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Why queryForObject Kotlin extension func returns nullable T? if it actually will throw EmptyResultDataAccessException?

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0

Using Kotlin and Spring 5 for some simple project.
I would like to get single record from database by id using queryForObject.
My query is a 'simple select by id':

jdbc.queryForObject("select id, name from example where id = ?", id)
 rs: ResultSet, _: Int -> NamedEnt(rs.getLong("id"), rs.getString("name") 

In JdbcOperationsExtensions.kt it is declared to return nullable type T?:

fun <T : Any> JdbcOperations.queryForObject(sql: String, vararg args: Any, function: (ResultSet, Int) -> T): T? =
 queryForObject(sql, RowMapper resultSet, i -> function(resultSet, i) , *args)

In practice when I pass not existing identifier, I face with:

org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0

Then I do not understand what is the point of returning nullable type? You either receive a single record or exception. Or I miss something?

kotlin spring-jdbc spring-kotlin

share|improve this question

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

add a comment | 

0

Using Kotlin and Spring 5 for some simple project.
I would like to get single record from database by id using queryForObject.
My query is a 'simple select by id':

jdbc.queryForObject("select id, name from example where id = ?", id)
 rs: ResultSet, _: Int -> NamedEnt(rs.getLong("id"), rs.getString("name") 

In JdbcOperationsExtensions.kt it is declared to return nullable type T?:

fun <T : Any> JdbcOperations.queryForObject(sql: String, vararg args: Any, function: (ResultSet, Int) -> T): T? =
 queryForObject(sql, RowMapper resultSet, i -> function(resultSet, i) , *args)

In practice when I pass not existing identifier, I face with:

org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0

Then I do not understand what is the point of returning nullable type? You either receive a single record or exception. Or I miss something?

kotlin spring-jdbc spring-kotlin

share|improve this question

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

add a comment | 

Using Kotlin and Spring 5 for some simple project.
I would like to get single record from database by id using queryForObject.
My query is a 'simple select by id':

jdbc.queryForObject("select id, name from example where id = ?", id)
 rs: ResultSet, _: Int -> NamedEnt(rs.getLong("id"), rs.getString("name") 

In JdbcOperationsExtensions.kt it is declared to return nullable type T?:

fun <T : Any> JdbcOperations.queryForObject(sql: String, vararg args: Any, function: (ResultSet, Int) -> T): T? =
 queryForObject(sql, RowMapper resultSet, i -> function(resultSet, i) , *args)

In practice when I pass not existing identifier, I face with:

org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0

Then I do not understand what is the point of returning nullable type? You either receive a single record or exception. Or I miss something?

kotlin spring-jdbc spring-kotlin

share|improve this question

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

jdbc.queryForObject("select id, name from example where id = ?", id)
 rs: ResultSet, _: Int -> NamedEnt(rs.getLong("id"), rs.getString("name") 

fun <T : Any> JdbcOperations.queryForObject(sql: String, vararg args: Any, function: (ResultSet, Int) -> T): T? =
 queryForObject(sql, RowMapper resultSet, i -> function(resultSet, i) , *args)

share|improve this question

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

share|improve this question

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

asked Mar 25 at 14:54

DerpDerp

1,41111 gold badge1717 silver badges3232 bronze badges

1 Answer
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1

JdbcOperationsExtensions.kt adds some extension functions to the org.springframework.jdbc.core.JdbcOperations interface (written in Java). If you look at the JavaDocs for queryForObject in that, it says:

@return the single mapped object (may be @code null if the given
@link RowMapper returned @code null)

See here for full source code of the JdbcOperations Java class.

So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned, hence the nullable type.

Except... as pointed out by @AlexeyRomanov, this particular overload of queryForObject takes in a lambda which returns T, so can't ever return null, so arguably this overload could return T, not T?. Maybe it's a bit inconsistent that this lambda in Kotlin can't return null, but the JavaDocs on the very similar overload in the Java class explicitly state that it (RowMapper) should be allowed to return null.

Regardless of that point, some other overloads of queryForObject simply call to down to the Java-written overload, and because it's written in Java, it's possible that it could return a null. So for them it does seem to make sense for it to be a nullable return value. In which case arguably it's a nice bit of consistency that all the overloads do in fact return the nullable T.

share|improve this answer

edited Mar 25 at 19:51

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned" Except they don't: function: (ResultSet, Int) -> T can't return null, so the RowMapper using it can't return null.
  
  – Alexey Romanov
  Mar 25 at 17:26
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ha! Well spotted @AlexeyRomanov: I missed that. I wonder if this is a small bug in that extension method then: maybe the lambda passed into queryForObject should be declared as having a return type of T?. Because the JavaDocs seem to suggest that the RowMapper should be allowed to return null.
  
  – Yoni Gibbs
  Mar 25 at 19:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I'd say so, yes. It should probably be inline too. Might be worth reporting...
  
  – Alexey Romanov
  Mar 25 at 21:28
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Agreed. Issue logged here.
  
  – Yoni Gibbs
  Mar 26 at 10:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @AlexeyRomanov Could you elaborate on why these extensions should be inline? Performance?
  
  – Sébastien Deleuze
  Mar 26 at 13:42
  

  
  
  
  

 | 
show 2 more comments

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1

JdbcOperationsExtensions.kt adds some extension functions to the org.springframework.jdbc.core.JdbcOperations interface (written in Java). If you look at the JavaDocs for queryForObject in that, it says:

@return the single mapped object (may be @code null if the given
@link RowMapper returned @code null)

See here for full source code of the JdbcOperations Java class.

So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned, hence the nullable type.

Except... as pointed out by @AlexeyRomanov, this particular overload of queryForObject takes in a lambda which returns T, so can't ever return null, so arguably this overload could return T, not T?. Maybe it's a bit inconsistent that this lambda in Kotlin can't return null, but the JavaDocs on the very similar overload in the Java class explicitly state that it (RowMapper) should be allowed to return null.

Regardless of that point, some other overloads of queryForObject simply call to down to the Java-written overload, and because it's written in Java, it's possible that it could return a null. So for them it does seem to make sense for it to be a nullable return value. In which case arguably it's a nice bit of consistency that all the overloads do in fact return the nullable T.

share|improve this answer

edited Mar 25 at 19:51

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned" Except they don't: function: (ResultSet, Int) -> T can't return null, so the RowMapper using it can't return null.
  
  – Alexey Romanov
  Mar 25 at 17:26
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ha! Well spotted @AlexeyRomanov: I missed that. I wonder if this is a small bug in that extension method then: maybe the lambda passed into queryForObject should be declared as having a return type of T?. Because the JavaDocs seem to suggest that the RowMapper should be allowed to return null.
  
  – Yoni Gibbs
  Mar 25 at 19:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I'd say so, yes. It should probably be inline too. Might be worth reporting...
  
  – Alexey Romanov
  Mar 25 at 21:28
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Agreed. Issue logged here.
  
  – Yoni Gibbs
  Mar 26 at 10:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @AlexeyRomanov Could you elaborate on why these extensions should be inline? Performance?
  
  – Sébastien Deleuze
  Mar 26 at 13:42
  

  
  
  
  

 | 
show 2 more comments

1

JdbcOperationsExtensions.kt adds some extension functions to the org.springframework.jdbc.core.JdbcOperations interface (written in Java). If you look at the JavaDocs for queryForObject in that, it says:

@return the single mapped object (may be @code null if the given
@link RowMapper returned @code null)

See here for full source code of the JdbcOperations Java class.

So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned, hence the nullable type.

Except... as pointed out by @AlexeyRomanov, this particular overload of queryForObject takes in a lambda which returns T, so can't ever return null, so arguably this overload could return T, not T?. Maybe it's a bit inconsistent that this lambda in Kotlin can't return null, but the JavaDocs on the very similar overload in the Java class explicitly state that it (RowMapper) should be allowed to return null.

Regardless of that point, some other overloads of queryForObject simply call to down to the Java-written overload, and because it's written in Java, it's possible that it could return a null. So for them it does seem to make sense for it to be a nullable return value. In which case arguably it's a nice bit of consistency that all the overloads do in fact return the nullable T.

share|improve this answer

edited Mar 25 at 19:51

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned" Except they don't: function: (ResultSet, Int) -> T can't return null, so the RowMapper using it can't return null.
  
  – Alexey Romanov
  Mar 25 at 17:26
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ha! Well spotted @AlexeyRomanov: I missed that. I wonder if this is a small bug in that extension method then: maybe the lambda passed into queryForObject should be declared as having a return type of T?. Because the JavaDocs seem to suggest that the RowMapper should be allowed to return null.
  
  – Yoni Gibbs
  Mar 25 at 19:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I'd say so, yes. It should probably be inline too. Might be worth reporting...
  
  – Alexey Romanov
  Mar 25 at 21:28
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Agreed. Issue logged here.
  
  – Yoni Gibbs
  Mar 26 at 10:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @AlexeyRomanov Could you elaborate on why these extensions should be inline? Performance?
  
  – Sébastien Deleuze
  Mar 26 at 13:42
  

  
  
  
  

 | 
show 2 more comments

JdbcOperationsExtensions.kt adds some extension functions to the org.springframework.jdbc.core.JdbcOperations interface (written in Java). If you look at the JavaDocs for queryForObject in that, it says:

@return the single mapped object (may be @code null if the given
@link RowMapper returned @code null)

See here for full source code of the JdbcOperations Java class.

So the Kotlin-written extension functions need to adhere to this and allow nulls to be returned, hence the nullable type.

Except... as pointed out by @AlexeyRomanov, this particular overload of queryForObject takes in a lambda which returns T, so can't ever return null, so arguably this overload could return T, not T?. Maybe it's a bit inconsistent that this lambda in Kotlin can't return null, but the JavaDocs on the very similar overload in the Java class explicitly state that it (RowMapper) should be allowed to return null.

Regardless of that point, some other overloads of queryForObject simply call to down to the Java-written overload, and because it's written in Java, it's possible that it could return a null. So for them it does seem to make sense for it to be a nullable return value. In which case arguably it's a nice bit of consistency that all the overloads do in fact return the nullable T.

share|improve this answer

edited Mar 25 at 19:51

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

@return the single mapped object (may be @code null if the given
@link RowMapper returned @code null)

share|improve this answer

edited Mar 25 at 19:51

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges

answered Mar 25 at 15:20

Yoni GibbsYoni Gibbs

1,97911 silver badge1818 bronze badges