Kotlin学习笔记：Kotlin中五种单例模式的实现

作者: 码途有道 | 来源:发表于2019-03-20 10:57 被阅读0次

1、饿汉式

在类被初始化时就已经在内存中创建了对象，以空间换时间，故不存在线程安全问题，但是会使类加载变慢。

Java实现

public class SingletonDemo {

    private SingletonDemo () {

    }

    // 在类加载时就完成了初始化，使得类加载较慢
    private static SingletonDemo instance = new SingletonDemo ();

    public static SingletonDemo getInstance() {
        return instance ;
    }
}

Kotlin实现

object SingletonDemo {

}

Kotlin编译成的Java字节

public final class SingletonDemo {
   public static final SingletonDemo instance ;

   static {
      SingletonDemo var0 = new SingletonDemo();
      instance = var0;
   }
}

2、懒汉式 - 非线程安全

在方法被调用后才创建对象，以时间换空间，在多线程环境下存在风险。

Java

public class SingletonDemo {

    private SingletonDemo () {

    }
   
    private static SingletonDemo instance ;

    public static SingletonDemo getInstance() {
        if (instance == null) {
            instance = new SingletonDemo ();
        }
        return instance ;
    }
}

Kotlin

class SingletonDemo {
    
    private constructor() {
    }

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

        fun get(): SingletonDemo {
            return instance!!
        }
    }
}

Kotlin编译成的Java字节

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

   private SingletonDemo() {
   }

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

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

         return SingletonDemo.instance;
      }

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

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

         return var10000;
      }

      private Companion() {
      }

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

3、懒汉式 - 线程安全

相比上种非线程安全模式，对getInstance()方法添加了同步锁，保证线程安全。但是此种方式在有多个线程同时执行时，每次执行getInstance()方法时都要先获得锁再去执行方法体，如果没有锁，就要等待，耗时长。

Java

public class SingletonDemo {
    private static SingletonDemo instance;
    private SingletonDemo(){}
    public static synchronized SingletonDemo getInstance(){
        if(instance== null){
            instance = new SingletonDemo();
        }
        return instance;
    }
}

Kotlin

class SingletonDemo {
    private constructor() {}
    
    companion object {
        private var instance: SingletonDemo? = null
            get() {
                if (field == null) {
                    field = SingletonDemo()
                }
                return field
            }
        
        @Synchronized
        fun get(): SingletonDemo{
            return instance!!
        }
    }
}

Kotlin编译成的Java字节

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

   private SingletonDemo() {
   }

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

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

         return SingletonDemo.instance;
      }

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

      @NotNull
      public final synchronized SingletonDemo get() {
         SingletonDemo var10000 = ((SingletonDemo.Companion)this).getInstance();
         if (var10000 == null) {
            Intrinsics.throwNpe();
         }

         return var10000;
      }

      private Companion() {
      }

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

4、双重检测锁

只有在对象需要被使用时才创建，第一次判断 instance == null为了避免非必要加锁，当第一次加载时才对实例进行加锁再实例化。这样既可以节约内存空间，又可以保证线程安全。（并且一定要加上volatile 关键字，用于防止指令重排序问题发生）

Java

public class SingletonDemo {
    private volatile static SingletonDemo instance;
    private SingletonDemo(){} 
    public static SingletonDemo getInstance(){
        if(instance == null){
            synchronized (SingletonDemo.class){
                if(instance == null){
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }
}

Kotlin

class SingletonDemo {
    private constructor() {}

    companion object {
        val instance: SingletonDemo by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
            SingletonDemo()
        }
    }
}

Kotlin编译成的Java字节

public final class SingletonDemo {
   @NotNull
   private static final Lazy instance$delegate;
   public static final SingletonDemo.Companion Companion = new SingletonDemo.Companion((DefaultConstructorMarker)null);

   private SingletonDemo() {
   }

   static {
      instance$delegate = LazyKt.lazy(LazyThreadSafetyMode.SYNCHRONIZED, (Function0)null.INSTANCE);
   }

   // $FF: synthetic method
   public SingletonDemo(DefaultConstructorMarker $constructor_marker) {
      this();
   }
   ...
   public static final class Companion {
      // $FF: synthetic field
      static final KProperty[] $$delegatedProperties = new KProperty[]{(KProperty)Reflection.property1(new PropertyReference1Impl(Reflection.getOrCreateKotlinClass(SingletonDemo.Companion.class), "instance", "getInstance()Lcom/homeprint/module/mine/viewmodel/SingletonDemo;"))};

      @NotNull
      public final SingletonDemo getInstance() {
         Lazy var1 = SingletonDemo.instance$delegate;
         KProperty var3 = $$delegatedProperties[0];
         return (SingletonDemo)var1.getValue();
      }

      private Companion() {
      }

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

这里面使用到了Kotlin的延迟属性 Lazy，Lazy 是接受一个 lambda 并返回一个 Lazy 实例的函数，返回的实例可以作为实现延迟属性的委托：第一次调用 get() 会执行已传递给 lazy() 的 lambda 表达式并记录结果，后续调用 get() 只是返回记录的结果。并且 Lazy 只能用于val修饰的对象。

此处在附上Kotlin可带参数的双重检测锁模式

class SingletonDemo {

    private constructor(args: Int) {
        
    }

    companion object {
        @Volatile
        private var instance: SingletonDemo? = null

        fun getInstance(args: Int) =
                instance ?: synchronized(this) {
                    instance ?: SingletonDemo(args).also { instance = it }
                }
    }
}

5、静态内部类

外部类加载时并不需要立即加载内部类，内部类不被加载则不会去初始化 instance ，就不会占内存。不仅确保了线程安全，也能保证单例的唯一性，同时也延迟了单例的实例化。但是有一个致命缺点，无法传递参数。

Java

public class SingletonDemo {
    private SingletonDemo(){
    }

    private static class SingletonHolder{
        private static SingletonDemo instance = new SingletonDemo();
    }
   
    public static SingletonDemo getInstance(){
        return SingletonHolder.instance;
    }
}

Kotlin

class SingletonDemo {
    private constructor() {}

    companion object {
        val instance = SingletonHolder.holder
    }

    private object SingletonHolder {
        val holder = SingletonDemo()
    }
}

Kotlin编译成的Java字节

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

   private SingletonDemo() {
   }

   static {
      instance = SingletonDemo.SingletonHolder.INSTANCE.getHolder();
   }

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

   ...
   private static final class SingletonHolder {
      @NotNull
      private static final SingletonDemo holder;
      public static final SingletonDemo.SingletonHolder INSTANCE;

      @NotNull
      public final SingletonDemo getHolder() {
         return holder;
      }

      static {
         SingletonDemo.SingletonHolder var0 = new SingletonDemo.SingletonHolder();
         INSTANCE = var0;
         holder = new SingletonDemo((DefaultConstructorMarker)null);
      }
   }

   ...
   public static final class Companion {
      @NotNull
      public final SingletonDemo getInstance() {
         return SingletonDemo.instance;
      }

      private Companion() {
      }

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

枚举

枚举在 java 中与普通类一样，都能拥有字段与方法，而且枚举实例创建是线程安全的，在任何情况下，它都是一个单例，只是在Android中枚举类太过于耗费资源。此处做一下补充。

Java

public enum SingletonDemo {
      INSTANCE(0xFF0000)

      private int rgb;
      SingletonDemo（int rgb）{
            this.rgb = rg
      }
}

// 调用方法
SingletonDemo .INSTANCE

Kotlin

enum class SingletonDemo (val rgb: Int) {
    INSTANCE(0xFF0000)
}

// 调用方法
SingletonDemo .INSTANCE

Kotlin学习笔记：Kotlin中五种单例模式的实现

1、饿汉式

2、懒汉式 - 非线程安全

3、懒汉式 - 线程安全

4、双重检测锁

5、静态内部类

枚举

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深度阅读

栏目导航

热点阅读

面试专题

Kotlin学习笔记：Kotlin中五种单例模式的实现

1、 饿汉式

2、 懒汉式 - 非线程安全

3、 懒汉式 - 线程安全

4、双重检测锁

5、静态内部类

枚举

推荐文章

相关文章

网友评论

延伸阅读

深度阅读

栏目导航

热点阅读

面试专题

1、饿汉式

2、懒汉式 - 非线程安全

3、懒汉式 - 线程安全