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Transaction Isolation Level

Read phenomenas:
The ANSI/ISO standard SQL speaks of three different phenomenas when a session reads data that another session might have changed:

1. Dirty read,
2. Non-repeatable read and
3. Phantom read

Dirty read:
If User1 can read data that User2 has written but not yet commited, this is called a dirty read. It is dirty because user2 might later decide to rollback the transaction which leads to the situation that User1 works with data that actually must be considered as non-existant.
Oracle doesn't allow dirty reads.

For eg.User1 Transaction.
UPDATE employee SET salary = 10000 WHERE empno = 'I-001'
User2 Transaction.
SELECT * FROM employee

(User2 Transaction sees data updated by User1 transaction. Those updates have not yet been committed.)

Non-repeatable read:
User1 reads data which is later changed and commited by User2. If User1 reads the same data again (after User2's commit) and finds it to have changed or to be deleted (according to User2's changes), this is called a non-repeatable read. It is called non-repeatable because the same select statement doesn't return the same data (within the same transaction).

For eg.User1 Transaction.
SELECT * FROM employee WHERE empno = 'I-001'
User2 Transaction.
UPDATE employee SET salary = 20000 WHERE empno = 'I-001'

(user2 updates rows viewed by User1 before user1 commits.) If user1 issues the same SELECT statement, the results will be different.

Phantom read:
User1 reads data (select) with a specific where condition. After this read, User2 inserts some data that meets the User1's where condition and commits the inserted data. When User1 issues a select statement with the same where condition, it finds new records. It is called phantom read because the new records seem to be of phantom origin.
A phantom read is thus a special case of a non-repeatable read.

For eg.User1 Transaction.
SELECT * FROM employee WHERE salary > 30000
User2 Transaction.
INSERT INTO employee(empno, firstnme, lastname, job,salary)
VALUES ('I-001', 'Sivaji','Chinnanan','Software Engineer',35000)

User2 inserts a row that would satisfy the query in User1 if it were issued again.

Depending on the scenario the above read phenomenas are used.
There are four isolation levels:

1. READ UNCOMMITTED
2. READ COMMITTED
3. REPEATABLE READ
4. SERIALIZABLE

READ UNCOMMITTED:
The READ UNCOMMITTED isolation level allows dirty reads.
When it's used, you can read an uncommitted transaction that might get rolled back later. This isolation level is also called dirty read. This is the lowest isolation level.

READ COMMITTED:
Each query executed by a transaction sees only data that was committed before the query began (Oracle default isolation level)

REPEATABLE READ:
When it's used, then dirty reads and nonrepeatable reads cannot occur. It means that locks will be placed on all data that is used in a query, and another transactions cannot update the data.

SERIALIZABLE:
Most restrictive isolation level. When it's used, then phantom values cannot occur. It prevents other users from updating or inserting rows into the data set until the transaction is complete.

Summary:

Isolation Levels	Dirty read	Non repeatable read	Phantom read
Read Uncommitted	Yes	Yes	Yes
Read Committed	No	Yes	Yes
Repeatable Read	No	No	Yes
Serializable	No	No	No

Follow the following instruction to set isolation levels in JDBC
con.setTransactionIsolation (Connection.TRANSACTION_READ_UNCOMMITTED);
con.setTransactionIsolation (Connection.TRANSACTION_READ_COMMITTED);
con.setTransactionIsolation (Connection.TRANSACTION_READ_REPEATABLE_READ);
con.setTransactionIsolation (Connection.TRANSACTION_READ_SERIALIZABLE);
con.setAutoCommit(false);