運算子==未定義引數型別long,null

嘗試將課程物件插入到 H2 資料庫中，我在該行遇到錯誤

if(course.getId()==null)

說沒有為引數型別 long,null 定義運算子。也試過

if(course.getId()==0)

同樣的問題。

這是課程物體/班級

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

private String name;

public Course() {
}

public Course(String name) {
    super();
    this.name = name;
}

public long getId() {
    return id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

@Override
public String toString() {
    return "Course [name="   name   "]";
}

}

這是DAO層

@Repository
@Transactional
public class CourseRepository {

@Autowired
EntityManager em;

public Course findById(Long id) {
    return em.find(Course.class, id);
}

public void deleteById(Long id) {
    em.remove(findById(id));
    
}

public Course save(Course course) {
        if(course.getId()==null){ 
        em.persist(course);
    }
    else {
        em.merge(course);
    }
    return course;
}

}

控制臺中的一個小片段

java.lang.IllegalStateException: Failed to execute CommandLineRunner
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:794) ~[spring-boot-2.5.5.jar:2.5.5]
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:775) ~[spring-boot-2.5.5.jar:2.5.5]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:345) ~[spring-boot-2.5.5.jar:2.5.5]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1343) ~[spring-boot-2.5.5.jar:2.5.5]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1332) ~[spring-boot-2.5.5.jar:2.5.5]
    at com.in28minutes.jpa.hibernate.demo.DemoApplication.main(DemoApplication.java:21) ~[classes/:na]
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because "this.id" is null
    at com.in28minutes.jpa.hibernate.demo.entity.Course.getId(Course.java:25) ~[classes/:na]
    at com.in28minutes.jpa.hibernate.demo.repository.CourseRepository.save(CourseRepository.java:29) ~[classes/:na]
    at com.in28minutes.jpa.hibernate.demo.repository.CourseRepository$$FastClassBySpringCGLIB$$6d4415f4.invoke(<generated>) ~[classes/:na]
    at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:779) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137) ~[spring-tx-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123) ~[spring-tx-5.3.10.jar:5.3.10]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388) ~[spring-tx-5.3.10.jar:5.3.10]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119) ~[spring-tx-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750) ~[spring-aop-5.3.10.jar:5.3.10]
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:692) ~[spring-aop-5.3.10.jar:5.3.10]
    at com.in28minutes.jpa.hibernate.demo.repository.CourseRepository$$EnhancerBySpringCGLIB$$862dd48f.save(<generated>) ~[classes/:na]
    at com.in28minutes.jpa.hibernate.demo.DemoApplication.run(DemoApplication.java:29) ~[classes/:na]
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:791) ~[spring-boot-2.5.5.jar:2.5.5]
    ... 5 common frames omitted

uj5u.com熱心網友回復：

Long id;    
public long getId() {
        return id;
    }

該錯誤是由您的回傳型別 long 而不是 Long 引起的。對于long回傳型別，需要通過方法將Long從Long轉換為long：

Long.longValue()

所以這就是為什么當 id 為 null 時你得到 nullpointerexception 的原因。只需將型別 long 更改為 Long 并重新檢查...

更改自：

long getId()

到

Long getId() 

標籤：爪哇 春天 jpa 弹簧数据-jpa

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