145 lines
4.0 KiB
Markdown
145 lines
4.0 KiB
Markdown
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# 第一章：Spring的基本应用
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## 四、依赖注入
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依赖注入（Dependency Injection）建成DI，与控制反转（IoC）的含义相同
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依赖注入的作用就是在使用Spring 框架创建对象时，动态地将其所依赖的对象注入Bean组件中，其实现方式通常有两种，一种是属性setter方法注入，另一种是构造方法注入，具体介绍如下:
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属性 setter方法注入：指Spring容器使用setter方法注入被依赖的实例。通过调用无参构造器或无参静态工厂方法实例化 Bean后，调用该Bean的setter 方法，即可实现基于 setter方法的依赖注入。
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构造方法注入：指Spring容器使用构造方法注入被依赖的实例。基于构造方法的依赖注入通过调用带参数的构造方法来实现，每个参数代表着一个依赖。
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在com.gzh的包下创建UserService接口并创建一个say方法，代码如下：
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```java
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public interface UserService {
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public void say();
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}
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```
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在com.gzh的包下创建UserServiceImpl实现类，代码如下：
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```java
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public class UserServiceImpl implements UserService{
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//声明UserDao的属性
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private UserDao userDao;
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//添加UserDao属性的setter方法，用于实现依赖注入
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public void setUserDao(UserDao userDao) {
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this.userDao = userDao;
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}
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//实现接口中的方法
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public void say(){
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//调用userDao中的say()方法，并执行输出语句
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this.userDao.say();
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System.out.println("UserService say Hellow World!");
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}
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}
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```
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在applicationContext.xml里面创建一个id为userService的实例
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```java
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<bean id="userService" class="com.gzh.UserServiceImpl">
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<!-- 将id为userDao的Bean实例注入到userService的实例中-->
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<property name="userDao" ref="userDao"/>
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</bean>
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```
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在com.gzh下创建TestDI实例
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```java
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public class TsetDI {
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public static void main(String[] args) {
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//1.初始化Spring容器，加载配置文件
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ApplicationContext applicationContext =new ClassPathXmlApplicationContext("applicationContext.xml");
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//2.通过容器获取UserService实例
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UserService userService =(UserService) applicationContext.getBean("userService");
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//3.调用实例中的say()方法
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userService.say();
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}
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}
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```
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## 1.IOC（控制反转）：类对象的创建由程序员转给了Spring框架
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Spring框架创建类对象的步骤：
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（1）导包5个
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commons_logging
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spring-context
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spring-beans
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spring-core
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spring-expression
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```java
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<dependency>
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<groupId>org.springframework</groupId>
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<artifactId>spring-context</artifactId>
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<version>4.3.6.RELEASE</version>
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</dependency>
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```
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（2）resources文件夹下创建applicationContext.xml
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```java
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<?xml version="1.0" encoding="UTF-8"?>
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<beans xmlns="http://www.springframework.org/schema/beans"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
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<bean id="userDao" class="com.gzh.UserDaoImpl"/>
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```
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（3）获取Spring框架创建的类对象
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```java
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//使用Spring框架创建对象调用方法
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ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
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UserDao userDao = (UserDao) applicationContext.getBean("userDao");
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userDao.say();
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```
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## 2.DI：依赖注入
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D：依赖（动词）
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```java
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class B{}
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class A{
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B b;
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void a(){
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b.b();
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}
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}
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//类的对象作为A类的成员变量，就是A依赖B。
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```
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I：注入：就是给成员变量赋值
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Spring 框架如何实现依赖注入？
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直接在配置文件中写如下代码：
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```java
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<bean id="userService" class="com.gzh.UserServiceImpl">
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<!-- 将id为userDao的Bean实例注入到userService的实例中-->
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<property name="userDao" ref="userDao"/>
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</bean>
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//name属性值是成员变量，ref属性值是id属性的值
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```
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#
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