Spring框架下Bean的方法注入(Method injection)

心已赠人 2022-05-23 03:42 190阅读 0赞

通常情况下我们在spring中定义的常用bean都是单例模式（singleton）的例如@Service、@Controller定义的bean，任何一个类都只有一个对象，只进行一次初始化这些类的属性永远是最后一次调用它时的值。但是针对不同的业务场景我们可能会定义一些bean，这些bean的属性在不同场景下它的属性值是不同的，因此这时候我们就不能使用单例模式了而使用原型模式（prototype），这种模式相当于new一个对象。参见spring官方文档。  
1.4.6. Method injection  
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.  
最后两句：假定有单例bean A和原型bean B，当A初始化的时候只有一次机会初始化B，但是A中可能在不同方法中对B进行了调用，这时候可能B中的部分属性发生了变化，而在A中的不同方法可能希望B中的属性是刚初始化时的值，但是容器并不能提供新的B类的实体，因此才有了下面的解决办法

A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean(“B”) call to the container ask for (a typically new) bean B instance every time bean A needs it. The following is an example of this approach:

利用ApplicationContextAware接口中的getBean方法向容器请求一个新的bean  
具体参考以下代码

package com.spring.util;
    
    import org.springframework.beans.BeansException;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.ApplicationContextAware;
    import org.springframework.stereotype.Component;
    
    /** * @author chengjian * @date 2018/5/16 **/
    @Component
    public class SpringBeanFactory implements ApplicationContextAware { 
        private  ApplicationContext applicationContext;
    
    
        @Override
        public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
            this.applicationContext=applicationContext;
        }
    
        public <T> T getBean(Class<T> clazz){
            try{
                return applicationContext.getBean(clazz);
            }catch (Exception e){
                e.printStackTrace();
            }
            return null;
        }
    }

我们创建了一个SpringBeanFactory，这个类实现了ApplicationContextAware方法，我们在类中新定义了一个基于泛型的getBean方法，通过这样的方式我们就能通过方法的方式从spring容器中获取到我们想要的bean。参考下面的代码

package com.spring.javatest.io;
    
    import com.github.javafaker.Faker;
    import com.spring.config.SpringConfig;
    import com.spring.javatest.entity.UserEntity;
    import com.spring.util.CommonUtil;
    import com.spring.util.SpringBeanFactory;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.ObjectOutput;
    import java.io.ObjectOutputStream;
    import java.util.Locale;
    
    /** * @author chengjian * @date 2018/5/15 **/
    @Component
    public class OutPutStreamTest {
        @Autowired
        private SpringBeanFactory springBeanFactory;
    
        public void fileOutPutSerializable() {
    
            try {
                Faker faker=new Faker(new Locale("zh-CN"));
                UserEntity user=new UserEntity();
                user.setUserid(CommonUtil.getUuid());
                user.setUserpwd("123456");
                user.setUsername("a001");
                user.setRealname(faker.name().fullName());
                String fileName = springBeanFactory.getBean(SpringConfig.class).getLocation() + "output";
                FileOutputStream f = new FileOutputStream(fileName);
                ObjectOutput s = new ObjectOutputStream(f);
                s.writeObject(user);
                s.flush();
                s.close();
                f.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    }

我们并没有通过@Autowired注解注入SpringConfig，而是通过方法注入的方式注入的bean