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1、饿汉式

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

2、懒汉式

2.1普通加锁模式

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

2.2占位符模式（推荐）

属于懒汉式单例，由于Java机制规定。内部类SingletonHolder仅仅有在getInstance()方法第一次调用的时候才会被载入（实现了lazy），并且其载入过程是线程安全的。内部类载入的时候实例化一次instance。

public class Singleton {
  private Singleton() { }
   privatestatic class SingletonHolder {  
//内部类。第一次使用时才载入，且仅仅能SingletonHolder类能訪问
//特别注意：static域中改动共享变量是线程安全的，由JVM保障
     static Singleton INSTANCE = new Singleton();
   }
  public static Singleton getInstance() {
     return SingletonHolder.INSTANCE;
   }
}

2.3双重检測

普通双重检測：

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

public class Singleton {
  private static volatile Singletoninstance = null;  //以volatilekeyword修饰防止指令重排
  private Singleton() { }  //构造函数为私有，防止被实例化
  public static Singleton getInstance() {
     if(instance == null) {     //双重检測
        synchronzied(Singleton.class) {
           if(instance == null) {
               instance = new Singleton();
           }
        }
     }
     return instance;
   }
}