Spring中@Import的三种情况

其他 2020-05-26 08:21:33 阅读次数: 0

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"?> <beans xmlns="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 "> <import resource="cms-validator-service.xml"/> <import resource="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"
 */
public interface LogService { void print(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 public class StdOutLogServiceImpl implements LogService { @Override public void print(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 public class FileLogServiceImpl implements LogService { private static final String FILE_NAME="d://LogService.txt"; @Override public void print(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 public class MysqlLogServiceImpl implements LogService { @Override public void print(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 public class StdOutConfig { @Bean(name="stdOutLogServiceImpl") public LogService stdOutLogServiceImpl(){ return new 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 public class FileLogConfig { @Bean(name="fileLogServiceImpl") public LogService fileLogServiceImpl(){ return new 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 public class MysqlLogConfig { @Bean(name="mysqlLogServiceImpl") public LogService mysqlLogServiceImpl(){ return new 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}) public class LogParentConfig { }

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" */ public class LogConfigurationAppTest { public static void main(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;

public class ApplicationProperties { private String connectionUrl; private String connectionName; public String getConnectionUrl() { return connectionUrl; } public void setConnectionUrl(String connectionUrl) { this.connectionUrl = connectionUrl; } public String getConnectionName() { return connectionName; } public void setConnectionName(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/dongguangming
app.name=dongguangming

2.1.2   建立@Configuration配置类

@Configuration
@PropertySource("classpath:conf/myapp.properties")
public class AppConfig { @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

public class LogImportSelector implements ImportSelector{ @Override public String[] selectImports(AnnotationMetadata importingClassMetadata) { return new 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) public class LogImportSelectorConfig { }

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" */ public class LogImportSelectorConfigurationAppTest { public static void main(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")) { return new String[] { "com.spring.config.AppConfig", "com.spring.config.StdOutConfig" }; } else if (logType.equalsIgnoreCase("File")) { return new String[] { "com.spring.config.AppConfig", "com.spring.config.FileLogConfig" }; } else if (logType.equalsIgnoreCase("Mysql")) { return new String[] { "com.spring.config.AppConfig", "com.spring.config.MysqlLogConfig" }; } else { return new 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")
public class LogImportSelectorConfig { }

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

public interface DeferredImportSelector extends ImportSelector { }

 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

@Bean
	LogBean logBean() {
		return new LogBean(); } @Bean(name = "fileLogServiceImpl") public LogService fileLogServiceImpl() { return new FileLogServiceImpl(" 来自LogImportSelectorConfig "); }

在文件配置类FileLogConfig.java中修改为

@Bean(name="fileLogServiceImpl")
	public LogService fileLogServiceImpl(){ return new FileLogServiceImpl("来自 FileLogConfig"); }

选择器实现类还是

public class LogImportSelector implements ImportSelector {。。。}

 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

public class LogImportSelector implements DeferredImportSelector {。。。}

 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

public interface ImportBeanDefinitionRegistrar {

	public void registerBeanDefinitions( 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

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