Kotlin单例

Kotlin单例

下面介绍一下kotlin 线程安全的几种单例写法。

0x01 饿汉模式

// Kotlin实现
object Singleton {
}

// 反编译Kotlin实现的Java代码
public final class Singleton {
   public static final Singleton INSTANCE;

   private Singleton() {
   }

   static {
      Singleton var0 = new Singleton();
      INSTANCE = var0;
   }
}

0x02 双重校验锁式

双重校验锁式（Double Check)

// Kotlin实现
class Singleton private constructor() {
    companion object {
        @Volatile
        private var instance: Singleton? = null

        @JvmStatic
        fun getInstance(): Singleton {
            if (instance == null) {
                synchronized(Singleton::class.java) {
                    if (instance == null) {
                        instance = Singleton()
                    }
                }
            }
            return instance!!
        }
    }
}

// 反编译Kotlin实现的Java代码
public final class Singleton {
   private static volatile Singleton instance;
   public static final Singleton.Companion Companion = new Singleton.Companion((DefaultConstructorMarker)null);

   private Singleton() {
   }

   // $FF: synthetic method
   public Singleton(DefaultConstructorMarker $constructor_marker) {
      this();
   }

   @JvmStatic
   @NotNull
   public static final Singleton getInstance() {
      return Companion.getInstance();
   }

   public static final class Companion {
      @JvmStatic
      @NotNull
      public final Singleton getInstance() {
         if (Singleton.instance == null) {
            Class var1 = Singleton.class;
            boolean var2 = false;
            boolean var3 = false;
            synchronized(var1) {
               int var4 = false;
               if (Singleton.instance == null) {
                  Singleton.instance = new Singleton((DefaultConstructorMarker)null);
               }

               Unit var6 = Unit.INSTANCE;
            }
         }

         Singleton var10000 = Singleton.instance;
         if (var10000 == null) {
            Intrinsics.throwNpe();
         }

         return var10000;
      }

      private Companion() {
      }

      // $FF: synthetic method
      public Companion(DefaultConstructorMarker $constructor_marker) {
         this();
      }
   }
}

0x03 静态内部类

//Java实现
public class SingletonDemo {
    private static class SingletonHolder{
        private static SingletonDemo instance=new SingletonDemo();
    }
    private SingletonDemo(){
        System.out.println("Singleton has loaded");
    }
    public static SingletonDemo getInstance(){
        return SingletonHolder.instance;
    }
}

// Kotlin实现
class Singleton private constructor() {
    companion object {
        @JvmStatic
        fun getInstance() = SingletonHolder.instance
    }
    private object SingletonHolder {
        @JvmStatic
        val instance: Singleton = Singleton()
    }
}

// 反编译Kotlin实现的Java代码
public final class Singleton {
   public static final Singleton.Companion Companion = new Singleton.Companion((DefaultConstructorMarker)null);
   private Singleton() {
   }
   @JvmStatic
   @NotNull
   public static final Singleton getInstance() {
      return Companion.getInstance();
   }
   private static final class SingletonHolder {
      @NotNull
      private static final Singleton instance;
      public static final Singleton.SingletonHolder INSTANCE;
      @NotNull
      public static final Singleton getInstance() {
         return instance;
      }
      static {
         Singleton.SingletonHolder var0 = new Singleton.SingletonHolder();
         INSTANCE = var0;
         instance = new Singleton((DefaultConstructorMarker)null);
      }
   }
   public static final class Companion {
      @JvmStatic
      @NotNull
      public final Singleton getInstance() {
         return Singleton.SingletonHolder.getInstance();
      }
      private Companion() {
      }
   }
}