What are the three methods of Spring dependency injection?

The three methods of Spring dependency injection (DI) are:

1, interface injection

2, Setter method injection

3. Constructor method injection

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Let me introduce to you how these three types of dependency injection work in Spring. realized.

First we need the following classes:

Interface Login.java

Interface implementation class LoginImpl.java

A processing class LoginAction.java

There is also a test class TestMain.java

LoginImpl.java as follows:

package com.spring.test.di;

public class LoginImpl implements Login{

public String getName(){

return "fengyun";

}

}

TestMain.java

package com.spring.test.di;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.FileSystemXmlApplicationContext;

public class TestMain {

/**

* @param args

*/

public static void main(String[] args) {// 得到ApplicationContext对象

ApplicationContext ctx = new FileSystemXmlApplicationContext(

"applicationContext.xml");

// 得到Bean

LoginAction loginAction = (LoginAction) ctx.getBean("loginAction");

loginAction.execute();

}

}

LoginAction.java will be slightly different depending on the injection method used.

Let’s look at the LoginAction.java class according to the injection method

Setter method injection:

package com.spring.test.di;

public class LoginAction {

private Login login;

public void execute() {

String name = login.getName();

System.out.print("My Name Is " + name);

}

/**

* @return the login

*/

public Logic getLogin() {

return login;

}

/**

* @param login

* the login to set

*/

public void setLogin(Login login) {

this.login = login;

}

}

defines a Login type variable login, in LoginAction does not instantiate login, but only its corresponding setter/getter method, because we are using Spring's dependency injection method here.

The applicationContext.xml configuration file is as follows:

<bean id="login" class="com.spring.test.di.LoginImpl"/>
<bean id="loginAction" class="com.spring.test.di.LoginAction">
<property name="login" ref="login"></property>
</bean>

Now you can run testMain.java, and we can see that My Name Is fengyun is printed on the console.

OK, this is spring’s setter method injection.

Constructor method injection

As the name suggests, constructor method injection means that we rely on the constructor method of LoginAction to achieve the purpose of DI, as shown below:

LoginAction.java

package com.spring.test.di;

public class LoginAction {

private Login login;

public LoginAction(Login login) {

this.login = login;

}

public void execute() {

String name = login.getName();

System.out.print("My Name Is " + name);

}

}

Here we have added a LoginAction constructor

applicationContext.xml configuration file is as follows:

<bean id="login" class="com.spring.test.di.LoginImpl"/>
<bean id="loginAction" class="com.spring.test.di.LoginAction">
<constructor­arg index="0" ref="login"></constructor­arg>
</bean>

We use constructorarg for configuration. The index attribute is used to indicate the order of parameters in the constructor. , if there are multiple parameters, configure them in order from 0,1...

We can now run testMain.java, and the result is exactly the same as using the Setter method to inject.

One thing to note is: if the constructor has multiple parameters, such as: parameter 1, parameter 2, and parameter 2 depends on parameter 1, in this case, you must pay attention to the order of the constructor, and the parameters must be 1 is placed before parameter 2.

Let’s continue talking about interface injection that we don’t commonly use. Let’s take LoginAction as an example. We have modified it as follows:

LoginAction.java

package com.spring.test.di;

public class LoginAction {

private Logic login;

public void execute() {

try {

Object obj = Class.forName("com.spring.test.di.LoginImpl")

.newInstance();

login = (Login) obj;

String name = login.getName();

System.out.print("My Name Is " + name);

} catch (Exception e) {

e.printStackTrace();

}

}

}

Configuration file:

<bean id="logic" class="com.spring.test.di.LoginImpl"/>
<bean id="loginAction" class="com.spring.test.di.LoginAction">
</bean>

The two most commonly used injection methods at work are Setter and constructor.

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