通過案例說明Spring中@Component的使用

1 @Component介紹

@Component大致可以分爲以下幾個註解：

a.@controller 控制器(注入服務),用於標註控制層組件(如struts中的action)

b.@service 服務(注入service),用於標註業務層組件

c.@repository dao(注入dao),用於標註數據訪問組件，即DAO組件

d.@component （把普通bean實體實例化到spring容器中，相當於配置文件中的<bean id="" class=""/>),泛指組件，當組件不好歸類的時候，我們可以使用這個註解進行標註

@Component,@Service,@Controller,@Repository註解的類，並把這些類納入進spring容器中管理。

在annotaion配置註解中用@Component來表示一個通用註釋用於說明一個類是一個spring容器管理的類。即就是該類已經拉入到spring的管理中了。而@Controller, @Service, @Repository是@Component的細化，這三個註解比@Component帶有更多的語義，它們分別對應了控制層、服務層、持久層的類。

2 案例：實現簡單的登錄

使用@Controller @Service @Repository @Component @Autowired

@Autowired ：當 Spring遇到一個在 setter 方法中使用的 @Autowired 註解，它會在方法中執行 byType 自動裝配。

2.1 新建web項目：LoginTest

2.2 設置數據庫sql語句：

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`userid` int(10) NOT NULL AUTO_INCREMENT,
`user_name` varchar(30) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`password` varchar(30) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
PRIMARY KEY (`userid`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 6 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES (1, 'Donta2', '123456');
INSERT INTO `user` VALUES (2, 'Donta445', '345666');
INSERT INTO `user` VALUES (3, 'Jerry', '01234');
INSERT INTO `user` VALUES (4, 'key', '456');
INSERT INTO `user` VALUES (5, 'book', '123');

SET FOREIGN_KEY_CHECKS = 1;

數據展示：

2.3 導入jar包4+2，log4j.properties , applicationContext.xml

Spring 的XML開發和註解開發導入jar包是相同的

依賴：（pom.xml）

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.huang</groupId>
	<artifactId>LoginTest</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>war</packaging>

	<name>MybatisTest1</name>
	<url>http://maven.apache.org</url>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
	</properties>

	<dependencies>
	<!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
    <scope>provided</scope>
</dependency>
	
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>3.8.1</version>
			<scope>test</scope>
		</dependency>

		<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
		<dependency>
			<groupId>org.mybatis</groupId>
			<artifactId>mybatis</artifactId>
			<version>3.4.6</version>
		</dependency>
		<dependency>
			<groupId>com.github.pagehelper</groupId>
			<artifactId>pagehelper</artifactId>
			<version>3.7.5</version>
		</dependency>
		<dependency>
			<groupId>com.github.jsqlparser</groupId>
			<artifactId>jsqlparser</artifactId>
			<version>0.9.1</version>
		</dependency>


		<!-- https://mvnrepository.com/artifact/org.slf4j/slf4j-log4j12 -->
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-log4j12</artifactId>
			<version>1.7.25</version>
			<scope>test</scope>
		</dependency>


		<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
		<dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<version>5.1.31</version>
		</dependency>

		<!-- https://mvnrepository.com/artifact/org.springframework/spring-aop -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-aop</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-core -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-core</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-beans -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-beans</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-jdbc -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-jdbc</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-aspects -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-aspects</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-expression -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-expression</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.springframework/spring-web -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-web</artifactId>
			<version>5.1.4.RELEASE</version>
		</dependency>

	</dependencies>
	<build>
		<plugins>
			<plugin>
                               //修改環境爲1.7
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.2</version>
				<configuration>
					<source>1.7</source>
					<target>1.7</target>
					<encoding>UTF-8</encoding>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.apache.tomcat.maven</groupId>
				<artifactId>tomcat7-maven-plugin</artifactId>
				<version>2.2</version>
			</plugin>
		</plugins>
	</build>
</project>

2.4 applicationContext.xml配置：

<?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"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="
 http://www.springframework.org/schema/beans 
 http://www.springframework.org/schema/beans/spring-beans.xsd
 http://www.springframework.org/schema/context 
 http://www.springframework.org/schema/context/spring-context.xsd">
<!-- 開啓註解模式 -->
 <context:annotation-config/>
<!--  開啓包掃描，開啓包掃描後，會默認開啓註解模式 -->
  <context:component-scan base-package="com.huang"></context:component-scan>
</beans>

2.5 Bean層代碼：設置實體User使用@Component

package com.huang.bean;

import org.springframework.stereotype.Component;

@Component
public class User {
 private int userid;
 private String password;
 private String userName;
 
public int getUserid() {
	return userid;
}

public void setUserid(int userid) {
	this.userid = userid;
}

public String getPassword() {
	return password;
}

public void setPassword(String password) {
	this.password = password;
}

public String getUserName() {
	return userName;
}

public void setUserName(String userName) {
	this.userName = userName;
}
@Override
public String toString() {
	return "User [userid=" + userid + ", password=" + password + ", userName=" + userName + "]";
}
}

2.6 Dao層頁面代碼：使用@Repository注入dao ,@Autowired注入外部資源User

package com.huang.dao;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;

import com.huang.bean.User;


@Repository
public class UserDao {
	@Autowired
	User user;
	
	public User login(String userName,String password) throws Exception {
		String  driver ="com.mysql.jdbc.Driver";
		String	url = "jdbc:mysql:///mybank?useUnicode=true&amp;characterEncoding=utf-8";
		String	usern ="root";
		String	passw ="root";
        Class.forName(driver);
        Connection conn =DriverManager.getConnection(url, usern, passw);
        Statement sta = conn.createStatement();
        String sql ="select * from user where user_name='"+userName+"'and password='"+password+"'";
        ResultSet res = sta.executeQuery(sql);
        while(res.next()) {
        	user.setUserid(res.getInt(1));
        	user.setPassword(res.getString(2));
        	user.setUserName(res.getString(3));
        }
		return user;
	}
	
	
}

2.7 Service頁面代碼：使用@service注入service，@Autowired注入外部資源UserService

package com.huang.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.huang.bean.User;
import com.huang.dao.UserDao;

@Service
public class UserService {
    @Autowired
	UserDao userdao;
    public User login(String userName,String password) {
    	try {
			return userdao.login(userName,password);
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			throw new RuntimeException("登錄錯誤");
		}
    }
}

2.8 controller 層代碼：使用@WebServlet（命名），使得jsp頁面提交到這個servlet中

package com.huang.controller;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.huang.bean.User;
import com.huang.service.UserService;

@WebServlet("/userController")
public class UserController extends HttpServlet {
	
	ApplicationContext applicationcontext;
	
	UserService userService ; 
	
	User user;
	
	private static final long serialVersionUID = 1L;
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		applicationcontext=	new ClassPathXmlApplicationContext("applicationContext.xml");
		userService = (UserService) applicationcontext.getBean("userService");
		user = (User) applicationcontext.getBean("user");
		request.setCharacterEncoding("utf-8");
		String userName = request.getParameter("userName");
		String password = request.getParameter("password");
		user= userService.login(userName,password);
		System.out.println(user);
		
		
	}

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}

}

2.8 jsp 代碼

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<form action="userController" method="post" >
賬戶：<input type ="text" name="userName">
密碼：<input type ="password" name="password">
<input type ="submit" value="提交">
</form>

</body>
</html>

測試結果：

輸入測試數據

輸出結果：

3 拓展：@Autowired

首先要知道另一個東西，default-autowire，它是在xml文件中進行配置的，可以設置爲byName、byType、constructor和autodetect；比如byName，不用顯式的在bean中寫出依賴的對象，它會自動的匹配其它bean中id名與本bean的set**相同的，並自動裝載。
@Autowired是用在JavaBean中的註解，通過byType形式，用來給指定的字段或方法注入所需的外部資源。
兩者的功能是一樣的，就是能減少或者消除屬性或構造器參數的設置，只是配置地方不一樣而已。
autowire四種模式的區別：
先看一下bean實例化和@Autowired裝配過程：
1. 一切都是從bean工廠的getBean方法開始的，一旦該方法調用總會返回一個bean實例，無論當前是否存在，不存在就實例化一個並裝配，否則直接返回。（Spring MVC是在什麼時候開始執行bean的實例化過程的呢？其實就在組件掃描完成之後）
2. 實例化和裝配過程中會多次遞歸調用getBean方法來解決類之間的依賴。
3. Spring幾乎考慮了所有可能性，所以方法特別複雜但完整有條理。
4. @Autowired最終是根據類型來查找和裝配元素的，但是我們設置了<beans default-autowire="byName"/>後會影響最終的類型匹配查找。因爲在前面有根據BeanDefinition的autowire類型設置PropertyValue值得一步，其中會有新實例的創建和註冊。就是那個autowireByName方法。

通過案例說明Spring中@Component的使用