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iBatis Error with Recursive Generics

From: "andrew cooke" <andrew@...>

Date: Wed, 3 Sep 2008 13:10:01 -0400 (CLT)

I have a base class for ORM that looks something like this:

public class Keyed<KeyType extends Comparable<KeyType>>
implements Comparable<Keyed<KeyType>>
{
  private KeyType key;
  protected final void setKey(final KeyType key) {...}
  ...
}

For some reason, iBatis does not like this, even thought it was fine (as
far as I can tell) with an earlier version that was also generic (but did
not include the recursive Comparable declaration).

The error when you try to map (sublasses of) this is given below (in full
for Google).

I tried tracking what was causing this, and it seems that the type handler
is not inferred correctly while reading the configuration.  Rather than
digging further I found a simple workaround - specify the type handler by
hand.

So my maps now look like:

<sqlMap namespace="entity">
  <resultMap id="entity-result" class="...Entity">
    <result property="key" column="entity_id"
     typeHandler="com.ibatis.sqlmap.engine.type.StringTypeHandler"/>
    <result property="title" column="title"/>
    <result property="description" column="description"/>
    ...
  </resultMap>
  ...
</sqlMap>

Note the explicit typeHandler.

Andrew


03-Sep-2008 13:06:32 com.isti.kpidb.KpiDb getEntities
SEVERE: Failed to read Entities
com.ibatis.common.jdbc.exception.NestedSQLException:
--- The error occurred in maps/entity.xml.
--- The error occurred while applying a result map.
--- Check the entity.entity-result.
--- Check the result mapping for the 'key' property.
--- Cause: com.ibatis.sqlmap.client.SqlMapException: No type handler could
be found to map the property 'key' to the column 'entity_id'.  One or both
of the types, or the combination of types is not supported.
	at
com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.executeQueryWithCallback(MappedStatement.java:204)
	at
com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.executeQueryForList(MappedStatement.java:139)
	at
com.ibatis.sqlmap.engine.mapping.statement.CachingStatement.executeQueryForList(CachingStatement.java:97)
	at
com.ibatis.sqlmap.engine.impl.SqlMapExecutorDelegate.queryForList(SqlMapExecutorDelegate.java:567)
	at
com.ibatis.sqlmap.engine.impl.SqlMapExecutorDelegate.queryForList(SqlMapExecutorDelegate.java:541)
	at
com.ibatis.sqlmap.engine.impl.SqlMapSessionImpl.queryForList(SqlMapSessionImpl.java:118)
	at
com.ibatis.sqlmap.engine.impl.SqlMapClientImpl.queryForList(SqlMapClientImpl.java:94)
	at com.isti.kpidb.KpiDb.queryForList(KpiDb.java:331)
	at com.isti.kpidb.KpiDb.queryForList(KpiDb.java:311)
	at com.isti.kpidb.KpiDb.getEntities(KpiDb.java:102)
	at com.isti.kpidb.demo.ListEntities.execute(ListEntities.java:23)
	at com.isti.kpidb.demo.Demo.processInput(Demo.java:125)
	at com.isti.kpidb.demo.Demo.mainLoop(Demo.java:84)
	at com.isti.kpidb.demo.Demo.<init>(Demo.java:58)
	at com.isti.kpidb.demo.Demo.main(Demo.java:183)
Caused by: com.ibatis.sqlmap.client.SqlMapException: No type handler could
be found to map the property 'key' to the column 'entity_id'.  One or both
of the types, or the combination of types is not supported.
	at
com.ibatis.sqlmap.engine.mapping.result.ResultMap.getPrimitiveResultMappingValue(ResultMap.java:613)
	at
com.ibatis.sqlmap.engine.mapping.result.ResultMap.getResults(ResultMap.java:345)
	at
com.ibatis.sqlmap.engine.execution.SqlExecutor.handleResults(SqlExecutor.java:384)
	at
com.ibatis.sqlmap.engine.execution.SqlExecutor.handleMultipleResults(SqlExecutor.java:300)
	at
com.ibatis.sqlmap.engine.execution.SqlExecutor.executeQuery(SqlExecutor.java:189)
	at
com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.sqlExecuteQuery(MappedStatement.java:221)
	at
com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.executeQueryWithCallback(MappedStatement.java:189)
	... 14 more

Probably due to Erasure

From: "andrew cooke" <andrew@...>

Date: Mon, 8 Sep 2008 19:36:57 -0400 (CLT)

I just realised what is causing this while looking through Wadler's book
to see if I could find something else (this is on p122).

The erased type for Keyed<KeyType extends Comparable<KeyType>>
 is going to be Comparable, rather than Object.  It would probably work OK
with Keyed<KeyType extends Object & Comparable<KeyType>>

Andrew

Confirmation - Type Erasure, not Recursion

From: "andrew cooke" <andrew@...>

Date: Thu, 11 Sep 2008 11:19:48 -0400 (CLT)

Just to confirm, the issue was type erasure, not the recursive generic
declaration.

Everything works fine (without explicit type handlers) if I use
  Keyed<KeyType extends Object & Comparable<KeyType>>
(with the leading "Object &")

Andrew

Still Not Simple

From: "andrew cooke" <andrew@...>

Date: Mon, 6 Oct 2008 15:43:49 -0400 (CLT)

It's not that simple, because iBatis will see a "Object" and use whatever
type it feels like.  This may not be consistent with the actual
implementation.

For example, I have a generic base class that has a "key" attribute.  When
used with Oracle, the integer key value was being set as a BigDecimal.  I
then got a class cast exception at some random moment when code that
compiled fine with <Integer> tried to access the BigDecimal key.

So the moral of the story is: either avoid generics, or give explicit type
handlers.  Since I like my Java code as it is, I'm going with the type
handlers for now.

Andrew

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