Spring单例和多例

使用bean的scope属性来控制单例和多例：

    <!-- bean 的 scope属性可以控制单例和多例
        singleton是默认值：单例的 ；
        prototype：   多例的；
        request:  在web应用中每次请求重新实例化；
        session:  在web应用中每次会话重新实例化;
     -->
    <bean id="people" class="com.spring.pojo.People" scope="singleton"></bean>
    <bean id="people2" class="com.spring.pojo.People" scope="prototype"></bean>

测试代码：

public class Test {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
//        String[] beans = applicationContext.getBeanDefinitionNames();
//        System.out.println(Arrays.toString(beans));
        People people1 = applicationContext.getBean("people",People.class);
        People people2 = applicationContext.getBean("people",People.class);
        System.out.println(people1==people2);
        
        People people3 = applicationContext.getBean("people2",People.class);
        People people4 = applicationContext.getBean("people2",People.class);
        System.out.println(people3==people4);
    }
}

控制台输出：

true
false

单例设计模式，懒汉式： 由于加了锁，所以效率低，于是产生了饿汉式

//单例设计模式：懒汉式
public class Teacher {
    private static Teacher teacher;
    private Teacher() {}
    public static Teacher getInstance() {
        if(teacher==null) {
            //考考虑到多线程，双重判断
            synchronized(Teacher.class) {
                if(teacher==null) {
                    teacher=new Teacher();
                }
            }
        }
        return teacher;
    }
}

单例设计模式，饿汉式：

//单例设计模式：饿汉式
public class Teacher {
    //在对象实例化里就赋值
    private static Teacher teacher = new Teacher();
    private Teacher() {}
    public static Teacher getInstance() {
        return teacher;
    }
}