Android- Activity的启动过程-3-Process

ProcessList 可以看做是 服务于 AMS 的工具类，用于 管理进程。
例如在启动 Activity 过程中，Activity 所在应用未启动时，将委托它先启动进程。

基于API 31 源码， 学习一下 ProcessList

1. system_server 内部调用 AMI 接口（实际是抽象类）

Framework 内部通过 ActivityManagerInternal 接口（在这里简称为AMI） 访问 AMS.
如在 ATMS(ActivityTaskManagerService) 里

void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,
            String hostingType) {
        try {
            if (Trace.isTagEnabled(TRACE_TAG_WINDOW_MANAGER)) {
                Trace.traceBegin(TRACE_TAG_WINDOW_MANAGER, "dispatchingStartProcess:"
                        + activity.processName);
            }
            // Post message to start process to avoid possible deadlock of calling into AMS with the
            // ATMS lock held.
            final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess,
                    mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead,
                    isTop, hostingType, activity.intent.getComponent());
            mH.sendMessage(m);
        } finally {
            Trace.traceEnd(TRACE_TAG_WINDOW_MANAGER);
        }
    }

2. AMS 实现 AMI

LocalService 实现了 AMI 里的函数，包括 startProcess 用于启动进程。
AMS 里创建 LocalService，并把它添加到 system_server 里的 LocalServices 里。
后续则可以通过 getService 获取到：

mInternal = new LocalService();
LocalServices.addService(ActivityManagerInternal.class, mInternal);

实际上 LocalService 它会再交给 ProcessList 处理，这里先调用AMS的 内部方法 startProcessLocked:

    @VisibleForTesting
    public final class LocalService extends ActivityManagerInternal
            implements ActivityManagerLocal {
       ....
        @Override
        public void startProcess(String processName, ApplicationInfo info, boolean knownToBeDead,
                boolean isTop, String hostingType, ComponentName hostingName) {
            try {
                if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "startProcess:"
                            + processName);
                }
                synchronized (ActivityManagerService.this) {
                    // If the process is known as top app, set a hint so when the process is
                    // started, the top priority can be applied immediately to avoid cpu being
                    // preempted by other processes before attaching the process of top app.
                    startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
                            new HostingRecord(hostingType, hostingName, isTop),
                            ZYGOTE_POLICY_FLAG_LATENCY_SENSITIVE, false /* allowWhileBooting */,
                            false /* isolated */);
                }
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            }
        }

注意这个 startProcessLocked 是用上了 同步锁的，需要等待锁，避免并发引起的问题。
实际我们会发现，多次调用了 startProcess， 但是 startProcessLocked有可能只调用了一次

3. AMS 调用 ProcessList

AMS不直接启动，而是委托调用 ProcessList.startProcessLocked,

AMS 的 startProcessLocked

    @GuardedBy("this")
    final ProcessRecord startProcessLocked(String processName,
            ApplicationInfo info, boolean knownToBeDead, int intentFlags,
            HostingRecord hostingRecord, int zygotePolicyFlags, boolean allowWhileBooting,
            boolean isolated) {
        return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
                hostingRecord, zygotePolicyFlags, allowWhileBooting, isolated, 0 /* isolatedUid */,
                null /* ABI override */, null /* entryPoint */,
                null /* entryPointArgs */, null /* crashHandler */);
    }

可以看出 ProcessList 的同名函数 startProcessLocked， 实际上也是受 AMS 里的锁作用的

4.参考

Android App Process 启动流程攻略 - 掘金 (juejin.cn)