Nodejs：在第一个连接回调中连接和“writeHead”

Question

当在第一个 connect 回调中调用“res.writeHead(...)”时，我总是收到错误“Error: Can't set headers after they are sent.”：

var http = require('http');
var connect = require('connect');

var simpleApp = connect();

simpleApp
    .use(function(req, res, next){

        res.writeHead(200, { 'content-type': 'text/html' });
        res.write('response powered by SIMPLE connect-object as middelware');

        console.log('Pre');
        next();
        console.log('Post');
    })
    .use(function(req, res, next){
        console.log('I am the header guy!');
        next();
    })
    .use('/admin', function(req, res, next){
        console.log('someone entered the admin area....');
        next();
    })
    .use(function(req, res){
        console.log('reached the tail of the "chain of responsibility!!!');
        res.end();
    });

http.createServer(simpleApp).listen(process.env.PORT || 3000);

console.log('Running on port "' + (process.env.PORT || 3000) + '"');

// just a save-guard to stop the process after some time...
setTimeout(function(){
    process.exit(0);
}, 20000);

这是错误信息：

Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (http.js:691:11)
    at ServerResponse.res.setHeader (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\patch.js:63:22)
    at next (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:156:13)
    at Object.handle (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\connectSampleApp.js:13:3)
    at next (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:193:15)
    at Function.app.handle (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:201:3)
    at Server.app (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\connect.js:65:37)
    at Server.EventEmitter.emit (events.js:98:17)
    at HTTPParser.parser.onIncoming (http.js:2108:12)

当我将“writeHead”代码移动到最后一个回调中时，一切都很好：

.use(function(req, res){
        console.log('reached the tail of the "chain of responsibility!!!');
        res.writeHead(200, { 'content-type': 'text/html' });
        res.write('response powered by SIMPLE connect-object as middelware');
        res.end();
    });

所以我的问题是：是否只允许在使用连接时在最后一个回调中使用 writeHead /write？

Answer 1

检查另一个问题。 res.writeHead 基本上在res.writeHead被调用后，header就不能再修改了，而next方法会尝试修改header导致异常。

所以你可以在第一个connect回调中修改header，但是你不能写body(res.write)。以下代码应该可以正常工作。简而言之，您可以修改标题但不能刷新它们。

var http = require('http');
var connect = require('connect');

var simpleApp = connect();

simpleApp
    .use(function(req, res, next){
        res.statusCode = 200;
        res.setHeader( 'content-type', 'text/html' );

        console.log('Pre');
        next();
        console.log('Post');
    })
    .use(function(req, res, next){
        console.log('I am the header guy!');
        next();
    })
    .use('/admin', function(req, res, next){
        console.log('someone entered the admin area....');
        next();
    })
    .use(function(req, res){
        res.write('response powered by SIMPLE connect-object as middelware');
        console.log('reached the tail of the "chain of responsibility!!!');
        res.end();
    });

http.createServer(simpleApp).listen(process.env.PORT || 3000);

console.log('Running on port "' + (process.env.PORT || 3000) + '"');

// just a save-guard to stop the process after some time...
setTimeout(function(){
    process.exit(0);
}, 20000);

Answer 2

您在第一部分代码中忘记了这一行：

res.end();

如果这不起作用，这里有 TJ Holowaychuk 的评论： https://github.com/senchalabs/connect/issues/683#issuecomment-10018892