Question

下面的朋友是我的代码，我正在尝试使用Spring运行依赖注入

我有一个接口，该接口的两个类实现。

一个bean.xml和一个主方法类。

接口IWriter.java

package DI;
    public interface IWriter {
    public void writer(String s);
}

Class Writer.java

     package DI;

     import org.springframework.stereotype.Service;

     @Service
     public class Writer implements IWriter {
        public void writer (String s){
            System.out.println(s);
        }
     }

Class NiceWriter.java

package DI;

import org.springframework.stereotype.Service;

@Service
public class NiceWriter implements IWriter {
    public void writer (String s){
        System.out.println("The string is " + s);
    }
}

另一个班级

package DI;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Service;

@Service
public class MySpringBeanWithDependency {

    @Autowired
    private IWriter writer;

    public void run() {
        String s = "This is my test";
        writer.writer(s);
    }
}

Main.java

package DI;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import DI.MySpringBeanWithDependency;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
        BeanFactory factory = context;
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) factory.getBean("mySpringBeanWithDependency");
        test.run();
    }
}

bean.xml

          <?xml version="1.0" encoding="UTF-8"?>
          <beans xmlns="http://www.springframework.org/schema/beans"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:aop="http://www.springframework.org/schema/aop"
           xmlns:context="http://www.springframework.org/schema/context"
          xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
       http://www.springframework.org/schema/aop
       http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
       http://www.springframework.org/schema/context
       http://www.springframework.org/schema/context/spring-context-2.5.xsd">

       <context:component-scan base-package="DI" />

       </beans>

当我运行代码时，Spring容器给出了Writer.java类方法的输出。我没有指定选择哪个实现。 Spring如何获得Writer.java的实现？

Answer 1

如果有多个接口实现，并且在这种情况下使用@Autowired，则绑定任何类。但是如果你想自动装配特定的实现，那么你可以使用

@Qualifier( "<implementing class name>" )

@Qualifier documentation

你必须了解的关于Spring的一些事情

管理所有的spring bean - 它们“活在”容器内，称为“应用程序上下文”。
每个应用程序都有一个指向该上下文的入口点。此外，还有一个应用程序上下文被引导的地方和所有bean - 自动装配。在Web应用程序中，这可以是启动侦听器。

通过将一个bean的实例放入另一个bean的实例中的所需字段中来实现自动装配。这两个类都应该是bean，即它们应该被定义为存在于应用程序上下文中。

Answer 2

按如下方式更改您的代码。

Class Writer.java

  package DI;

     import org.springframework.stereotype.Service;

     @Service("writer")
     public class Writer implements IWriter {
     public void writer (String s){
      System.out.println(s);
     }
    }

Class NiceWriter.java

    package DI;

    import org.springframework.stereotype.Service;

   @Service("niceWriter")
   public class NiceWriter implements IWriter {
   public void writer (String s){
    System.out.println("The string is " + s);
   }
  }

另一个班级

     package DI;

      import org.springframework.beans.factory.annotation.Autowired;
      import org.springframework.beans.factory.annotation.Qualifier;
      import org.springframework.stereotype.Service;

      @Service
      public class MySpringBeanWithDependency {

     @Autowired
     @Qualifier("writer")//if you need to autowire Writer service   
     private IWriter writer;

     @Autowired
    @Qualifier("niceWriter")//if you need to autowire NiceWriter service
    private IWriter niceWriter


       public void run() {
       String s = "This is my test";
        writer.writer(s);
      }
    }

Answer 3

试试这个。

Class Writer.java

  package DI;

     import org.springframework.stereotype.Service;

     @Service("writer")
     public class Writer implements IWriter {
     public void writer (String s){
      System.out.println(s);
     }
    }

Class NiceWriter.java

    package DI;

    import org.springframework.stereotype.Service;

   @Service("niceWriter")
   public class NiceWriter implements IWriter {
   public void writer (String s){
    System.out.println("The string is " + s);
   }
  }

另一个班级

     package DI;

      import org.springframework.beans.factory.annotation.Autowired;
      import org.springframework.beans.factory.annotation.Qualifier;
      import org.springframework.stereotype.Service;

      @Service
      public class MySpringBeanWithDependency {

     @Autowired
     private IWriter writer;

     @Autowired
    private IWriter niceWriter


       public void run() {
       String s = "This is my test";
        writer.writer(s);
      }
    }

Answer 4

我想使用application.properties再显示一个选项。

好处：

添加/更改接口实现时不需要更改代码
与单元测试和其他环境配合良好

基于@ConditionalOnProperty属性的示例代码

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.stereotype.Service;

@Service
@ConditionalOnProperty(value = "writer.type", havingValue = "default")
public class Writer implements IWriter {
    @Override
    public void writer(String s) {
        System.out.println("The string is " + s);
    }
}

和

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.stereotype.Service;

@Service
@ConditionalOnProperty(value = "writer.type", havingValue = "nice")
public class NiceWriter implements IWriter {
    @Override
    public void writer(String s) {
        System.out.println("Nice string is " + s);
    }
}

当application.properties包含writer.type=nice时，将为IWriter界面实例化NiceWriter。

除了@ConditionalOnProperty之外，还有其他选项，例如Conditional，@ConditionalOnExpression。

Answer 5

这取决于用例，您可以使用spring概要文件，自定义注释或使用@Qualifier（按名称注入）来选择接口的实现，等效于JSR-330的@Named注释

Spring正在挑选一个接口实现，而不是自己的？

5 个答案: