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在Android中存在着C和Java兩個完全不同的世界，前者直接建立在Linux的基礎上，後者直接建立在JVM的基礎上。zygote的中文名字爲“受精卵”，這個名字很好的詮釋了zygote進程的作用。作爲java世界的孵化者，zygote本身是一個native程序，是由init根據init.rc文件中的配置項創建的。
@/system/core/rootdir/init.rc
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 netdzygote的實現在app_main.cpp中：
@frameworks/base/cmds/app_process/app_main.cpp
int main(int argc, char* const argv[])
{
//針對ARM平臺的特殊邏輯
#ifdef __arm__
/*
* b/7188322 - Temporarily revert to the compat memory layout
* to avoid breaking third party apps.
*
* THIS WILL GO AWAY IN A FUTURE ANDROID RELEASE.
*
* http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=7dbaa466
* changes the kernel mapping from bottom up to top-down.
* This breaks some programs which improperly embed
* an out of date copy of Android's linker.
*/
char value[PROPERTY_VALUE_MAX];
property_get("ro.kernel.qemu", value, "");
bool is_qemu = (strcmp(value, "1") == 0);
if ((getenv("NO_ADDR_COMPAT_LAYOUT_FIXUP") == NULL) && !is_qemu) {
int current = personality(0xFFFFFFFF);
if ((current & ADDR_COMPAT_LAYOUT) == 0) {
personality(current | ADDR_COMPAT_LAYOUT);
setenv("NO_ADDR_COMPAT_LAYOUT_FIXUP", "1", 1);
execv("/system/bin/app_process", argv);
return -1;
}
}
unsetenv("NO_ADDR_COMPAT_LAYOUT_FIXUP");
#endif
// 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;
}
mArgLen--;
AppRuntime runtime;
const char* argv0 = argv[0];
// Process command line arguments
// ignore argv[0]
argc--;
argv++;
// Everything up to '--' 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) {//根據傳入的參數，初始化啓動zygote所需的參數
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) == 0) {
niceName = arg + 12;
} else {
className = arg;
break;
}
}
if (niceName && *niceName) {
setArgv0(argv0, niceName);
set_process_name(niceName);//設置本進程的名稱爲zygote，至此進程有app_process變爲了zygote
}
runtime.mParentDir = parentDir;
if (zygote) {//根據我們傳入的參考，這裏的zygote值爲TRUE
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer ? "start-system-server" : "");
} else if (className) {//可以看出除了zygote，RuntimeInit也是在這裏啓動的
// Remainder of args get passed to startup class main()
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;
}
}
可以看出start是AndroidRuntime中的方法。通過start函數前面的註釋我們瞭解到它的主要作用是：啓動Android運行時環境，包括啓動虛擬機和調用className參數所指定的類的main()方法（即：Java中的main方法）。
/*
* 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 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) {
/* 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)));
}
//環境變量ANDROID_ROOT是否已經設置，如果未設置，則設置其值爲"/system"
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) != 0) {//啓動Java虛擬機
return;
}
onVmCreated(env);//空函數
/*
* Register android functions.
*/
if (startReg(env) < 0) {//註冊Android JNI函數
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;
jstring optionsStr;
stringClass = env->FindClass("java/lang/String");//JNI中調用java中的String類
assert(stringClass != NULL);
//創建包含2個元素的String數組，這裏相當於Java中的String strArray[] = new String[2]
strArray = env->NewObjectArray(2, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);//classNameStr的值爲"com.android.internal.os.ZygoteInit"
assert(classNameStr != NULL);
env->SetObjectArrayElement(strArray, 0, classNameStr);
optionsStr = env->NewStringUTF(options);//optionsStr的值爲"start-system-server"
env->SetObjectArrayElement(strArray, 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);//將"com.android.internal.os.ZygoteInit"中的"."替換成"/"供JNI調用
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");//ZygoteInit類中的main()方法
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);//通過JNI調用main()方法
#if 0
if (env->ExceptionCheck())
threadExitUncaughtException(env);
#endif
}
}
free(slashClassName);
//如果JVM退出。這兩句代碼一般來說執行不到
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");
}- 啓動JVM。
- 註冊Android JNI函數。
- 調用ZygoteInit的main()方法。
創建Java虛擬機
start()中與創建虛擬機相關的代碼如下：
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) {//啓動Java虛擬機
return;
}
onVmCreated(env);//空函數@/libnativehelper/jniInvocation.cpp
bool JniInvocation::Init(const char* library) {
#ifdef HAVE_ANDROID_OS
char default_library[PROPERTY_VALUE_MAX];
property_get("persist.sys.dalvik.vm.lib", default_library, "libdvm.so");
#else
const char* default_library = "libdvm.so";
#endif
if (library == NULL) {
library = default_library;
}
handle_ = dlopen(library, RTLD_NOW);
if (handle_ == NULL) {
ALOGE("Failed to dlopen %s: %s", library, dlerror());
return false;
}
if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetDefaultJavaVMInitArgs_),
"JNI_GetDefaultJavaVMInitArgs")) {
return false;
}
if (!FindSymbol(reinterpret_cast<void**>(&JNI_CreateJavaVM_),
"JNI_CreateJavaVM")) {
return false;
}
if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetCreatedJavaVMs_),
"JNI_GetCreatedJavaVMs")) {
return false;
}
return true;
}在初始化完虛擬機環境後，接下來調用startVm()來創建虛擬機。
@/frameworks/base/core/jni/AndroidRuntime.cpp
/*
* 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("-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 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;
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;
}
}
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;
}
property_get("dalvik.vm.stack-trace-file", stackTraceFileBuf, "");
property_get("dalvik.vm.check-dex-sum", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
checkDexSum = true;
}
property_get("log.redirect-stdio", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
logStdio = true;
}
strcpy(enableAssertBuf, "-ea:");
property_get("dalvik.vm.enableassertions", enableAssertBuf+4, "");
strcpy(jniOptsBuf, "-Xjniopts:");
property_get("dalvik.vm.jniopts", jniOptsBuf+10, "");
/* route exit() to our handler */
opt.extraInfo = (void*) runtime_exit;
opt.optionString = "exit";
mOptions.add(opt);
/* route fprintf() to our handler */
opt.extraInfo = (void*) runtime_vfprintf;
opt.optionString = "vfprintf";
mOptions.add(opt);
/* register the framework-specific "is sensitive thread" hook */
opt.extraInfo = (void*) runtime_isSensitiveThread;
opt.optionString = "sensitiveThread";
mOptions.add(opt);
opt.extraInfo = NULL;
/* enable verbose; standard options are { jni, gc, class } */
//options[curOpt++].optionString = "-verbose:jni";
opt.optionString = "-verbose:gc";
mOptions.add(opt);
//options[curOpt++].optionString = "-verbose:class";
/*
* The default starting and maximum size of the heap. Larger
* values should be specified in a product property override.
*/
strcpy(heapstartsizeOptsBuf, "-Xms");
property_get("dalvik.vm.heapstartsize", heapstartsizeOptsBuf+4, "4m");
opt.optionString = heapstartsizeOptsBuf;
mOptions.add(opt);
strcpy(heapsizeOptsBuf, "-Xmx");
property_get("dalvik.vm.heapsize", heapsizeOptsBuf+4, "16m");
opt.optionString = heapsizeOptsBuf;
mOptions.add(opt);
// Increase the main thread's interpreter stack size for bug 6315322.
opt.optionString = "-XX:mainThreadStackSize=24K";
mOptions.add(opt);
// Set the max jit code cache size. Note: size of 0 will disable the JIT.
strcpy(jitcodecachesizeOptsBuf, "-Xjitcodecachesize:");
property_get("dalvik.vm.jit.codecachesize", jitcodecachesizeOptsBuf+19, NULL);
if (jitcodecachesizeOptsBuf[19] != '\0') {
opt.optionString = jitcodecachesizeOptsBuf;
mOptions.add(opt);
}
strcpy(heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");
property_get("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf+20, "");
if (heapgrowthlimitOptsBuf[20] != '\0') {
opt.optionString = heapgrowthlimitOptsBuf;
mOptions.add(opt);
}
strcpy(heapminfreeOptsBuf, "-XX:HeapMinFree=");
property_get("dalvik.vm.heapminfree", heapminfreeOptsBuf+16, "");
if (heapminfreeOptsBuf[16] != '\0') {
opt.optionString = heapminfreeOptsBuf;
mOptions.add(opt);
}
strcpy(heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");
property_get("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf+16, "");
if (heapmaxfreeOptsBuf[16] != '\0') {
opt.optionString = heapmaxfreeOptsBuf;
mOptions.add(opt);
}
strcpy(heaptargetutilizationOptsBuf, "-XX:HeapTargetUtilization=");
property_get("dalvik.vm.heaptargetutilization", heaptargetutilizationOptsBuf+26, "");
if (heaptargetutilizationOptsBuf[26] != '\0') {
opt.optionString = heaptargetutilizationOptsBuf;
mOptions.add(opt);
}
property_get("ro.config.low_ram", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
opt.optionString = "-XX:LowMemoryMode";
mOptions.add(opt);
}
/*
* Enable or disable dexopt features, such as bytecode verification and
* calculation of register maps for precise GC.
*/
property_get("dalvik.vm.dexopt-flags", dexoptFlagsBuf, "");
if (dexoptFlagsBuf[0] != '\0') {
const char* opc;
const char* val;
opc = strstr(dexoptFlagsBuf, "v="); /* verification */
if (opc != NULL) {
switch (*(opc+2)) {
case 'n': val = "-Xverify:none"; break;
case 'r': val = "-Xverify:remote"; break;
case 'a': val = "-Xverify:all"; break;
default: val = NULL; break;
}
if (val != NULL) {
opt.optionString = val;
mOptions.add(opt);
}
}
opc = strstr(dexoptFlagsBuf, "o="); /* optimization */
if (opc != NULL) {
switch (*(opc+2)) {
case 'n': val = "-Xdexopt:none"; break;
case 'v': val = "-Xdexopt:verified"; break;
case 'a': val = "-Xdexopt:all"; break;
case 'f': val = "-Xdexopt:full"; break;
default: val = NULL; break;
}
if (val != NULL) {
opt.optionString = val;
mOptions.add(opt);
}
}
opc = strstr(dexoptFlagsBuf, "m=y"); /* register map */
if (opc != NULL) {
opt.optionString = "-Xgenregmap";
mOptions.add(opt);
/* turn on precise GC while we're at it */
opt.optionString = "-Xgc:precise";
mOptions.add(opt);
}
}
/* enable debugging; set suspend=y to pause during VM init */
/* use android ADB transport */
opt.optionString =
"-agentlib:jdwp=transport=dt_android_adb,suspend=n,server=y";
mOptions.add(opt);
ALOGD("CheckJNI is %s\n", checkJni ? "ON" : "OFF");
if (checkJni) {
/* extended JNI checking */
opt.optionString = "-Xcheck:jni";
mOptions.add(opt);
/* set a cap on JNI global references */
opt.optionString = "-Xjnigreflimit:2000";
mOptions.add(opt);
/* with -Xcheck:jni, this provides a JNI function call trace */
//opt.optionString = "-verbose:jni";
//mOptions.add(opt);
}
char lockProfThresholdBuf[sizeof("-Xlockprofthreshold:") + sizeof(propBuf)];
property_get("dalvik.vm.lockprof.threshold", propBuf, "");
if (strlen(propBuf) > 0) {
strcpy(lockProfThresholdBuf, "-Xlockprofthreshold:");
strcat(lockProfThresholdBuf, propBuf);
opt.optionString = lockProfThresholdBuf;
mOptions.add(opt);
}
/* Force interpreter-only mode for selected opcodes. Eg "1-0a,3c,f1-ff" */
char jitOpBuf[sizeof("-Xjitop:") + PROPERTY_VALUE_MAX];
property_get("dalvik.vm.jit.op", propBuf, "");
if (strlen(propBuf) > 0) {
strcpy(jitOpBuf, "-Xjitop:");
strcat(jitOpBuf, propBuf);
opt.optionString = jitOpBuf;
mOptions.add(opt);
}
/* Force interpreter-only mode for selected methods */
char jitMethodBuf[sizeof("-Xjitmethod:") + PROPERTY_VALUE_MAX];
property_get("dalvik.vm.jit.method", propBuf, "");
if (strlen(propBuf) > 0) {
strcpy(jitMethodBuf, "-Xjitmethod:");
strcat(jitMethodBuf, propBuf);
opt.optionString = jitMethodBuf;
mOptions.add(opt);
}
if (executionMode == kEMIntPortable) {
opt.optionString = "-Xint:portable";
mOptions.add(opt);
} else if (executionMode == kEMIntFast) {
opt.optionString = "-Xint:fast";
mOptions.add(opt);
} else if (executionMode == kEMJitCompiler) {
opt.optionString = "-Xint:jit";
mOptions.add(opt);
}
if (checkDexSum) {
/* perform additional DEX checksum tests */
opt.optionString = "-Xcheckdexsum";
mOptions.add(opt);
}
if (logStdio) {
/* convert stdout/stderr to log messages */
opt.optionString = "-Xlog-stdio";
mOptions.add(opt);
}
if (enableAssertBuf[4] != '\0') {
/* accept "all" to mean "all classes and packages" */
if (strcmp(enableAssertBuf+4, "all") == 0)
enableAssertBuf[3] = '\0';
ALOGI("Assertions enabled: '%s'\n", enableAssertBuf);
opt.optionString = enableAssertBuf;
mOptions.add(opt);
} else {
ALOGV("Assertions disabled\n");
}
if (jniOptsBuf[10] != '\0') {
ALOGI("JNI options: '%s'\n", jniOptsBuf);
opt.optionString = jniOptsBuf;
mOptions.add(opt);
}
if (stackTraceFileBuf[0] != '\0') {
static const char* stfOptName = "-Xstacktracefile:";
stackTraceFile = (char*) malloc(strlen(stfOptName) +
strlen(stackTraceFileBuf) +1);
strcpy(stackTraceFile, stfOptName);
strcat(stackTraceFile, stackTraceFileBuf);
opt.optionString = stackTraceFile;
mOptions.add(opt);
}
/* extra options; parse this late so it overrides others */
property_get("dalvik.vm.extra-opts", extraOptsBuf, "");
parseExtraOpts(extraOptsBuf);
/* Set the properties for locale */
{
char langOption[sizeof("-Duser.language=") + 3];
char regionOption[sizeof("-Duser.region=") + 3];
strcpy(langOption, "-Duser.language=");
strcpy(regionOption, "-Duser.region=");
readLocale(langOption, regionOption);
opt.extraInfo = NULL;
opt.optionString = langOption;
mOptions.add(opt);
opt.optionString = regionOption;
mOptions.add(opt);
}
/*
* We don't have /tmp on the device, but we often have an SD card. Apps
* shouldn't use this, but some test suites might want to exercise it.
*/
opt.optionString = "-Djava.io.tmpdir=/sdcard";
mOptions.add(opt);
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");
goto bail;
}
result = 0;
bail:
free(stackTraceFile);
return result;
} 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("-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 heaptargetutilizationOptsBuf[sizeof("-XX:HeapTargetUtilization=")-1 + PROPERTY_VALUE_MAX];
char jitcodecachesizeOptsBuf[sizeof("-Xjitcodecachesize:")-1 + PROPERTY_VALUE_MAX];@system/core/include/cutils/properties.h
/* System properties are *small* name value pairs managed by the
** property service. If your data doesn't fit in the provided
** space it is not appropriate for a system property.
**
** WARNING: system/bionic/include/sys/system_properties.h also defines
** these, but with different names. (TODO: fix that)
*/
#define PROPERTY_KEY_MAX PROP_NAME_MAX
#define PROPERTY_VALUE_MAX PROP_VALUE_MAX[dalvik.vm.heapconcurrentstart]: [2097152]
[dalvik.vm.heapgrowthlimit]: [96m]
[dalvik.vm.heapidealfree]: [8388608]
[dalvik.vm.heapsize]: [384m]
[dalvik.vm.heapstartsize]: [8m]
[dalvik.vm.heaputilization]: [0.25]
[dalvik.vm.stack-trace-file]: [/data/anr/traces.txt] /*
* 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");
goto bail;
}註冊JNI函數
創建好了虛擬機，接下來要給虛擬機註冊一些JNI函數。不知道各位讀者有沒有想過，這裏爲什麼要註冊JNI函數呢？沒錯，因爲在基於虛擬機的Java世界裏，Java並不是萬能的，它用到的很多方法（例如：音頻、視頻相關、Binder等）都需要以native的方式實現，而這些方法就需要虛擬機以JNI的方式進行加載。
@/frameworks/base/core/jni/AndroidRuntime.cpp
/*
* Register android native functions with the VM.
*/
/*static*/ 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.)
*/
<span style="white-space:pre"> </span>//設置Thread類的線程創建函數爲javaCreateThreadEtc
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;
}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;
}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_FileUtils),
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{ register_android_debug_JNITest, "register_android_debug_JNITest" }int register_android_debug_JNITest(JNIEnv* env)
{
return jniRegisterNativeMethods(env, "android/debug/JNITest",
gMethods, NELEM(gMethods));
}到這裏JNI註冊就講完了。
ZygoteInit初始化
在JNI註冊完成後，讓我們再回頭繼續看AndroidRuntime中start函數：env->CallStaticVoidMethod(startClass, startMeth, strArray);//通過JNI調用main()方法@/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();//啓動性能統計
registerZygoteSocket();//註冊zygote用的socket
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();//初始化，主要進行framework中一些類和資源的預加載
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();//結束統計並生成結果文件
// Do an initial gc to clean up after startup
gc();//強制進行一次回收
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
// If requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("start-system-server")) {
startSystemServer();//啓動system_server進程
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
runSelectLoop();//變成守護進程，接收socket信息進行處理
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}- 註冊Zygote用的socket
- 類和資源的預加載
- 啓動system_server進程
- 進入一個死循環，等待接收和處理socket事件
接下來將對它們一一進行分析。
registerZygoteSocket()方法
registerZygoteSocket()方法的實現如下：
/**
* Registers a server socket for zygote command connections
*
* @throws RuntimeException when open fails
*/
private static void registerZygoteSocket() {
if (sServerSocket == null) {
int fileDesc;
try {
String env = System.getenv(ANDROID_SOCKET_ENV);//從環境變量中獲取socket的fd
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);
}
}
}類和資源的預加載
在Zygote中通過preload()方法完成類和資源的加載，它的實現如下：
static void preload() {
preloadClasses();//加載類
preloadResources();//加載資源
preloadOpenGL();//加載OpenGL
} /**
* Performs Zygote process initialization. Loads and initializes
* commonly used classes.
*
* Most classes only cause a few hundred bytes to be allocated, but
* a few will allocate a dozen Kbytes (in one case, 500+K).
*/
private static void preloadClasses() {
final VMRuntime runtime = VMRuntime.getRuntime();
InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream(
PRELOADED_CLASSES);//加載preloaded-classes這個文件中定義的需要預加載的類
if (is == null) {
Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");
} else {
Log.i(TAG, "Preloading classes...");
long startTime = SystemClock.uptimeMillis();
// Drop root perms while running static initializers.
setEffectiveGroup(UNPRIVILEGED_GID);
setEffectiveUser(UNPRIVILEGED_UID);
// Alter the target heap utilization. With explicit GCs this
// is not likely to have any effect.
float defaultUtilization = runtime.getTargetHeapUtilization();
runtime.setTargetHeapUtilization(0.8f);
// Start with a clean slate.
System.gc();
runtime.runFinalizationSync();
Debug.startAllocCounting();
try {
BufferedReader br
= new BufferedReader(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);//以反射的方式加載類
if (Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {
if (false) {
Log.v(TAG,
" GC at " + Debug.getGlobalAllocSize());
}
System.gc();
runtime.runFinalizationSync();
Debug.resetGlobalAllocSize();
}
count++;
} catch (ClassNotFoundException e) {
Log.w(TAG, "Class not found for preloading: " + line);
} catch (Throwable t) {
Log.e(TAG, "Error preloading " + line + ".", t);
if (t instanceof Error) {
throw (Error) t;
}
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
}
throw new RuntimeException(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);
// Restore default.
runtime.setTargetHeapUtilization(defaultUtilization);
// Fill in dex caches with classes, fields, and methods brought in by preloading.
runtime.preloadDexCaches();
Debug.stopAllocCounting();
// Bring back root. We'll need it later.
setEffectiveUser(ROOT_UID);
setEffectiveGroup(ROOT_GID);
}
}
}# Classes which are preloaded by com.android.internal.os.ZygoteInit.
# Automatically generated by frameworks/base/tools/preload/WritePreloadedClassFile.java.
# MIN_LOAD_TIME_MICROS=1250
# MIN_PROCESSES=10
android.R$styleable
android.accounts.Account
android.accounts.Account$1
android.accounts.AccountManager
android.accounts.AccountManager$12
android.accounts.AccountManager$13
android.accounts.AccountManager$6
android.accounts.AccountManager$AmsTask
android.accounts.AccountManager$AmsTask$1
android.accounts.AccountManager$AmsTask$Response /**
* Preload any class that take longer to load than MIN_LOAD_TIME_MICROS us.
*/
static final int MIN_LOAD_TIME_MICROS = 1250;
/**
* Preload any class that was loaded by at least MIN_PROCESSES processes.
*/
static final int MIN_PROCESSES = 10;這裏我簡單估算了一下，在Android4.4中預加載這些類需要4秒鐘作用，這對於系統啓動來說是一個比較長的時間，因此在進行系統啓動速度的優化時，這裏可以作爲一個優化大點。
在看preloadClass的代碼時有些讀者看的setEffectiveUser的幾句代碼不明白什麼意思：
private static void preloadClasses() {
......
// Drop root perms while running static initializers.
setEffectiveGroup(UNPRIVILEGED_GID);
setEffectiveUser(UNPRIVILEGED_UID);
......
} finally {
......
// Bring back root. We'll need it later.
setEffectiveUser(ROOT_UID);
setEffectiveGroup(ROOT_GID);
}
}
} /**
* Sets effective user ID.
*/
private static void setEffectiveUser(int uid) {
int errno = setreuid(ROOT_UID, uid);
if (errno != 0) {
Log.e(TAG, "setreuid() failed. errno: " + errno);
}
} /**
* The Linux syscall "setreuid()"
* @param ruid real uid
* @param euid effective uid
* @return 0 on success, non-zero errno on fail
*/
static native int setreuid(int ruid, int euid);有效用戶ID
有效用戶ID（Effective UID，即EUID）與有效用戶組ID（Effective Group ID，即EGID）在創建與訪問文件的時候發揮作用；具體來說，創建文件時，系統內核將根據創建文件的進程的EUID與EGID設定文件的所有者/組屬性，而在訪問文件時，內核亦根據訪問進程的EUID與EGID決定其能否訪問文件。
真實用戶ID
真實用戶ID（Real UID,即RUID）與真實用戶組ID（Real GID，即RGID）用於辨識進程的真正所有者，且會影響到進程發送信號的權限。沒有超級用戶權限的進程僅在其RUID與目標進程的RUID相匹配時才能向目標進程發送信號，例如在父子進程間，子進程從父進程處繼承了認證信息，使得父子進程間可以互相發送信號。
暫存用戶ID
暫存用戶ID（Saved UID，即SUID）於以提升權限運行的進程暫時需要做一些不需特權的操作時使用，這種情況下進程會暫時將自己的有效用戶ID從特權用戶（常爲root）對應的UID變爲某個非特權用戶對應的UID，而後將原有的特權用戶UID複製爲SUID暫存；之後當進程完成不需特權的操作後，進程使用SUID的值重置EUID以重新獲得特權。在這裏需要說明的是，無特權進程的EUID值只能設爲與RUID、SUID與EUID（也即不改變）之一相同的值。
文件系統用戶ID
文件系統用戶ID（File System UID，即FSUID）在Linux中使用，且只用於對文件系統的訪問權限控制，在沒有明確設定的情況下與EUID相同（若FSUID爲root的UID，則SUID、RUID與EUID必至少有一亦爲root的UID），且EUID改變也會影響到FSUID。設立FSUID是爲了允許程序（如NFS服務器）在不需獲取向給定UID賬戶發送信號的情況下以給定UID的權限來限定自己的文件系統權限。
這段代碼轉自http://zh.wikipedia.org/wiki/%E7%94%A8%E6%88%B7ID那這裏這樣做的用意何在？我猜這裏是爲了保證預加載的類是所有的用戶都是可用的。
預加載資源的代碼如下：
/**
* Load in commonly used resources, so they can be shared across
* processes.
*
* These tend to be a few Kbytes, but are frequently in the 20-40K
* range, and occasionally even larger.
*/
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);//預加載Drawable
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);//預加載Color
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();
}
}除了加載類和資源，還會加載OpenGL的一些東西：
private static void preloadOpenGL() {
if (!SystemProperties.getBoolean(PROPERTY_DISABLE_OPENGL_PRELOADING, false)) {
EGL14.eglGetDisplay(EGL14.EGL_DEFAULT_DISPLAY);
}
}在創建socket和資源加載前後有這麼兩句：
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();//啓動性能統計
registerZygoteSocket();//註冊zygote用的socket
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();//初始化，主要進行framework中一些類和資源的預加載
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();//結束統計並生成結果文件啓動system_server
/**
* 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
);
/* Hardcoded command line to start the system server */
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",
"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(//以fork的方式創建system_server進程
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) {//pid==0說明在子進程中，父進程爲Zygote
handleSystemServerProcess(parsedArgs);
}
return true;
}runSelectLoop()方法
在創建system_server後，Zygote調用runSelectLoop（）進入到一個死循環中：
/**
* Runs the zygote process's select loop. Accepts new connections as
* they happen, and reads commands from connections one spawn-request's
* worth at a time.
*
* @throws MethodAndArgsCaller in a child process when a main() should
* be executed.
*/
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());//registerZygoteSocket中創建的socket的描述符
peers.add(null);
int loopCount = GC_LOOP_COUNT;
while (true) {//死循環
int index;//selectReadable方法監控的句柄的下標（fdArray中的下標）
/*
* Call gc() before we block in select().
* It's work that has to be done anyway, and it's better
* to avoid making every child do it. It will also
* madvise() any free memory as a side-effect.
*
* Don't call it every time, because walking the entire
* heap is a lot of overhead to free a few hundred bytes.
*/
if (loopCount <= 0) {//Zygote每循環GC_LOOP_COUNT（這裏的值是10）次就會進行一次內存回收
gc();
loopCount = GC_LOOP_COUNT;
} else {
loopCount--;
}
try {
fdArray = fds.toArray(fdArray);
index = selectReadable(fdArray);//內部由select()實現，在沒有客戶端事件時會堵塞
} catch (IOException ex) {
throw new RuntimeException("Error in select()", ex);
}
if (index < 0) {
throw new RuntimeException("Error in select()");
} else if (index == 0) {//index==0表示selcet接收到的是Zygote的socket的事件
ZygoteConnection newPeer = acceptCommandPeer();
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {//調用ZygoteConnection對象的runOnce方法，ZygoteConnection是在index == 0時被添加到peers的
boolean done;
done = peers.get(index).runOnce();
if (done) {
peers.remove(index);
fds.remove(index);
}
}
}
}下面是selcetReadable方法的代碼：
/**
* Invokes select() on the provider array of file descriptors (selecting
* for readability only). Array elements of null are ignored.
*
* @param fds non-null; array of readable file descriptors
* @return index of descriptor that is now readable or -1 for empty array.
* @throws IOException if an error occurs
*/
static native int selectReadable(FileDescriptor[] fds) throws IOException;Zygote接收到socket客戶端的鏈接後會將其（客戶端Socket）保存到一個ZygoteConnection對象中，然後保存到peers
/**
* Waits for and accepts a single command connection. Throws
* RuntimeException on failure.
*/
private static ZygoteConnection acceptCommandPeer() {
try {
return new ZygoteConnection(sServerSocket.accept());
} catch (IOException ex) {
throw new RuntimeException(
"IOException during accept()", ex);
}
}