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浅谈Spring中@Import注解的作用和使用

2020年06月14日  | 移动技术网IT编程  | 我要评论

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@import用来导入@configuration注解的配置类、声明@bean注解的bean方法、导入importselector的实现类或导入importbeandefinitionregistrar的实现类。

@import注解的作用

查看import注解源码

/**
 * indicates one or more {@link configuration @configuration} classes to import.
 *
 * <p>provides functionality equivalent to the {@code <import/>} element in spring xml.
 * only supported for classes annotated with {@code @configuration} or declaring at least
 * one {@link bean @bean} method, as well as {@link importselector} and
 * {@link importbeandefinitionregistrar} implementations.
 *
 * <p>{@code @bean} definitions declared in imported {@code @configuration} classes
 * should be accessed by using {@link org.springframework.beans.factory.annotation.autowired @autowired}
 * injection. either the bean itself can be autowired, or the configuration class instance
 * declaring the bean can be autowired. the latter approach allows for explicit,
 * ide-friendly navigation between {@code @configuration} class methods.
 *
 * <p>may be declared at the class level or as a meta-annotation.
 *
 * <p>if xml or other non-{@code @configuration} bean definition resources need to be
 * imported, use {@link importresource @importresource}
 *
 * @author chris beams
 * @since 3.0
 * @see configuration
 * @see importselector
 * @see importresource
 */
@target(elementtype.type)
@retention(retentionpolicy.runtime)
@documented
public @interface import {

 /**
 * the @{@link configuration}, {@link importselector} and/or
 * {@link importbeandefinitionregistrar} classes to import.
 */
 class<?>[] value();
}

分析类注释得出结论：

  • 声明一个bean
  • 导入@configuration注解的配置类
  • 导入importselector的实现类
  • 导入importbeandefinitionregistrar的实现类

@import注解的使用

声明一个bean

package com.example.demo.bean;

public class testbean1 {
}

package com.example.demo;

import com.example.demo.bean.testbean1;
import org.springframework.context.annotation.configuration;
import org.springframework.context.annotation.import;

@import({testbean1.class})
@configuration
public class appconfig {
}

导入@configuration注解的配置类

package com.example.demo.bean;

public class testbean2 {
}

package com.example.demo.bean;

import org.springframework.context.annotation.bean;
import org.springframework.context.annotation.configuration;

@configuration
public class testconfig {
  @bean
  public testbean2 gettestbean2(){
    return new testbean2();
  }
}

package com.example.demo;

import com.example.demo.bean.testbean1;
import com.example.demo.bean.testconfig;
import org.springframework.context.annotation.configuration;
import org.springframework.context.annotation.import;

@import({testbean1.class,testconfig.class})
@configuration
public class appconfig {
}

导入importselector的实现类

package com.example.demo.bean;

public class testbean3 {
}

package com.example.demo.bean;

import org.springframework.context.annotation.importselector;
import org.springframework.core.type.annotationmetadata;

public class testimportselector implements importselector {
  @override
  public string[] selectimports(annotationmetadata importingclassmetadata) {
    return new string[]{"com.example.demo.bean.testbean3"};
  }
}

package com.example.demo;

import com.example.demo.bean.testbean1;
import com.example.demo.bean.testconfig;
import com.example.demo.bean.testimportselector;
import org.springframework.context.annotation.configuration;
import org.springframework.context.annotation.import;

@import({testbean1.class,testconfig.class,testimportselector.class})
@configuration
public class appconfig {
}

导入importbeandefinitionregistrar的实现类

package com.example.demo.bean;

public class testbean4 {
}

package com.example.demo.bean;

import org.springframework.beans.factory.support.beandefinitionregistry;
import org.springframework.beans.factory.support.rootbeandefinition;
import org.springframework.context.annotation.importbeandefinitionregistrar;
import org.springframework.core.type.annotationmetadata;

public class testimportbeandefinitionregistrar implements importbeandefinitionregistrar {
  @override
  public void registerbeandefinitions(annotationmetadata importingclassmetadata, beandefinitionregistry registry) {
    rootbeandefinition rootbeandefinition = new rootbeandefinition(testbean4.class);
    registry.registerbeandefinition("testbean4", rootbeandefinition);
  }
}

package com.example.demo;

import com.example.demo.bean.testbean1;
import com.example.demo.bean.testconfig;
import com.example.demo.bean.testimportbeandefinitionregistrar;
import com.example.demo.bean.testimportselector;
import org.springframework.context.annotation.configuration;
import org.springframework.context.annotation.import;

@import({testbean1.class,testconfig.class,testimportselector.class,testimportbeandefinitionregistrar.class})
@configuration
public class appconfig {
}

最后，我们来看下导入结果：

package com.example.demo;

import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.boot.test.context.springboottest;
import org.springframework.context.annotation.annotationconfigapplicationcontext;
import org.springframework.test.context.junit4.springrunner;

import java.util.arrays;

@runwith(springrunner.class)
@springboottest
public class demoapplicationtests {

  @test
  public void test() {
    annotationconfigapplicationcontext annotationconfigapplicationcontext = new annotationconfigapplicationcontext(appconfig.class);
    string[] beandefinitionnames = annotationconfigapplicationcontext.getbeandefinitionnames();
    system.out.println("--------------------------------------------------------");
    for (string beandefinitionname: beandefinitionnames) {
      system.out.println(beandefinitionname);
    }
    system.out.println("--------------------------------------------------------");
  }
}

打印结果如下：

--------------------------------------------------------
org.springframework.context.annotation.internalconfigurationannotationprocessor
org.springframework.context.annotation.internalautowiredannotationprocessor
org.springframework.context.annotation.internalrequiredannotationprocessor
org.springframework.context.annotation.internalcommonannotationprocessor
org.springframework.context.event.internaleventlistenerprocessor
org.springframework.context.event.internaleventlistenerfactory
appconfig
com.example.demo.bean.testbean1
com.example.demo.bean.testconfig
gettestbean2
com.example.demo.bean.testbean3
testbean4
--------------------------------------------------------

可以看出testbean1,testbean2,testbean3,testbean4通过不同的4种导入方法被导入springioc容器中。 

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