另一个 "can't set headers after they're sent"

Question

我收到 "can't set headers after they're sent" 错误，据我所知，这可能是由于我的服务器不止一次发送响应，但我的代码似乎并非如此，至少在表面。我查询 dbpedia 以获得一些资源，然后将其发回给我的客户。这是代码：

app.get("/",function(req,res,next){
    if (req.query.titolo){
        var response ;
        let [ artist, title ] = getArtistTitle(req.query.titolo);
        title = title.replace(" ","_");//for dbpedia resource names
        const prefixes = {
            dbo: "http://dbpedia.org/ontology/",
            db: "http://dbpedia.org/resource/"
        }
        DbPediaClient.setOptions('application/sparql-results+json',prefixes);
        DbPediaClient.query("SELECT ?abstract WHERE { db:"+title+
        " dbo:abstract ?abstract. FILTER langMatches(lang(?abstract),'en') }")
        .then((data)=>{
            response = data["results"]["bindings"][0]["abstract"]["value"] ;
            return res.json({abstract: response}) ;
        })
        .catch((error)=>{
            console.log(error);
        });
    }
});

我使用 virtuoso-sparql-client 查询 http://dbpedia.org/sparql（DbPediaClient 在此函数之外初始化）。我已经尝试使用 res.send 而不是 res.json，但它仍然给出相同的错误。顺便说一下，它是这样的：

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:491:11)
    at ServerResponse.setHeader (_http_outgoing.js:498:3)
    at ServerResponse.header (/home/lorenzo/Scrivania/ProgettoTechweb/AlphaTube/node_modules/express/lib/response.js:767:10)
    at ServerResponse.send (/home/lorenzo/Scrivania/ProgettoTechweb/AlphaTube/node_modules/express/lib/response.js:170:12)
    at DbPediaClient.query.then (/home/lorenzo/Scrivania/ProgettoTechweb/AlphaTube/server.js:43:15)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

如果您需要更多信息，请询问。

Answer 1

Express 是一个处理 request/response 循环的框架，它通过迭代匹配 HTTP 请求中指定路径的路由并让您的代码在每个匹配中发送响应或调用 next() 如果它没有'发送一个。这里的问题是您有以下问题：

app.get("/",function(req,res,next){
    res.sendFile(...);
    next();
});

app.get("/",function(req,res,next){
    if (req.query.titolo){
        ...
        res.json(...);
    }
});

这段代码表示

Whenever I get an HTTP request to / then send some HTML, then if the request contains titolo in the query, send some JSON as well.

Express 告诉您不能再发送任何东西，因为您已经发送了 HTML。

一个解决方案是将 API 路线移动到 HTML:

上方

app.get("/",function(req,res,next){
    if (req.query.titolo){
        ...
        res.json(...);
    } else {
      next();
    }
});

app.get("/",function(req,res){
    res.sendFile(...);
});

这意味着

Whenever I get an HTTP request to /, if the request contains titolo in the query, send some JSON, otherwise send some HTML.

但我的建议是将 API 路由移动到不与静态文件的路由冲突的不同路径，这对于 HTTP 服务器来说更为典型。

另一个 "can't set headers after they're sent"

Another "can't set headers after they're sent"

sparql

node.js

express