Android中使用Retrofit登录时遭遇401 Unauthorized错误求助

Hey there, sorry to hear you're stuck with this frustrating 401 issue—let's dig into why Retrofit might be failing while raw OkHttp and Postman work perfectly. Most of the time, this boils down to subtle differences in how the request is constructed, so let's break down the key areas to check:

1. Compare Request Headers Side-by-Side #

Postman often adds default headers or uses ones you might have forgotten to include in Retrofit. The first step is to log every header your Retrofit request sends and match it against Postman's:

• Add an OkHttp logging interceptor to print headers:

  HttpLoggingInterceptor loggingInterceptor = new HttpLoggingInterceptor();
  loggingInterceptor.setLevel(HttpLoggingInterceptor.Level.HEADERS);
  
  OkHttpClient client = new OkHttpClient.Builder()
      .addInterceptor(loggingInterceptor)
      .build();
  

• Check critical headers like Authorization, Content-Type, User-Agent, or even Accept-Encoding—Postman might auto-include these, but Retrofit won't unless you explicitly set them.
• For example, if you're using form data, ensure Retrofit uses @FormUrlEncoded with @Field parameters, and that the Content-Type header matches Postman's application/x-www-form-urlencoded (not application/json by mistake).

2. Verify Authentication Credential Format #

• If using Basic Auth: Postman automatically encodes your username/password into a Base64 string, but Retrofit won't do this unless you handle it. Use OkHttp's Credentials helper:

  String credentials = Credentials.basic("yourUsername", "yourPassword");
  // Add this header to your Retrofit request
  @Headers("Authorization: " + credentials)
  

• If using Bearer Token: Double-check that you're prepending Bearer to your token (e.g., Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...—missing this prefix is a super common mistake).

3. Check for Request Body Discrepancies #

Even a tiny difference in the request body can trigger a 401. Use the logging interceptor to print the full body Retrofit sends, then compare it to Postman's:

• Ensure all required parameters are present (no missing fields).
• Check for case sensitivity—if your backend expects username but you're sending UserName, it might reject the credentials.
• If Postman uses form data, don't use @Body in Retrofit (which sends JSON)—stick to @FormUrlEncoded and @Field.

4. Cookie/Session Persistence #

Postman automatically saves cookies from previous requests, but Retrofit doesn't by default. If your login relies on session cookies:

• Add a cookie jar to your OkHttpClient to persist cookies:

  CookieJar cookieJar = new PersistentCookieJar(new SetCookieCache(), new SharedPrefsCookiePersistor(context));
  OkHttpClient client = new OkHttpClient.Builder()
      .cookieJar(cookieJar)
      .build();
  

• As a quick test, copy the session cookie from Postman and manually add it to your Retrofit request header to see if that fixes the 401.

5. Rule Out Custom Interference #

If you've added custom interceptors to your OkHttpClient, they might be modifying the request in unexpected ways (e.g., overwriting headers, altering parameters). Temporarily remove all custom interceptors and test again—if the 401 goes away, you know the issue is in your interceptor logic.

6. Confirm Base URL & Endpoint Match #

Double-check that your Retrofit base URL plus endpoint exactly matches what you're using in Postman. A missing trailing slash, extra path segment, or incorrect environment (test vs. production) can lead to unexpected 401s if the endpoint redirects to an unauthenticated route.

Final Tip #

The fastest way to narrow this down is to get the full raw request from Retrofit (using the logging interceptor) and compare it line-by-line with Postman's raw request. 9 times out of 10, you'll spot a tiny mismatch that's causing the authentication failure.

内容的提问来源于stack exchange，提问作者Morshed Islam