Spring Boot官方例子《Developing Your First Spring Boot Application》无法运行

官方的第一个例子就卡住了：
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#getting-started.first-application
按照要求，一步一步走：

查看Java版本和MVN版本：

$ java -version
openjdk version "17.0.4.1" 2022-08-12 LTS
OpenJDK Runtime Environment (build 17.0.4.1+1-LTS)
OpenJDK 64-Bit Server VM (build 17.0.4.1+1-LTS, mixed mode, sharing)

$ mvn -v
Apache Maven 3.8.5 (3599d3414f046de2324203b78ddcf9b5e4388aa0)
Maven home: usr/Users/developer/tools/maven/3.8.5
Java version: 17.0.4.1, vendor: BellSoft, runtime: /Users/developer/sdkman/candidates/java/17.0.4.1-librca

创建POM文件(我放在了D:\dev\2023\000000_learn\230310_spring\pom.xml)：

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.example</groupId><artifactId>myproject</artifactId><version>0.0.1-SNAPSHOT</version><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>3.0.4</version></parent><!-- Additional lines to be added here... -->
</project>

添加类依赖，在pom.xml的下面添加：

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

创建源码文件：src/main/java/MyApplication.java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;@RestController
@SpringBootApplication
public class MyApplication {@RequestMapping("/")String home() {return "Hello World!";}public static void main(String[] args) {SpringApplication.run(MyApplication.class, args);}
}

运行程序：

$ mvn spring-boot:run.   ____          _            __ _ _/\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \\\/  ___)| |_)| | | | | || (_| |  ) ) ) )'  |____| .__|_| |_|_| |_\__, | / / / /=========|_|==============|___/=/_/_/_/:: Spring Boot ::  (v3.0.4)
....... . . .
....... . . . (log output here)
....... . . .
........ Started MyApplication in 0.906 seconds (process running for 6.514)

多么美好，但是我的运行结果是：

D:\dev\2023\000000_learn\230310_spring>mvn spring-boot:run
[INFO] Scanning for projects...
[INFO]
[INFO] -----------------------< com.example:myproject >------------------------
[INFO] Building myproject 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] >>> spring-boot-maven-plugin:3.0.4:run (default-cli) > test-compile @ myproject >>>
[INFO]
[INFO] --- maven-resources-plugin:3.3.0:resources (default-resources) @ myproject ---
[INFO] skip non existing resourceDirectory D:\dev\2023\000000_learn\230310_spring\src\main\resources
[INFO] skip non existing resourceDirectory D:\dev\2023\000000_learn\230310_spring\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.10.1:compile (default-compile) @ myproject ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to D:\dev\2023\000000_learn\230310_spring\target\classes
[INFO]
[INFO] --- maven-resources-plugin:3.3.0:testResources (default-testResources) @ myproject ---
[INFO] skip non existing resourceDirectory D:\dev\2023\000000_learn\230310_spring\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.10.1:testCompile (default-testCompile) @ myproject ---
[INFO] No sources to compile
[INFO]
[INFO] <<< spring-boot-maven-plugin:3.0.4:run (default-cli) < test-compile @ myproject <<<
[INFO]
[INFO]
[INFO] --- spring-boot-maven-plugin:3.0.4:run (default-cli) @ myproject ---
[INFO] Attaching agents: [].   ____          _            __ _ _/\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \\\/  ___)| |_)| | | | | || (_| |  ) ) ) )'  |____| .__|_| |_|_| |_\__, | / / / /=========|_|==============|___/=/_/_/_/:: Spring Boot ::                (v3.0.4)2023-03-10T20:21:20.631+08:00  INFO 21024 --- [           main] MyApplication                            : Starting MyApplication using Java 17.0.2 with PID 21024 (D:\dev\2023\000000_learn\230310_spring\target\classes started by jianmin.liu in D:\dev\2023\000000_learn\230310_spring)
2023-03-10T20:21:20.631+08:00  INFO 21024 --- [           main] MyApplication                            : No active profile set, falling back to 1 default profile: "default"
2023-03-10T20:21:20.678+08:00  WARN 21024 --- [           main] ionWarningsApplicationContextInitializer :** WARNING ** : Your ApplicationContext is unlikely to start due to a @ComponentScan of the default package.2023-03-10T20:21:20.983+08:00  WARN 21024 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanDefinitionStoreException: Failed to read candidate component class: URL [jar:file:D:\app\apache-maven-3.8.6\alimvn\org\springframework\boot\spring-boot-autoconfigure\3.0.4\spring-boot-autoconfigure-3.0.4.jar!/org/springframework/boot/autoconfigure/r2dbc/ConnectionFactoryConfigurations$PoolConfiguration.class]
2023-03-10T20:21:20.983+08:00  INFO 21024 --- [           main] .s.b.a.l.ConditionEvaluationReportLogger :Error starting ApplicationContext. To display the condition evaluation report re-run your application with 'debug' enabled.
2023-03-10T20:21:20.999+08:00 ERROR 21024 --- [           main] o.s.boot.SpringApplication               : Application run failedorg.springframework.beans.factory.BeanDefinitionStoreException: Failed to read candidate component class: URL [jar:file:D:\app\apache-maven-3.8.6\alimvn\org\springframework\boot\spring-boot-autoconfigure\3.0.4\spring-boot-autoconfigure-3.0.4.jar!/org/springframework/boot/autoconfigure/r2dbc/ConnectionFactoryConfigurations$PoolConfiguration.class]at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.scanCandidateComponents(ClassPathScanningCandidateComponentProvider.java:463) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.findCandidateComponents(ClassPathScanningCandidateComponentProvider.java:317) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ClassPathBeanDefinitionScanner.doScan(ClassPathBeanDefinitionScanner.java:276) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ComponentScanAnnotationParser.parse(ComponentScanAnnotationParser.java:128) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassParser.doProcessConfigurationClass(ConfigurationClassParser.java:289) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:243) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:196) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:164) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:398) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:283) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:344) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:115) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:747) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:565) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:146) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:732) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:434) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:310) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:1304) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:1293) ~[spring-boot-3.0.4.jar:3.0.4]at MyApplication.main(MyApplication.java:15) ~[classes/:na]
Caused by: java.lang.IllegalStateException: Could not evaluate condition on org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryConfigurations$PoolConfiguration due to io/r2dbc/spi/ValidationDepth not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (e.g. if you put a @ComponentScan in the default package by mistake)at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:54) ~[spring-boot-autoconfigure-3.0.4.jar:3.0.4]at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:88) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:71) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.isConditionMatch(ClassPathScanningCandidateComponentProvider.java:518) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.isCandidateComponent(ClassPathScanningCandidateComponentProvider.java:501) ~[spring-context-6.0.6.jar:6.0.6]at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.scanCandidateComponents(ClassPathScanningCandidateComponentProvider.java:436) ~[spring-context-6.0.6.jar:6.0.6]... 20 common frames omitted
Caused by: java.lang.NoClassDefFoundError: io/r2dbc/spi/ValidationDepthat java.base/java.lang.Class.getDeclaredFields0(Native Method) ~[na:na]at java.base/java.lang.Class.privateGetDeclaredFields(Class.java:3297) ~[na:na]at java.base/java.lang.Class.getDeclaredField(Class.java:2608) ~[na:na]at org.springframework.boot.context.properties.bind.DefaultBindConstructorProvider$Constructors.isInnerClass(DefaultBindConstructorProvider.java:147) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.DefaultBindConstructorProvider$Constructors.getCandidateConstructors(DefaultBindConstructorProvider.java:137) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.DefaultBindConstructorProvider$Constructors.getConstructors(DefaultBindConstructorProvider.java:105) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.DefaultBindConstructorProvider.getBindConstructor(DefaultBindConstructorProvider.java:44) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.ValueObjectBinder$ValueObject.get(ValueObjectBinder.java:190) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.ValueObjectBinder.bind(ValueObjectBinder.java:67) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:476) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:590) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:576) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:474) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:414) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:343) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:332) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:262) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:223) ~[spring-boot-3.0.4.jar:3.0.4]at org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryConfigurations$PooledConnectionFactoryCondition.getMatchOutcome(ConnectionFactoryConfigurations.java:139) ~[spring-boot-autoconfigure-3.0.4.jar:3.0.4]at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47) ~[spring-boot-autoconfigure-3.0.4.jar:3.0.4]... 26 common frames omitted
Caused by: java.lang.ClassNotFoundException: io.r2dbc.spi.ValidationDepthat java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) ~[na:na]at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) ~[na:na]at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[na:na]... 46 common frames omitted[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  10.108 s
[INFO] Finished at: 2023-03-10T20:21:21+08:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:3.0.4:run (default-cli) on project myproject: Process terminated with exit code: 1 -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

查了很多都说是因为JAVA版本不对，但实际上是缺少对package的声明。
只要在src/main/java/MyApplication.java的第一行加上：

package com.example.myproject;

就解决了。

搞笑的是，官方文档也提出了，在写程序的时候“最好”要写上default package，但是他却没有这么做。。

6.2.1. Using the “default” Package
When a class does not include a package declaration, it is considered to be in the “default package”. The use of the “default package” is generally discouraged and should be avoided. It can cause particular problems for Spring Boot applications that use the @ComponentScan, @ConfigurationPropertiesScan, @EntityScan, or @SpringBootApplication annotations, since every class from every jar is read.