Week 5 Mapping IDL to C and Java

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Week 5Mapping IDL to C and Java
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IDL Language Mappings

• How do we code the behavior for an IDL interface
  in a real programming language
• Mapping to a language involves following a set of
  rules for the target programming language
• Follow the rules, and all will be fine-

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IDL-C Mapping

• By an example

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Stock-Online IDL
module Stock // // The exception DBError is
used to indicate some sort of failure in the //
back-end database. All operations that access
the database // can raise a DBError. It
contains a single string which represents // a
textual description of the error. // exception
DBError string reason // Consts/Typedefs
for zero terminated strings const short
subscriberName 30 1 typedef
stringltsubscriberNamegt TSubscriberName const
short subscriberAddr 60 1 typedef
stringltsubscriberAddrgt TSubscriberAddr const
short stockCode 4 1 typedef
stringltstockCodegt TStockCode const short
MAX_ENTRIES 20
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Stock-Online IDL
// structures struct TSubscriber
TSubscriberName sName TSubscriberAddr sAdd
r unsigned long sCredit struct
TStockHolding unsigned long
stock_id unsigned long amount //
Sequence Typedefs to handle return of Stock
Holdings // (0-20 possible) typedef
sequenceltTStockHolding, MAX_ENTRIESgt
TStockHoldingSeq
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Stock-Online IDL
interface SubscriberServices void
CreateAccount( in TSubscriber subscriberInfo,
out unsigned long subAccNo ) raises
(DBError) void UpdateAccount( in unsigned long
subAccNo, in unsigned long sCredit) raises
(DBError) void QueryStockValueByID(in unsigned
long stockID, out float currentVal, out
float highVal, out float lowVal ) raises
(DBError) void QueryStockValueByCode(in
TStockCode stockCode,out float currentVal,
out float highVal, out float lowVal )
raises (DBError) void BuyStock (in unsigned
long subAccNo, in unsigned long stockID, in
unsigned long amount ) raises (DBError) void
SellStock (in unsigned long subAccNo, in unsigned
long stockID, in unsigned long amount )
raises (DBError) void GetHoldingStatement
(in unsigned long subAccNo, in unsigned long
startStID, out TStockHoldingSeq stockList )
raises (DBError) //end SubscriberServices
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Servant Class
include "stock.hh //generated by IDL
compiler namespace Stock class
SubscriberServices_i public virtual
SubscriberServicesBOAImpl public SubscriberSer
vices_i() // programmer defined SubscriberServi
ces_i() virtual void CreateAccount
(const TSubscriber subscriberInfo,
CORBAULong subAccNo, CORBAEnviron
ment IT_env) throw (DBError)
virtual void UpdateAccount (CORBAULong
subAccNo, CORBAULong sCredit, CORBA
Environment IT_env) throw (DBError)
virtual void QueryStockValueByID ( CORBAULong
stockID, CORBAFloat currentVal,
CORBAFloat highVal, CORBAFloat
lowVal, CORBAEnvironment IT_env)
throw (DBError)
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Servant Class (cont)
virtual void QueryStockValueByCode (const char
stockCode, CORBAFloat currentVal,
CORBAFloat highVal, CORBAFloat
lowVal, CORBAEnvironment IT_env)
throw (DBError) virtual void BuyStock
( CORBAULong subAccNo, CORBAULong
stockID, CORBAULong amount,
CORBAEnvironment IT_env) throw
(DBError) virtual void SellStock ( CORBAULong
subAccNo, CORBAULong stockID,
CORBAULong amount, CORBAEnvironment
IT_env) throw (DBError) virtual void
GetHoldingStatement ( CORBAULong subAccNo,
CORBAULong startStID, TStockHoldingS
eq stockList, CORBAEnvironment IT_env)
throw (DBError) // end class and
namespace
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Object Implementation

• Lets look at the C to implement some of these
  functions
• Focus on
• handling of parameters
• memory management for in/out parameters
• exceptions

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CreateAccount - Client Code
StockTSubscriber subInfo // Allocate strings
to pass to server subInfo.sName
CORBAstring_alloc(StocksubscriberName )
subInfo.sAddr CORBAstring_alloc(Stocksubsc
riberAddr ) // get name and address values
somehow - omitted try subObject-gtCreateAccoun
t (subInfo, subAccNo ) catch (const
StockDBError ex) cerr ltlt " (Database error
occured)" ltlt endl cerr ltlt " " ltlt ex.reason ltlt
endl catch (CORBASystemException
ex) cerr ltlt " (system exception -
transaction failed)" ltlt endl cerr ltlt " " ltlt ex
ltlt endl
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CreateAccount - Simple Server Code
void StockSubscriberServices_i CreateAccount
(const StockTSubscriber subscriberInfo,
CORBAULong subAccNo, CORBAEnvironment
IT_env) // display the input
parameters TRACE ("Create " ltlt
subscriberInfo.sName ltlt " " ltlt subscriberInfo.
sAddr ltlt " " ltlt subscriberInfo.sCredit )
// set the output parameter subAccNo 100
// silly example, always return 100 to
client //exception throwing code
omitted TRACE ("Exit Create Account" )
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QueryByCode - Client
char sCodeStockstockCode float currVal,
highVal, lowVal cout ltlt "Query Stock Value By
Code" ltlt endl cout ltlt "Enter Stock Code (max 4
chars) " cin.ignore(256, '\n') cin.getline(sCo
de, StockstockCode) try subObj-gtQueryStockV
alueByCode (sCode, currVal, highVal, lowVal )
cout ltlt "Query Transaction Successful" ltlt
endl cout ltlt "Stock Code " ltlt sCode ltlt "
Current " ltlt currVal ltlt " High " ltlt highVal
ltlt " Low " ltlt lowVal ltlt endl catch
(const StockDBError ex) // lots missing
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QueryByCode - Server
void StockSubscriberServices_i
QueryStockValueByCode (const char stockCode,
CORBAFloat currentVal, CORBAFloat
highVal, CORBAFloat lowVal,
CORBAEnvironment IT_env) // display input
value TRACE ("Query by Code" ltlt stockCode
) // set output values to silly numbers -
example only currentVal highVal lowVal 99
TRACE ("Exit Query by Code" )
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GetHoldingStatement - Client
unsigned long startStID 0 int i cout ltlt
"STATEMENT LISTING" ltlt endl subAccNo 99 //
just an example - needs to be set StockTStockHo
ldingSeq list NULL //initialize the
sequence to NULL try subObject-gtGetHoldingSta
tement (subAccNo, startStID, list ) cout ltlt
"Stock ID \t " ltlt "Amount" ltlt endl cout ltlt
"----------------------------" ltlt endl //
Print out each entry in the sequence returned
from the server for (i 0 i lt list-gtlength()
i ) cout ltlt (list)i.stock_id ltlt "\t\t"
ltlt (list) i.amount ltlt endl catch
// etc
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GetHoldingStatement - Server
void StockSubscriberServices_i
GetHoldingStatement (CORBAULong subAccNo,
CORBAULong startStID, StockTStockHoldingSeq
stockList, CORBAEnvironment IT_env)
TRACE ("Statement" ltlt subAccNo ltlt "starting
at" ltlt startStID ) CStockHolding list20
// non-ORB version of TStockHoldingSeq short
len // query the database to get the results -
details not important int result
GetHoldingList ( subAccNo, startStID, list, len)
if (result ! SUCCESS ) throw ex //
setting exception values omitted stockList
new TStockHoldingSeq // ORB frees
this... stockList-gtlength(len) for (int i 0
i lt len i ) (stockList)i.stock_id
listi.stock_id (stockList)i.amount
listi.amount TRACE ("Exit Get Holdings "
)
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IDL-Java Mapping
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Mapping of Module

• IDL module is mapped to a Java package with the
  samename.
• All IDL type declarations within the module are
  mapped tothe corresponding Java class or
  interface declarationswithin the generated
  package.
• //IDL
• module finance     
• interface cash     ...     
• //Java
• package finance     
• public interface cash     ...

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Mapping of Basic Types

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Mapping to Basic Types

• Holder Classes
• Accommodate the passing of out and inout
  parameters in Java
• Predefined and user-defined types
• User-defined types generated by IDL compiler
• Predefined types provided by CORBA class library
• Predefined holder classes are defined in
  org.omg.CORBA
• Client creates object framework and passes it on
  to the server. Server fills the items and returns
  the same

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Mapping to Basic Types
Example Package org.omg.CORBA final public class
IntHolder public int value public
IntHolder() public IntHolder(int
initial) valueinitial
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Mapping to Basic Types

• Generic Pattern
• Holder class for a user defined TYPE is defined
  according to the following pattern
• final public class TypeHolder
• public Type value
• public TypeHolder()
• public TypeHolder(Type initial)
• public void
• _read(org.omg.CORBA.portable.InputStream i)
  
• public void _write(org.omg.CORBA.portable.O
  utputStream o)
• public org.omg.CORBA.TypeCode _type()

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Mapping to Basic Types

• Generic Pattern
• The _read() and _write() methods are used for
  marshalling code
• The _type() method provides a easy way to access
  the TypeCode of a user-defined type

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Mapping to Basic Types

• String Types
• IDL defines strings that are bounded or unbounded
• All IDL strings are mapped to the Java object of
  class java.lang.String
• The stub code generated from the IDL checks the
  correctness of the string bounded at run time and
  raises the exception CORBAMARSHALL if exceeded
• Character range violations cause a
  CORBADATA_CONVERSION exception
• Bounds violations cause a CORBABAD_PARAM
  exception to be raised

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Mapping to Basic Types

• String Types
• The holder class for strings and wide strings is
  defined in the package org.omg.package as
  follows
• final public class StringHolder
• public java.lang.String value
• public StringHolder()
• public StringHolder(java.lang.String initial)
• valueinitial

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Mapping for Constants

• IDL constants are generally mapped to a static
  final variable that has a value of the constant.
• static ensures that instances of that class have
  the same value
• final ensures that the value cannot be overridden

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Mapping for Constants

• //IDL Code
• interface Tester
• const short MaxSlots 8
• //generated Java
• public interface Tester extends
  org.omg.CORBA.Object
• final public static short MaxSlots (short) 8
•  

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Mapping for Constants

• Constants not within an interface
• Constants not declared within an interface are
  mapped to public interface with the same name as
  the constant and containing a field, namely
  value, that holds the constants value
• Example
• // IDL Code
• module Exampleconst long aLongerOne-123
• // Generated Java
• package Example
• public interface aLongerOne
• int value (int) (-123L)

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Mapping for Complex Types

• Helpers
• All user defined (complex) IDL types have an
  additional abstract helper Java class with the
  suffix appended to the type name generated.
• Generic pattern
• abstract public class lttypenamegtHelper
• public static void
• insert(org.omg.CORBA.Any a, lttypenamegt t)
• public static lttypenamegtextract (Any a)
• public static org.omg.CORBA.TypeCode type()
• public static String id()
• // stuff missing
• public static lttypenamegt //for interface
  only narrow(org.omg.CORBA.Object obj)

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Mapping for Complex Types

• Struct
• IDLs struct is mapped to a Java final class that
  provides fields for the members of the struct and
  some constructors
• The class is named after the struct
• There is a constructor which has a parameter for
  each member of the struct and initializes the
  object properly
• A second constructor, null, creates the object.
  The values of the structure members have to be
  filled in later

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Mapping for Complex Types

• Struct example
• // IDL Code
• Struct TestStruct
• short a_short
• long a_long
• This is mapped to the following
• // Java Code
• final public class TestStruct
• public short a_short
• public int a_long
• public TestStruct()
• public TestStruct (short a_short, int a_long)
• this.a_shorta_short
• this.a_longa_long

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Typedefs

• Typedef
• Java has no aliasing for types
• Helper classes are generated for all typedefs
• The base type has to be used where the typedef
  name is expected in the Java implementation
• An exception to this rule is that new types are
  generated for IDL array and sequence typedefs

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Typedefs

• Arrays
• IDL arrays are mapped to Java arrays
• No Java type or class are generated
• Example
• //IDL array
• typedef long long_array105
• //Java- a programmer has to declare and allocate
  the array in the Java application code
• int a_long_array
• a_long_array new int105

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Sequences

• Sequence
• Sequences are mapped similarly to arrays
• The bound of bounded sequences is checked at run
  time and the exception
• BAD_PARAM is raised if it is violated. Its the
  application programmers
• responsibility to declare and create a Java
  array of the corresponding
• member type.
•  //IDL
• typedef sequenceltlong, 10gtbounded_10_seq
• typedef sequenceltlonggtunbounded_seq
•      

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Sequences

• final public class unboundedHolder implements
                          org.omg.CORBA.portable.St
  reamable     
• public int value     public
  UnboundedDataHolder()     
• public UnboundedDataHolder(int initial) ...
• public void _read(org.omg.CORBA.portable.InputStr
  eam i) ...     
• public void _write(org.omg.CORBA.portable.OutputS
  tream o) ...     
• public org.omg.CORBA.TypeCode _type() ...
• final public class boundedHolder implements
             org.omg.CORBA.portable.Streamable
      
• public int value     public
  BoundedDataHolder()     
• public BoundedDataHolder(int initial) ...
      
• public void _read(org.omg.CORBA.portable.InputStr
  eam i) ...     
• public void _write(org.omg.CORBA.portable.OutputS
  tream o) ...     
• public org.omg.CORBA.TypeCode _type() ...

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Mapping for Complex Types

• Operations to Methods
• IDL operations are mapped to Java methods of the
  same name.
• IDL void return type is mapped according to Java
  void.
• IDL in parameters are mapped according to the
  mapping for IDL types
• IDL inout and out parameters are mapped to the
  Holder classes which aregenerated for their IDL
  type.
• IDL operations can explicitly raise one or more
  user-defined exceptions and can also implicitly
  raise system exceptions.
• Raise clause of an IDL operation is mapped to a
  throw clause on the equivalent Java method.

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Interfaces

• Example
• //IDL
• Exception SomethingWrong
• string reason
• long id
•  
• interface Tester
• boolean test(in string name, inout boolean flag,
  out long id)
• raises(SomethingWrong)
• //Java
• Public interface Tester extends
  org.omg.CORBA.Object
• Public boolean test( java.lang.String name,
• org.omg.CORBA.BooleanHolder flag,
• org.omg.CORBA.LongHolder id )
• throws SomethingWrong

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Interfaces

• Interfaces
• An interface type is mapped to a Java public
  interface of the same name
• An IDL interface type is mapped to a Java public
  interface of the same name.
• Attributes are mapped to a pair of Java accessor
  and modifier methods. These methods have the same
  name as the IDL attribute and are overloaded.
  There is no modifier method for IDL read only
  attributes.
•  

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Mapping for Complex Types

• Example
• IDL
• module Example     
• interface Face         
• long method (in long arg) raises (e)
•    attribute long assignable
• readonly attribute long nonassignable     
• Java
• package Example  
• public interface Face extends org.omg.CORBA.Objec
  t     
• int method(int arg) throws Example.e
• int assignable()
• void assignable(int i)
• int nonassignable()
•  

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Mapping for Complex Types

• //Helper class
• public class FaceHelper      // standard helper
  methods
• public static void insert(org.omg.CORBA.Any a,
  Face t)
• ...    
• public static Face extract (Any a) ...    
• public static org.omg.CORBA.TypeCode type()...
• public static String id() ...    
• public static Face read(org.omg.CORBA.port
  able.InputStream istream) ...
• public static void write(org.omg.CORBA.por
  table.OutputStream ostream, Facevalue)
  ...     
• // interface specific narrow method    
• public static Face narrow(org.omg.CORBA.Object
  obj)
• ...  
•  

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Mapping for Complex Types

• // Holder class
• final public class FaceHolder implements
  org.omg.CORBA.portable.Streamable     
• public Face value     
• public FaceHolder()     
• public FaceHolder(Face initial) ...     
• public void _read(org.omg.CORBA.portable.InputStr
  eam i) ...     
• public void _write(org.omg.CORBA.portable.Output
  Stream o)
• ...     
• public org.omg.CORBA.TypeCode _type() ...

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Mapping for Exceptions

• IDL Exception
• Mapped very similarly to structs.
• Mapped to Java class that provides instance
  variables for the fields of exception and
  constructors.
• User-defined exceptions system exceptions

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Mapping for Exceptions

• Example
• // IDL code
• Exception SomethingWrong
• string reason
• long id

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Mapping for Exceptions

• The generated Java code is
• final public class SomethingWrong extends
• org.omg.CORBA.UserException
• public java.lang.String reason
• public int id
• public SomethingWrong ( )
• public SomethingWrong (java.lang.String reason,
  int id) this.reasonreason
• this.idid

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Mapping for Exceptions

• The generated holder class is
• final public class SomethingWrongHolder
  implements
• org.omg.CORBA.portable.Streamable
• public SomethingWrong value
• public SomethingWrongHolder ( )
• public SomethingWrongHolder (SomethingWrong
  value)
• this.value value
• // stuff missing

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Mapping for Exceptions

• The table below lists all CORBA system exceptions
  and their mapping to Java counterparts

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Memory Management Issues

• Java provides automatic garbage collection
• Greatly simplifies the memory management issues
  associated with the IDL to Java mapping
• However, an CORBA client programmer should
  understand where memory needs to be allocated for
  IDL operation parameters.

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Semantics of in, out and inout

• in
• passed-by-value
• passed-by-reference
• out inout
• passed-by-reference

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Parameters

• In Parameters
• All IDL basic types except type Any and String
  are passed-by-value
• Struct, union, sequence, array and Any are
  passed-by-reference as in the form of holder
  class
• Out/inout parameters
• All IDL basic and user defined types are
  passed-by-reference in the form of holder class

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Parameter Passing Example

• IDL
• module Example
• interface Modes
• long operation(in long inArg, out long outArg,
  inout long inoutArg)

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Parameter Passing Example

• Java
• package Example
• public interface Modes
• long operation(int inArg, IntHolder outArg,
  IntHolder inoutArg)

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Example Client

• // set the in actual value
• int inArg 99
• // prepare to receive out
• IntHolder outHolder new IntHolder()
• // set up the in side of the inout
• IntHolder inoutHolder new IntHolder(199)
• // make the invocation
• int result target.operation(inArg, outHolder,
  inoutHolder)
• // use the value of the outHolder
• outHolder.value...
• // use the value of the inoutHolder
• inoutHolder.value...