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This article introduces the relevant knowledge of "how to solve the callback function problem in the Java loop". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

The problem occurs in the callback function inside the loop, with a very simple example:

For x in xrange (3):

1.

Print "requests begin:%s" x

two。

Def callback (respon):

3.

Print x

4.

Print respon.body

5.

Client.fetch ("http://httpbin.org/get?x=%s" x, callback)

This example ignores the wait implementation waiting for the callback function to complete (failure to implement this will result in exiting without getting a result when running as a single file), and AsyncTestCase in tornado.testing provides the relevant functionality

The httpbin.org/get address returns the requested json object, such as:

{

1.

"args": {

two。

"x": "0"

3.

}

4.

"headers": {

5.

"Accept-Encoding": "gzip"

6.

"Connection": "close"

7.

"Host": "httpbin.org"

8.

"X-Request-Id": "95df3c15-7ed0-4a6d-830d-fb9629e66515"

9.

}

10.

"origin": "192.81.129.91"

11.

"url": "http://httpbin.org/get?x=0""

twelve。

}

But in fact, due to the special feature of the callback function: accessing the current value of a local variable in the closure. Easy to know, in the first request

When the url of http://httpbin.org/get?x=0 returns, the loop is already over, and x is already 2, so in fact, although the json returned by httpbin.org tells us that the x in the get parameter is 0, the x accessed in the closure is already 2.

I thought of two solutions. One is to use the variable space when the callback function is constructed, and this parameter is generated when the function is constructed, such as:

Client = AsyncHTTPClient (self.io_loop)

1.

For x in xrange (3):

two。

Def callback (respon,num=x):

3.

Print x, num

4.

Print respon.body

5.

If num = = 2:

6.

Self.stop ()

7.

Client.fetch ("http://httpbin.org/get?x=%s" x, wrap (x))

One is to wrap another layer of closures (which can also be placed outside the for):

Client = AsyncHTTPClient (self.io_loop)

1.

For x in xrange (3):

two。

Def wrap (number):

3.

Num = number

4.

Def callback (respon):

5.

Print x, num

6.

Print respon.body

7.

If num = = 2:

8.

Self.stop ()

9.

Return callback

10.

Client.fetch ("http://httpbin.org/get?x=%s" x, wrap (x))

11.

# wrap is placed outside the for:

twelve。

Client = AsyncHTTPClient (self.io_loop)

13.

Def wrap (number):

14.

Num = number

15.

Def callback (respon):

16.

Print x, num

17.

Print respon.body

18.

If num = = 2:

19.

Self.stop ()

20.

Return callback

21.

For x in xrange (3):

twenty-two。

Client.fetch ("http://httpbin.org/get?x=%s" x, wrap (x))

After thinking about it, the memory footprint of closures should be inevitable? When each item (x) of the loop body is a large memory object, the memory footprint is equivalent to looping through a list without an iterator, and I don't know if there is a more elegant solution.

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