???????????????
public class Singleton{
private Singleton(){}
public static Singleton getInstance(){
return InnerClassSingleton.singleton;
}
private class InnerClassSingleton{
protected static Singleton singleton = new Singleton();
}
}
?????????????????????????????????????????????????????????????????????????????
?????????????
????· ?????????????????????????????????п?????????
????· ????java?????????????????????????п??????????????
???????л???????л????????????????л??????????????????
private static void singleSerializable() {
try (FileOutputStream fileOutputStream=new FileOutputStream(new File("myObjectFilee.txt"));
ObjectOutputStream objectOutputStream=new ObjectOutputStream(fileOutputStream);) {
//            SingletonObject singletonObject = SingletonObject.getInstance();
//            InnerClassSingleton singletonObject = InnerClassSingleton.getInstance();
EnumSingleton singletonObject = EnumSingleton.INSTANCE;
objectOutputStream.writeObject(singletonObject);
objectOutputStream.close();
fileOutputStream.close();
System.out.println(singletonObject.hashCode());
} catch (IOException e) {
e.printStackTrace();
}
try (FileInputStream fileInputStream=new FileInputStream(new File("myObjectFilee.txt"));
ObjectInputStream objectInputStream=new ObjectInputStream(fileInputStream);) {
//            SingletonObject singleTest=(SingletonObject) objectInputStream.readObject();
//            InnerClassSingleton singleTest=(InnerClassSingleton) objectInputStream.readObject();
EnumSingleton singleTest=(EnumSingleton) objectInputStream.readObject();
objectInputStream.close();
fileInputStream.close();
System.out.println(singleTest.hashCode());
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
??????????????
ObjectInputStream?е?readOrdinaryObject??
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
if (unshared && rep.getClass().isArray()) {
rep = cloneArray(rep);
}
if (rep != obj) {
handles.setObject(passHandle?? obj = rep);
}
}
??????????????readResolve????
????protected Object readResolve(){
????System.out.println("??????readResolve??????");
????return  InnerClassSingleton.getInstance();
????}
??????????????????????????????
private static void attack() {
try {
Class<?> classType = InnerClassSingleton.class;
Constructor<?> constructor = classType.getDeclaredConstructor(null);
constructor.setAccessible(true);
InnerClassSingleton singleton = (InnerClassSingleton) constructor.newInstance();
InnerClassSingleton singleton2 = InnerClassSingleton.getInstance();
System.out.println(singleton == singleton2);  //false
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
??????????? : ??й??????н?????????ж??
????private InnerClassSingleton() {
????synchronized (InnerClassSingleton.class) {
????if (false == flag) {
????flag = !flag;
????} else {
????throw new RuntimeException("???????????????");
????}
????}
????}
?????????????????????
???????????????????
????· ???????л???
????· ???????????????????÷??????????????????????????????
????· ???????
????public enum Singleton {
????INSTANCE;
????private Singleton(){}
????}
????Hibernate????????
???????ThreadLocal????
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;
public class HibernateSessionFactory {
private static String CONFIG_FILE_LOCATION = "/hibernate.cfg.xml";
private static final ThreadLocal threadLocal = new ThreadLocal();
private static Configuration configuration = new Configuration();
private static org.hibernate.SessionFactory sessionFactory;
private static String configFile = CONFIG_FILE_LOCATION;
static {
try {
configuration.configure(configFile);
sessionFactory = configuration.buildSessionFactory();
} catch (Exception e) {
System.err.println("%%%% Error Creating SessionFactory %%%%");
e.printStackTrace();
}
}
private HibernateSessionFactory() {
}
public static Session getSession() throws HibernateException {
Session session = (Session) threadLocal.get();
if (session == null || !session.isOpen()) {
if (sessionFactory == null) {
rebuildSessionFactory();
}
session = (sessionFactory != null) ? essionFactory.openSession() : null;
threadLocal.set(session);
}
return session;
}
// Other methods...
}