kotlin单例实现以及对应的java代码

一、kotlin代码转换为java代码

kotlin代码转换.png

二、kotlin单例——object实现

object SingleDemo{
    fun helloWold(){
        println("Hello World")
    }
}

编写了此kotlin代码之后，我们用上述的方式来转换成java代码：

public final class SingleDemo {
   public static final SingleDemo INSTANCE;

   public final void helloWold() {
      String var1 = "Hello World";
      boolean var2 = false;
      System.out.println(var1);
   }

   private SingleDemo() {
   }

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

可以看到，java代码的实现就是单例的饿汉式

三、kotlin单例——懒汉式

class Singleton private constructor() {
    companion object {
        private var instance: Singleton? = null
            get() {
                if (field == null) {
                    field = Singleton()
                }
                return field
            }

        fun getSingletonInstance(): Singleton {
            return instance!!
        }
    }
}

转换为java代码（核心部分）：

public static final class Companion {
    private final Singleton getInstance()
    {
        if (Singleton.instance == null) {
            Singleton.instance = new Singleton ((DefaultConstructorMarker)null);
        }

        return Singleton.instance;
    }

    private final void setInstance(Singleton var1)
    {
        Singleton.instance = var1;
    }

    @NotNull
    public final Singleton getSingletonInstance()
    {
        Singleton var10000 =((Singleton.Companion)this).getInstance();
        if (var10000 == null) {
            Intrinsics.throwNpe();
        }

        return var10000;
    }

    private Companion()
    {
    }

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

四、kotlin单例——懒汉式安全实现

class Singleton private constructor() {
    companion object {
        private var instance: Singleton? = null
            get() {
                if (field == null) {
                    field = Singleton()
                }
                return field
            }

        @Synchronized
        fun getSingletonInstance(): Singleton {
            return instance!!
        }
    }
}

转换为java代码，其实就是对java版的getSingletonInstance()方法加上了synchronized关键字，这里就不贴出来了

五、kotlin单例——双重检测锁

class Singleton private constructor() {
    companion object {
        val instance : Singleton by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
            Singleton()
        }
    }
}

转换为java代码之后，发现主要是lazy()函数实现的双重检测所，进一步跟踪lazy得到如下源码：

// 未初始化的变量 相当与 NULL
internal object UNINITIALIZED_VALUE

private class SynchronizedLazyImpl<out T>(initializer: () -> T, lock: Any? = null) : Lazy<T>, Serializable {
    private var initializer: (() -> T)? = initializer
    // 禁止指令重排序
    @Volatile private var _value: Any? = UNINITIALIZED_VALUE
    // 对象锁
    private val lock = lock ?: this

    override val value: T
        get() {
            val _v1 = _value
            // 不为空就返回
            if (_v1 !== UNINITIALIZED_VALUE) {
                @Suppress("UNCHECKED_CAST")
                return _v1 as T
            }
            // 加锁
            return synchronized(lock) {
                val _v2 = _value
                 // 再次判空
                if (_v2 !== UNINITIALIZED_VALUE) {
                    @Suppress("UNCHECKED_CAST") (_v2 as T)
                } else {
                    val typedValue = initializer!!()
                    _value = typedValue
                    initializer = null
                    typedValue
                }
            }
        }
        //... 省略一些方法
}

六、kotlin单例——静态内部类

class Singleton private constructor() {
    companion object {
        val instance = Holder.holder
    }

    private object Holder{
        val holder = Singleton
    }
}

这里感觉跟kotlin的object饿汉式差别不大，都是在最开始实例化，只是中间嵌套了一层静态类，我们转换成java代码看一下：

public final class Singleton {
   @NotNull
   private static final Singleton.Companion instance;
   public static final Singleton.Companion Companion = new Singleton.Companion((DefaultConstructorMarker)null);

   private Singleton() {
   }

   static {
      instance = Singleton.Holder.INSTANCE.getHolder();
   }

   @Metadata(
      mv = {1, 1, 16},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0018\u0002\n\u0002\b\u0003\bÂ\u0002\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002R\u0011\u0010\u0003\u001a\u00020\u0004¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\u0007"},
      d2 = {"Lcom/example/hellworld/Singleton$Holder;", "", "()V", "holder", "Lcom/example/hellworld/Singleton$Companion;", "getHolder", "()Lcom/example/hellworld/Singleton$Companion;", "HellWorld.app"}
   )
   private static final class Holder {
      @NotNull
      private static final Singleton.Companion holder;
      public static final Singleton.Holder INSTANCE;

      @NotNull
      public final Singleton.Companion getHolder() {
         return holder;
      }

      static {
         Singleton.Holder var0 = new Singleton.Holder();
         INSTANCE = var0;
         holder = Singleton.Companion;
      }
   }

   @Metadata(
      mv = {1, 1, 16},
      bv = {1, 0, 3},
      k = 1,
      d1 = {"\u0000\f\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0005\b\u0086\u0003\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002R\u0011\u0010\u0003\u001a\u00020\u0000¢\u0006\b\n\u0000\u001a\u0004\b\u0004\u0010\u0005¨\u0006\u0006"},
      d2 = {"Lcom/example/hellworld/Singleton$Companion;", "", "()V", "instance", "getInstance", "()Lcom/example/hellworld/Singleton$Companion;", "HellWorld.app"}
   )
   public static final class Companion {
      @NotNull
      public final Singleton.Companion getInstance() {
         return Singleton.instance;
      }

      private Companion() {
      }

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