How to start a new process for Android?

We need start a new process when we tap application launcher or start a new service which is in a different process. This artical will describe how a new process is created but no matter whoever the caller is.

1.startProcessLocked in ActivityManagerService.java

    Ignore rest of the function and focus on below code.

 

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int pid = Process.start("android.app.ActivityThread",  
        mSimpleProcessManagement ? app.processName : null, uid, uid,  
        gids, debugFlags, null);    

[java]
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argsForZygote.add("--runtime-init");  
argsForZygote.add("--setuid=" + uid);  
argsForZygote.add("--setgid=" + gid);   

argsForZygote.add("--runtime-init");
argsForZygote.add("--setuid=" + uid);
argsForZygote.add("--setgid=" + gid);

 

    The first sentence means that we need to init runtime when create this Process, the purpose of this initialization will be discussed later.

    We should know the communication between ActivityManagerService and  zygote relies on socket, AM writes the arguments of the new process into the buffer for zygote socket.

    Then the starter's work is done, let's turn over into the zygote at looked at the other socket communication side.

3.

    The zygote process will be in a loop in order to detect any connect into the zygote socket and fork the request process after being started by init deamon process, all this work is running in function runSelectLoopMode in ZygoteInit.java and calls runOnce
function in ZygoteConnection.java to fork new process.

4.runOnce in ZygoteConnection.java

 

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pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid,  
        parsedArgs.gids, parsedArgs.debugFlags, rlimits);  

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if (pid == 0) {  
    // in child   
    handleChildProc(parsedArgs, descriptors, newStderr);  
    // should never happen   
    return true;  
} else { /* pid != 0 */  
    // in parent...pid of < 0 means failure
  
    return handleParentProc(pid, descriptors, parsedArgs);  
}    

if (pid == 0) {
// in child
handleChildProc(parsedArgs, descriptors, newStderr);
// should never happen
return true;
} else { /* pid != 0 */
// in parent...pid of < 0 means failure
return handleParentProc(pid, descriptors, parsedArgs);
}

   

    As we know, parent process and child process will execute the code simultaneously after fork operation, therefore, the parent process will get the real pid of child process and call handleParentProc method, meanwhile, the child process will get a zero pid
value and call handleChildProc.

    We ignore the handleParentProc in which there is nothing important but cleanup of parent process.

The belowing operations are in the new process.

5.handleChildProc in ZygoteConnection.java

    Function handleChildProc will check if the process starter needs the runtime initialization which is set in step 2. Here need to init runtime while every process is being created.

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if (parsedArgs.runtimeInit) {  
    RuntimeInit.zygoteInit(parsedArgs.remainingArgs);  
}      

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public static final void zygoteInit(String[] argv)  
           throws ZygoteInit.MethodAndArgsCaller {  
       // TODO: Doing this here works, but it seems kind of arbitrary. Find
  
       // a better place. The goal is to set it up for applications, but not
  
       // tools like am.   
       System.setOut(new AndroidPrintStream(Log.INFO, "System.out"));  
       System.setErr(new AndroidPrintStream(Log.WARN, "System.err"));  
  
       commonInit();  
       zygoteInitNative();  
  
       int curArg = 0;  
       for ( /* curArg */ ; curArg < argv.length; curArg++) {  
           String arg = argv[curArg];  
  
           if (arg.equals("--")) {  
               curArg++;  
               break;  
           } else if (!arg.startsWith("--")) {  
               break;  
           } else if (arg.startsWith("--nice-name=")) {  
               String niceName = arg.substring(arg.indexOf('=') + 1);  
               Process.setArgV0(niceName);  
           }  
       }  
  
       if (curArg == argv.length) {  
           Slog.e(TAG, "Missing classname argument to RuntimeInit!");  
           // let the process exit   
           return;  
       }  
  
       // Remaining arguments are passed to the start class's static main
  
  
       String startClass = argv[curArg++];  
       String[] startArgs = new String[argv.length - curArg];  
  
       System.arraycopy(argv, curArg, startArgs, 0, startArgs.length);  
       invokeStaticMain(startClass, startArgs);  
   }  

public static final void zygoteInit(String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
// TODO: Doing this here works, but it seems kind of arbitrary. Find
// a better place. The goal is to set it up for applications, but not
// tools like am.
System.setOut(new AndroidPrintStream(Log.INFO, "System.out"));
System.setErr(new AndroidPrintStream(Log.WARN, "System.err"));

commonInit();
zygoteInitNative();

int curArg = 0;
for ( /* curArg */ ; curArg < argv.length; curArg++) {
String arg = argv[curArg];

if (arg.equals("--")) {
curArg++;
break;
} else if (!arg.startsWith("--")) {
break;
} else if (arg.startsWith("--nice-name=")) {
String niceName = arg.substring(arg.indexOf('=') + 1);
Process.setArgV0(niceName);
}
}

if (curArg == argv.length) {
Slog.e(TAG, "Missing classname argument to RuntimeInit!");
// let the process exit
return;
}

// Remaining arguments are passed to the start class's static main

String startClass = argv[curArg++];
String[] startArgs = new String[argv.length - curArg];

System.arraycopy(argv, curArg, startArgs, 0, startArgs.length);
invokeStaticMain(startClass, startArgs);
}

6.1 zygoteInitNative

    This function is a native function which spawns a pool thread to detect binder IPCs. Its prototype in JNI layer is underlying:

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static void com_android_internal_os_RuntimeInit_zygoteInit(JNIEnv* env, jobject clazz)  
{  
    gCurRuntime->onZygoteInit();  
}  

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virtual void onZygoteInit()  
{  
    sp<ProcessState> proc = ProcessState::self();  
    if (proc->supportsProcesses()) {  
        LOGV("App process: starting thread pool./n");  
        proc->startThreadPool();  
    }         
}  

virtual void onZygoteInit()
{
sp<ProcessState> proc = ProcessState::self();
if (proc->supportsProcesses()) {
LOGV("App process: starting thread pool./n");
proc->startThreadPool();
}
}

6.2 invokeStaticMain

After creating process and corresponding pool thread for binder IPC, the last job here is to call the "main" method of the process's first class. It should be "android.app.ActivityThread" for AM to start a new activity or service of different processes.

ActivityThread instance is the main thread of the new process.

7.main method in ActivityThread.java

   

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public static final void main(String[] args) {  
       SamplingProfilerIntegration.start();  
  
       Process.setArgV0("<pre-initialized>");  
  
       Looper.prepareMainLooper();  
       if (sMainThreadHandler == null) {  
           sMainThreadHandler = new Handler();  
       }  
  
       ActivityThread thread = new ActivityThread();  
       thread.attach(false);  
  
       if (false) {  
           Looper.myLooper().setMessageLogging(new  
                   LogPrinter(Log.DEBUG, "ActivityThread"));  
       }  
  
       Looper.loop();  
  
       if (Process.supportsProcesses()) {  
           throw new RuntimeException("Main thread loop unexpectedly exited");  
       }  
  
       thread.detach();  
       String name = (thread.mInitialApplication != null)  
           ? thread.mInitialApplication.getPackageName()  
           : "<unknown>";  
       Slog.i(TAG, "Main thread of " + name + " is now exiting");  
   }  

public static final void main(String[] args) {
SamplingProfilerIntegration.start();

Process.setArgV0("<pre-initialized>");

Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}

Looper.loop();

if (Process.supportsProcesses()) {
throw new RuntimeException("Main thread loop unexpectedly exited");
}

thread.detach();
String name = (thread.mInitialApplication != null)
? thread.mInitialApplication.getPackageName()
: "<unknown>";
Slog.i(TAG, "Main thread of " + name + " is now exiting");
}
}

In above code, we can not find any sentence create the new activity or service. where is the operation hiding?

The implementation is very complicated, the above code has a sentence "thread.attach(false)" where all the stuff is hiding.

I will discuss how activity starts in later artical.