如何使用 Google Volley 在 Android / Java 中将 HTTP GET 请求的响应标头字段返回到主方法？

我正在使用 google volley 在 android 中进行网络连接。我将发出一个 http GET 请求并需要返回一个响应标头值。我在堆栈溢出上找到了一些答案来访问标头字段，但不知道如何将其返回到我的调用点。 请看看我的代码，我在其中输入了四个数字来解释我的问题。

在（1）我可以打印出我需要的值。我尝试将其保存在类属性 (2) 中，并且 IDE 中没有错误。如果我想从那里返回它 (3)，我会在 (4) 处得到一个 NullPointerException。也许这是一个先读后写的问题。 那么如何得到(1)到(4)的值呢？多谢！

public class Login {

String phpsessid = null;

public Login() {}

public String getSessionId(Context context) {
    RequestQueue queue = Volley.newRequestQueue(context);
    StringRequest sr = new StringRequest(Request.Method.GET, "any url",
            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    System.out.println(response);
                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                }
            }) {
        @Override
        protected Response<String> parseNetworkResponse(NetworkResponse response) {
            System.out.println(response.headers.get("Set-Cookie")); (1)
            phpsessid = response.headers.get("Set-Cookie"); (2)
            return super.parseNetworkResponse(response);
        }
    };
    queue.add(sr);
    return phpsessid; (3)
}

}

in main:

Login login = new Login();
String result = login.getSessionId(this.getContext);
System.out.println(result);  (4)

当您将请求添加到队列时，该请求就会被执行异步地。这意味着它的执行顺序与您读取它的顺序不同 - 它发生在另一个线程上，并最终在完成后返回。

protected Response<String> parseNetworkResponse(NetworkResponse response) {
    System.out.println(response.headers.get("Set-Cookie")); (1)
    phpsessid = response.headers.get("Set-Cookie"); (2)
    return super.parseNetworkResponse(response);
}

这将返回body响应的内容 - 从我在代码中读到的内容来看，您似乎想返回“Set-Cookie”标头的值。你可以这样做：

protected Response<String> parseNetworkResponse(NetworkResponse networkResponse) {
    String sessionId = response.headers.get("Set-Cookie");
    com.android.volley.Response<String> result = com.android.volley.Response.success(sessionId,
            HttpHeaderParser.parseCacheHeaders(networkResponse));
    return result;
}

这会将“Set-Cookie”标头的值返回给您onResponse method:

new Response.Listener<String>() {
    @Override
    public void onResponse(String response) {
        System.out.println(response);
    }
}

更好的想法可能是在创建成功/失败监听器时传递一个成功/失败监听器getSessionId称呼。通过这种方式，您可以轻松地访问调用类中的结果：

public void getSessionId(Context context, final Response.Listener<String> successListener, Response.ErrorListener failureListener) {
    RequestQueue queue = Volley.newRequestQueue(context); // This should be a singleton!
    StringRequest sr = new StringRequest(Request.Method.GET, "any url",
            successListener, 
            failureListener) {
        @Override
        protected Response<String> parseNetworkResponse(NetworkResponse networkResponse) {
            String sessionId = response.headers.get("Set-Cookie");
            com.android.volley.Response<String> result = com.android.volley.Response.success(sessionId,
                    HttpHeaderParser.parseCacheHeaders(networkResponse));
            return result;
        }
    };
    queue.add(sr);
}

Edit:

现在，您可以按如下方式调用它：

Login login = new Login();
login.getSessionId(this, new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    // You can access member variables from here.
                    // This will only get called after the network activity has happened!
                    phpsessid = response;
                    // All the work you need to do after your session ID has updated, you can put into a method and call it from here
                    // In your original example, this would be (4)
                    onSessionIdUpdated();
                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    // We'll just ignore these for now - you should handle errors though!
                }
            });
// Anything you put here will happen immediately after the getSessionId call above, and notably *before* onResponse or onErrorResponse is called (ignoring potential race conditions here for now).