进程fork的一些知识
在分析app进程fork时，先来简单普及进程fork相关的一些知识，后面会用到
fork子进程与父进程关系
引用一个大神的描述 [fork出的子进程和父进程] :
http://blog.csdn.net/theone10211024/article/details/13774669
由fork创建的新进程被称为子进程（child process）。该函数被调用一次，但返回两次。两次返回的区别是子进程的返回值是0，而父进程的返回值则是新进程（子进程）的进程 id。将子进程id返回给父进程的理由是：因为一个进程的子进程可以多于一个，没有一个函数使一个进程可以获得其所有子进程的进程id。对子进程来说，之所以fork返回0给它，是因为它随时可以调用getpid()来获取自己的pid；也可以调用getppid()来获取父进程的id。(进程id 0总是由交换进程使用，所以一个子进程的进程id不可能为0 )。
fork之后，操作系统会复制一个与父进程完全相同的子进程，虽说是父子关系，但是在操作系统看来，他们更像兄弟关系，这2个进程共享代码空间，但是数据空间是互相独立的，子进程数据空间中的内容是父进程的完整拷贝，指令指针也完全相同，子进程拥有父进程当前运行到的位置（两进程的程序计数器pc值相同，也就是说，子进程是从fork返回处开始执行的），但有一点不同，如果fork成功，子进程中fork的返回值是0，父进程中fork的返回值是子进程的进程号，如果fork不成功，父进程会返回错误。
可以这样想象，2个进程一直同时运行，而且步调一致，在fork之后，他们分别作不同的工作，也就是分岔了。这也是fork为什么叫fork的原因
那现在开始进行分析
向Zygote进程发送fork app进程请求
先来看ActivityManagerService的startProcessLocked方法，这是最开始入口
private final void startProcessLocked(ProcessRecord app, String hostingType,
String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
......省略代码
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);
......省略代码
}
代码是在太长，我们只看关键的地方，Process.start这个方法开始进行fork，ok那来看看它的内容,代码很长，可以直接看下面关于本段代码总结
public static final ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int debugFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String[] zygoteArgs) {
try {
return startViaZygote(processClass, niceName, uid, gid, gids,
debugFlags, mountExternal, targetSdkVersion, seInfo,
abi, instructionSet, appDataDir, zygoteArgs);
} catch (ZygoteStartFailedEx ex) {
Log.e(LOG_TAG,
"Starting VM process through Zygote failed");
throw new RuntimeException(
"Starting VM process through Zygote failed", ex);
}
}
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 argsForZygote = new ArrayList();
// --runtime-init, --setuid=, --setgid=,
// and --setgroups= must go first
argsForZygote.add("--runtime-init");
argsForZygote.add("--setuid=" + uid);
argsForZygote.add("--setgid=" + gid);
if ((debugFlags & Zygote.DEBUG_ENABLE_JNI_LOGGING) != 0) {
argsForZygote.add("--enable-jni-logging");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_SAFEMODE) != 0) {
argsForZygote.add("--enable-safemode");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_DEBUGGER) != 0) {
argsForZygote.add("--enable-debugger");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_CHECKJNI) != 0) {
argsForZygote.add("--enable-checkjni");
}
if ((debugFlags & Zygote.DEBUG_ENABLE_ASSERT) != 0) {
argsForZygote.add("--enable-assert");
}
if (mountExternal == Zygote.MOUNT_EXTERNAL_MULTIUSER) {
argsForZygote.add("--mount-external-multiuser");
} else if (mountExternal == Zygote.MOUNT_EXTERNAL_MULTIUSER_ALL) {
argsForZygote.add("--mount-external-multiuser-all");
}
argsForZygote.add("--target-sdk-version=" + targetSdkVersion);
//TODO optionally enable debuger
//argsForZygote.add("--enable-debugger");
// --setgroups is a comma-separated list
if (gids != null && gids.length > 0) {
StringBuilder sb = new StringBuilder();
sb.append("--setgroups=");
int sz = gids.length;
for (int i = 0; i < sz; i++) {
if (i != 0) {
sb.append(',');
}
sb.append(gids[i]);
}
argsForZygote.add(sb.toString());
}
if (niceName != null) {
argsForZygote.add("--nice-name=" + niceName);
}
if (seInfo != null) {
argsForZygote.add("--seinfo=" + seInfo);
}
if (instructionSet != null) {
argsForZygote.add("--instruction-set=" + instructionSet);
}
if (appDataDir != null) {
argsForZygote.add("--app-data-dir=" + appDataDir);
}
argsForZygote.add(processClass);
if (extraArgs != null) {
for (String arg : extraArgs) {
argsForZygote.add(arg);
}
}
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
}
上面的startViaZygote方法，所做的事情是：把参数最终放到一个列表中，接着调用zygoteSendArgsAndGetResult方法，该方法中的第一个参数是调用了openZygoteSocketIfNeeded(abi)方法，那我们先来看下这方法的内容
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);
}
}
......省略代码
}
ZygoteState.connect(ZYGOTE_SOCKET)接着看下这方法
/*与ZygoteInit的server socket建立链接通信*/
public static ZygoteState connect(String socketAddress) throws IOException {
DataInputStream zygoteInputStream = null;
BufferedWriter zygoteWriter = null;
final LocalSocket zygoteSocket = new LocalSocket();
try {
zygoteSocket.connect(new LocalSocketAddress(socketAddress,
LocalSocketAddress.Namespace.RESERVED));
zygoteInputStream = new DataInputStream(zygoteSocket.getInputStream());
zygoteWriter = new BufferedWriter(new OutputStreamWriter(
zygoteSocket.getOutputStream()), 256);
} catch (IOException ex) {
try {
zygoteSocket.close();
} catch (IOException ignore) {
}
throw ex;
}
String abiListString = getAbiList(zygoteWriter, zygoteInputStream);
Log.i("Zygote", "Process: zygote socket opened, supported ABIS: " + abiListString);
return new ZygoteState(zygoteSocket, zygoteInputStream, zygoteWriter,
Arrays.asList(abiListString.split(",")));
}
上面的代码其实就是与ZygoteInit类中的ServerSocket建立连接，socket连接起来了，那就可以进行通信了。现在我们在返回到zygoteSendArgsAndGetResult方法
private static ProcessStartResult zygoteSendArgsAndGetResult(
ZygoteState zygoteState, ArrayList args)
throws ZygoteStartFailedEx {
try {
/**
* See com.android.internal.os.ZygoteInit.readArgumentList()
* Presently the wire format to the zygote process is:
* a) a count of arguments (argc, in essence)
* b) a number of newline-separated argument strings equal to count
*
* After the zygote process reads these it will write the pid of
* the child or -1 on failure, followed by boolean to
* indicate whether a wrapper process was used.
*/
final BufferedWriter writer = zygoteState.writer;
final DataInputStream inputStream = zygoteState.inputStream;
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();
// Should there be a timeout on this?
ProcessStartResult result = new ProcessStartResult();
result.pid = inputStream.readInt();
/*pid小于0代表有问题，＝＝0代表是子进程，》0代表是父进程*/
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
result.usingWrapper = inputStream.readBoolean();
return result;
} catch (IOException ex) {
zygoteState.close();
throw new ZygoteStartFailedEx(ex);
}
}
代码也很简单，上面提到过client与server已经建立了socket连接，那这个方法，会把所有的参数通过socket发送到ZygoteInit的ServerSocket，发送完毕后，就等待ServerSocket把结果返回
Zygote进程处理fork请求
上一节提到过，client发送的建立socket连接最终会在ZygoteInit中创建一个ZygoteConnection对象，收到client发送的fork请求，会调用ZygoteConnection对象的runOnce方法，因此来看这方法
boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {
.......省略代码
pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,
parsedArgs.appDataDir);
......省略代码
try {
/*子进程执行pid＝＝0情况，父进程执行else情况*/
if (pid == 0) {
/*子进程*/
// in child
IoUtils.closeQuietly(serverPipeFd);
serverPipeFd = null;
handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
// should never get here, the child is expected to either
// throw ZygoteInit.MethodAndArgsCaller or exec().
return true;
} else {
// in parent...pid of < 0 means failure
IoUtils.closeQuietly(childPipeFd);
childPipeFd = null;
/*Process中的io流监听的pid等信息都是通过下面的代码发出去的*/
return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
}
} finally {
IoUtils.closeQuietly(childPipeFd);
IoUtils.closeQuietly(serverPipeFd);
}
}
Zygote.forkAndSpecialize这个方法会调用native方法来fork app进程，fork成功后，子进程就复制了基本上父进程所有的数据等，这在本节开始的时候科普过这个知识，子进程fork出的pid＝＝0，因此if(pid == 0){}else{}这段代码就特别有意思了，pid＝＝0是子进程执行，else是父进程执行，父进程执行的代码我就不贴了，它主要是把fork成功的pid返回给client端，这时候ActivityManagerService的startProcessLocked就可以继续执行。
我们还是来看下子进程执行的代码，最终会执行handleChildProc方法
private void handleChildProc(Arguments parsedArgs,
FileDescriptor[] descriptors, FileDescriptor pipeFd, PrintStream newStderr)
throws ZygoteInit.MethodAndArgsCaller {
......省略代码
if (parsedArgs.runtimeInit) {
if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
pipeFd, parsedArgs.remainingArgs);
} else {
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion,
parsedArgs.remainingArgs, null /* classLoader */);
}
} else {
......省略代码
}
}
从传递过来的参数可以定位最终调用了RuntimeInit.zygoteInit方法，那就来看下
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
redirectLogStreams();
commonInit();
nativeZygoteInit();
applicationInit(targetSdkVersion, argv, classLoader);
}
该方法我们只关注applicationInit(targetSdkVersion, argv, classLoader)这个方法
private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
// If the application calls System.exit(), terminate the process
// immediately without running any shutdown hooks. It is not possible to
// shutdown an Android application gracefully. Among other things, the
// Android runtime shutdown hooks close the Binder driver, which can cause
// leftover running threads to crash before the process actually exits.
nativeSetExitWithoutCleanup(true);
// We want to be fairly aggressive about heap utilization, to avoid
// holding on to a lot of memory that isn't needed.
VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
final Arguments args;
try {
args = new Arguments(argv);
} catch (IllegalArgumentException ex) {
Slog.e(TAG, ex.getMessage());
// let the process exit
return;
}
// Remaining arguments are passed to the start class's static main
invokeStaticMain(args.startClass, args.startArgs, classLoader);
}
private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
Class cl;
try {
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
throw new ZygoteInit.MethodAndArgsCaller(m, argv);
}
最终我们关注invokeStaticMain这个方法，该方法最终会抛出一个ZygoteInit.MethodAndArgsCaller(m, argv)异常，这个异常会把ActivityThread的main方法反射出来。
还记得上一节ZygoteInit的main方法吗
public static void main(String argv[]){
try{
.....省略代码
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
main方法最终会把MethodAndArgsCaller异常给捕获到，捕获到后其实最终就是调用ActivityThread的main方法
通过抛异常的方式来进行调用，主要目的是把当前线程的堆栈信息给置空
ok，关于Zygote进程和app进程fork过程解析到此为止