• 懒汉式:线程安全，开销大

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

• 双重检查锁：线程安全，根据需求使用

public class Singleton{
  private volatile static Singleton singleton;
  private Singleton(){
  }
  public static final Singleton getInstance(){
      if(singleton==null){
        synchronized(Singleton.class){
          if(singleton==null){
          singleton=new Singleton();
          }
        }
      }
   return singleton;
  }
}

• 静态内部类锁：线程安全，比较推荐

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

• 饿汗式：线程安全，但没有实现懒加载

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

• 枚举:线程安全，但比较少人用

public class Singleton{
public enum EasySingleton{
    INSTANCE;
}
}

• 参考文章
  • 具体JVM机制
  • 具体代码讲解