Three commonly used injection methods in Spring

Three commonly used injection methods in Spring

1. Construction method, using reflection, completes injection through the construction method.

Let’s take a brief look at the structure of the test project. It is built with maven and has four packages:

• entity: storage entity, with only one User class in it

• dao: data access, one interface, two implementation classes

• service: service layer, one interface, one implementation class, the implementation class depends on IUserDao

• test: Test package

Register UserService in the spring configuration file, and inject UserDaoJdbc into a userService through the constructor-arg tag. Parameter constructor

<!-- 注册userService -->
<bean id="userService" class="com.lyu.spring.service.impl.UserService">
  <constructor-arg ref="userDaoJdbc"></constructor-arg>
</bean>
<!-- 注册jdbc实现的dao -->
<bean id="userDaoJdbc" class="com.lyu.spring.dao.impl.UserDaoJdbc"></bean>

If there is only one constructor with parameters and the parameter type matches the type of the injected bean, it will be injected into the constructor.

public class UserService implements IUserService {
  private IUserDao userDao;
  
  public UserService(IUserDao userDao) {
    this.userDao = userDao;
  }
  
  public void loginUser() {
    userDao.loginUser();
  }
}
@Test
public void testDI() {
  ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
  // 获取bean对象
  UserService userService = ac.getBean(UserService.class, "userService");
  // 模拟用户登录
  userService.loginUser();
}

Test print result: jdbc-login successful

Note: The loginUser method that simulates user login actually just prints an output statement. The output of the class implemented by jdbc is: jdbc- The login is successful. The output of the class implemented by mybatis is: mybatis-login successful.

2. Setter also uses reflection, but the injection is completed through setter.

The configuration file is as follows:

<!-- 注册userService -->
<bean id="userService" class="com.lyu.spring.service.impl.UserService">
  <!-- 写法一 -->
  <!-- <property name="UserDao" ref="userDaoMyBatis"></property> -->
  <!-- 写法二 -->
  <property name="userDao" ref="userDaoMyBatis"></property>
</bean>
<!-- 注册mybatis实现的dao -->
<bean id="userDaoMyBatis" class="com.lyu.spring.dao.impl.UserDaoMyBatis"></bean>

Note: Both of the above writing methods are acceptable. Spring will convert the first letter of each word in the name value to uppercase, and then splice it in front. Use "set" to form a method name, then search for the method in the corresponding class, and call it through reflection to achieve injection.

Remember: the name attribute value has nothing to do with the member variable name in the class or the parameter name of the set method. It is only related to the corresponding set method name. The following writing method can run successfully

public class UserService implements IUserService {
  private IUserDao userDao1;
  
  public void setUserDao(IUserDao userDao1) {
    this.userDao1 = userDao1;
  }
  
  public void loginUser() {
    userDao1.loginUser();
  }
}

One more thing to note: If you inject properties through the set method, spring will instantiate the object through the default empty parameter constructor, so if you write a constructor with parameters in the class, it must The constructor method with empty parameters must be written, otherwise spring will not be able to instantiate the object, resulting in an error.

3. Based on annotations, commonly used ones are "@Autowried" and "@Resource".

Sample code

// candidateBeans 为上一步通过类型匹配到的多个bean，该 Map 中至少有两个元素。
protected String determineAutowireCandidate(Map<String, Object> candidateBeans, DependencyDescriptor descriptor) {
    //  requiredType 为匹配到的接口的类型
   Class<?> requiredType = descriptor.getDependencyType();
   // 1. 先找 Bean 上有@Primary 注解的，有则直接返回
   String primaryCandidate = this.determinePrimaryCandidate(candidateBeans, requiredType);
   if (primaryCandidate != null) {
       return primaryCandidate;
   } else {
       // 2.再找 Bean 上有 @Order，@PriorityOrder 注解的，有则返回
       String priorityCandidate = this.determineHighestPriorityCandidate(candidateBeans, requiredType);
       if (priorityCandidate != null) {
           return priorityCandidate;
       } else {
           Iterator var6 = candidateBeans.entrySet().iterator();
           String candidateBeanName;
           Object beanInstance;
           do {
               if (!var6.hasNext()) {
                   return null;
               }
               // 3. 再找 bean 的名称匹配的
               Entry<String, Object> entry = (Entry)var6.next();
               candidateBeanName = (String)entry.getKey();
               beanInstance = entry.getValue();
           } while(!this.resolvableDependencies.values().contains(beanInstance) && !this.matchesBeanName(candidateBeanName, descriptor.getDependencyName()));
           return candidateBeanName;
       }
   }
}

Recommended tutorial: "Java Tutorial"

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