从rabbit模块启动

从rabbit模块启动 看rabbit.erl的  start()-> 函数
start() ->
    try
        ok = prepare(),
        ok = rabbit_misc:start_applications(?APPS)
    after
        %%give the error loggers some time to catch up
        timer:sleep(100)

    end.

prepare()准备环境 然后调用rabbit_misc:start_applications(?APPS)
-define(APPS, [os_mon, mnesia, rabbit]).
看看rabbit_misc:start_applications

start_applications(Apps) ->
    manage_applications(fun lists:foldl/3,
                        fun application:start/1,
                        fun application:stop/1,
                        already_started,
                        cannot_start_application,
                        Apps).

start_applications调用的是manage_applications 传递的参数是
                        fun lists:foldl/3,
                        fun application:start/1,
                        fun application:stop/1,
                        already_started,
                        cannot_start_application,
                        [os_mon, mnesia, rabbit]  

这个比较精妙，仔细看看！分别对应到Iterate, Do, Undo, SkipError, ErrorTag, Apps，如下

manage_applications(Iterate, Do, Undo, SkipError, ErrorTag, Apps) ->
    Iterate(fun (App, Acc) ->
                    case Do(App) of
                        ok -> [App | Acc];
                        {error, {SkipError, _}} -> Acc;
                        {error, Reason} ->
                            lists:foreach(Undo, Acc),
                            throw({error, {ErrorTag, App, Reason}})
                    end
            end, [], Apps),
    ok.

实际如下

manage_applications(lists:foldl/3, application:start/1, application:stop, already_started, cannot_start_application, [os_mon, mnesiarabbit]) ->
    lists:foldl(fun (os_mon, Acc) ->
                    case application:start(os_mon) of
                        ok -> [os_mon | Acc];
                        {error, {already_started, _}} -> Acc;
                        {error, Reason} ->
                            lists:foreach(application:stop, Acc),
                            throw({error, {cannot_start_application, os_mon, Reason}})
                    end
            end, [], [os_mon, mnesia, rabbit]),
    ok.  依次启动os_mon, mnesia, rabbit 
使用application:start(rabbit) 会调用rabbit:start()/2