Spring详细内容-Spring 中的自动装配 En（3）

红太狼 2023-09-24 19:11 47阅读 0赞

# 1. Autowiring in Spring #

*  Autowiring feature of spring framework enables you to `inject the object dependency` `implicitly`. It internally uses `setter`or `constructor injection`.
 *  Autowiring `can't be used to inject primitive and string values`. It works with reference only.

## 1)Advantage of Autowiring ##

It requires the less code because we don’t need to write the code to **inject the dependency explicitly**.

## 2)Disadvantage of Autowiring ##

*  No control of programmer.
 *  It can’t be used for **primitive and string values**.

## 3)Autowiring Modes ##

<table> 
 <thead> 
  <tr> 
   <th>No.</th> 
   <th>Mode</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1)</td> 
   <td>no</td> 
   <td>It is the <code>default autowiring mode</code>. It means no autowiring bydefault.</td> 
  </tr> 
  <tr> 
   <td>2)</td> 
   <td>byName</td> 
   <td>The byName mode <code>injects the object dependency</code> according to name of the bean. In such case, <code>property name</code> and <code>bean name</code> must be same. It internally calls<code> setter method</code>.</td> 
  </tr> 
  <tr> 
   <td>3)</td> 
   <td>byType</td> 
   <td>The byType mode injects the object dependency according to <code>type</code>. So property name and bean name can be different. It internally <code>calls setter method</code>.</td> 
  </tr> 
  <tr> 
   <td>4)</td> 
   <td>constructor</td> 
   <td>The constructor mode injects the dependency by calling the constructor of the class. It calls the constructor having large number of parameters.</td> 
  </tr> 
 </tbody> 
</table>

### Example of Autowiring ###

Use autowire attribute of bean element to apply the autowire modes

> This class contains a constructor and method only.

public class B {
        
        B(){
        System.out.println("b is created");}
        void print(){
        System.out.println("hello b");}
    }

> This class contains reference of B class and constructor and method.

public class A {
        
        B b;
        A(){
        System.out.println("a is created");}
    
        public B getB() {
        
            return b;
        }
    
        public void setB(B b) {
        
            this.b = b;
        }
        void print(){
        System.out.println("hello a");}
        void display(){
        
            print();
            b.print();
        }
    }

<?xml version="1.0" encoding="UTF-8"?>
    <beans
            xmlns="http://www.springframework.org/schema/beans"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns:p="http://www.springframework.org/schema/p"
            xsi:schemaLocation="http://www.springframework.org/schema/beans
                   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
        <bean id="a" class="com.amy.A" autowire="byName"></bean>
        <bean id="b"  class="com.amy.B"></bean>
    </beans>

> This class gets the bean from the applicationContext.xml file and calls the display method.

public class Test {
        
        public static void main(String[] args) {
        
           ApplicationContext context = new ClassPathXmlApplicationContext( "applicationContext.xml");
            A a= (A) context.getBean("a");
            a.display();
        }
    }

public class Test {
        
        public static void main(String[] args) {
        
           ApplicationContext context = new ClassPathXmlApplicationContext( "applicationContext.xml");
            A a= context.getBean("a",A.class);
            a.display();
        }
    }

> two methods are same

public class Test {
        
        public static void main(String[] args) {
        
            Resource resource = new ClassPathResource("applicationContext.xml");
            BeanFactory factory = new XmlBeanFactory(resource);
            A a= (A) factory.getBean("a");
            a.display();
        }
    }

a is created
    b is created
    hello a
    hello b

#### 1) byName autowiring mode ####

*  In case of byName autowiring mode, **bean id and reference name must be same.**
 *  It **internally uses setter injection**.

**Correct**

**Incorrect**

a is created
    b is created
    hello a
    Exception in thread "main" java.lang.NullPointerException
    	at com.amy.A.display(A.java:21)
    	at com.amy.Test.main(Test.java:20)

#### 2）byType autowiring mode ####

*  In case of byType autowiring mode, `bean id and reference name may be different`.
 *  But there must be **only one bean of a type**.

*  it works fine **because you have created an instance of B type.**
 *  It doesn’t matter that you have different bean name than reference name.
 *  But, if you **have multiple bean of one type**, it will `not work` and `throw exception`.

#### 3) constructor autowiring mode ####

*  In case of constructor autowiring mode, spring container injects the dependency by `highest parameterized constructor`.
 *  If you `have 3 constructors`in a class, `zero-arg`, `one-arg` and `two-arg` then injection will be performed by `calling the two-arg constructor`.

#### 4) no autowiring mode ####

*  In case of no autowiring mode, **spring container doesn’t inject the dependency by autowiring.**

a is created
    b is created
    hello a
    Exception in thread "main" java.lang.NullPointerException
    	at com.amy.A.display(A.java:21)
    	at com.amy.Test.main(Test.java:20)

# 2.Dependency Injection with Factory Method in Spring #

*  Spring framework provides facility to `inject bean using factory method`.
 *  To do so, we can use `two attributes of bean element`.

## 1) Factory method ##

**A method that returns instance of a class** is called `factory method`.

public class A {
          
    	public static A getA(){
        //factory method  
    	    return new A();  
    	}  
    }

## 2) Factory Method Types ##

There can be three types of factory method:

1.  `static factory method` that returns `instance of its own class`
2.  `static factory method` that returns `instance of another class`
3.  `non-static factory method` that returns `instance of another class`

> ### (1） `static factory method` that returns `instance of its own class` ###

public class A {
        
       private static  final A obj = new A();
       private A(){
        
           System.out.println("private constructor");
       }
       public static A getA(){
        
           System.out.println("factory method ");
           return obj;
       }
       public void msg(){
        
           System.out.println("hello user");
       }
    }

private constructor
    factory method 
    hello user

> ### (2） `static factory method` that returns `instance of another class` ###

public interface Printable {
        
        void print();
    }

public class A implements Printable{
        
       @Override
        public  void print(){
        
           System.out.println("hello a");
       }
    
    }

public class B implements Printable{
        
        @Override
        public void print(){
        
            System.out.println("hello b");}
    }

public class PrintableFactory {
        
        public static Printable getPrintable(){
        
            return new A();
        }
    }

<?xml version="1.0" encoding="UTF-8"?>
    <beans
            xmlns="http://www.springframework.org/schema/beans"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns:p="http://www.springframework.org/schema/p"
            xsi:schemaLocation="http://www.springframework.org/schema/beans
                   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
        <bean id="pf" class="com.amy.PrintableFactory" factory-method="getPrintable"></bean>
    </beans>

public class Test {
        
        public static void main(String[] args) {
        
           ApplicationContext context = new ClassPathXmlApplicationContext( "applicationContext.xml");
            Printable p= (A) context.getBean("pf");
            p.print();
        }
    }

hello a

> ### (3）`non-static factory method` that returns `instance of another class` ###

public interface Printable {
        
        void print();
    }

public class A implements Printable{
        
       @Override
        public  void print(){
        
           System.out.println("hello a");
       }
    
    }

public class B implements Printable{
        
        @Override
        public void print(){
        
            System.out.println("hello b");}
    }

> There are difference between static factory method and non-static factory method

public class PrintableFactory {
        
        //public  static Printable getPrintable(){/
        public  Printable getPrintable(){
        //non-static factory method
            return new A();
        }
    }

<?xml version="1.0" encoding="UTF-8"?>
    <beans
            xmlns="http://www.springframework.org/schema/beans"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns:p="http://www.springframework.org/schema/p"
            xsi:schemaLocation="http://www.springframework.org/schema/beans
                   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    //static
    //<bean id="pf" class="com.amy.PrintableFactory" factory-//method="getPrintable"></bean>
    //</beans>
    //non-static
        <bean id="pf" class="com.amy.PrintableFactory" ></bean>
        <bean id="p" class="com.amy.PrintableFactory" factory-method="getPrintable" factory-bean="pf"></bean>
    </beans>