StackOverflowError在Kotlin中使用Singleton

任何人都可以告訴我，爲什麼我要使用這個Kotlin類獲得java.lang.StackOverflowError？ 41號線是if (instance == null) {StackOverflowError在Kotlin中使用Singleton

class TokenHelper protected constructor() { 
    var token: String? = null 
    var appId: String? = null 
    var installationId: String? = null 
    var userId: String? = null 

    companion object { 
     var instance: TokenHelper? = null 
      get() { 
       if (instance == null) { 
        instance = TokenHelper() 
       } 
       return instance 
      } 
    } 
}

堆棧跟蹤：

04-11 19:07:42.188 16142-16142/com.foo.bar.debug E/AndroidRuntime: FATAL EXCEPTION: main 
    Process: com.foo.bar.debug, PID: 16142 
    java.lang.StackOverflowError: stack size 8MB 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:0) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
     at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41 
04-11 19:07:42.271 16142-16142/com.foo.bar.debug D/Error: ERR: exClass=java.lang.StackOverflowError 
04-11 19:07:42.271 16142-16142/com.foo.bar.debug D/Error: ERR: exMsg=stack size 8MB 
04-11 19:07:42.271 16142-16142/com.foo.bar.debug D/Error: ERR: file=TokenHelper.kt 
04-11 19:07:42.271 16142-16142/com.foo.bar.debug D/Error: ERR: class=com.foo.bar.helper.TokenHelper$Companion 
04-11 19:07:42.271 16142-16142/com.foo.bar.debug D/Error: ERR: method=getInstance line=0 
04-11 19:07:42.282 16142-16142/com.foo.bar.debug D/Error: ERR: stack=java.lang.StackOverflowError: stack size 8MB 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:0) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(TokenHelper.kt:41) 
    at com.foo.bar.helper.TokenHelper$Companion.getInstance(
04-11 19:07:42.283 16142-16142/com.foo.bar.debug D/Error: ERR: TOTAL BYTES WRITTEN: 880768 
04-11 19:07:45.527 16142-16142/com.foo.bar.debug D/Process: killProcess, pid=16142 
04-11 19:07:45.528 16142-16142/com.foo.bar.debug D/Process: com.android.internal.os.RuntimeInit$UncaughtHandler.uncaughtException:113 java.lang.ThreadGroup.uncaughtException:693 java.lang.ThreadGroup.uncaughtException:690

的Android 2.0工作室，科特林1.0.1-2

來源

2016-04-11 user3105453

如果你想讓'TokenHelper'成爲一個singleton，只需將它聲明爲'object'並將'instance'屬性全部放在一起即可。編譯器會照顧你的樣板。 –

@KirillRakhman我相信你的評論是真正的答案。現在它是一個對象，我使用它像'TokenHelper.INSTANCE.getToken（）'（在Java類中）。 – user3105453

我將其添加爲答案。 –

從原來的答案評論改編：

在科特林，你並不需要所有的樣板申報單。當你的聲明類，只是使它成爲object像這樣：TokenHelper.token或從Java TokenHelper.INSTANCE.getToken()：

object TokenHelper { 
    var token: String? = null 
    var appId: String? = null 
    var installationId: String? = null 
    var userId: String? = null 
}

如下然後訪問它。

來源

2016-04-14 13:25:05

你調用遞歸instance getter方法。改變你的getter定義中使用field代替：

var instance: TokenHelper? = null 
    get() { 
     if (field == null) { 
      field = TokenHelper() 
     } 
     return field 
    }

相關摘自文檔：

類在科特林不能有領域。但是，在使用自定義訪問器時，有時需要使用才能擁有後臺字段。對於這些目的，科特林提供可使用字段識別符訪問的自動支持字段：
var counter = 0 // the initializer value is written directly to the backing field 
    set(value) { 
    if (value >= 0) 
     field = value 
    } 
字段的標識符只能在屬性的訪問中使用。

來源

2016-04-11 17:19:17 miensol

謝謝。我剛剛搜索了「Kotlin Singleton」並提出了這個解決方案（https://medium.com/@adinugroho/singleton-in-kotlin-502f80fd8a63#.64ssyu1by）： '私人對象持有者val INSTANCE = TokenHelper（） } 同伴對象{ VAL實例：通過TokenHelper懶{} Holder.INSTANCE} ' 你認爲這是科特林兼容嗎？ Btw：我的初始代碼是由AS在生成「Convert to Kotlin」命令時生成的。 – user3105453

是的，它符合標準。 IDE轉換器適用於非常簡單的情況 - 它的輸出應該在使用前始終檢查。 – miensol

救了我！謝謝。 –

StackOverflowError在Kotlin中使用Singleton

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