(the notes of the article are from the video of station b. when you meet the crazy God, you can carry the notes as a study for your reference and study together)
Automatic assembly instructions

• Automatic assembly is a way to use spring to meet bean dependencies
• spring will find the bean that a bean depends on in the application context.

There are three assembly mechanisms for bean s in Spring:

1. Explicit configuration in xml;
2. Explicit configuration in java;
3. Implicit bean discovery mechanism and automatic assembly.

Here we mainly talk about the third kind: automatic assembly bean.

Spring's automatic assembly needs to be implemented from two perspectives, or two operations:

1. Component scanning: spring will automatically discover the bean s created in the application context;
2. Autowiring: spring automatically satisfies the dependencies between bean s, which is what we call IoC/DI;

The combination of component scanning and automatic assembly exerts great power to minimize the configuration of display.

It is recommended to use annotations instead of automatic assembly xml configuration

**
**

Test environment construction

1. Create a new project

2. Create two new entity classes. Cat Dog has a method called

public class Cat {
   public void shout() {
       System.out.println("miao~");
  }
}

public class Dog {
   public void shout() {
       System.out.println("wang~");
  }
}

3. Create a new User class User

public class User {
   private Cat cat;
   private Dog dog;
   private String str;
}

4. Writing Spring configuration files

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

   <bean id="dog" class="com.qf.pojo.Dog"/>
   <bean id="cat" class="com.qf.pojo.Cat"/>

   <bean id="user" class="com.qf.pojo.User">
       <property name="cat" ref="cat"/>
       <property name="dog" ref="dog"/>
       <property name="str" value="wei"/>
   </bean>
</beans>

5. Testing

public class MyTest {
   @Test
   public void testMethodAutowire() {
       ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
       User user = (User) context.getBean("user");
       user.getCat().shout();
       user.getDog().shout();
  }
}

The result is output normally and the environment is OK

byName

autowire byName

In the process of manually configuring xml, errors such as missing letters and case often occur, so it is impossible to check them, which reduces the development efficiency.

Using automatic assembly will avoid these errors and simplify the configuration.

Test:

1. Modify the bean configuration and add an attribute autowire = "byName"

<bean id="user" class="com.qf.pojo.User" autowire="byName">
   <property name="str" value="wei"/>
</bean>

2. Test again, and the result is still output successfully!

3. We modify the bean id of cat to catXXX

4. Test again and execute the null pointer Java lang.NullPointerException. Because if you find the wrong set method according to byName rule, the real setCat will not be executed and the object will not be initialized, so a null pointer error will be reported when calling.

Summary:

When a bean node has the attribute autowire byName.

1. All set method names in its class, such as setCat, will be searched to obtain the string with set removed and lowercase, that is, cat.
2. Go to the spring container to find whether there is an object with this string name id.
3. If yes, take out the injection; If not, a null pointer exception is reported.

byType

autowire byType

Using autowire byType first needs to ensure that objects of the same type are unique in the spring container. If it is not unique, it will report an exception that is not unique.

NoUniqueBeanDefinitionException

Test:

1. Modify the bean configuration of user: autowire = "byType"

2. Test, normal output

3. Register a cat bean object!

<bean id="dog" class="com.qf.pojo.Dog"/>
<bean id="cat" class="com.qf.pojo.Cat"/>
<bean id="cat2" class="com.qf.pojo.Cat"/>

<bean id="user" class="com.qf.pojo.User" autowire="byType">
   <property name="str" value="wei"/>
</bean>

4. Test, error: NoUniqueBeanDefinitionException

5. Delete cat2 and change the bean name of cat! Test! Because it is assembled by type, no exception will be reported and the final result will not be affected. Even removing the id attribute does not affect the result.

This is automatic assembly according to type!

Using annotations

jdk1.5 start supporting annotations, spring 2 5 begin to fully support annotations.

Preparation: inject attributes by annotation.

1. Introduce the context file header into the spring configuration file

xmlns:context="http://www.springframework.org/schema/context"

http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd

2. Enable annotation attribute support!

<context:annotation-config/>

@Autowired

• @Autowired is automatically transferred by type and does not support id matching.
• You need to import the package of spring AOP!

Test:

1. Remove the set method in the User class and use the @ Autowired annotation

public class User {
   @Autowired
   private Cat cat;
   @Autowired
   private Dog dog;
   private String str;

   public Cat getCat() {
       return cat;
  }
   public Dog getDog() {
       return dog;
  }
   public String getStr() {
       return str;
  }
}

2. The contents of the configuration file are displayed

<context:annotation-config/>

<bean id="dog" class="com.qf.pojo.Dog"/>
<bean id="cat" class="com.qf.pojo.Cat"/>
<bean id="user" class="com.qf.pojo.User"/>

3. Test, output results successfully!

@Autowired(required=false) Description: false, the object can be null; true, the object must be saved and cannot be null.

//If the allowed object is null, set required = false, and the default is true
@Autowired(required = false)
private Cat cat;

@Qualifier

• @Autowired is automatically assembled according to the type. With @ Qualifier, it can be automatically assembled according to byName
• @Qualifier cannot be used alone.

Test steps:

1. Modify the content of the configuration file to ensure that there are objects of type. And the name is not the default name of the class!

<bean id="dog1" class="com.qf.pojo.Dog"/>
<bean id="dog2" class="com.qf.pojo.Dog"/>
<bean id="cat1" class="com.qf.pojo.Cat"/>
<bean id="cat2" class="com.qf.pojo.Cat"/>

2. No Qualifier test is added, and an error is reported directly

3. Add a Qualifier annotation to the attribute

@Autowired
@Qualifier(value = "cat2")
private Cat cat;
@Autowired
@Qualifier(value = "dog2")
private Dog dog;

Test, successful output!

@Resource

• @If the Resource has a specified name attribute, first search the assembly by name according to the attribute;
• Secondly, assemble in the default byName mode;
• If none of the above is successful, it will be assembled automatically by byType.
• If they are not successful, an exception is reported.

Entity class:

public class User {
   //If the allowed object is null, set required = false, and the default is true
   @Resource(name = "cat2")
   private Cat cat;
   @Resource
   private Dog dog;
   private String str;
}

applicationContext.xml

<bean id="dog" class="com.qf.pojo.Dog"/>
<bean id="cat1" class="com.qf.pojo.Cat"/>
<bean id="cat2" class="com.qf.pojo.Cat"/>

<bean id="user" class="com.qf.pojo.User"/>

Test result: OK

Configuration file 2: beans XML, delete cat2

<bean id="dog" class="com.qf.pojo.Dog"/>
<bean id="cat1" class="com.qf.pojo.Cat"/>

Only annotations are retained on entity classes

@Resource
private Cat cat;
@Resource
private Dog dog;

Result: OK

Conclusion: byName search first failed; The byType search is successful.

Summary

@Similarities and differences between Autowired and @ Resource:

1. Both @ Autowired and @ Resource can be used to assemble bean s. Can be written on fields or setter methods.

2. @ Autowired is assembled by type by default (belonging to the spring specification). By default, dependent objects must exist. If null value is allowed, its required attribute can be set to false, such as @ Autowired(required=false). If we want to use name assembly, it can be used in combination with @ Qualifier annotation

3. @ Resource (belonging to J2EE complex return), assembled by name by default, and the name can be specified through the name attribute. If the name attribute is not specified, when the annotation is written on the field, the default is to take the field name to search by name. If the annotation is written on the setter method, the default is to take the attribute name for assembly. Assemble by type when no bean matching the name is found. However, it should be noted that once the name attribute is specified, it will only be assembled according to the name.

Their functions are the same. They all inject objects by annotation, but the execution order is different@ Autowired byType first, @ Resource byName first.