new LocalSocketImpl(fd);
impl.listen(LISTEN_BACKLOG);
localAddress = impl.getSockAddress();
}
}
registerZygoteSocket很简单，就是创建一个服务端的socket。
预加载类和资源——preload
我们先来看一下preload函数实现：
static void preload()
{
preloadClasses();
preloadResources();
preloadOpenGL();
}
preload函数里面分别调用了三个预加载函数，我们分别来分析一下这几个函数的实现。
首先是preloadClasses，函数实现如下：
private static final int UNPRIVILEGED_UID = 9999;
private static final int UNPRIVILEGED_GID = 9999;
private static final int ROOT_UID = 0;
private static final int ROOT_GID = 0;
private static void preloadClasses()
{
final VMRuntime runtime = VMRuntime.getRuntime();
InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream(PRELOADED_CLASSES);
if (is == null) {
Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");
} else {
Log.i(TAG, "Preloading classes...");
long startTime = SystemClock.uptimeMillis();
setEffectiveGroup(UNPRIVILEGED_GID);
setEffectiveGroup(UNPRIVILEGED_UID);
float defaultUtilization = runtime.getTargetHeapUtilization();
runtime.setTargetHeapUtilization(0.8f);
System.gc();
runtime.runFinalizationSync();
Debug.startAllocCounting();
try {
// 创建一个缓冲区为256字符的输入流
BufferedReader br = new BufferdReader(new InputStreamReader(is), 256);
int count = 0;
String line;
while ((line = br.readLine()) != null) {
// skip comments and blank lines.
line = line.trim();
if (line.startsWith("#") || line.equals("")) {
continue;
}
try {
if (false) {
Log.v(TAG, "Preloading " + line + "...");
}
Class.forName(line);
count ++;
} catch (ClassNotFoundException e) {
Log.w(TAG, "Class not found for preloading: " + line);
} catch (UnsatisfiedLinkError e) {
Log.w(TAG, "Problem preloading " + line + ": " + e);
} catch(Throwable t) {
Log.e(TAG, "Error preloading " + line + ".", t);
}
}
Log.i(TAG, "...preloaded " + count + " classes in " + (SystemClock.uptimeMillis()-startTime) + "ms.");
} catch (IOException e) {
Log.e(TAG, "Error reading " + PRELOADED_CLASSES + ".", e);
} finally {
IoUtils.closeQuietly(is);
runtime.setTargetHeapUtilization(defaultUtilization);
runtime.preloadDexCaches();
Debug.stopAllocCounting();
setEffectiveUser(ROOT_UID);
setEffectiveGroup(ROOT_GID);
}
}
}
preloadClasses看起来很简单，但是实际上它有很多的类需要加载。可以查看一下/frameworks/base/preloaded-classes文件，这里面都是需要预加载的类。
接下来，分析一下preloadResources函数的源码：
private static final boolean PRELOAD_RESOURCES = true;
private static void preloadResources()
{
final VMRuntime runtime = VMRuntime.getRuntime();
Debug.startAllocCounting();
try {
System.gc();
runtime.runFinalizationSync();
mResources = Resources.getSystem();
mResources.startPreloading();
if (PRELOAD_RESOURCES) {
Log.i(TAG, "Preloading resources...");
long startTime = SystemClock.uptimeMillis();
TypedArray ar = mResources.obtainTypedArray(com.android.internal.R.array.preloaded_drawables);
int N = preloadDrawables(runtime, ar);
ar.recycle();
Log.i(TAG, "...preloaded " + N + " resources in " + (SystemClock.uptimeMillis()-startTime) + "ms.");
startTime = SystemClock.uptimeMillis();
ar = mResources.obtainTypedArray(com.android.internal.R.array.preloaded_color_state_lists);
N = preloadColorstateLists(runtime, ar);
ar.recycle();
Log.i(TAG, "...preloaded " + N + " resources in " + (SystemClock.uptimeMillis() - startTime) + "ms.");
}
mResources.finishPreloading();
} catch (RuntimeException e) {
Log.w(TAG, "Failure preloading resources", e);
} finally {
Debug.stopAllocCounting();
}
}
接下来，是预加载OpenGL。源码如下：
private static void preloadOpenGL()
{
if (!SystemProperties.getBoolean(PROPERTY_DISABLE_OPENGL_PRELOADING, false)) {
EGL14.eglGetDisplay(EGL14.EGL_DEFAULT_DISPLAY);
}
}
启动system_server
现在我们要分析第三个关键点：startSystemServer。这个函数会创建java世界中系统Service所驻留的进程system_server，该进程是framework的核心。如何system_server挂掉，会导致zygote自杀。我们来看一下startSystemServer()实现源码。
/**
* Prepare the arguments and fork for the system server process.
*/
private static boolean startSystemServer() throws MethodAndArgsCaller, RuntimeException
{
long capabilities = posixCapabilitiesAsBits(
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_RESOURCE,
OsConstants.CAP_SYS_TIME,
OsConstants.CAP_SYS_TTY_CONFIG
);
// 设置参数
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1032,3001,3002,3003,3006,3007",
"--capabilities=" + capabilities + "," + capabilities,
"--runtime-init",
"--nice-name=system_server", // 进程名为system_server
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities
);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
if (pid == 0) {
handleSystemServerProcess(parsedArgs);
}
return true;
}
有求必应之等待请求——runSelectLoop
zygote从startSystemServer返回后，将进入第四个关键的函数:runSelectLoop。我们来看一下这个函数的实现：
static final int GC_LOOP_COUNT = 10;
private static void runSelectLoop() throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
FileDescriptor[] fdArray = new FileDescriptor[4];
fds.add(sServerSocket.getFileDescriptor());
peers.add(null);
int loopCount = GC_LOOP_COUNT;
while (true) {
int index;
if (loopCount <= 0) {
gc();
loopCount = GC_LOOP_COUNT;
} else {
loopCount --;
}
try {
fdArray = fds.toArray(fdArray);
index = selectReadable(fdArray);
} catch(IOException ex) {
throw new RuntimeException("Error in select()", ex);
}
if (index < 0) {
throw new RuntimeException("Error in select()");
} else if (index == 0) {
ZygoteConnection newPeer = acceptCommandPeer();
peers.add(newPeer);
}
}
}