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Use of serialVersionUID

SerialVersionUId play an vital role in the serialization. SerialVersionUId is used for version control of object.Consequence of not specifying serialversionUID is that when you add or modify and field in class then already serialized class will not able to recover because serialVersionUID generated for new class and old serialized object will be different.

Java serialization process relies on correct serialVersionUID for recovering state of serialized object and throw  java.io.InvalidClassException in case of  serialVersionUID mismatch.

The serialVersionUID is a universal version identifier for a Serializable class. Deserialization uses this number to ensure that a loaded class corresponds exactly to a serialized object. If no match is found, then an InvalidClassException is thrown.

Guidelines for serialVersionUID :

* always include it as a field, for example: “private static final long serialVersionUID = 7526472295622776147L; ” include this field even in the first version of the class, as a reminder of its importance
* do not change the value of this field in future versions, unless you are knowingly making changes to the class which will render it incompatible with old serialized objects
* new versions of Serializable classes may or may not be able to read old serialized objects; it depends upon the nature of the change; provide a pointer to Sun’s guidelines for what constitutes a compatible change, as a convenience to future maintainers

In Windows, generate serialVersionUID using the JDK’s graphical tool like so :

* use Control Panel | System | Environment to set the classpath to the correct directory
* run serialver -show from the command line
* point the tool to the class file including the package, for example, finance.stock.Account – without the .class
* (here are the serialver docs for both Win and Unix)

readObject and writeObject :

* readObject implementations always start by calling default methods
* deserialization must be treated as any constructor : validate the object state at the end of deserializing – this implies that readObject should almost always be implemented in Serializable classes, such that this validation is performed.
* deserialization must be treated as any constructor : if constructors make defensive copies for mutable object fields, so must readObject
* when serializing a Collection, store the number of objects in the Collection as well, and use this number to read them back in upon deserialization; avoid tricks using null

Other points :

* use javadoc’s @serial tag to denote Serializable fields
* the .ser extension is conventionally used for files representing serialized objects
* no static or transient fields undergo default serialization
* extendable classes should not be Serializable, unless necessary
* inner classes should rarely, if ever, implement Serializable
* container classes should usually follow the style of Hashtable, which implements Serializable by storing keys and values, as opposed to a large hash table data structure

/**
* Determines if a de-serialized file is compatible with this class.
*
* Maintainers must change this value if and only if the new version
* of this class is not compatible with old versions. See Sun docs
* for * /serialization/spec/version.doc.html> details.
*
* Not necessary to include in first version of the class, but
* included here as a reminder of its importance.
*/
private static final long serialVersionUID = 7526471155622776147L;

[Post New]posted Monday, May 09, 2005 10:44 PM private message
Quote [Up]
*g* as you mention “Java RMI”… it says:

“The downside of using serialVersionUID is that, if a significant change is made (for example, if a field is added to the class definition), the suid will not reflect this difference. This means that the deserialization code might not detect an incompatible version of a class.”

and furtheron suggests to implement readObject & friends and writing a version “manually”.

All this in order to gain performance:

“Setting serialVersionUID is a simple, and often surprisingly noticeable, performance improvement. If you don’t set serialVersionUID, the serialization mechanism has to compute it. This involves going through all the fields and methods and computing a hash. If you set serialVersionUID, on the other hand, the serialization mechanism simply looks up a single value.”

Ref – http://sprasanth.blogspot.com/2009/07/what-is-use-of-setting-serialversionuid.html

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Oracle ADF- Creating ADF Business Component View Objects on More that one Entity Object — Techartifact

One of the common requirements when building an application on relational database tables is the ability to incorporate attribute from different sources into a data view which is more representative of the application requirement, rather than the database schema. ADF Business Components provides a powerful feature which can be used, for example, when viewing Emp data, you would want to displayed Department Name from the Dept table rather than just DeptId, which is less intuitive to the end users. This ADF Insider Essentials explains how you can do this.

Reference- ADF insider essential

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Pl/Sql Exception

Exceptions

 

Oracle includes about 20 predefined exceptions (errors) – we can allow Oracle to raise these implicitly.

For errors that don’t fall into the predefined categories – declare in advance and allow oracle to raise an exception.

For problems that are not recognised as an error by Oracle – but still cause some difficulty within your application – declare a User Defined Error and raise it explicitly
i.e IF x >20 then RAISE …

Syntax:

EXCEPTION

WHEN exception1 [OR exception2...]] THEN

...

[WHEN exception3 [OR exception4...] THEN

...]

[WHEN OTHERS THEN

...]

Where exception is the exception_name e.g. WHEN NO_DATA_FOUND… Only one handler is processed before leaving the block.

Trap non-predefined errors by declaring them You can also associate the error no. with a name so that you can write a specific handler.
This is done with the PRAGMA EXCEPION_INIT pragma.

PRAGMA (pseudoinstructions) indicates that an item is a ‘compiler directive’ Running this has no immediate effect but causes all subsequent references to the exception name to be interpreted as the associated Oracle Error.

–

 

Trapping a non-predefined Oracle server exception

 

DECLARE

-- name for exception

e_emps_remaining EXCEPTION

PRAGMA_EXCEPTION_INIT (

e_emps_remaining, -2292);

v_deptno dept.deptno%TYPE :=&p_deptno;

 

BEGIN

DELETE FROM dept

WHERE deptno = v_deptno

COMMIT;

EXCEPTION

WHEN e_emps_remaining THEN

DBMS_OUTPUT.PUT_LINE ('Cannot remove dept '||

TO_CHAR(v_deptno) || '. Employees exist. ');

END;

When an exception occurs you can identify the associated error code/message with two supplied functions SQLCODE and SQLERRM

SQLCODE – Number

SQLERRM – message

 

An example of using these:

DECLARE
v_error_code NUMBER;
v_error_message VARCHAR2(255);

 

BEGIN

 

...

 

EXCEPTION

WHEN OTHERS THEN

ROLLBACK;

v_error_code := SQLCODE

v_error_message := SQLERRM

INSERT INTO t_errors

VALUES ( v_error_code, v_error_message);

END;

Trapping user-defined exceptions

DECLARE the exception

RAISE the exception

Handle the raised exception

 

e.g.

DECLARE

e_invalid_product EXCEPTION

BEGIN

update PRODUCT

SET descrip = '&prod_descr'

WHERE prodid = &prodnoumber';

IF SQL%NOTFOUND THEN

RAISE e_invalid_product;

END IF;

COMMIT;

EXCEPTION

WHEN e_invalid_product THEN

DBMS_OUTPUT.PUT_LINE ('INVALID PROD NO');

END;

Propagation of Exception handling in sub blocks

If a sub block does not have a handler for a particular error it will propagate to the enclosing block – where it can be caught by more general exception handlers.

RAISE_APPLICATION_ERROR (error_no, message[,{TRUE|FALSE}]);

 

This procedure  allows user defined error

messages from stored sub programs – call only from stored sub prog.

error_no = a user defined no (between -20000 and -20999)

 

TRUE = stack errors

FALSE = keep just last

 

This can either be used in the executable section of code or

the exception section

 

e.g.

EXCEPTION

WHEN NO_DATA_FOUND THEN

RAISE_APPLICATION_ERROR (-2021,

‘manager not a valid employee’);

END;

Standard Exceptions, from the the STANDARD package

Oracle Exception Name

Oracle Error

Explanation
DUP_VAL_ON_INDEX ORA-00001 You attempted to create a duplicate value in a field restricted by a unique index.
TIMEOUT_ON_RESOURCE ORA-00051 A resource timed out, took too long.
TRANSACTION_BACKED_OUT ORA-00061 The remote portion of a transaction has rolled back.
INVALID_CURSOR ORA-01001 The cursor does not yet exist. The cursor must be OPENed before any FETCH cursor or CLOSE cursor operation.
NOT_LOGGED_ON ORA-01012 You are not logged on.
LOGIN_DENIED ORA-01017 Invalid username/password.
NO_DATA_FOUND ORA-01403 No data was returned
TOO_MANY_ROWS ORA-01422 You tried to execute a SELECT INTO statement and more than one row was returned.
ZERO_DIVIDE ORA-01476 Divide by zero error.
INVALID_NUMBER ORA-01722 Converting a string to a number was unsuccessful.
STORAGE_ERROR ORA-06500 Out of memory.
PROGRAM_ERROR ORA-06501 Generic "Contact Oracle support" message.
VALUE_ERROR ORA-06502 You tried to perform an operation and there was a error on a conversion, truncation, or invalid constraining of numeric or character data.
ROWTYPE_MISMATCH ORA-06504  
CURSOR_ALREADY_OPEN ORA-06511 The cursor is already open.
ACCESS_INTO_NULL ORA-06530  
COLLECTION_IS_NULL ORA-06531  
       

Named exceptions

An exception can be given a name. In order to create such a named exceptions, two steps are necessary: first, it needs a line that reads exception_name exception. Then, this exception must be connected to a number with the pragma exception_init(exception_name, exc-number). The number must be in the range -20999 through -20000 (these are negative numbers!). 
create or replace package exc as

  some_exc exception;

  pragma exception_init(some_exc, -20005);

  procedure raise_exc;
end exc;
/

create or replace package body exc as

  procedure raise_exc is begin 
    raise some_exc;
  end raise_exc;

end exc;
/
create or replace package use_exc as
  procedure do;
end use_exc;
/

create or replace package body use_exc as

  procedure do is begin

    exc.raise_exc;

    exception 
      when exc.some_exc then dbms_output.put_line('some_exc');
      when others       then dbms_output.put_line('others'  );

  end do;

end use_exc;
/
set serveroutput on
begin
  use_exc.do;
end;
/

The output:

some_exc

Reference -http://www.adp-gmbh.ch/ora/plsql/exception.html