Spring入门：The IoC Container，实践篇（下）

WEBJ2EE

发布于 2019-09-17 15:57:47

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发布于 2019-09-17 15:57:47

文章被收录于专栏：WebJ2EEWebJ2EE

8. 基于【注解（Annotation）】的容器配置

8.1. 应该用 XML 还是 Annotation？

各有特点，应由开发人员根据情况挑选适合自己的方案。
注解配置更简洁，会侵入业务逻辑；（XML 恰好反过来）；
Spring 支持注解与 XML 配置混合使用；
Spring 通过 BeanPostProcessor 实现基于注解的配置；
<context:annotation-config/> 可隐式注册那些用于实现注解配置的 BeanPostProcessor：AutowiredAnnotationBeanPostProcessor, CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, RequiredAnnotationBeanPostProcessor
<context:annotation-config/> 的影响范围近限它所在的容器；
基于注解注入的执行时机要比基于XML的注入靠前；

8.2. @Required

应用于 Bean 属性的 setter 方法上。实现必填的依赖项控制；
Spring 5.1 中废弃了，建议使用【构造器注入】或【InitializingBean.afterPropertiesSet()】解决；

示例1：

package webj2ee;

import org.springframework.beans.factory.annotation.Required;

public class ExampleBean {
    private String name;

    @Required
    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "ExampleBean{" +
                "name='" + name + '\'' +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
    <bean id="exampleBean" class="webj2ee.ExampleBean"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        ExampleBean exampleBean = context.getBean("exampleBean", ExampleBean.class);
        System.out.println(exampleBean);
        context.close();
    }
}

8.3. @Autowired

@Autowired 注解可用于构造函数、setter方法、字段；
@Autowired 可以收集 ApplicationContext 中匹配某种特殊类型的所有 Bean；（注1：可收集为 Map、Set、Array）（注2：可通过 @Order 注解，调整顺序）
@Autowired 标记的方法或字段，默认是 required 的，可通过 @Autowired(required = false) 调整为非必填字段。也可以通过@Nullable、java.util.Optional （JDK8）作为替代方案；
当 @Autowired 标记到构造函数上时：如果只有一个构造函数，实际没必要通过@Autowired标注。如果存在多个构造函数，最多只能有一个构造函数被标记为（required=true）；多个构造函数可同时被标记为（required=false），都将作为候选，满足最大依赖项的构造函数将会被选用。
构造函数参数中的【多元素注入（arrays、set、map）】的注入比较特殊，如果没有找到可注入项，则会被解析为空（Empty）
@Autowired 可以直接解析一些Spring 内部依赖项：BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource。

示例1：（只有一个构造函数，没必要标记 @Autowired）

package webj2ee;
public class MovieFinder {
    @Override
    public String toString() {
        return "MovieFinder{}";
    }
}

package webj2ee;

public class SimpleMovieLister {
    private MovieFinder movieFinder;

    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    @Override
    public String toString() {
        return "SimpleMovieLister{" +
                "movieFinder=" + movieFinder +
                '}';
    }

    // ...
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="movieFinder" class="webj2ee.MovieFinder"/>
    <bean id="simpleMovieLister" class="webj2ee.SimpleMovieLister"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        SimpleMovieLister simpleMovieLister = context.getBean("simpleMovieLister", SimpleMovieLister.class);
        System.out.println(simpleMovieLister);
        context.close();
    }
}

示例2：（@Autowired + setter）

package webj2ee;
public class MovieFinder {
    @Override
    public String toString() {
        return "MovieFinder{}";
    }
}

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;

public class SimpleMovieLister {
    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    @Override
    public String toString() {
        return "SimpleMovieLister{" +
                "movieFinder=" + movieFinder +
                '}';
    }

    // ...
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="movieFinder" class="webj2ee.MovieFinder"/>
    <bean id="simpleMovieLister" class="webj2ee.SimpleMovieLister"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        SimpleMovieLister simpleMovieLister = context.getBean("simpleMovieLister", SimpleMovieLister.class);
        System.out.println(simpleMovieLister);
        context.close();
    }
}

示例3：（@Autowired + required=false）

package webj2ee;
public class MovieFinder {
    @Override
    public String toString() {
        return "MovieFinder{}";
    }
}

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;

public class SimpleMovieLister {
    private MovieFinder movieFinder;

    @Autowired(required = false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    
//    @Autowired
//    public void setMovieFinder(@Nullable MovieFinder movieFinder) {
//        this.movieFinder = movieFinder;
//    }

//    @Autowired
//    public void setMovieFinder(Optional<MovieFinder>  movieFinder) {
//        this.movieFinder = movieFinder.orElse(null);
//    }

    @Override
    public String toString() {
        return "SimpleMovieLister{" +
                "movieFinder=" + movieFinder +
                '}';
    }

    // ...
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="simpleMovieLister" class="webj2ee.SimpleMovieLister"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        SimpleMovieLister simpleMovieLister = context.getBean("simpleMovieLister", SimpleMovieLister.class);
        System.out.println(simpleMovieLister);
        context.close();
    }
}

示例4：（@Autowired + Set、Array、Map）

package webj2ee;
public interface MovieCatalog {
}

package webj2ee;

import org.springframework.core.annotation.Order;

@Order(1)
public class Comedy implements MovieCatalog {
    @Override
    public String toString() {
        return "Comedy{}";
    }
}

package webj2ee;

import org.springframework.core.annotation.Order;

@Order(2)
public class Dracula implements MovieCatalog {
    @Override
    public String toString() {
        return "Dracula{}";
    }
}

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;

import java.util.Arrays;
import java.util.Map;
import java.util.Set;

public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogsArr;

    @Autowired
    private Set<MovieCatalog> movieCatalogsSet;

    @Autowired
    private Map<String, MovieCatalog> movieCatalogsMap;

    @Override
    public String toString() {
        return "MovieRecommender{" +
                "movieCatalogsArr=" + Arrays.toString(movieCatalogsArr) +
                ", movieCatalogsSet=" + movieCatalogsSet +
                ", movieCatalogsMap=" + movieCatalogsMap +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="dracula" class="webj2ee.Dracula"/>
    <bean id="comedy" class="webj2ee.Comedy"/>
    <bean id="movieRecommender" class="webj2ee.MovieRecommender"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        MovieRecommender movieRecommender = context.getBean("movieRecommender", MovieRecommender.class);
        System.out.println(movieRecommender);
        context.close();
    }
}

示例5：（@Autowired + Constructors + Multi-Elements）

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;

import java.util.Arrays;
import java.util.Map;
import java.util.Set;

public class MovieRecommender {
    private MovieCatalog[] movieCatalogsArr;
    private Set<MovieCatalog> movieCatalogsSet;
    private Map<String, MovieCatalog> movieCatalogsMap;

    @Autowired(required = true)
    public MovieRecommender(
            MovieCatalog[] movieCatalogsArr,
            Set<MovieCatalog> movieCatalogsSet,
            Map<String, MovieCatalog> movieCatalogsMap) {
        this.movieCatalogsArr = movieCatalogsArr;
        this.movieCatalogsSet = movieCatalogsSet;
        this.movieCatalogsMap = movieCatalogsMap;
    }

    @Override
    public String toString() {
        return "MovieRecommender{" +
                "\nmovieCatalogsArr=" + Arrays.toString(movieCatalogsArr) +
                ", \nmovieCatalogsSet=" + movieCatalogsSet +
                ", \nmovieCatalogsMap=" + movieCatalogsMap +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="movieRecommender" class="webj2ee.MovieRecommender"/>
</beans>

示例6：（@Autowired + ApplicationContext ...）

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;

public class SimpleMovieLister {

    @Autowired
    private ApplicationContext context;

    @Override
    public String toString() {
        return "SimpleMovieLister{" +
                "context=" + context +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="simpleMovieLister" class="webj2ee.SimpleMovieLister"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        SimpleMovieLister simpleMovieLister = context.getBean("simpleMovieLister", SimpleMovieLister.class);
        System.out.println(simpleMovieLister);
        context.close();
    }
}

8.4. @Autowired + @Primary

@Autowired 是按【类型】装配，如果存在多个同类型候选Bean，就会报错；
可通过 @Primary 在一堆同类型 Bean 中标记其中某一个是主要的；

package webj2ee;

import org.springframework.beans.factory.annotation.Autowired;

public class MovieRecommender {
    @Autowired
    private MovieCatalog movieCatalog;

    @Override
    public String toString() {
        return "MovieRecommender{" +
                "movieCatalog=" + movieCatalog +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>

    <bean id="dracula" class="webj2ee.Dracula" primary="true"/>
    <bean id="comedy" class="webj2ee.Comedy"/>
    <bean id="movieRecommender" class="webj2ee.MovieRecommender"/>
</beans>

package webj2ee;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("context.xml");
        MovieRecommender movieRecommender = context.getBean("movieRecommender", MovieRecommender.class);
        System.out.println(movieRecommender);
        context.close();
    }
}

8.5. @Autowired + @Qualifier

@Qualifier 用于进一步精确限制候选 Bean 范围；

标准用法：

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>

    <bean id="dracula" class="webj2ee.Dracula">
        <qualifier value="level1"></qualifier>
    </bean>

    <bean id="comedy" class="webj2ee.Comedy">
        <qualifier value="level2"></qualifier>
    </bean>

    <bean id="movieRecommender" class="webj2ee.MovieRecommender"/>
</beans>

package webj2ee;

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

public class MovieRecommender {
    @Autowired
    @Qualifier("level1")
    private MovieCatalog movieCatalog;

    @Override
    public String toString() {
        return "MovieRecommender{" +
                "movieCatalog=" + movieCatalog +
                '}';
    }
}

@Qualifier(value) 的回退匹配特性：如果在候选 Bean 中没有找到复合 qualifier value 的 Bean，则会去找一个 id 与 qualifier value 相等的 Bean；但！不要将 @Qualifier(value) 的这种回退匹配的特性用于实现 byName 的依赖注入，应考虑其语义化特征。如果要实现 byName，应当选择@Resource。

8.6. @Resource

隶属 JSR-250 规范。
可应用于字段、Bean 的 setter 方法。
语义是 by-name。
@Resource 的回退匹配策略，会寻找主类型匹配元素 Bean。
@Resource 也可以解析像 BeanFactory, ApplicationContext, ResourceLoader, ApplicationEventPublisher, MessageSource 这些 Spring 依赖。

示例：

package webj2ee;

import org.springframework.context.ApplicationContext;

import javax.annotation.Resource;

public class SimpleMovieLister {
    private MovieFinder movieFinder;

    @Resource
    private ApplicationContext context;

    @Resource(name = "movieFinder")
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    @Override
    public String toString() {
        return "SimpleMovieLister{" +
                "movieFinder=" + movieFinder +
                ", context=" + context +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans ...>
    <context:annotation-config/>
    <bean id="movieFinder" class="webj2ee.MovieFinder"/>
    <bean id="simpleMovieLister" class="webj2ee.SimpleMovieLister"/>
</beans>

9. Classpath Scanning and Managed Components

前面提到的 @Autowired、@Resource 等只是解决了注入层面问题，但还是要在 XML 中手写 Bean 定义。
@Component 是标记 Bean 的基础注解；其衍生有：@Repository（持久层）, @Service（服务层）, @Controller（展示层）。
<context:component-scan> 用于实现 Bean 的扫描与注册；
<context:component-scan> 隐式启用了 <context:annotation-config>；
可通过 include-filter、exclude-filter 定制扫描策略；

<beans>
    <context:component-scan base-package="org.example">
        <context:include-filter type="regex"
                expression=".*Stub.*Repository"/>
        <context:exclude-filter type="annotation"
                expression="org.springframework.stereotype.Repository"/>
    </context:component-scan>
</beans>

下一波就轮到写

AOP实践篇咯

参考：

Spring Framework Runtime： https://docs.spring.io/spring/docs/4.2.x/spring-framework-reference/html/overview.html The IoC Container： https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#spring-core Inversion of Control Containers and the Dependency Injection pattern： https://martinfowler.com/articles/injection.html 《Java Web 高级编程技术》《Spring 入门经典》《精通 Spring 4.x 企业应用开发实战》《Spring5 高级编程》《Spring实战（第3版）》

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原始发表：2019-09-14，如有侵权请联系 cloudcommunity@tencent.com 删除

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