为什么spring-boot-data-jdbc查询方法不需要Args构造函数

Question

在我看来，使用spring-boot-data-jdbc的实体上的所有查询方法都需要一个无参数的构造函数？有原因吗？这里有一个简单的例子给出了这个例外。使用H2数据库

原因：org.SpringFramework.Beans.BeanInstantiationException：无法实例化[Com.Example.RelationalDataAccess.Person]：未找到默认构造函数；嵌套异常是java.lang.NosuchMethodException：com.example.relationalDataAccess.Person.（）

通常是Person实体-

import org.springframework.data.annotation.Id;

public class Person {
    

    @Id
    private long id;
    private  String firstName;
    private  String lastName;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

然后是存储库

public interface PersonRepository extends CrudRepository<Person, Long>{

    @Query("select * from person where first_name= :firstName")
    List<Person> findByFirstName(@Param("firstName") String firstName);

}

下面是模式

create table person (
                        id integer identity not null primary key,
                        first_name varchar(30),
                        last_name varchar(30)
);

@SpringBootApplication
public class Application implements CommandLineRunner {

    @Autowired
    private PersonRepository repository;


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

    @Override
    public void run(String... arg0) throws Exception {
        Person person = new Person("John", "Doe");
        repository.save(person);
        System.out.println("FIND BY FIRST_NAME ----->> "+ repository.findByFirstName("John"));
    }
}

spring.jpa.hibernate.ddl-auto=none
spring.h2.console.enabled=true
spring.datasource.url=jdbc:h2:mem:persondb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=test
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
logging.level.org.springframework.jdbc.core.JdbcTemplate=DEBUG
logging.level.org.springframework.data=INFO

下面是App的堆栈痕迹--

java.lang.IllegalStateException: Failed to execute CommandLineRunner
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:798) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:779) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:322) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at com.example.relationaldataaccess.Application.main(Application.java:21) ~[classes/:na]
Caused by: org.springframework.data.mapping.model.MappingInstantiationException: Failed to instantiate com.example.relationaldataaccess.Person using constructor NO_CONSTRUCTOR with arguments 
    at org.springframework.data.mapping.model.ReflectionEntityInstantiator.createInstance(ReflectionEntityInstantiator.java:65) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:87) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.data.relational.core.conversion.BasicRelationalConverter.createInstance(BasicRelationalConverter.java:147) ~[spring-data-relational-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.core.convert.BasicJdbcConverter$ReadingContext.createInstanceInternal(BasicJdbcConverter.java:527) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.core.convert.BasicJdbcConverter$ReadingContext.mapRow(BasicJdbcConverter.java:387) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.core.convert.BasicJdbcConverter.mapRow(BasicJdbcConverter.java:323) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.core.convert.EntityRowMapper.mapRow(EntityRowMapper.java:67) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.repository.support.JdbcQueryLookupStrategy$PostProcessingRowMapper.mapRow(JdbcQueryLookupStrategy.java:152) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.jdbc.core.RowMapperResultSetExtractor.extractData(RowMapperResultSetExtractor.java:94) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.RowMapperResultSetExtractor.extractData(RowMapperResultSetExtractor.java:61) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.JdbcTemplate$1.doInPreparedStatement(JdbcTemplate.java:679) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:617) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:669) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:694) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate.query(NamedParameterJdbcTemplate.java:176) ~[spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.data.jdbc.repository.query.AbstractJdbcQuery.lambda$getQueryExecution$2(AbstractJdbcQuery.java:127) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.jdbc.repository.query.StringBasedJdbcQuery.execute(StringBasedJdbcQuery.java:85) ~[spring-data-jdbc-2.0.4.RELEASE.jar:2.0.4.RELEASE]
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor$QueryMethodInvoker.invoke(QueryExecutorMethodInterceptor.java:195) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.doInvoke(QueryExecutorMethodInterceptor.java:152) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.invoke(QueryExecutorMethodInterceptor.java:130) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:367) ~[spring-tx-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:118) ~[spring-tx-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139) ~[spring-tx-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:95) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212) ~[spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at com.sun.proxy.$Proxy48.findByFirstName(Unknown Source) ~[na:na]
    at com.example.relationaldataaccess.Application.run(Application.java:28) ~[classes/:na]
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:795) ~[spring-boot-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    ... 5 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.relationaldataaccess.Person]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.example.relationaldataaccess.Person.<init>()
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:146) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.data.mapping.model.ReflectionEntityInstantiator.createInstance(ReflectionEntityInstantiator.java:62) ~[spring-data-commons-2.3.4.RELEASE.jar:2.3.4.RELEASE]
    ... 36 common frames omitted
Caused by: java.lang.NoSuchMethodException: com.example.relationaldataaccess.Person.<init>()
    at java.base/java.lang.Class.getConstructor0(Class.java:3349) ~[na:na]
    at java.base/java.lang.Class.getDeclaredConstructor(Class.java:2553) ~[na:na]
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:139) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    ... 37 common frames omitted

2020-09-23 19:31:04.053  INFO 29948 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown initiated...
2020-09-23 19:31:04.059  INFO 29948 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown completed.

Process finished with exit code 1

Answer 1

我认为这条错误信息是误导的。Spring Data JDBC不需要No-Args-Constructor，但它需要一种设置所有属性的方法，并且不访问私有字段。它们毕竟是私有的，因此只能由拥有它们的类访问。您的id字段不满足此要求。

执行下列操作之一应可解决此问题：

• 使id字段至少为package private，即无修饰符
• 为ID字段添加一个setter。至少再次包私有
• 向构造函数的参数添加ID。
• 添加一个将所有三个字段都作为参数的构造函数。您可能必须将@PersistenceConstructor添加到该构造函数。