Retrofit在Android 5.0以下设备请求失败：不支持协议问题求助

Got it, let's break down why this is happening and how to fix it:

Your error SSL23_GET_SERVER_HELLO:unsupported protocol is caused by older Android versions (pre-5.0, API level <21) not enabling TLS 1.2 by default. Most modern websites (including the Hepsiburada one you're targeting) have phased out support for insecure, outdated protocols like SSLv3, TLS 1.0, and TLS 1.1. So when your app tries to connect from an older device, the server rejects the handshake because it doesn't recognize the protocol being used.

Here's the step-by-step fix:

1. Customize OkHttp to enforce modern TLS protocols #

Retrofit relies on OkHttp for network calls, so we need to tweak the OkHttpClient instance to force TLS 1.2 support on older devices.

First, ensure you have OkHttp in your build.gradle (it's usually included with Retrofit, but explicit is better):

implementation 'com.squareup.okhttp3:okhttp:4.11.0' // Use the latest stable version available

2. Update your Retrofit initialization code #

Replace your existing Retrofit setup with this code that includes a custom OkHttpClient:

private static final String TAG = "TESTTESTTESTTEST";
private static String url = "https://www.hepsiburada.com/lassa-235-65r17-108h-xl-competush-" + "l-p-OTLST216410?magaza=LastikArt%C4%B1&utm_source=pc&utm_medium=cimri&utm_campaign" + "=c&utm_content=c&utm_term=5083&wt_pc=cimri.c.5083.pc/";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Create a connection spec that uses only secure modern protocols
    ConnectionSpec modernTlsSpec = new ConnectionSpec.Builder(ConnectionSpec.MODERN_TLS)
            .tlsVersions(TlsVersion.TLS_1_2, TlsVersion.TLS_1_3)
            .cipherSuites(
                    CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
                    CipherSuite.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
                    CipherSuite.TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
            )
            .build();

    // Build OkHttpClient with the custom spec
    OkHttpClient okHttpClient = new OkHttpClient.Builder()
            .connectionSpecs(Collections.singletonList(modernTlsSpec))
            // Uncomment below if you need to support legacy servers (not recommended)
            // .connectionSpecs(Arrays.asList(modernTlsSpec, ConnectionSpec.COMPATIBLE_TLS))
            .build();

    // Initialize Retrofit with our custom OkHttpClient
    Retrofit retrofit = new Retrofit.Builder()
            .addConverterFactory(ScalarsConverterFactory.create())
            .baseUrl("https://www.google.com/")
            .client(okHttpClient) // This line is crucial!
            .build();

    IGetHtml iGetHtml = retrofit.create(IGetHtml.class);
    Call<String> stringCall = iGetHtml.getHtml(url);
    stringCall.enqueue(new Callback<String>() {
        @Override
        public void onResponse(Call<String> call, Response<String> response) {
            if (response.isSuccessful()) {
                String responseString = response.body();
                Log.e(TAG, "onResponse: " + response.body());
            }
        }
        @Override
        public void onFailure(Call<String> call, Throwable t) {
            Log.e(TAG, "onFailure: " + t);
        }
    });
}

3. What this does exactly #

• ConnectionSpec.MODERN_TLS defines a set of secure protocols and cipher suites that are widely accepted by modern servers.
• By explicitly specifying tlsVersions(TlsVersion.TLS_1_2, TlsVersion.TLS_1_3), we override the default protocol settings on older Android devices, forcing them to use these supported protocols.
• If you absolutely need to connect to some older servers that still use legacy protocols, you can add ConnectionSpec.COMPATIBLE_TLS as a fallback (commented in the code), but this is not ideal for security.

Quick checks to avoid other issues #

• Ensure your minSdkVersion is set correctly in build.gradle (if you're supporting pre-5.0 devices, this fix is non-negotiable).
• Double-check that your IGetHtml interface is correctly defined to handle the full URL (since you're passing the full URL to getHtml(), make sure the interface uses @Url annotation):

public interface IGetHtml {
    @GET
    Call<String> getHtml(@Url String url);
}

内容的提问来源于stack exchange，提问作者Emil Mammadov