Mike的筆記
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[Android] FD leak

發表於 更新於

分享實際遇到的案例。

至於什麼是FD leak? 此文章已有詳細介紹https://juejin.cn/post/7058098018683191310，就不贅述。
可簡單理解成打開thread, cursor, 檔案, io等，皆會產生FD檔，當超過系統限制(基本是1024)時，便會造成錯誤。

偵測

要知道問題發生在哪，有沒有解決，需要每段時間去偵測FD數量變化。
參考https://blog.csdn.net/qunimaode/article/details/126996454

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public static void logFdCount() {
    int count = 0;
    try {
        Process process = Runtime.getRuntime().exec("ls /proc/" + android.os.Process.myPid() + "/fd");
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
        String s = bufferedReader.readLine();
        while (s != null) {
            s = bufferedReader.readLine();
            count++;
        }
        bufferedReader.close();

        bufferedReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
        s = bufferedReader.readLine();
        while (s != null) {
            s = bufferedReader.readLine();
            count++;
        }
        bufferedReader.close();
        process.destroy();
    } catch (IOException ex) {
        Log.e("Mike", "Exception: " + ex.getLocalizedMessage(), ex);
    }
    Log.d("Mike", "fd count=" + count);
}

Thread

有用Executors.newSingleThreadExecutor()，但對於每次使用MyService時，都建一次新的，是沒有達成共用的。當api呼叫的量達到一定程度，便會造成FD leak。

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public static MyService getMyService(String baseUrl) {
    OkHttpClient.Builder httpBuilder = new OkHttpClient.Builder();
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(baseUrl)
            .client(httpBuilder.build())
            .addConverterFactory(GsonConverterFactory.create())
            .callbackExecutor(Executors.newSingleThreadExecutor())
            .build();

    return retrofit.create(MyService.class);
}

當時的解法是共用threadPool，這樣即便有打向不同服務的api，也可共用和重複利用thread。

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public static final Executor THREAD_POOL_EXECUTOR = Executors.newCachedThreadPool(new ThreadFactory() {
    private final AtomicInteger mCount = new AtomicInteger(1);

    @Override
    public Thread newThread(@NonNull Runnable runnable) {
        return new Thread(runnable, "work-" + mCount.getAndIncrement());
    }
});

public static MyService getMyService(String baseUrl) {
    OkHttpClient.Builder httpBuilder = new OkHttpClient.Builder();
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(baseUrl)
            .client(httpBuilder.build())
            .addConverterFactory(GsonConverterFactory.create())
            .callbackExecutor(THREAD_POOL_EXECUTOR)
            .build();

    return retrofit.create(MyService.class);
}

Thread + Looper

為了讓ProgressDialog不因主執行緒做太多事而卡頓，改放至副執行緒。

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public static void showLoadingDialog(Activity activity) {
    new Thread(new Runnable() {
        @Override
        public void run() {
            Looper.prepare();
            progressDialog = ProgressDialog.show(activity, null, "Loading...");
            Looper.loop();
        }
    }).start();
}

當呼叫Looper.loop()便會讓執行緒卡著，持續等待接收message，就算事後dismiss ProgressDialog，一樣不會終止。要避免這樣的寫法。

通則是UI操作要維持在主執行緒，業務邏輯就該放在副執行緒。