Spring中@Import的三种情况
我们在使用Spring框架中，特别是框架级的功能，经常看到有@Import导入功能，
我就介绍下它能导入什么，首先声明下@Import是注解，导入类型可分为三类：
1. 导入配置 @Configuration，类似于spring早期版本2.5的import xml文件一样，
<?xml version="1.0" encoding="UTF-8"?><beansxmlns="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-2.5.xsd
">
<importresource="cms-validator-service.xml"/>
<importresource="cms-validator-dao.xml"/>
</beans>
只是现在注解抢了风头，但目的一样，用于使用所有标有@configuration注解的配置。
下面我就写个小例子，怎么建java项目就略了
先建java主包com.spring, 然后分别建子包
com.spring.service, com.spring.service.impl, com.spring.config, com.spring.test
1.1 建立服务接口
package com.spring.service;/**
*
* @author dgm
* @describe "日志服务接口"
*/
publicinterfaceLogService{
voidprint(String message);
}
1.2 建立服务实现类，分三种情况，控制台、文件和数据库mysql
package com.spring.service.impl;import org.springframework.stereotype.Component;
import com.spring.service.LogService;
/**
* @author dgm
* @describe "日志到控制台"
*/
@Component
publicclassStdOutLogServiceImplimplementsLogService{
@Override
publicvoidprint(String message){
System.out.println(message);
System.out.println("写日志到控制台！");
}
}
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import org.springframework.stereotype.Component;
import com.spring.service.LogService;
/**
*
* @author dgm
* @describe "日志到文件"
*/
@Component
publicclassFileLogServiceImplimplementsLogService{
privatestaticfinal String FILE_NAME="d://LogService.txt";
@Override
publicvoidprint(String message){
try {
File file = new File(FILE_NAME);
FileWriter fw = null;
// true:表示是追加的标志
fw = new FileWriter(file, true);
fw.write(message+"n");
fw.close();
System.out.println(message);
System.out.println("写日志入文件！");
} catch (IOException e) {
}
}
}
/**
* @author dgm
* @describe "写日志入mysql数据库"
*/
@Component
publicclassMysqlLogServiceImplimplementsLogService{
@Override
publicvoidprint(String message){
System.out.println(message);
System.out.println("写日志入数据库");
}
}
1.3 写配置类，三个服务实现类对应三个@Configuration
package com.spring.config;import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.spring.service.LogService;
import com.spring.service.impl.StdOutLogServiceImpl;
@Configuration
publicclassStdOutConfig{
@Bean(name="stdOutLogServiceImpl")
public LogService stdOutLogServiceImpl(){
returnnew StdOutLogServiceImpl();
}
}
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.spring.service.LogService;
import com.spring.service.impl.FileLogServiceImpl;
@Configuration
publicclassFileLogConfig{
@Bean(name="fileLogServiceImpl")
public LogService fileLogServiceImpl(){
returnnew FileLogServiceImpl();
}
}
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.spring.service.LogService;
import com.spring.service.impl.MysqlLogServiceImpl;
@Configuration
publicclassMysqlLogConfig{
@Bean(name="mysqlLogServiceImpl")
public LogService mysqlLogServiceImpl(){
returnnew MysqlLogServiceImpl();
}
}
然后@Import注解登场了
import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;
@Configuration
@Import({StdOutConfig.class, FileLogConfig.class, MysqlLogConfig.class})
publicclassLogParentConfig{
}
1.4 建立测试类看效果
import org.springframework.context.annotation.AnnotationConfigApplicationContext;import com.spring.config.LogParentConfig;
import com.spring.service.*;
/**
* @author dgm
* @describe "java configuration bean"
*/
publicclassLogConfigurationAppTest{
publicstaticvoidmain(String[] args){
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
LogParentConfig.class);
//控制台
LogService obj = (LogService) context.getBean("stdOutLogServiceImpl");
System.out.println(obj);
obj.print("控制台输出");
//file
obj = (LogService) context.getBean("fileLogServiceImpl");
System.out.println(obj);
obj.print("文件输出");
//mysql
obj = (LogService) context.getBean("mysqlLogServiceImpl");
System.out.println(obj);
obj.print("数据库mysql");
context.close();
}
}
输出效果
2. 导入实现ImportSelector接口或子接口DeferredImportSelector的类
@Import annotation can also be configured with an ImportSelector implementation to select @Configuration classes programmatically, based on some selection criteria.
下面我也演示下，这个很重要，框架里和spring扩展开发用的多，先建立备用子包com.spring.bean和com.spring.importSelector，然后建立配置文件目录conf
2.1 实现了ImportSelector
2.1.1 建立辅助类ApplicationProperties.java和外置配置文件myapp.properties
package com.spring.bean;publicclassApplicationProperties{
private String connectionUrl;
private String connectionName;
public String getConnectionUrl(){
return connectionUrl;
}
publicvoidsetConnectionUrl(String connectionUrl){
this.connectionUrl = connectionUrl;
}
public String getConnectionName(){
return connectionName;
}
publicvoidsetConnectionName(String connectionName){
this.connectionName = connectionName;
}
@Override
public String toString(){
return"ApplicationProperties [connectionUrl=" + connectionUrl
+ ", connectionName=" + connectionName + "]";
}
}
然后在conf目录下建立配置文件myapp.properties，内容如下：
app.url=https://github.com/dongguangmingapp.name=dongguangming
2.1.2 建立@Configuration配置类
@Configuration@PropertySource("classpath:conf/myapp.properties")
publicclassAppConfig{
@Autowired
ConfigurableEnvironment environment;
@Bean
ApplicationProperties appProperties(){
ApplicationProperties bean = new ApplicationProperties();
bean.setConnectionUrl(environment.getProperty("app.url"));
bean.setConnectionName(environment.getProperty("app.name"));
return bean;
}
}
2.1.3 建立实现了ImportSelector接口的导入类，返回列表里的值是有标志@Configuration
publicclassLogImportSelectorimplementsImportSelector{@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
returnnew String[]{"com.spring.config.AppConfig","com.spring.config.LogParentConfig"};
}
}
2.1.4 建立有@import功能的配置类，导入2.1.3的实现类
package com.spring.config;import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import com.spring.importSelector.LogImportSelector;
@Configuration
@Import(LogImportSelector.class)
publicclassLogImportSelectorConfig{
}
2.1.5 编写测试类
import org.springframework.context.annotation.AnnotationConfigApplicationContext;import com.spring.bean.ApplicationProperties;
import com.spring.config.LogImportSelectorConfig;
import com.spring.service.*;
/**
* @author dgm
* @describe "java configuration bean"
*/
publicclassLogImportSelectorConfigurationAppTest{
publicstaticvoidmain(String[] args){
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
LogImportSelectorConfig.class);
// 控制台
LogService obj = (LogService) context.getBean("stdOutLogServiceImpl");
System.out.println(obj);
obj.print("控制台输出");
// file
obj = (LogService) context.getBean("fileLogServiceImpl");
System.out.println(obj);
obj.print("文件输出");
// mysql
obj = (LogService) context.getBean("mysqlLogServiceImpl");
System.out.println(obj);
obj.print("数据库mysql");
//
ApplicationProperties ap = context.getBean(ApplicationProperties.class);
System.out.println(ap);
context.close();
}
}
输出效果：
效果不错，也能完成bean的注册
还有一种基于注解的变体，我也示例下，先建个子包com.spring.annotation
建立自定义注解：
@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(LogImportSelector.class)
/**
* @author dgm
* @describe "自定义Enable功能"
*/
public@interface EnableLogService {
//默认日志输出到控制台
String logType()default "stdout";
@AliasFor("value")
String[] basePackages() default {};
@AliasFor("basePackages")
String[] value() default {};
}
然后修改导入选择器实现类，根据启用日志功能时传的参数绝对加载哪个bean
AnnotationAttributes attributes = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(
EnableLogService.class.getName(), false));
System.out.println(attributes);
//根据日志类型确定加载bean
String logType = attributes.getString("logType");
if (logType.equalsIgnoreCase("StdOut")) {
returnnew String[] { "com.spring.config.AppConfig",
"com.spring.config.StdOutConfig" };
} elseif (logType.equalsIgnoreCase("File")) {
returnnew String[] { "com.spring.config.AppConfig",
"com.spring.config.FileLogConfig" };
} elseif (logType.equalsIgnoreCase("Mysql")) {
returnnew String[] { "com.spring.config.AppConfig",
"com.spring.config.MysqlLogConfig" };
} else {
returnnew String[] { "com.spring.config.AppConfig",
"com.spring.config.LogParentConfig" };
}
修改配置类，追加自定义注解@EnableLogService，并设置参数为file（可选stdout，file，mysql）
@Configuration//@Import(LogImportSelector.class)
@EnableLogService(logType="file")
publicclassLogImportSelectorConfig{
}
修改测试类，此时不再是三种日志实现的bean都加载，按配置参数加载
LogService obj = (LogService) context.getBean("fileLogServiceImpl");System.out.println(obj);
obj.print("文件输出");
就因为配置了@EnableLogService(logType="file")，只加载了一个日志实现bean
2.2 实现了 DeferredImportSelector
publicinterfaceDeferredImportSelectorextendsImportSelector{}
可是看出它是2.1的子接口
The configuration class directly registered with the application context given preference over imported one. That means a bean of type T, configured in the main configuration will be used instead of a bean of the same type T from imported configuration. That applies to ImportSelector as well. On the other hand, DeferredImportSelector applies after all other configuration beans have been processed.
我们可以比较下实现两种接口的区别
在主选择器的配置类LogImportSelectorConfig.java中增加
@BeanLogBean logBean(){
returnnew LogBean();
}
@Bean(name = "fileLogServiceImpl")
public LogService fileLogServiceImpl(){
returnnew FileLogServiceImpl(" 来自LogImportSelectorConfig ");
}
在文件配置类FileLogConfig.java中修改为
@Bean(name="fileLogServiceImpl")public LogService fileLogServiceImpl(){
returnnew FileLogServiceImpl("来自 FileLogConfig");
}
选择器实现类还是
publicclassLogImportSelectorimplementsImportSelector{。。。}
执行测试代码
LogBean bean = context.getBean(LogBean.class);bean.printMessage();
此时修改选择器实现的接口改为DeferredImportSelector，其它不改
publicclassLogImportSelectorimplementsDeferredImportSelector{。。。}
再次执行测试
2.3 导入实现了ImportBeanDefinitionRegistrar接口的类
可以先瞄下接口的如何定义和定义了什么
publicinterfaceImportBeanDefinitionRegistrar{publicvoidregisterBeanDefinitions(
AnnotationMetadata importingClassMetadata,
BeanDefinitionRegistry registry);
}
This Interface is to be implemented by types that register additional bean definitions when processing @Configuration classes.
具体可参考还记得我以前写的博文Spring Bean注册的几种方式https://blog.csdn.net/dong19891210/article/details/105798650吗，详细看第5.2小节，这里就不再重复啰嗦写了。
其实你搞懂了Bean，spring本身、及衍生的第三方扩展， 99.99%的问题都不再是问题了！！！
小结：一图
务必掌握好2和3，写扩展很有用，甚至spring本身都在大量使用。
spring围绕着bean运转的，注册的几种方式，每种注册方式的条件性选择
最后请慢慢学会忘记xml格式的配置文件，现在或往后都是注解式了，虽然xml配置并不影响功能！
附部分注解图一张：
参考：
0. @Import Annotation in Spring Framework
https://javabeat.net/use-import-importing-javaconfig-files-spring-projects/
1. Spring向容器注册Bean的高级应用 https://cloud.tencent.com/developer/article/1497795
2. how spring import annotation parse（要翻墙） https://laptrinhx.com/spring-import-annotation-source-parsing-3074679655/
注意我说的墙不是下面这样的墙
以上是 Spring中@Import的三种情况 的全部内容， 来源链接： utcz.com/z/516783.html
