org.springframework.beans.factory.NoSuchBeanDefinitionException：没有可用的 'com.example.simple.RealApplication' 类型的合格 bean

Question

我无法理解为什么我的 Bean RealApplication 没有注册/找到。

文件DemoApplication.java:

package com.example.simple;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

@SpringBootApplication
public class DemoApplication
{
    public static void main(String[] args)
    {
        SpringApplication.run(DemoApplication.class, args);

        AnnotationConfigApplicationContext ctx = 
            new AnnotationConfigApplicationContext();
        ctx.refresh();

        RealApplication app = ctx.getBean(RealApplication.class);
        app.run();

        ctx.close();
    }
}

文件RealApplication.java:

package com.example.simple;

import org.springframework.stereotype.Component;

@Component
public class RealApplication
{
    public void run()
    {
        System.out.println("Hello World!");
    }
}

然而，奇怪的是，这段代码有效：

文件DemoApplication.java:

package com.example.simple;

import org.springframework.boot.SpringApplication;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class DemoApplication
{
    public static void main(String[] args)
    {
        SpringApplication.run(DemoApplication.class, args);

        AnnotationConfigApplicationContext ctx = 
            new AnnotationConfigApplicationContext("com.example.simple");

        RealApplication app = ctx.getBean(RealApplication.class);
        app.run();

        ctx.close();
    }
}

class RealApplication 未更改。

输出为：

Hello World!

我的 maven pom.xml 看起来像这样：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.6.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>simple-boot</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/javax.inject/javax.inject -->
        <dependency>
            <groupId>javax.inject</groupId>
            <artifactId>javax.inject</artifactId>
            <version>1</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

出于某些奇怪的原因，当我只使用 @SpringBootApplication 注释时，组件扫描没有拾取 RealApplication Bean，即使两个 classes 是同一个包。

但是当我在构造函数中指定要扫描的包时它能够找到它 AnnotationConfigApplicationContext

Answer 1

您显示的代码有误吗？您显示的代码总是创建两个应用程序上下文，一个是 AnnotationConfigApplicationContext，另一个是使用 spring-boot 方式创建的，其中涉及使用 SpringApplication 和 @SpringBootApplication.

而且您总是从 AnnotationConfigApplicationContext 创建的上下文中获取 RealApplication bean，而不是从 spring 引导创建的上下文中获取。要测试 RealApplication bean 是否真的在后一种情况下创建，您必须从 SpringApplication.run():

返回的上下文中获取 bean

ApplicationContext context = SpringApplication.run(DemoApplication.class, args);
RealApplication app = context.getBean(RealApplication.class);
app.run();

org.springframework.beans.factory.NoSuchBeanDefinitionException：没有可用的 'com.example.simple.RealApplication' 类型的合格 bean

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.simple.RealApplication' available

java

spring-boot

component-scan