Spring Boot应用CommandLineRunner执行失败（空指针异常）求助

我正在开发一个Spring Boot应用，主类实现了CommandLineRunner接口，需要执行的代码放在重写的run()方法中。启动应用时遇到了NullPointerException，Stack Overflow上的现有解决方案没能解决我的问题，请求帮助排查并解决。

代码片段 #

应用主类代码 #

package com.First_Spring.Database_My_Spring;
import com.First_Spring.Database_My_Spring.Entities.Person;
import com.First_Spring.Database_My_Spring.JDBC.Person_JDBC_DAO;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DatabaseMySpringApplication implements CommandLineRunner{
	@Autowired
	Person_JDBC_DAO person_jdbc_dao;

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

	@Override
	public void run(String... args) throws Exception {
		System.out.println( person_jdbc_dao.find_All() );
	}
}

application.properties配置 #

spring.h2.console.enabled=true
spring.jpa.hibernate.ddl-auto=create
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

pom.xml依赖 #

<?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>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.3.1.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.First_Spring</groupId>
	<artifactId>Database_My_Spring</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>Database_My_Spring</name>
	<description>Demo project for Spring Boot</description>
	<properties>
		<java.version>11</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jdbc</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
		<dependency>
			<groupId>com.h2database</groupId>
			<artifactId>h2</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
		</dependency>
	</dependencies>
	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>
</project>

启动错误日志 #

2020-06-19 19:54:40.072 INFO 22134 --- [ main] ConditionEvaluationReportLoggingListener : Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2020-06-19 19:54:40.081 ERROR 22134 --- [ main] o.s.boot.SpringApplication : Application run failed
java.lang.IllegalStateException: Failed to execute CommandLineRunner
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:798) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:779) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:322) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	at com.First_Spring.Database_My_Spring.DatabaseMySpringApplication.main(DatabaseMySpringApplication.java:18) ~[classes/:na]
Caused by: java.lang.NullPointerException: null
	at com.First_Spring.Database_My_Spring.JDBC.Person_JDBC_DAO.find_All(Person_JDBC_DAO.java:18) ~[classes/:na]
	at com.First_Spring.Database_My_Spring.JDBC.Person_JDBC_DAO$$FastClassBySpringCGLIB$$f0cd1232.invoke(<generated>) ~[classes/:na]
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:771) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:749) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139) ~[spring-tx-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:749) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:691) ~[spring-aop-5.2.7.RELEASE.jar:5.2.7.RELEASE]
	at com.First_Spring.Database_My_Spring.JDBC.Person_JDBC_DAO$$EnhancerBySpringCGLIB$$e3e35047.find_All(<generated>) ~[classes/:na]
	at com.First_Spring.Database_My_Spring.DatabaseMySpringApplication.run(DatabaseMySpringApplication.java:25) ~[classes/:na]
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:795) ~[spring-boot-2.3.1.RELEASE.jar:2.3.1.RELEASE]
	... 5 common frames omitted

问题分析与解决方案 #

从错误栈可以看出，NullPointerException发生在Person_JDBC_DAO的find_All()方法第18行，而且Spring已经通过CGLIB生成了DAO的代理类，说明person_jdbc_dao本身已经被Spring正确注入，问题出在DAO内部的某个依赖对象为null，最常见的情况是**JdbcTemplate未被正确注入**。

步骤1：确保Person_JDBC_DAO被Spring容器管理 #

在Person_JDBC_DAO类上添加@Repository注解，让Spring自动扫描并将该类注册为容器中的Bean：

package com.First_Spring.Database_My_Spring.JDBC;

import com.First_Spring.Database_My_Spring.Entities.Person;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Repository;
import java.util.List;

@Repository
public class Person_JDBC_DAO {

    // 推荐使用构造函数注入（Spring 4.3+无需额外添加@Autowired）
    private final JdbcTemplate jdbcTemplate;

    public Person_JDBC_DAO(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    public List<Person> find_All() {
        String sql = "SELECT * FROM person";
        // 使用注入的jdbcTemplate执行查询，避免空指针
        return jdbcTemplate.query(sql, (rs, rowNum) -> {
            Person person = new Person();
            person.setId(rs.getInt("id"));
            person.setName(rs.getString("name"));
            // 补充其他属性的映射逻辑
            return person;
        });
    }
}

步骤2：检查find_All()方法的实现 #

确保方法中没有使用未初始化的对象，比如不要手动实例化JdbcTemplate，一定要通过Spring依赖注入获取实例。

步骤3：优化依赖配置 #

你的pom.xml同时引入了spring-boot-starter-data-jdbc和spring-boot-starter-data-jpa，如果仅使用JDBC操作数据库，可以移除spring-boot-starter-data-jpa依赖，避免潜在的配置冲突。

步骤4：验证包扫描范围 #

@SpringBootApplication默认扫描主类所在包及其子包，你的Person_JDBC_DAO在com.First_Spring.Database_My_Spring.JDBC包下，属于主类包的子包，扫描范围是正常的。如果DAO在其他包，需要添加@ComponentScan指定额外的扫描路径。

按照以上步骤调整后，重新启动应用即可解决空指针问题。

内容的提问来源于stack exchange，提问作者Sunny Khandare