【設計モード】Springで使用する設計モード

    <bean id="student" class="com.happy.MyStudent">
       <property name="age" value="18"/>
   </bean>

	public class SpringDesignModeDemo {
	    public static void main(String[] args) {
	        ApplicationContext context = new FileSystemXmlApplicationContext("../applicationcontext.xml");
	        MyStudent myStudent = (MyStudent) context.getBean("student");
	        myStudent.getAge();
	    }

<bean id="student" class="com.happy.MyStudent" scope="singleton"/>

 @Scope(value = "singleton")

//    ConcurrentHashMap（    ）        
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "'beanName' must not be null");
        synchronized (this.singletonObjects) {
            //              
            Object singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
                //...       
                try {
                    singletonObject = singletonFactory.getObject();
                }
                //...      
                //           ，         。
                addSingleton(beanName, singletonObject);
            }
            return (singletonObject != NULL_OBJECT ? singletonObject : null);
        }
    }
    //           
    protected void addSingleton(String beanName, Object singletonObject) {
            synchronized (this.singletonObjects) {
                this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
            }
    }
}