相关文章链接：
4. Android Framework - 开机启动 Zygote 进程
相关源码文件：
/frameworks/base/services/core/java/android/os/Process.java
/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/frameworks/base/core/java/com/android/internal/os/ZygoteConnection.java
/frameworks/base/core/java/com/android/internal/os/Zygote.java
/frameworks/base/core/jni/com_android_internal_os_Zygote.cpp
/frameworks/base/core/java/android/app/ActivityThread.java
/frameworks/base/core/java/android/app/LoadedApk.java
1. AMS 与 Zygote 通信过程
无论是点击桌面图标调用 startActivitySafely 还是直接调用 startActivity 在源码中都是调用的 startActivityForResult 方法。由于 Activity 的启动流程有些复杂，因此本文先从进程的 fork 创建过程来入手分析。
void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
// Is this activity's process already running?
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
r.task.stack.setLaunchTime(r);
// 判断进程是否创建
if (app != null && app.thread != null) {
...
}
// 当前进程没有创建则先创建进程
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
try {
// 设置参数
...
// 设置 entryPoint = "android.app.ActivityThread"
boolean isActivityProcess = (entryPoint == null);
if (entryPoint == null) entryPoint = "android.app.ActivityThread";
// 启动创建进程
Process.ProcessStartResult startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
app.info.dataDir, entryPointArgs);
synchronized (mPidsSelfLocked) {
// 缓存 pid 和 进程信息
this.mPidsSelfLocked.put(startResult.pid, app);
...
}
...
} catch (RuntimeException e) {
...
}
}
private static ProcessStartResult startViaZygote(final String processClass,
final String niceName,
final int uid, final int gid,
final int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String[] extraArgs)
throws ZygoteStartFailedEx {
synchronized(Process.class) {
ArrayList<String> argsForZygote = new ArrayList<String>();
// 添加创建进程的基本参数
argsForZygote.add("--runtime-args");
argsForZygote.add("--setuid=" + uid);
argsForZygote.add("--setgid=" + gid);
...
argsForZygote.add("--target-sdk-version=" + targetSdkVersion);
...
// 向 zygote 发起创建进程的请求，通过 socket 的方式
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
}
// 打开 zygot socket 连接
private static ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
try {
primaryZygoteState = ZygoteState.connect(ZYGOTE_SOCKET);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);
}
}
if (primaryZygoteState.matches(abi)) {
return primaryZygoteState;
}
// The primary zygote didn't match. Try the secondary.
if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {
try {
secondaryZygoteState = ZygoteState.connect(SECONDARY_ZYGOTE_SOCKET);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);
}
}
if (secondaryZygoteState.matches(abi)) {
return secondaryZygoteState;
}
throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}
private static ProcessStartResult zygoteSendArgsAndGetResult(
ZygoteState zygoteState, ArrayList<String> args)
throws ZygoteStartFailedEx {
try {
// 获取 socket 的 output 与 input
final BufferedWriter writer = zygoteState.writer;
final DataInputStream inputStream = zygoteState.inputStream;
// 把数据传送给 zygote 进程
writer.write(Integer.toString(args.size()));
writer.newLine();
int sz = args.size();
for (int i = 0; i < sz; i++) {
String arg = args.get(i);
if (arg.indexOf('\n') >= 0) {
throw new ZygoteStartFailedEx(
"embedded newlines not allowed");
}
writer.write(arg);
writer.newLine();
}
writer.flush();
// 处理 zygote fork 进程返回的结果
ProcessStartResult result = new ProcessStartResult();
result.pid = inputStream.readInt();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
result.usingWrapper = inputStream.readBoolean();
return result;
} catch (IOException ex) {
// 有异常则关闭连接
zygoteState.close();
throw new ZygoteStartFailedEx(ex);
}
}
activity 的启动过程中会掉用 startSpecificActivityLocked 方法，首先会根据 processName 和 uid 来判断进程是否有创建，如果没有创建进程则需要先创建进程，processName 默认情况是包名，uid 是由 PMS 在启动的过程中解析计算好的，其具体的计算赋值过程可以参考《Android 系统服务 - PMS 的启动过程》
如果当前进程没有创建，则调用 Process 的 startViaZygote 方法去创建进程，就是向 Zygote 进程发起创建进程的请求，这里跨进程通信采用的是 Socket 套接字的方式。注意，其中有一个重要的参数 entryPoint 是 android.app.ActivityThread 。
2. Zygote fork 创建进程
// 循环读取处理请求
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
fds.add(sServerSocket.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();
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}
boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {
try {
// 读取 AMS 发过来的参数
args = readArgumentList();
} catch (IOException ex) {
...
return true;
}
try {
parsedArgs = new Arguments(args);
// fork 创建进程
pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,
parsedArgs.appDataDir);
} catch (ErrnoException ex) {
...
}
try {
if (pid == 0) {
// child process 处理
handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
return true;
} else {
// parent process 处理
return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
}
} finally {
IoUtils.closeQuietly(childPipeFd);
IoUtils.closeQuietly(serverPipeFd);
}
}
public static int forkAndSpecialize(int uid, int gid, int[] gids, int debugFlags,
int[][] rlimits, int mountExternal, String seInfo, String niceName, int[] fdsToClose,
String instructionSet, String appDataDir) {
int pid = nativeForkAndSpecialize(
uid, gid, gids, debugFlags, rlimits, mountExternal, seInfo, niceName, fdsToClose,
instructionSet, appDataDir);
...
return pid;
}
static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArray javaGids,
jint debug_flags, jobjectArray javaRlimits,
jlong permittedCapabilities, jlong effectiveCapabilities,
jint mount_external,
jstring java_se_info, jstring java_se_name,
bool is_system_server, jintArray fdsToClose,
jstring instructionSet, jstring dataDir) {
// 调用系统函数 fork
pid_t pid = fork();
...
return pid;
}
private void handleChildProc(Arguments parsedArgs,
FileDescriptor[] descriptors, FileDescriptor pipeFd, PrintStream newStderr)
throws ZygoteInit.MethodAndArgsCaller {
// 关闭 zygote 进程继承过来的 socket 连接
closeSocket();
ZygoteInit.closeServerSocket();
...
if (parsedArgs.invokeWith != null) {
...
} else {
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion,
parsedArgs.remainingArgs, null /* classLoader */);
}
}
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
// common 初始化
commonInit();
// 初始化打开 binder 驱动
nativeZygoteInit();
// 抛异常，然后执行 ActivityThread.main 方法
applicationInit(targetSdkVersion, argv, classLoader);
}
Zygote 启动后会循环处理客户端发过来的请求，当接收到请求后会调用 ZygoteConnection 的 runOnce 方法，解析到客户端的参数后会调用 native 底层的 fork 的方法，至此进程才真正创建完毕。进程 fork 完毕后，Zygote 进程会将信息返回给 AMS，新创建的进程会调用 commonInit 、nativeZygoteInit 和 applicationInit 三个核心方法：
-
commonInit：通用初始化方法
-
nativeZygoteInit：打开底层 binder 驱动
-
applicationInit：调用 ActivityThread 的 main 方法
3. Application 的创建与绑定
public static void main(String[] args) {
// 创建主线程 Looper
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
// 进入 loop 循环
Looper.loop();
}
private void attach(boolean system) {
if (!system) {
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
// 像 AMS 发起绑定请求
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
// Ignore
}
...
} else {
...
}
}
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
// 通过 pid 查询到进程信息，之前 fork 后有缓存
ProcessRecord app;
if (pid != MY_PID && pid >= 0) {
synchronized (mPidsSelfLocked) {
app = mPidsSelfLocked.get(pid);
}
} else {
app = null;
}
...
// 绑定 thread
app.makeActive(thread, mProcessStats);
...
try {
// 回调 bindApplication 方法
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
} catch (Exception e) {
...
return false;
}
return true;
}
private void handleBindApplication(AppBindData data) {
try {
// 创建绑定 Application
Application app = data.info.makeApplication(data.restrictedBackupMode, null);
mInitialApplication = app;
} finally {
StrictMode.setThreadPolicy(savedPolicy);
}
}
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
if (mApplication != null) {
return mApplication;
}
Application app = null;
// 查找到 application 的 class
String appClass = mApplicationInfo.className;
if (forceDefaultAppClass || (appClass == null)) {
// 默认是 android.app.Application
appClass = "android.app.Application";
}
try {
// 先创建 ClassLoader
java.lang.ClassLoader cl = getClassLoader();
if (!mPackageName.equals("android")) {
initializeJavaContextClassLoader();
}
// 创建一个 ContextImpl
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
// 创建和绑定 Application
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app);
} catch (Exception e) {
if (!mActivityThread.mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to instantiate application " + appClass
+ ": " + e.toString(), e);
}
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
if (instrumentation != null) {
try {
// 调用 Application 的 onCreate 方法
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!instrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
}
...
return app;
}
// 创建 application 并且调用 attach 方法
static public Application newApplication(Class<?> clazz, Context context)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
Application app = (Application)clazz.newInstance();
app.attach(context);
return app;
}
进程 fork 后会执行 ActivityThread 的 main 方法，该方法又会向 AMS 发起绑定 IApplicationThread 请求，这里 IApplicationThread 是一个可以跨进程通信的 Binder 对象，然后 AMS 又会调用 IApplicationThread 的 bindApplication 方法去创建应用的 Application，等应用的 Application 创建完毕后，才会真正的开始创建启动 Activity 。
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