69      技術社區[雲棲]

單例模式--餓漢式、懶漢式、線程安全的懶漢式、高性能安全的懶漢式

深入學習單例設計模式

餓漢式單例：開發時較為常用。

class Single1 {
	private Single1(){}
	private static Single1 single = new Single1();
	public static Single1 getInstance() {
 	      return single;
	}
	public static void main(String[] args) {
		Single1 s = Single1.getInstance();
		Single1 s1 = Single1.getInstance();
		System.out.println(s==s1);
	}
}

多線程會出現問題的懶漢式單例：

class Single2{
private Single2(){}
private static Single2 single = null;
public static Single2 getInstance(){
if (single == null) {
single = new Single2();
}
return single;
}
public static void main(String[] args) {
Single2 s = Single2.getInstance();
Single2 s1 = Single2.getInstance();
System.out.println(s==s1);
}
}

 解決多線程安全問題的懶漢式單例

class Single3{
private Single3(){}
private static Single3 single = null;
public static synchronizedSingle3 getInstance(){
if (single == null) {
single = new Single3();
}
return single;
}
public static void main(String[] args) {
Single3 s = Single3.getInstance();
Single3 s1 = Single3.getInstance();
System.out.println(s==s1);
}
}

懶漢式的目的是為了提高性能，synchronized卻降低了性能

class Single4{
private Single4(){}
private static Single4 single = null;
public static Single4 getInstance(){
if (single == null) {
synchronized(Single4.class) {
if (single == null)
{
single = new Single4();
}
}
}
return single;
}
public static void main(String[] args) {
Single4 s = Single4.getInstance();
Single4.s1 = Single4.getInstance();
System.out.println(s==s1);
}
}

比較推薦的還是第一種餓漢式單例（Hibernate4中官方源碼）：

public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
            new Configuration().configure().buildSessionFactory(
			    new StandardServiceRegistryBuilder().build() );
        }
        catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

}

未完待續...

最後更新：2017-04-02 06:51:52

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