单例模式

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实现方式

懒汉模式

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

饿汉模式

public class Singleton {      ////类加载时实例化对象    private static Singleton instance = new Singleton();      private Singleton (){}      public static Singleton getInstance() {      return instance;      }  }

双重校验锁（DCL，即 double-checked locking）

public class Singleton {      private volatile static Singleton singleton;//延迟加载      private Singleton (){}      public static Singleton getSingleton() {      if (singleton == null) {          synchronized (Singleton.class) {//同步锁        if (singleton == null) {              singleton = new Singleton();          }          }      }      return singleton;      }  }

登记式

public class Singleton {      private static final class  Holder { //静态内部类         static final Singleton instance = new Singleton();      }      private Singleton() {}     public static Singleton getInstance() {          return Holder.instance;      }  }

枚举方式

public enum Singleton {      INSTANCE;      public void whateverMethod() {      }  }

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