Javascript 无法加载XMLHttpRequesthttp://localhost:3000/get

我从另一个运行在tomcat中的程序调用一个NodeJS服务，当我使用下面的代码时，一切都很好

var express = require('express')
  , routes = require('./routes')
  , user = require('./routes/user')
  , http = require('http')
  , path = require('path'),

var app = express();

// all environments
app.set('port', process.env.PORT || 3000);
app.set('views', __dirname + '/views');
app.set('view engine', 'jade');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(express.methodOverride());
app.use(app.router);
app.use(express.static(path.join(__dirname, 'public')));

// development only
if ('development' == app.get('env')) {
  app.use(express.errorHandler());
}

app.get('/get', function (req, res) {
    // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', '*');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);

      res.send('Hello World!');
    });


http.createServer(app).listen(app.get('port'), function(){
  console.log('Express server listening on port ' + app.get('port'));
});

但为了尽量减少代码，我在其中引入了中间件，如下所示

var express = require('express')
  , routes = require('./routes')
  , user = require('./routes/user')
  , http = require('http')
  , path = require('path'),

var app = express();
var allowCrossDomain = function(req, res, next) {
        // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', '*');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);
   next();
}

// all environments
app.set('port', process.env.PORT || 3000);
app.set('views', __dirname + '/views');
app.set('view engine', 'jade');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(express.methodOverride());
app.use(app.router);
app.use(allowCrossDomain);
app.use(express.static(path.join(__dirname, 'public')));

// development only
if ('development' == app.get('env')) {
  app.use(express.errorHandler());
}

app.get('/get', function (req, res) {
      res.send('Hello World!');
    });


http.createServer(app).listen(app.get('port'), function(){
  console.log('Express server listening on port ' + app.get('port'));
});

---

此时，我无法加载XMLHttpRequest。请求的资源上不存在“Access Control Allow Origin”标头。因此不允许访问源代码“”。，我不知道我在哪里丢失了它。

这是我的解决方法，希望对您有所帮助

var express = require('express')
  , routes = require('./routes')
  , user = require('./routes/user')
  , http = require('http')
  , path = require('path'),

var app = express();


// all environments

app.set('views', __dirname + '/views');
app.set('view engine', 'jade');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(express.methodOverride());
app.use(app.router);

app.use(express.static(path.join(__dirname, 'public')));

// development only


app.get('/get', function (req, res) {
      res.end('Hello World!');
    });



  app.listen(3000,function(){
  console.log("It's Started on PORT 3000");
});

var express = require('express'),
    routes = require('./routes'),
    user = require('./routes/user'),
    http = require('http'),
    path = require('path'),

var app = express();
var allowCrossDomain = function(req, res, next) {
        // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', '*');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);
   next();
}

// all environments

app.set('views', __dirname + '/views');
app.set('view engine', 'jade');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(express.methodOverride());
app.use(app.router);
app.use(allowCrossDomain);
app.use(express.static(path.join(__dirname, 'public')));

// development only
if ('development' == app.get('env')) {
  app.use(express.errorHandler());
}

app.get('/get', function (req, res) {
      res.end('Hello World!');
    });


   app.listen(3000,function(){
  console.log("It's Started on PORT 3000");
}); 

---

感谢Ravindra的即时响应，但不幸的是它无法工作。同时更改app.set（'port'，process.env.port | 3000）；到app.listen（3000）；