public class Singleton { 
 
  /* 持有私有静态实例，防止被引用，此处赋值为null，目的是实现延迟加载 */ 
  private static Singleton instance = null; 
 
  /* 私有构造方法，防止被实例化 */ 
  private Singleton() { 
  } 
 
  /* 1:懒汉式，静态工程方法，创建实例 */ 
  public static Singleton getInstance() { 
    if (instance == null) { 
      instance = new Singleton(); 
    } 
    return instance; 
  } 
} 

Singleton.getInstance().method(); 

/*2.懒汉式变种，解决线程安全问题**/ 
 public static synchronized Singleton getInstance() { 
   if (instance == null) { 
     instance = new Singleton(); 
   } 
   return instance; 
 } 

/*加上synchronized，但是每次调用实例时都会加载**/ 
 public static Singleton getInstance() { 
   synchronized (Singletonclass) { 
     if (instance == null) { 
       instance = new Singleton(); 
     } 
   } 
   return instance; 
 } 

Singleton.getInstance().method(); 

/*3.双重锁定:只在第一次初始化的时候加上同步锁*/ 
 public static Singleton getInstance() { 
   if (instance == null) { 
     synchronized (Singletonclass) { 
       if (instance == null) { 
         instance = new Singleton(); 
       } 
     } 
   } 
   return instance; 
 } 

instance = new Singleton(); 

Singleton.getInstance().method(); 

public class SingletonInner { 
 
  /** 
   * 内部类实现单例模式 
   * 延迟加载，减少内存开销 
   * 
   * @author xuzhaohu 
   * 
   */ 
  private static class SingletonHolder { 
    private static SingletonInner instance = new SingletonInner(); 
  } 
 
  /** 
   * 私有的构造函数 
   */ 
  private SingletonInner() { 
 
  } 
 
  public static SingletonInner getInstance() { 
    return SingletonHolderinstance; 
  } 
 
  protected void method() { 
    Systemoutprintln("SingletonInner"); 
  } 
} 

SingletonInner.getInstance().method(); 

/** 
 * @function:单例模式枚举实现 
 * @author xuzhaohu 
 * 
 */ 
public enum SingletonEnum { 
  /** 
   * 从Java5开始支持; 
   * 无偿提供序列化机制; 
   * 绝对防止多次实例化，即使在面对复杂的序列化或者反射攻击的时候; 
   */ 
 
  instance; 
 
  private String others; 
 
  SingletonEnum() { 
 
  } 
 
  public void method() { 
    Systemoutprintln("SingletonEnum"); 
  } 
 
  public String getOthers() { 
    return others; 
  } 
 
  public void setOthers(String others) { 
    thisothers = others; 
  } 
} 

SingletonEnum.instance.method();