单例模式--饿汉式、懒汉式、线程安全的懒汉式、高性能安全的懒汉式

单例模式学习小结

饿汉式单例：开发时较为常用。

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);

}

}

多线程会出现问题的懒汉式单例：

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);

}

}

解决多线程安全问题的懒汉式单例

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);

}

}

懒汉式的目的是为了提高性能，synchronized却降低了性能

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);

}

}

比较推荐的还是第一种饿汉式单例：

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;
    }

}