Java spring / hibernate 问题：查询给了我一个错误，但我不知道为什么

Question

对于一项任务，我应该返回机场数量最多的前 10 个国家/地区。我用 Spring Boot 编写了一个应用程序并尝试了许多不同的方法，但我无法让它工作。我的猜测是我的查询不正确，但我不确定。我在谷歌上搜索了几天，但无法弄清楚问题所在。

正如您在错误中看到的那样，它显示了每个国家/地区的机场数量，但无法将 arrayList 转换为 List？？

当我运行应用程序时出现此错误：

Exception in thread "restartedMain" java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
Caused by: org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.util.ArrayList<?>] to type [@org.springframework.data.jpa.repository.Query java.util.List<com.example.lunatech.datamodels.Airport>] for value '[1, 32, 62, 3, 1, 8, 13, 57, 24, 713, 4, 138, 1908, 1, 35, 11, 2, 16, 122, 50, 75, 2, 4, 9, 1, 3, 2, 192, 3, 3839, 63, 3, 35, 31, 20, 2454, 1, 267, 36, 50, 75, 24, 9, 474, 26, 330, 700, 124, 134, 10, 1, 1, 12, 146, 703, 9, 70, 2, 37, 62, 91, 26, 69, 3, 5, 181, 52, 106, 29, 13, 6, 8, 789, 32, 489, 3, 27, 10, 2, 8, 1, 82, 1, 14, 6, 4, 73, 49, 3, 5, 38, 9, 154, 32, 8, 64, 461, 47, 28, 3, 329, 1, 76, 135, 85, 200, 1, 24, 17, 148, 239, 50, 14, 21, 4, 2, 67, 249, 4, 8, 3, 106, 19, 4, 2, 1, 24, 15, 33, 40, 9, 24, 59, 36, 1, 8, 7, 2, 70, 33, 13, 31, 76, 29, 1, 11, 1, 21, 2, 7, 3, 9, 29, 206, 105, 40, 224, 21, 23, 1, 39, 30, 80, 162, 47, 1, 1, 205, 25, 63, 164, 54, 565, 117, 125, 170, 2, 62, 5, 71, 3, 52, 5, 2, 28, 26, 920, 7, 75, 37, 16, 31, 178, 9, 1, 33, 24, 12, 2, 14, 23, 28, 40, 2, 27, 1, 28, 16, 10, 23, 4, 7, 69, 15, 11, 21, 15, 6, 119, 3, 1, 32, 69, 137, 19, 6, 21501, 20, 174, 1, 5, 592, 3, 9, 42, 32, 2, 4, 25, 1, 445, 76, 83, 1]'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.Long] to type [@org.springframework.data.jpa.repository.Query com.example.lunatech.datamodels.Airport]
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:47)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:192)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:213)
    at org.springframework.data.repository.core.support.QueryExecutionResultHandler.postProcessInvocationResult(QueryExecutionResultHandler.java:167)
    at org.springframework.data.repository.core.support.QueryExecutionResultHandler.postProcessInvocationResult(QueryExecutionResultHandler.java:77)
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.invoke(QueryExecutorMethodInterceptor.java:138)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:80)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123)
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388)
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:145)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215)
    at com.sun.proxy.$Proxy101.countAirportByIso_country(Unknown Source)
    at com.example.lunatech.LunatechApplication.testDAO(LunatechApplication.java:34)
    at com.example.lunatech.LunatechApplication.main(LunatechApplication.java:21)
    ... 5 more
Caused by: org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.Long] to type [@org.springframework.data.jpa.repository.Query com.example.lunatech.datamodels.Airport]
    at org.springframework.core.convert.support.GenericConversionService.handleConverterNotFound(GenericConversionService.java:322)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:195)
    at org.springframework.core.convert.support.CollectionToCollectionConverter.convert(CollectionToCollectionConverter.java:89)
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41)
    ... 27 more

控制器：

public AirportController(AirportsService airportsService) {
    this.airportsService = airportsService;
}

@GetMapping("/top")
public ResponseEntity getCountriesWithMostAirports() {

    Optional<List<Airport>> airportList = airportsService.countriesWithMostAirports();
    if (airportList.isPresent())
        return ResponseEntity.ok(airportList.get());

    return ResponseEntity.badRequest().body("Something went wrong");
}

数据模型：

private String iso_country;

机场道：

public interface AirportDAO extends JpaRepository<Airport, Long> {
    @Query("SELECT COUNT(a) FROM Airport a GROUP BY a.iso_country ")
    List<Airport> countAirportByIso_country();
}

机场服务：

   public Optional<List<Airport>> countriesWithMostAirports(){
        List<Airport> countryList = airportDAO.countAirportByIso_country();
        return Optional.of(countryList);
    }

Answer 1

"SELECT COUNT(a) FROM Airport a GROUP BY a.iso_country " 返回一个非 Airport 对象的长值列表。

所以你道应该是这样的

public interface AirportDAO extends JpaRepository<Airport, Long> {
    @Query("SELECT COUNT(a) FROM Airport a GROUP BY a.iso_country ")
    List<Long> countAirportByIso_country();
}

但是如果我的理解是正确的，你需要的是计数和国家名称。 此外，您的结果必须订购并限制为 10 类似：

public interface AirportDAO extends JpaRepository<Airport, Long> {
    @Query("SELECT COUNT(a),iso_country   FROM Airport a GROUP BY a.iso_country order by count(a) limit 10")
    List<Object[]> countAirportByIso_country();
}