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

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

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

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

enum Singleton{
 INSTANCE;
 
 public void doSomething(){
 ...
 }
}

/**
 *通过反射破坏单例模式 
 */
public class Demo01 {

 public static void main(String[] args) throws Exception {
 Singleton s1 = Singleton.getInstance();
 Singleton s2 = Singleton.getInstance();
 
 System.out.println(s1 == s2);
 
 Class<Singleton> clazz = (Class<Singleton>) Class.forName("com.singleton.Singleton");
 Constructor<Singleton> constructor = clazz.getDeclaredConstructor(null);
 constructor.setAccessible(true);
 Singleton s3 = constructor.newInstance();
 
 System.out.println(s1 == s3);
 }
}

class Singleton{
 private static Singleton instance = new Singleton();
 
 private Singleton(){
 //防止反射破坏单利模式的方法,打开注释部分
// if(instance != null){
//  throw new RuntimeException();
// }
 }
 
 public static Singleton getInstance(){
 return instance;
 }
}

/**
 * 反序列化打破单例模式
 */
public class Demo02 {

 public static void main(String[] args) throws Exception {
 Singleton s1 = Singleton.getInstance();
 Singleton s2 = Singleton.getInstance();
 
 System.out.println(s1 == s2);
 
 FileOutputStream fos = new FileOutputStream("d://test.txt");
 ObjectOutputStream oos = new ObjectOutputStream(fos);
 oos.writeObject(s1);
 
 oos.close();
 fos.close();
 
 ObjectInputStream ois = new ObjectInputStream(new FileInputStream("d://test.txt"));
 Singleton s3 = (Singleton) ois.readObject();
 
 System.out.println(s1 == s3);
 }
}
class Singleton implements Serializable{
 private static Singleton instance = new Singleton();
 
 public static Singleton getInstance(){
 return instance;
 }
 
 //反序列化时，如果对象已经存在，将调用这个方法
// private Object readResolve() throws ObjectStreamException{
// return instance;
// 
// }
}