如果DB不支持默认参数,则会抛出异常：

ProcedureCall procedure = getSession().createStoredProcedureCall("my_procedure");

procedure.registerParameter("my_nullable_param",ParameterMode.IN)
         .bindValue(null);

// execute
procedure.getOutputs();

从Hibernate的源码将参数绑定到底层CallableStatement：

public abstract class AbstractParameterRegistrationImpl {

  ..

  @Override
  public void prepare(CallableStatement statement,int startIndex) throws sqlException {

    if ( mode == ParameterMode.INOUT || mode == ParameterMode.IN ) {
      if ( bind == null || bind.getValue() == null ) {
        // the user did not bind a value to the parameter being processed.  That might be ok *if* the
        // procedure as defined in the database defines a default value for that parameter.
        // Unfortunately there is not a way to reliably know through JDBC Metadata whether a procedure
        // parameter defines a default value.  So we simply allow the procedure execution to happen
        // assuming that the database will complain appropriately if not setting the given parameter
        // bind value is an error.
        log.debugf("Stored procedure [%s] IN/INOUT parameter [%s] not bound; assuming procedure defines default value",procedureCall.getProcedureName(),this);
      } else {
         typeToUse.nullSafeSet( statement,bind.getValue(),startIndex,session() );
      }
    }
  }

  ..
}

以上评论如下：

The user did not bind a value to the parameter being processed. That
might be ok if the procedure as defined in the database defines a
default value for that parameter. Unfortunately there is not a way to
reliably know through JDBC Metadata whether a procedure parameter
defines a default value. So we simply allow the procedure execution
to happen assuming that the database will complain appropriately if
not setting the given parameter bind value is an error.

我正在解释为JPA(特别是Hibernate)不支持设置空参数.看起来他们正在努力支持默认参数值,而在适当时替换空值.他们选择支持前者.看起来像后者需要支持(可空值)必须使用java.sql.CallableStatement：

getSession().doWork(new Work() {

  @Override
  public void execute(Connection conn) throws sqlException {

    CallableStatement stmt = conn.prepareCall("{ call my_prodecure(:my_nullable_param) }");

    if(stringVariableThatIsNull != null) {
       stmt.setString("my_nullable_param",stringVariableThatIsNull);
    } else {
       stmt.setNull("my_nullable_param",Types.VARCHAR);
    }

    stmt.execute();
    stmt.close();

  }    
});

tl; dr我们仍然被迫处理低级JDBC,因为JPA或Hibernate似乎没有处理可空参数.它们支持过程参数默认值,而不是替换空值.