Spring中基于XML实例化Bean的四种方式

1.无参数构造器（默认构造器） （最常用）

spring在创建bean的时候自动调用无参构造器来实例化，相当于new Bean1()

Bean1.java

public class Bean1 {
    //默认无参构造
}

applicationContext.xml

<!-- 1.默认构造器实例化对象 -->
<bean id="bean1" class="com.alilaoye.demo02_instantiate_bean.way1.Bean1"/>

Bean1Test.java

public class Bean1Test {
    @Test
    public void test() {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        Bean1 bean1 = (Bean1)applicationContext.getBean("bean1");
        System.out.println(bean1);
    }
}

2.静态工厂方法

Bean2.java

public class Bean2 {
    //默认无参构造
}

Bean2Factory.java

public class Bean2Factory {
    public static Bean2 getBean2() {
        //在做实例化的时候，可以做其他的事情，即可以在这里写初始化其他对象的代码
        //...
        return new Bean2();
    }
}

applicationContext.xml

<!-- 1.默认构造器实例化对象 -->
<!-- 2.静态工厂获取实例化对象 -->
<!-- class:直接指定到静态工厂类, factory-method: 指定生产实例的方法, spring容器在实例化工厂类的时候会自动调用该方法并返回实例对象 -->
<bean id="bean2" class="com.alilaoye.demo02_instantiate_bean.way2.Bean2Factory" factory-method="getBean2"/>

Bean2Test.java

public class Bean2Test {
    @Test
    public void test() {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        Bean2 bean2 = (Bean2)applicationContext.getBean("bean2");
        System.out.println(bean2);
    }
}

3.实例工厂方法

Bean3.java

public class Bean3 {
    //默认无参构造
}

Bean3Factory.java

public class Bean3Factory {
    public Bean3 getBean3() {
        return new Bean3();
    }
}

applicationContext.xml

<!-- 3：实例工厂的方式实例化bean -->
<bean id="bean3Factory" class="com.alilaoye.demo02_instantiate_bean.way3.Bean3Factory"/>
<!-- factory-bean相当于ref：引用一个bean对象 -->
<bean id="bean3" factory-bean="bean3Factory" factory-method="getBean3"/>

Bean3Test.java

public class Bean3Test {
    @Test
    public void test() {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        Bean3 bean3 = (Bean3)applicationContext.getBean("bean3");
        System.out.println(bean3);
    }
}

4.FactoryBean方式

Bean4.java

public class Bean4 {
    //默认无参构造
}

Bean4Factory.java

spring在实例化对象的时候回判断是否该对象实现了FactoryBean接口,如果实现了就直接调用

public class Bean4Factory implements FactoryBean<Bean4> {
    //产生对象
    @Override
    public Bean4 getObject() throws Exception {
        return new Bean4();
    }

    @Override
    public Class<?> getObjectType() {
        return null;
    }
}

applicationContext.xml

<!-- 4:FactoryBean-->
<!--spring在实例化对象的时候会判断是否该对象实现了FactoryBean接口,如果实现了就直接调用getObject()方法,并返回执行结果-->
<bean id="bean4" class="com.alilaoye.demo02_instantiate_bean.way4.Bean4Factory"/>

Bean3Test.java

public class Bean4Test {
    @Test
    public void test() {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        Bean4 bean4 = (Bean4)applicationContext.getBean("bean4");
        System.out.println(bean4);
    }
}