独立模块中的 spring-boot-starter-parent 导致其 bean 在依赖模块中看不到答案

【问题标题】：spring-boot-starter-parent in independent module causes its beans not be seen in dependent modules独立模块中的 spring-boot-starter-parent 导致其 bean 在依赖模块中看不到
【发布时间】：2017-08-10 09:37:30
【问题描述】：

我有一个聚合器 POM，它引用了 2 个模块“appA”和“appB”。 'appB' 依赖于 'appA' 作为依赖项 'appB' 有 'spring-boot-starter-parent' 作为父 POM 'appA' 在包 "com.multipom.config" 中定义了一个 bean ('hwBean')

当“appB”应用程序运行时，只有在“appA”不使用时它才会看到“hwBean” 'spring-boot-starter-parent' 作为父 POM。当“appA”使用“spring-boot-starter-parent”作为父 POM 时，“appA”中定义的“hwBean”bean 在“appB”中不可用。这是一个spring-boot错误吗？

在 AppA 模块中

---- com.multipom.App_A.java ------

package com.multipom;

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

---- com.multipom.config.ApplicationConfig.java

package com.multipom.config;
@Configuration
public class ApplicationConfig {
    @Bean
    public String hwBean () {
        return "Hello World";
    }
}

在 AppB 模块中

---- com.multipom.App_B.java ------

package com.multipom;

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

root/pom.xml

<groupId>com.splitpom</groupId>
<artifactId>root</artifactId>
<version>1.0.0</version>
<packaging>pom</packaging>
<modules>
    <module>./appA</module>
    <module>./appB</module>
</modules>

appB/pom.xml

<groupId>com.multipom</groupId>
<artifactId>appB</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>appB</name>
<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>1.5.2.RELEASE</version>
  <relativePath/> <!-- lookup parent from repository -->
</parent>

<dependencies>
  <dependency>
    <groupId>com.multipom</groupId>
    <artifactId>appA</artifactId>
    <version>0.0.1-SNAPSHOT</version>
  </dependency>
</dependencies>

appA/pom.xml（案例 1）

<groupId>com.multipom</groupId>
<artifactId>appA</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>appA</name>
<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>1.5.2.RELEASE</version>
  <relativePath/> <!-- lookup parent from repository -->
</parent>

appA/pom.xml（案例 2）

<groupId>com.multipom</groupId>
<artifactId>appA</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>appA</name>
<parent>
  <groupId>com.splitpom</groupId>
  <artifactId>root</artifactId>
  <version>1.0.0</version>
</parent>

$ java -jar appA-0.0.1-SNAPSHOT.jar

2017-03-18 11:23:23.340 TRACE 26149 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appA/target/appA-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/config/ApplicationConfig.class]
2017-03-18 11:23:23.415 DEBUG 26149 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Identified candidate component class: URL [jar:file:/multipom/appA/target/appA-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/config/ApplicationConfig.class]
2017-03-18 11:23:23.416 TRACE 26149 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appA/target/appA-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_A.class]
2017-03-18 11:23:23.430 TRACE 26149 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Ignored because not matching any filter: URL [jar:file:/multipom/appA/target/appA-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_A.class]
2017-03-18 11:23:24.084 DEBUG 26149 --- [           main] a.ConfigurationClassBeanDefinitionReader : Registering bean definition for @Bean method com.multipom.config.ApplicationConfig.hwBean()

案例1：如果'appA'使用'spring-boot-starter-parent'作为父POM（注意这里'appB'没有从'appA'找到'hwBean'

$ java -jar appB-0.0.1-SNAPSHOT.jar

2017-03-18 11:24:31.461 TRACE 26203 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]
2017-03-18 11:24:31.546 TRACE 26203 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Ignored because not matching any filter: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]

案例 2：如果 'appA' 不使用 'spring-boot-starter-parent' 作为父 POM（请注意，'appB' 确实从 'appA' 中找到了 'hwBean'。

$ java -jar appB-0.0.1-SNAPSHOT.jar   
2017-03-18 12:03:28.261 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]
2017-03-18 12:03:28.316 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Ignored because not matching any filter: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]
2017-03-18 12:03:28.316 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/App_A.class]
2017-03-18 12:03:28.324 DEBUG 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Identified candidate component class: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/App_A.class]
2017-03-18 12:03:28.325 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/config/ApplicationConfig.class]
2017-03-18 12:03:28.330 DEBUG 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Identified candidate component class: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/config/ApplicationConfig.class]
2017-03-18 12:03:28.348 DEBUG 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning
2017-03-18 12:03:28.373 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]
2017-03-18 12:03:28.384 DEBUG 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Identified candidate component class: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/multipom/App_B.class]
2017-03-18 12:03:28.385 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/App_A.class]
2017-03-18 12:03:28.388 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Ignored because not matching any filter: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/App_A.class]
2017-03-18 12:03:28.389 TRACE 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Scanning URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/config/ApplicationConfig.class]
2017-03-18 12:03:28.390 DEBUG 27097 --- [           main] o.s.c.a.ClassPathBeanDefinitionScanner   : Identified candidate component class: URL [jar:file:/multipom/appB/target/appB-0.0.1-SNAPSHOT.jar!/BOOT-INF/lib/appA-0.0.1-SNAPSHOT.jar!/com/multipom/config/ApplicationConfig.class]
2017-03-18 12:03:29.123 DEBUG 27097 --- [           main] a.ConfigurationClassBeanDefinitionReader : Registering bean definition for @Bean method com.multipom.config.ApplicationConfig.hwBean()

【问题讨论】：

标签： spring-boot parent-pom

【解决方案1】：

问题在于 spring-boot-maven-plugin 打包依赖 jar 的方式。在默认设置中，此插件将 jar 打包为特殊的可执行 jar 格式，该格式将您的所有类放在 /BOOT-INF/classes/... 下。如果您的 jar 用作另一个模块中的依赖项，您不希望那。相反，您的 jar 必须是普通的扁平 jar，以便依赖应用程序可以发现所有包含的 bean。解决方案是将以下内容添加到spring-boot-maven-plugin 部分（在 appA/pom.xml 中）：

    <executions>
            <execution>
                <goals>
                    <goal>repackage</goal>
                </goals>
                <configuration>
                    <classifier>exec</classifier>
                </configuration>
            </execution>
        </executions>

【讨论】：