Spring构造函数依赖项注入-带vs.不带自动装配

Question

我遇到了此问题中描述的问题 ，已经通过在构造函数中添加@Autowired批注来解决该问题 。 现在，我想知道为什么会有所帮助。 之间有什么区别

public RegistrationController(UserDao userDao) {
    this.userDao = userDao;
}

和

@Autowired
public RegistrationController(UserDao userDao) {
    this.userDao = userDao;
}

在这两种情况下，都将userDao注入Controller。 我发现的唯一区别是，仅在第二个示例中，使用@persistenceContext注释标记的entityManager仅被注入到userDao中。 但是我不知道为什么。 有什么线索吗？ 还有其他区别吗？

编辑：我的servlet上下文看起来像这样：

<beans  xmlns="http://www.springframework.org/schema/beans"
        xmlns:context="http://www.springframework.org/schema/context"
        xmlns:mvc="http://www.springframework.org/schema/mvc"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:p="http://www.springframework.org/schema/p"
        xmlns:tx="http://www.springframework.org/schema/tx"
        xsi:schemaLocation="
            http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
            http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
            http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd">

    <!-- 
    Adds some default beans (HandlerAdapter, HandlerMapping, Binding Initializer...). It also turn on some annotations. 
    Explanation in https://stackoverflow.com/questions/28851306/spring-framework-what-is-the-purpose-of-mvcannotation-driven

    WITHOUT THIS, @RequestMapping ANNOTATIONS ARE LOADED, BUT MAPPING DO NOT WORK!!
    -->
    <mvc:annotation-driven />

<!--    Set loading annotations from classes
    <context:component-scan base-package="com.fido.pia"/>-->

    <!--manual homepage-->
    <mvc:view-controller path="/" view-name="home"/>

    <!--view resolver-->
    <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/view/" />
        <property name="suffix" value=".jsp" />
    </bean>

    <!--static resources - request will be handeled by ResourceHttpRequestHandler-->
    <mvc:resources mapping="/resources/**" location="/resources/" />

    <!--database config-->
    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName" value="com.mysql.jdbc.Driver" />
        <property name="url" value="jdbc:mysql://localhost:3306/pia" />
        <property name="username" value="root" />
        <property name="password" value="" />
    </bean>

    <!--entity manager factory-->
    <bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
        <property name="packagesToScan" value="com.fido.pia.*/**" />
        <property name="dataSource" ref="dataSource" />
        <property name="jpaVendorAdapter">
            <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
                <!--<property name="generateDdl" value="true" />-->
                <property name="showSql" value="true" />
            </bean>
        </property>
    </bean>

    <!-- Transactions -->
    <bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
        <property name="entityManagerFactory" ref="entityManagerFactory" />
    </bean>

    <!-- enable the configuration of transactional behavior based on annotations -->
    <tx:annotation-driven transaction-manager="transactionManager" />

    <!--Set loading annotations from classes-->
    <context:component-scan base-package="com.fido.pia"/>
</beans>

编辑2：控制器：

package com.fido.pia;

import com.fido.pia.dao.UserDao;
import com.fido.pia.model.User;
import java.io.IOException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
@RequestMapping("/register")
public class RegistrationController {

    private UserDao userDao;

    @Autowired
    public RegistrationController(UserDao userDao) {
        this.userDao = userDao;
    }

    @RequestMapping(method = RequestMethod.POST)
    @Transactional
    public void registrationSubmit(HttpServletRequest request, HttpServletResponse response) 
            throws IOException{
        String username = request.getParameter("first_name");

        userDao.save(new User("test"));

        response.sendRedirect("/");
    }
}

道：

package com.fido.pia.dao;

import com.fido.pia.model.User;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Repository;


@Repository
public class UserDao {

    @PersistenceContext
    protected EntityManager entityManager;

    public User save(User row) {
        if(row.isNew()) {
            entityManager.persist(row);
            return row;
        } else {
            return entityManager.merge(row);
        }
    }   
}

Answer 1

存在两种形式的注射对象的最大用途！

- First 
  @Autowired, 
 - Second in constructor 
 @Autowired
 constructor(Object object)
 - other
 @Autowired
 setObjectMehod(Object object)

当为两个以上的对象实现接口时 ，有必要限定要插入的对象。

总是将要注入组件的每个对象 ，最常见的@ Repository，@ Services，@ Controller和@Component都具有适当的作用域。

注入具有相同的目的，只是对象所需要的方式和需求不同