很长一段时间，我都很天真的认为python，特别是以gevent为表明的库，才是协程的乐土。Java里是无法实现协程，更别说实现stackless python这样能够pickle的协程的。Bong！咱真的是太井底之蛙了。
Java不但能够实现协程，并且还有不少个实现版本。不彻底列表以下：html
还有一个据（做者）说是最NB的kilim （https://github.com/kilim/kilim）
这些协程库的实现方式都是相似的，都是经过jvm字节码生成达到pause/resume的目的。在这篇文章中，RIFE的做者很清楚地讲明白了其实现方式：
http://www.artima.com/lejava/articles/continuations.htmljava
Geert Bevin: At the byte-code level, when the method pops variables
from the stack, exactly the same operation will be performed on the
parallel stack. We just add a piece of byte code that mimics what goes
on exactly. Then, when a continuation has to be resumed, there's a bit
of added byte-code that interacts with the state of the stack, and
puts all the variables in the correct location so that they are
present when the executing thread passes that point. At this point,
it's as if nothing happened—execution resumed as if nothing happened.python
The second problem is how to arrive at the point to resume a
continuation. You want to skip over code that you don't want to
execute. That's easily done in byte code, because you can maintain a
tableswitch in byte code that allows you to jump to a particular
location. You can create a unique label for each continuation point,
jump to that label in the switch table, and then you know exactly what
the stack for that label of variables was. You can restore that stack,
and continue executing from that point on.git
kiliam的做者用代码解释得更加清楚（http://www.malhar.net/sriram/kilim/thread_of_ones_own.pdf）：github
// original
void a() throws Pasuable {
x = ...
b(); // b is pausable
print (x);
}
通过代码加强以后是web
// transformed code
void a(Fiber f) {
switch (f.pc) { // prelude
case 0: goto START;
case 1: goto CALL_B}
START:
x = ...
CALL_B: // pre_call
f.down()
b(f);
f.up() // post-call
switch (f.status) {
case NOT_PAUSING_NO_STATE:
goto RESUME
case NOT_PAUSING_HAS_STATE:
restore state
goto RESUME
case PAUSING_NO_STATE :
capture state, return
case PAUSING_HAS_STATE:
return
}
RESUME:
print (x);
}
由于这些框架都是以java对象的方式来存储call stack state和programe counter的，因此要作到序列化存储一个执行中的状态（continuation）一点都不困难。RIFE在其web框架就狠狠地利用了clonable的状态来实现复杂的wizard（回到任意时刻地过去从新执行之类的）。
看过了这些实现以后，我不由以为持久化一个continuation好像没啥了不得的，为何会是stackless python的pypy两家的独门绝技呢？基于CPython的两个协程实现，一个greenlet一个fibers是否能够实现状态的持久化？状态持久化能不能不用更高效的serializer来实现呢？apache