org.josql.expressions

Class BindVariable

java.lang.Object

org.josql.expressions.Expression

org.josql.expressions.ValueExpression

org.josql.expressions.BindVariable

public class BindVariable
extends ValueExpression

This class represents a "bind variable" used within a SQL statement. A bind variable can be either:

• Named - a named bind variable is prefixed with ":", the value of the variable is set in the Query object via: Query.setVariable(String,Object). The parser used (javacc generated) is a little picky about "reserved keywords" and as such you cannot use the following as the names of bind variables: select, from, limit, execute, on, all, results, where, having, order, by, group. If you must use one of reserved names then suffix it with "$", i.e. "execute$", "limit$" and so on.
• Anonymous - an anonymous bind variable is represented as "?". The value of the variable is set in the Query object via: Query.setVariable(int,Object). Anonymous variables are assigned an internal integer index starting at 1 and they increase by 1.
• Special - there are 3 types of "special" bind variable:
  • :_currobj - Indicates the current object within the execution scope.
  • :_allobjs - Indicates the current set of objects within the execution scope.
  • :_query - Indicates the Query object.
  • :_grpby - Indicates the Group By object. If there are more than 1 group by objects then :_grpby1 ... :_grpby2 ... :_grpbyX can be used to identify which group by object you mean. Remember that the group by object only has meaning if there is a group by statement.
  • :_grpbys - Indicates the set of group bys, this will be a list of lists, one list for each group by key, and each group by column will be in that list.
  • :_groupby - A synonym for :_grpby.
  • :_groupbys - A synonym for :_grpbys.

  Special bind variables are mainly used in function calls however they can also be used just about anywhere.

Examples

   SELECT :_query,
          :_allobjs,
          :_currobj
   FROM java.lang.Object

   SELECT name
   FROM   java.io.File
   WHERE  length(:_currobj, name) > :length
   AND    length > avg(:_query,:_allobjs,length)
   AND    path LIKE '%' + ?
 

Accessors

It is also possible for bind variables (including the special variables) to have accessors. For example:

   SELECT :_query.variables
   FROM   java.lang.Object
 

Would cause all the bind variables in the query to be returned. Also, if the ? in the next query is an instance of java.lang.String.

   SELECT ?.length
   FROM   java.lang.Object
 

Field Summary

Fields

Modifier and Type	Field and Description
static String	SPECIAL_NAME_PREFIX

Constructor Summary

Constructors

Constructor and Description
BindVariable()

Method Summary

Modifier and Type	Method and Description
boolean	equals(Object o)
Object	evaluate(Object o, Query q) Evaluates the value of this bind variable.
String	getAccessor()
Class	getExpectedReturnType(Query q) Get the expected return type.
String	getName()
Object	getValue(Object o, Query q) Gets the value of this bind variable.
boolean	hasFixedResult(Query q) Will always return false since a bind variable cannot be fixed.
void	init(Query q) Initialises this bind variable.
boolean	isAnonymous()
boolean	isTrue(Object o, Query q) Returns whether the value of this bind variable represents a true value.
void	setAccessor(String a)
void	setAnonymous(boolean v)
void	setName(String name)
String	toString() Returns a string version of this bind variable.

Methods inherited from class org.josql.expressions.Expression

isBracketed, setBracketed

Methods inherited from class java.lang.Object

clone, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

SPECIAL_NAME_PREFIX

public static final String SPECIAL_NAME_PREFIX

See Also:

Constant Field Values

Constructor Detail

BindVariable

public BindVariable()

Method Detail

equals

public boolean equals(Object o)

Overrides:

equals in class Object

getAccessor

public String getAccessor()

setAccessor

public void setAccessor(String a)

getExpectedReturnType

public Class getExpectedReturnType(Query q)
                            throws QueryParseException

Get the expected return type. The exact class returned here is dependent (obviously) on what the bind variable represents. Wherever possible it attempts to get the most specific class for the variable. It is generally better to set the variables prior to executing the: Query.parse(String) method to ensure that the correct class is returned here.

Specified by:

getExpectedReturnType in class Expression

Parameters:

q - The Query object.

Returns:

The return type class or java.lang.Object.class if the class cannot be determined.

Throws:

QueryParseException - If the type cannot be determined.

init

public void init(Query q)
          throws QueryParseException

Initialises this bind variable. If the bind variable is "anonymous" then a name is gained for it from the Query object. If there is a value then it is gained from the Query object and cached. Also, if there is an accessor defined then it is inited where possible.

Specified by:

init in class Expression

Parameters:

q - The Query object.

Throws:

QueryParseException - If the bind variable cannot be inited.

getName

public String getName()

isAnonymous

public boolean isAnonymous()

setAnonymous

public void setAnonymous(boolean v)

setName

public void setName(String name)

getValue

public Object getValue(Object o,
                       Query q)
                throws QueryExecutionException

Gets the value of this bind variable.

Overrides:

getValue in class ValueExpression

Parameters:

o - The current object. Note that this variable isn't used in this method.

q - The Query object.

Returns:

The value.

Throws:

QueryExecutionException - If something goes wrong during the accessing of the value.

isTrue

public boolean isTrue(Object o,
                      Query q)
               throws QueryExecutionException

Returns whether the value of this bind variable represents a true value. See: ArithmeticExpression.isTrue(Object,Query) for details of how the return value is determined.

Specified by:

isTrue in class Expression

Parameters:

o - The current object. Not used in this method.

q - The Query object.

Returns:

true if the bind variable evaluates to true.

Throws:

QueryExecutionException - If a problem occurs during evaluation.

evaluate

public Object evaluate(Object o,
                       Query q)
                throws QueryExecutionException

Evaluates the value of this bind variable. This is just a thin-wrapper around: getValue(Object,Query).

Specified by:

evaluate in class ValueExpression

Parameters:

o - The current object, not used in this method.

q - The Query object.

Returns:

The value of this bind variable.

Throws:

QueryExecutionException - If there is a problem getting the value.

toString

public String toString()

Returns a string version of this bind variable. Returns in the form: ? | ":" [ "_" ] Name

Specified by:

toString in class Expression

Returns:

A string version of the bind variable.

hasFixedResult

public boolean hasFixedResult(Query q)

Will always return false since a bind variable cannot be fixed.

Specified by:

hasFixedResult in class Expression

Parameters:

q - The Query object.

Returns:

false always.