String ua = new WebView(context).getSettings().getUserAgentString();

// 錯誤示例：使用 Activity Context
new WebView(MyActivity.this); // 持有 Activity 引用

// 正確做法：
new WebView(getApplicationContext());

WebView webView = new WebView(context);
String ua = webView.getSettings().getUserAgentString();
// 忘記銷毀導(dǎo)致原生資源泄漏（尤其 Android 5.0 以下）
webView.destroy(); // 必須調(diào)用

// 非主線程調(diào)用會崩潰
new Thread(() -> {
    new WebView(context); 
}).start();

// API 17+
String ua = WebSettings.getDefaultUserAgent(context);

// 推薦使用 Application Context
WebSettings.getDefaultUserAgent(getApplicationContext());

// 避免使用 Activity Context（可能間接持有引用）

// 需要 API 17+ 兼容處理
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
    ua = WebSettings.getDefaultUserAgent(context);
} else {
    // 回退方案
}

// 帶緩存的 UA 獲取工具類
public class UAUtils {
    private static String cachedUA;
    
    public static synchronized String getDefaultUA(Context context) {
        if (cachedUA != null) return cachedUA;
        
        Context appContext = context.getApplicationContext();
        
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
            cachedUA = WebSettings.getDefaultUserAgent(appContext);
        } else {
            // 低版本回退方案
            cachedUA = getLegacyUA(appContext);
        }
        
        // 添加自定義標(biāo)識（可選）
        return cachedUA + " MyApp/2.4.0";
    }
    
    private static String getLegacyUA(Context appContext) {
        WebView webView = null;
        try {
            webView = new WebView(appContext);
            return webView.getSettings().getUserAgentString();
        } finally {
            if (webView != null) webView.destroy();
        }
    }
}

String ua = System.getProperty("http.agent");

// 某些設(shè)備可能返回 null
if (ua == null) {
    ua = "Dalvik/2.1.0 (Linux; U; Android)";
}

// 無法標(biāo)識為現(xiàn)代瀏覽器，可能觸發(fā)安全限制
// 某些支付/認證系統(tǒng)會拒絕非標(biāo)準(zhǔn) UA

// 性能測試代碼示例
void runPerformanceTest() {
    // WebView 方式
    long start1 = SystemClock.elapsedRealtime();
    new WebView(context).destroy();
    long cost1 = SystemClock.elapsedRealtime() - start1;
    
    // WebSettings 方式
    long start2 = SystemClock.elapsedRealtime();
    WebSettings.getDefaultUserAgent(context);
    long cost2 = SystemClock.elapsedRealtime() - start2;
    
    // System 屬性方式
    long start3 = SystemClock.elapsedRealtime();
    System.getProperty("http.agent");
    long cost3 = SystemClock.elapsedRealtime() - start3;
    
    Log.d("Performance", String.format(
        "WebView: %dms, WebSettings: %dms, System: %dms", 
        cost1, cost2, cost3
    ));
}

// 推薦的標(biāo)準(zhǔn)實現(xiàn)
public String getUserAgent(Context context) {
    // 1. 優(yōu)先使用WebSettings API
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
        return WebSettings.getDefaultUserAgent(context.getApplicationContext());
    }
    
    // 2. 低版本使用帶緩存的WebView方案
    return LegacyUAHelper.getUA(context);
}

// 低版本專用工具類
private static class LegacyUAHelper {
    private static String cachedUA;
    
    static String getUA(Context context) {
        if (cachedUA != null) return cachedUA;
        
        final Context appContext = context.getApplicationContext();
        if (Looper.myLooper() == Looper.getMainLooper()) {
            cachedUA = createUA(appContext);
        } else {
            // 非主線程需切到主線程執(zhí)行
            Handler handler = new Handler(Looper.getMainLooper());
            CountDownLatch latch = new CountDownLatch(1);
            handler.post(() -> {
                cachedUA = createUA(appContext);
                latch.countDown();
            });
            latch.await(2, TimeUnit.SECONDS);
        }
        return cachedUA;
    }
    
    private static String createUA(Context appContext) {
        WebView webView = null;
        try {
            webView = new WebView(appContext);
            return webView.getSettings().getUserAgentString();
        } finally {
            if (webView != null) {
                webView.destroy();
                // Android 5.0+ 需要額外處理
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                    CookieManager.getInstance().flush();
                }
            }
        }
    }
}

// OkHttp 攔截器示例
public class UserAgentInterceptor implements Interceptor {
    private final String userAgent;
    
    public UserAgentInterceptor(Context context) {
        this.userAgent = UAUtils.getDefaultUA(context);
    }
    
    @Override
    public Response intercept(Chain chain) throws IOException {
        Request request = chain.request()
            .newBuilder()
            .header("User-Agent", userAgent)
            .build();
        return chain.proceed(request);
    }
}

// 初始化
OkHttpClient client = new OkHttpClient.Builder()
    .addInterceptor(new UserAgentInterceptor(context))
    .build();

// 安全設(shè)置 WebView UA
webView.getSettings().setUserAgentString(
    WebSettings.getDefaultUserAgent(getApplicationContext()) 
    + " MyApp/2.4.0"
);

// 重要：確保使用 Application Context
webView.setWebViewClient(new WebViewClient() {
    // 實現(xiàn)必要回調(diào)
});

if (ActivityManager.isLowRamDeviceStatic()) {
    // 低內(nèi)存設(shè)備避免創(chuàng)建 WebView
    String ua = System.getProperty("http.agent");
    if (ua != null) {
        ua = ua.replace("Dalvik", "Mozilla/5.0 (Linux; Android)")
              + " AppleWebKit/400 (KHTML, like Gecko)";
    }
} else {
    // 正常獲取流程
}

// 錯誤1：在列表適配器中創(chuàng)建 WebView
@Override
public View getView(int position, View convertView, ViewGroup parent) {
    String ua = new WebView(context).getSettings().getUserAgentString();
    // 導(dǎo)致快速滑動時 OOM
}

// 錯誤2：使用 Activity Context 且不銷毀
void getUserAgent() {
    WebView webView = new WebView(MyActivity.this); // 內(nèi)存泄露
    String ua = webView.getSettings().getUserAgentString();
}

// 錯誤3：在高頻循環(huán)中調(diào)用
for (int i = 0; i < 100; i++) {
    String ua = new WebView(context).getSettings().getUserAgentString();
    // 導(dǎo)致內(nèi)存急劇飆升
}

// 適用于任何場景的 UA 獲取方案
public static String getOptimizedUserAgent(Context context) {
    // 1. 現(xiàn)代設(shè)備使用官方API
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
        return WebSettings.getDefaultUserAgent(context.getApplicationContext());
    }
    
    // 2. 低版本設(shè)備使用增強的系統(tǒng)UA
    String baseUA = System.getProperty("http.agent");
    if (baseUA == null) baseUA = "";
    
    return baseUA
        .replace("Dalvik", "Mozilla/5.0 (Linux; Android")
        .replace("; U;", ";")
        .replace("(Linux; U;", "(Linux;")
        + " AppleWebKit/400 (KHTML, like Gecko) Mobile";
}