Hey, let's break down what's going wrong here and get your Bkash API call working correctly.

The Problem Breakdown #

You're stuck with two frustrating issues:

• When you add a trailing slash to your Base URL (https://www.bkashcluster.com:9081/dreamwave/merchant/trxcheck/sendmsg/), Retrofit appends your query parameters right after that slash. This creates a URL like https://www.bkashcluster.com:9081/dreamwave/merchant/trxcheck/sendmsg/?user=XX&pass=r@12... — the extra slash after sendmsg is what's triggering the 500 Internal Server Error, since the API expects sendmsg to be followed directly by the ? and parameters.
• If you remove the trailing slash from the Base URL, your app crashes because the manual URL string you're building doesn't play nice with Retrofit's URL resolution logic.

The Solution #

The cleanest way to fix this is to adjust your Base URL structure and let Retrofit handle parameter stitching instead of doing it manually. Here's how to implement it:

Step 1: Fix the Base URL

Trim the Base URL down to the parent directory of sendmsg, making sure it ends with a slash (this is key for Retrofit's proper URL resolution):

.baseUrl("https://www.bkashcluster.com:9081/dreamwave/merchant/trxcheck/")

Step 2: Update Your API Interface

Instead of using a raw @Url parameter, define the sendmsg endpoint in the @GET annotation and use @Query for each parameter. This lets Retrofit handle URL construction correctly, avoiding extra slashes or encoding mistakes:

@GET("sendmsg")
Call<List<Transaction>> getBkashTrasactionCode(
    @Query("user") String user,
    @Query("pass") String pass,
    @Query("msisdn") String msisdn,
    @Query("trxid") String trxid
);

Step 3: Adjust the Request Call

Remove the manual URL string concatenation and pass each parameter directly to the API method. We'll also add small tweaks to make the code more robust:

private void requestDataForBkashTransaction() {
    Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("https://www.bkashcluster.com:9081/dreamwave/merchant/trxcheck/")
        .addConverterFactory(GsonConverterFactory.create())
        .build();

    InstituteService api = retrofit.create(InstituteService.class);
    // Let Retrofit handle parameter stitching automatically
    Call<List<Transaction>> call = api.getBkashTrasactionCode(
        "Exampll",
        "12345",
        "0160000000",
        "6BM3KRWHLB"
    );

    call.enqueue(new retrofit2.Callback<List<Transaction>>() {
        @Override
        public void onResponse(Call<List<Transaction>> call, retrofit2.Response<List<Transaction>> response) {
            if (!response.isSuccessful()) {
                // Show a clearer error message for debugging
                Toast.makeText(PaymentActivity.this, "Error Code: " + response.code(), Toast.LENGTH_LONG).show();
                return;
            }
            List<Transaction> transactions = response.body();
            // Add null check to avoid crashes if response body is empty
            if (transactions != null) {
                for (Transaction transaction : transactions) {
                    String content = transaction.getTrxId();
                    textView.append(content);
                }
            }
        }

        @Override
        public void onFailure(Call<List<Transaction>> call, Throwable t) {
            // Give feedback when the request fails entirely
            Toast.makeText(PaymentActivity.this, "Request Failed: " + t.getMessage(), Toast.LENGTH_LONG).show();
        }
    });
}

Why This Works #

• By setting the Base URL to trxcheck/, Retrofit correctly appends sendmsg without adding an extra slash, resulting in the proper endpoint path: trxcheck/sendmsg.
• Using @Query annotations is safer and more maintainable than manual URL concatenation — Retrofit automatically handles parameter encoding and ensures the URL matches what the API expects.
• The added null checks and error messages make your code more resilient and help with debugging if something goes wrong.

If you absolutely need to stick with the @Url approach (not recommended), you could adjust your URL string to include sendmsg before the parameters:

String urlString = "sendmsg?user=Exampll&pass=12345&msisdn=0160000000&trxid=6BM3KRWHLB";

But the @Query method is far cleaner and less error-prone.

内容的提问来源于stack exchange，提问作者Hafizur Rahman Shaik