我们都知道Activity的生命周期onRestart第一次启动Activity不会执行，当Activity执行过onStop之后，再恢复页面时会执行onRestart，从源码Android9.0看它时怎么实现的
Activity.java

final void performResume(boolean followedByPause, String reason) {
    performRestart(true /* start */, reason);
    ...
    mInstrumentation.callActivityOnResume(this);
    ...
}

final void performRestart(boolean start, String reason) {
    ...
    //mStopped 只有执行过performStop才会设置成true，所以第一次不会执行
    if (mStopped) {  
        ...
        //start 传过来的是true,一定会会执行，但是第一次启动Activity走不是这里
        if (start) { 
            performStart(reason);
        }
    }
}

第一次onStart是在哪里调用的呢？
AMS发送过来的事务ClientTransaction，在TransactionExecutor中执行

//TransactionExecutor.java
public void execute(ClientTransaction transaction) {
  executeCallbacks(transaction);
  executeLifecycleState(transaction);
}

private void executeLifecycleState(ClientTransaction transaction) {
  ...
  cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
  ...
}

private void cycleToPath(ActivityClientRecord r, int finish,
            boolean excludeLastState) {
  //该start == on_create = 1
  final int start = r.getLifecycleState();
  //finish == on_resume = 3
  final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
  performLifecycleSequence(r, path);
}

public IntArray getLifecyclePath(int start, int finish, boolean excludeLastState) {
  ...
  if (finish >= start) { // 3 >= 1
    for (int i = start + 1; i <= finish; i++) {
        // on_start, on_resume, on_restart = 7不在这里
        mLifecycleSequence.add(i); 
    }
  } 
  ...
  if (excludeLastState && mLifecycleSequence.size() != 0) {
      //把on_resume又删除了
     mLifecycleSequence.remove(mLifecycleSequence.size() - 1);
  }
}

private void performLifecycleSequence(ActivityClientRecord r, IntArray path) {
    ...
    switch(state) {
        case ON_START:
            //这个mTransactionHandler就是ActivityThread
            mTransactionHandler.handleStartActivity(r, mPendingActions);
            break;
    }
}

这里就是Activity第一次启动时onStart被调用的地方