android应用程序启动流程

当在launcher进程（home进程）中点击一个应用的图标时其会调用startActivity，然后会通过Binder与system_server系统进程跨进程通讯。system_server会调用Process.start开始启动一个新进程。Process.start会先调用Porcess.StartViaZygote函数

Porcess.StartViaZygote函数会进一步调用Process.zygoteSendArgsAndGetResult函数打开一个socket连接。

在zygote进程中会监控此socket连接，监听返回后会调用

if (parsedArgs.mInvokeWith != null || parsedArgs.mStartChildZygote
                        || !multipleOK || peer.getUid() != Process.SYSTEM_UID) {
                    // Continue using old code for now. TODO: Handle these cases in the other path.
                    //调用forkAndSpecialize去fork一个新进程
                    pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid,
                            parsedArgs.mGids, parsedArgs.mRuntimeFlags, rlimits,
                            parsedArgs.mMountExternal, parsedArgs.mSeInfo, parsedArgs.mNiceName,
                            fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
                            parsedArgs.mInstructionSet, parsedArgs.mAppDataDir,
                            parsedArgs.mIsTopApp, parsedArgs.mPkgDataInfoList,
                            parsedArgs.mAllowlistedDataInfoList, parsedArgs.mBindMountAppDataDirs,
                            parsedArgs.mBindMountAppStorageDirs);

                    try {
                        if (pid == 0) {
                            // in child
                            //如果是子进程就调用handleParentProc（）
                            zygoteServer.setForkChild();

                            zygoteServer.closeServerSocket();
                            IoUtils.closeQuietly(serverPipeFd);
                            serverPipeFd = null;

                            return handleChildProc(parsedArgs, childPipeFd,
                                    parsedArgs.mStartChildZygote);
                        } else {
                            // In the parent. A pid < 0 indicates a failure and will be handled in
                            // handleParentProc.
                            IoUtils.closeQuietly(childPipeFd);
                            childPipeFd = null;
                            handleParentProc(pid, serverPipeFd);
                            return null;
                        }
                    } finally {
                        IoUtils.closeQuietly(childPipeFd);
                        IoUtils.closeQuietly(serverPipeFd);
                    }
                } 

handlerChildProc最终会调用findStaticMain去加载android.app.ActivityThread类并通过反射调用其对应的静态main方法

protected static Runnable findStaticMain(String className, String[] argv,
            ClassLoader classLoader) {
        Class<?> cl;

        try {
            //加载android.app.ActivityThread类
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }

        Method m;
        try {
            //得到android.app.ActivityThread类的静态main方法Method对象
            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);
        }
        //最后通过反射调用android.app.ActivityThread类的静态main方法
        return new MethodAndArgsCaller(m, argv);  
    }