1, 概述

在android中,进程的概念被弱化了,主要是四大组件,但是四大组件也运行于进程中;AMS负责管理和调度进程,

主要体现在启动进程,动态地根据组件调整进程在mLruProcesses列表中的位置,还可以调整进程的优先级,

这2项都和系统的内存回收相关。并且,一个进程里面可以运行多个apk(通过Shareuid的方法),

一个apk也可以运行在多个进程里(通过配置Android:process)

2, 进程启动

进程看不见摸不着,是在启动四大组件之前启动的。就是说,启动四大组件时会查看组件所在的进程是否启动,如果没有启动就首先启动该进程。

并且都会调用AMS的startProcessLocked方法来启动进程。

启动进程实在AMS的startProcessLocked方法中,当客户端调用startActivity时,会直接调用Activity.java的对应函数

private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) { long startTime = SystemClock.elapsedRealtime(); ••• 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); ••• }

直接看最主要的Process.start方法,

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<String> argsForZygote = new ArrayList<String>(); // --runtime-args, --setuid=, --setgid=, // and --setgroups= must go first argsForZygote.add("--runtime-args"); 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_JIT) != 0) { argsForZygote.add("--enable-jit"); } if ((debugFlags & Zygote.DEBUG_GENERATE_DEBUG_INFO) != 0) { argsForZygote.add("--generate-debug-info"); } if ((debugFlags & Zygote.DEBUG_ENABLE_ASSERT) != 0) { argsForZygote.add("--enable-assert"); } if (mountExternal == Zygote.MOUNT_EXTERNAL_DEFAULT) { argsForZygote.add("--mount-external-default"); } else if (mountExternal == Zygote.MOUNT_EXTERNAL_READ) { argsForZygote.add("--mount-external-read"); } else if (mountExternal == Zygote.MOUNT_EXTERNAL_WRITE) { argsForZygote.add("--mount-external-write"); } 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); } }

ProcessStartResult方法主要是保存所要启动进程的各种信息,

然后通过zygoteSendArgsAndGetResult方法将这些信息发送到zygoteState进程中。

private static ProcessStartResult zygoteSendArgsAndGetResult( ZygoteState zygoteState, ArrayList<String> 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(); if (result.pid < 0) { throw new ZygoteStartFailedEx("fork() failed"); } result.usingWrapper = inputStream.readBoolean(); return result; } catch (IOException ex) { zygoteState.close(); throw new ZygoteStartFailedEx(ex); } }

zygoteSendArgsAndGetResult通过socket向zygoteState进程发送创建进程的请求,

实际创建进程的是zygoteState进程中。在zygoteState进程中创建进程在此就不论述了。

进程的创建主要分为3大步:

1, system_server进程：AMS通过通过Process.start()方法发起创建新进程请求，

   会先收集各种新进程uid、gid、nice-name等相关的参数，然后通过socket通道发送给zygote进程,向zygoteState发送启动进程的请求

2, zygote进程: runSelectLoop方法创建并初始化进程，handleChildProc()方法回调新进程的main方法。

3, 新进程:进程初始化(启动组件)