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，

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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

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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！

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附部分注解图一张：

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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

 

原文链接:https://www.cnblogs.com/dongguangming/p/12963060.html
如有疑问请与原作者联系

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