Spring中@Import的三種情況

 

我們在使用Spring框架中,特別是框架級的功能,經常看到有@Import導入功能,

我就介紹下它能導入什麼,首先聲明下@Import是註解,導入類型可分爲三類

1.   導入配置 @Configuration,類似於spring早期版本2.5的import xml文件一樣,

<?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>  

 只是現在註解搶了風頭,但目的一樣,用於使用所有標有@configuration註解的配置。

下面我就寫個小例子,怎麼建java項目就略了

先建java主包com.spring, 然後分別建子包

com.spring.service,  com.spring.service.impl, com.spring.config, com.spring.test

1.1  建立服務接口

package com.spring.service;

/**
 * 
 * @author dgm
 * @describe "日誌服務接口"
 */
public interface LogService {

    void print(String message);
}

1.2  建立服務實現類,分三種情況,控制檯、文件和數據庫mysql

package com.spring.service.impl;

import org.springframework.stereotype.Component;
import com.spring.service.LogService;

/**
 * @author dgm
 * @describe "日誌到控制檯"
 */
@Component
public class StdOutLogServiceImpl implements LogService {

	@Override
	public void print(String message) {
        System.out.println(message);
        System.out.println("寫日誌到控制檯!");
	}
}


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 "日誌到文件"
 */
@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:表示是追加的標誌
			fw = new FileWriter(file, true);
			fw.write(message+"\n");
			fw.close();

	        System.out.println(message);
			System.out.println("寫日誌入文件!");
		} catch (IOException e) {
		}
	}
}


/**
 * @author dgm
 * @describe "寫日誌入mysql數據庫"
 */
@Component
public class MysqlLogServiceImpl implements LogService {

	@Override
	public void print(String message) {
        System.out.println(message);
        System.out.println("寫日誌入數據庫");
	}
}

1.3  寫配置類,三個服務實現類對應三個@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();
	}
}

然後@Import註解登場了

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  建立測試類看效果

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);
		//控制檯
		LogService obj = (LogService) context.getBean("stdOutLogServiceImpl");
		System.out.println(obj);
		obj.print("控制檯輸出");
		//file
		obj = (LogService) context.getBean("fileLogServiceImpl");
		System.out.println(obj);
		obj.print("文件輸出");
		//mysql
		obj = (LogService) context.getBean("mysqlLogServiceImpl");
		System.out.println(obj);
		obj.print("數據庫mysql");
		
		context.close();
	}
}

輸出效果

 

2.  導入實現ImportSelector接口或子接口DeferredImportSelector的類

@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,這個很重要,框架裏和spring擴展開發用的多,先建立備用子包com.spring.bean和com.spring.importSelector,然後建立配置文件目錄conf

2.1  實現了ImportSelector

2.1.1    建立輔助類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 + "]";
	}
}

然後在conf目錄下建立配置文件myapp.properties,內容如下:

app.url=https://github.com/dongguangming
app.name=dongguangming

2.1.2   建立@Configuration配置類

@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  建立實現了ImportSelector接口的導入類,返回列表裏的值是有標誌@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功能的配置類,導入2.1.3的實現類

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  編寫測試類

 

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);
		// 控制檯
		LogService obj = (LogService) context.getBean("stdOutLogServiceImpl");
		System.out.println(obj);
		obj.print("控制檯輸出");
		// file
		obj = (LogService) context.getBean("fileLogServiceImpl");
		System.out.println(obj);
		obj.print("文件輸出");
		// mysql
		obj = (LogService) context.getBean("mysqlLogServiceImpl");
		System.out.println(obj);
		obj.print("數據庫mysql");

		//
		ApplicationProperties ap = context.getBean(ApplicationProperties.class);
		System.out.println(ap);

		context.close();
	}
}

 輸出效果:

效果不錯,也能完成bean的註冊

還有一種基於註解的變體,我也示例下,先建個子包com.spring.annotation

建立自定義註解:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(LogImportSelector.class)
/**
 * @author dgm
 * @describe "自定義Enable功能"
 */
public @interface EnableLogService {
	//默認日誌輸出到控制檯
	String logType() default "stdout";

	@AliasFor("value")
	String[] basePackages() default {};

	@AliasFor("basePackages")
	String[] value() default {}; 
}

然後修改導入選擇器實現類,根據啓用日誌功能時傳的參數絕對加載哪個bean

AnnotationAttributes attributes = AnnotationAttributes
				.fromMap(importingClassMetadata.getAnnotationAttributes(
						EnableLogService.class.getName(), false));
		System.out.println(attributes);

		//根據日誌類型確定加載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" };
		}

 修改配置類,追加自定義註解@EnableLogService,並設置參數爲file(可選stdout,file,mysql)

@Configuration
//@Import(LogImportSelector.class)
@EnableLogService(logType="file")
public class LogImportSelectorConfig {

}

修改測試類,此時不再是三種日誌實現的bean都加載,按配置參數加載

LogService obj = (LogService) context.getBean("fileLogServiceImpl");
		System.out.println(obj);
		obj.print("文件輸出");

 

就因爲配置了@EnableLogService(logType="file"),只加載了一個日誌實現bean

 2.2  實現了 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.

我們可以比較下實現兩種接口的區別

在主選擇器的配置類LogImportSelectorConfig.java中增加

@Bean
	LogBean logBean() {
		return new LogBean();
	}

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

 

在文件配置類FileLogConfig.java中修改爲

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

選擇器實現類還是

public class LogImportSelector implements ImportSelector  {。。。}

 執行測試代碼

LogBean bean = context.getBean(LogBean.class);
	    bean.printMessage();

 

此時修改選擇器實現的接口改爲DeferredImportSelector,其它不改

public class LogImportSelector implements DeferredImportSelector  {。。。}

 再次執行測試

 

2.3   導入實現了ImportBeanDefinitionRegistrar接口的類

可以先瞄下接口的如何定義和定義了什麼

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.

具體可參考還記得我以前寫的博文Spring Bean註冊的幾種方式https://blog.csdn.net/dong19891210/article/details/105798650嗎,詳細看第5.2小節,這裏就不再重複囉嗦寫了。

其實你搞懂了Bean,spring本身、及衍生的第三方擴展, 99.99%的問題都不再是問題了!!!

 

小結:一圖

務必掌握好2和3,寫擴展很有用,甚至spring本身都在大量使用。

spring圍繞着bean運轉的,註冊的幾種方式,每種註冊方式的條件性選擇

 

最後請慢慢學會忘記xml格式的配置文件,現在或往後都是註解式了,雖然xml配置並不影響功能!

 

附部分註解圖一張:

 

參考:

0.  @Import Annotation in Spring Framework

https://javabeat.net/use-import-importing-javaconfig-files-spring-projects/ 

1.  Spring向容器註冊Bean的高級應用  https://cloud.tencent.com/developer/article/1497795

2. how spring import annotation parse(要翻牆) https://laptrinhx.com/spring-import-annotation-source-parsing-3074679655/

注意我說的牆不是下面這樣的牆

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