ddu-单例模式

饿汉式

  public class Singleton {
  private static final Singleton singleton=new Singleton();
 public Singleton getInstance(){
  return singleton;
   }
 }

懒汉式

   class Singleton2{
   private  static Singleton2 mInstance ;

   public static Singleton2 getInstance(){
   if (mInstance==null){
     return new Singleton2();
       }
     return mInstance;
      }
   }

方法加锁

  class Singleton3{
     private static Singleton3 mInstance;

     public static synchronized Singleton3 getInstance(){
     if (mInstance==null){
     mInstance= new Singleton3();
 }
 return mInstance;
   }
 }

Double Check Lock(DCL)

 class Singleton4{
 private static Singleton4 mInstance;

 public static  Singleton4 getInstance(){
 if (mInstance==null){
     synchronized(Singleton4.class){
         mInstance= new Singleton4();
     }
 }
 return mInstance;
   }
 }

静态内部类单例模式

   class Singleton5{

     private static Singleton5 getInstance(){
     return Singleton5Holder.singleton5;
     }
 }
 class Singleton5Holder{
 public static Singleton5 singleton5=new Singleton5();
 }