Erlang中atom数据类型能够做的唯一的运算就是比较;在erlang中模块名和方法名都是原子; Erlang M-F-A方法调用可以做的非常灵活,我们在shell里面操练一下:
Eshell V5.9 (abort with ^G)
1> lists:seq(1,5).
[1,2,3,4,5]
2> L=lists.
lists
3> S=seq.
seq
4> L:S(1,5).
[1,2,3,4,5]
5> L:seq(1,5).
[1,2,3,4,5]
6> L2=list_to_atom("list" ++"s").
lists
7> L2:seq(1,5).
[1,2,3,4,5]
8> apply(list_to_atom("li"++"sts"),seq,[1,5]).
[1,2,3,4,5]
9>
Erlang atom 不参与垃圾回收,一旦创建就不会被移除掉;一旦超出atom的数量限制(默认是1048576) VM就会终止掉.对于一个会持续运行很久的系统,把任意字符串转成atom是很危险的,内存会慢慢被吃光.如果使用的原子是在预期范围内的,比如协议模块的名称,那么可以使用list_to_existing_atom来进行防范,这个方法所产出的atom必须是之前已经创建过的.
%% list_to_existing_atom demo
Eshell V5.9 (abort with ^G)
1> list_to_existing_atom("player_1").
** exception error: bad argument %尝试调用 list_to_existing_atom("player_1").由于player_1的原子之前没有被创建过
in function list_to_existing_atom/1 %这里报错了 异常是bad argument
called as list_to_existing_atom("player_1")
2> list_to_atom("player_1"). %创建一下player_1
player_1
3> list_to_existing_atom("player_1"). %再次调用list_to_existing_atom就是对的了
player_1
4> list_to_existing_atom("player_2").
** exception error: bad argument
in function list_to_existing_atom/1
called as list_to_existing_atom("player_2")
5> player_2. %也可以这样创建原子
player_2
6> list_to_existing_atom("player_2"). %这时调用也是对的
player_2
7>
我们可以使用 string:tokens( binary_to_list(erlang:system_info(info)),"\n")在shell中看一下atom的使用情况,输出的片段中恰好包含了原子内存使用的情况,当前数量和数量限制;想看完整的输出?穿越到这里
%% list_to_atom demo limit
Eshell V5.9 (abort with ^G)
1> string:tokens( binary_to_list(erlang:system_info(info)),"\n").
["=memory","total: 4331920","processes: 438877",
"processes_used: 438862","system: 3893043","atom: 146321",
"atom_used: 119102","binary: 327936","code: 1929551",
"ets: 123308","=hash_table:atom_tab","size: 4813",
"used: 3508","objs: 6410","depth: 7",
"=index_table:atom_tab","size: 7168","limit: 1048576",
"entries: 6410","=hash_table:module_code","size: 97",
"used: 52","objs: 72","depth: 4","=index_table:module_code",
"size: 1024","limit: 65536","entries: 72",
[...]|...]
2> [list_to_atom("player_"++integer_to_list(Item)) || Item <- lists:seq(1,1000000) ].
[player_1,player_2,player_3,player_4,player_5,player_6,
player_7,player_8,player_9,player_10,player_11,player_12,
player_13,player_14,player_15,player_16,player_17,player_18,
player_19,player_20,player_21,player_22,player_23,player_24,
player_25,player_26,player_27,player_28,player_29|...]
3> string:tokens( binary_to_list(erlang:system_info(info)),"\n").
["=memory","total: 93955448","processes: 37830214",
"processes_used: 37830214","system: 56125234",
"atom: 20296629","atom_used: 20279627","binary: 360656",
"code: 1965264","ets: 124260","=hash_table:atom_tab",
"size: 823117","used: 627221","objs: 1006479","depth: 7",
"=index_table:atom_tab","size: 1006592","limit: 1048576",
"entries: 1006479","=hash_table:module_code","size: 97",
"used: 54","objs: 74","depth: 4","=index_table:module_code",
"size: 1024","limit: 65536","entries: 74",
[...]|...]
4> [list_to_atom("player_"++integer_to_list(Item)) || Item <- lists:seq(1,1000000) ].
[player_1,player_2,player_3,player_4,player_5,player_6,
player_7,player_8,player_9,player_10,player_11,player_12,
player_13,player_14,player_15,player_16,player_17,player_18,
player_19,player_20,player_21,player_22,player_23,player_24,
player_25,player_26,player_27,player_28,player_29|...]
5> string:tokens( binary_to_list(erlang:system_info(info)),"\n").
["=memory","total: 98839096","processes: 42712630",
"processes_used: 42712630","system: 56126466",
"atom: 20296629","atom_used: 20279627","binary: 361888",
"code: 1965264","ets: 124260","=hash_table:atom_tab",
"size: 823117","used: 627221","objs: 1006479","depth: 7",
"=index_table:atom_tab","size: 1006592","limit: 1048576", %注意再次调用的时候这里没有变化
"entries: 1006479","=hash_table:module_code","size: 97",
"used: 54","objs: 74","depth: 4","=index_table:module_code",
"size: 1024","limit: 65536","entries: 74",
[...]|...]
6>
我们挑战一下atom的数量上限, [list_to_atom("player_"++integer_to_list(Item)) || Item <- lists:seq(1,100000000) ].只要运行这个就可以了,不久我们就看到下面的提示:
所以在How to Crash Erlang 一文中,无节制使用atom名列前茅:
Run out of atoms. Atoms in Erlang are analogous to symbols in Lisp--that is, symbolic, non-string identifiers that make code more readable, like green or unknown_value--with one exception. Atoms in Erlang are not garbage collected. Once an atom has been created, it lives as long as the Erlang node is running. An easy way to crash the Erlang virtual machine is to loop from 1 to some large number, calling integer_to_list and then list_to_atom on the current loop index. The atom table will fill up with unused entries, eventually bringing the runtime system to halt.
Why is this is allowed? Because garbage collecting atoms would involve a pass over all data in all processes, something the garbage collector was specifically designed to avoid. And in practice, running out of atoms will only happen if you write code that's generating new atoms on the fly.
 
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