为什么 Spring Boot 需要数据源？

Question

我打开了一些奇怪的东西。

为什么Spring Boot在像这样在包中创建Application类时不需要数据源：

主要

------java

----------教

-------- SpringBootApplication.class

以及为什么在创建应用程序类时不需要数据源，而没有像这样的任何包：

主要

------java

-------- SpringBootApplication.class

在这两种方式中我都有相同的课程：

@SpringBootApplication
public class SpringBootWithoutDbExamplApplication {

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

}

和同一个pom：

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

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <excludes>
                        <exclude>
                            <groupId>org.projectlombok</groupId>
                            <artifactId>lombok</artifactId>
                        </exclude>
                    </excludes>
                </configuration>
            </plugin>
        </plugins>
    </build>

但是当我在没有包的情况下运行项目时出现错误：

2021-04-14 22:53:54.274 ERROR 5872 --- [           main] o.s.boot.SpringApplication               : Application run failed

org.springframework.beans.factory.BeanDefinitionStoreException: Failed to read candidate component class: URL [jar:file:C:\Users\altuk\.m2\repository\org\springframework\boot\spring-boot-autoconfigure\2.4.4\spring-boot-autoconfigure-2.4.4.jar!/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration.class]; nested exception is java.lang.IllegalStateException: Could not evaluate condition on org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration due to org/springframework/jdbc/datasource/embedded/EmbeddedDatabaseType not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (e.g. if you put a @ComponentScan in the default package by mistake)
    at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.scanCandidateComponents(ClassPathScanningCandidateComponentProvider.java:452) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.findCandidateComponents(ClassPathScanningCandidateComponentProvider.java:315) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ClassPathBeanDefinitionScanner.doScan(ClassPathBeanDefinitionScanner.java:276) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ComponentScanAnnotationParser.parse(ComponentScanAnnotationParser.java:132) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassParser.doProcessConfigurationClass(ConfigurationClassParser.java:296) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:250) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:207) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:175) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:331) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:247) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:311) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:112) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:746) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:564) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:144) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:769) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:326) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1313) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1302) ~[spring-boot-2.4.4.jar:2.4.4]
    at SpringBootWithoutDbExamplApplication.main(SpringBootWithoutDbExamplApplication.java:8) ~[classes/:na]
Caused by: java.lang.IllegalStateException: Could not evaluate condition on org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration due to org/springframework/jdbc/datasource/embedded/EmbeddedDatabaseType not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (e.g. if you put a @ComponentScan in the default package by mistake)
    at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:54) ~[spring-boot-autoconfigure-2.4.4.jar:2.4.4]
    at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:88) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:71) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.isConditionMatch(ClassPathScanningCandidateComponentProvider.java:513) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.isCandidateComponent(ClassPathScanningCandidateComponentProvider.java:496) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.scanCandidateComponents(ClassPathScanningCandidateComponentProvider.java:430) ~[spring-context-5.3.5.jar:5.3.5]
    ... 21 common frames omitted
Caused by: java.lang.NoClassDefFoundError: org/springframework/jdbc/datasource/embedded/EmbeddedDatabaseType
    at org.springframework.boot.jdbc.EmbeddedDatabaseConnection.<clinit>(EmbeddedDatabaseConnection.java:55) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration$EmbeddedDatabaseCondition.getMatchOutcome(DataSourceAutoConfiguration.java:136) ~[spring-boot-autoconfigure-2.4.4.jar:2.4.4]
    at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47) ~[spring-boot-autoconfigure-2.4.4.jar:2.4.4]
    ... 27 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:602) ~[na:na]
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178) ~[na:na]
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522) ~[na:na]
    ... 30 common frames omitted

Answer 1

关于可能导致答案的 Spring Boot 约定需要注意的一点。

当它启动时，它必须构建一个应用程序上下文，因此它需要“了解”在哪里搜索 bean。

按照惯例，spring boot 会搜索带有@SpringBootApplication注解的类所在的包中的bean以及下面的所有包。

例子：

如果您将带有@SpringBootApplication 注释的类放在com.myorg.myapp 中 它将在以下位置查找 bean：

com.myorg.myapp
com.myorg.myapp.abc
com.myorg.myapp.abc.def.any.inner.place

它不会在以下位置搜索 bean：

com.myorg.anotherapp
com.myorg
org

我相信你已经明白了。

现在，当您将带有 @SpringBootApplication 注释的主类放在默认包中时，就像在说 spring - 扫描所有内容

我认为这会导致问题，因为 spring 开始在不应该搜索的地方搜索。

除此之外，这会对性能产生严重影响，因为如果您的应用程序具有甚至没有 bean 的第三方依赖项 - spring 也会在那里搜索。

因此，最重要的是，您应该始终将主类放在您的应用程序独有的包中。

Answer 2

可能是因为 spring boot 自动配置。 尝试通过属性禁用它。

spring:
  autoconfigure:
    exclude:
      - org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration