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JDBC Interview Questions
11) I have the choice of manipulating database data using
a byte[] or a java.sql.Blob. Which has best performance?
java.sql.Blob, since it does not extract any data from the database until you explicitly ask it
to. The Java platform 2 type Blob wraps a database locator (which is essentially a pointer to
byte). That pointer is a rather large number (between 32 and 256 bits in size) - but the
effort to extract it from the database is insignificant next to extracting the full blob content.
For insertion into the database, you should use a byte[] since data has not been uploaded
to the database yet. Thus, use the Blob class only for extraction.
Conclusion:
use the java.sql.Blob class for extraction whenever you can.
12) I have the choice of manipulating database data using
a String or a java.sql.Clob. Which has best performance?
java.sql.Clob, since it does not extract any data from the database until you explicitly ask it
to. The Java platform 2 type Clob wraps a database locator (which is essentially a pointer to
char). That pointer is a rather large number (between 32 and 256 bits in size) - but the
effort to extract it from the database is insignificant next to extracting the full Clob content.
For insertion into the database, you should use a String since data need not been
downloaded from the database. Thus, use the Clob class only for extraction.
Conclusion:
particular table cell, use the java.sql.Clob class for extraction whenever you can.
Unless you always intend to extract the full textual data stored in the
13) Do I need to commit after an INSERT call in JDBC or
does JDBC do it automatically in the DB?
If your autoCommit flag (managed by the Connection.setAutoCommit method) is false, you
are required to call the commit() method - and vice versa.
14) How can I retrieve only the first n rows, second n rows
of a database using a particular WHERE clause ? For
example, if a SELECT typically returns a 1000 rows, how do
first retrieve the 100 rows, then go back and retrieve the
next 100 rows and so on ?
Use the Statement.setFetchSize method to indicate the size of each database fetch. Note
that this method is only available in the Java 2 platform. For Jdk 1.1.X and Jdk 1.0.X, no
standardized way of setting the fetch size exists. Please consult the Db driver manual.
15) What does ResultSet actually contain? Is it the actual
data of the result or some links to databases? If it is the
actual data then why can't we access it after connection is
closed?
A ResultSet is an interface. Its implementation depends on the driver and hence ,what it
"contains" depends partially on the driver and what the query returns.
For example with the Odbc bridge what the underlying implementation layer contains is an
ODBC result set. A Type 4 driver executing a stored procedure that returns a cursor - on an
oracle database it actually returns a cursor in the database. The oracle cursor can however
be processed like a ResultSet would be from the client.
Closing a connection closes all interaction with the database and releases any locks that
might have been obtained in the process.
16) What are SQL3 data types?
The next version of the ANSI/ISO SQL standard defines some new datatypes, commonly
referred to as the SQL3 types. The primary SQL3 types are:
STRUCT:
java.sql.Struct type.
This is the default mapping for any SQL structured type, and is manifest by the
REF:
to a SQL statement. Mapped to the java.sql.Ref type.
Serves as a reference to SQL data within the database. Can be passed as a parameter
BLOB:
Holds binary large objects. Mapped to the java.sql.Blob type.
CLOB:
Contains character large objects. Mapped to the java.sql.Clob type.
ARRAY:
You can retrieve, store and update SQL3 types using the corresponding getXXX(), setXXX(),
and updateXXX() methods defined in ResultSet interface
Can store values of a specified type. Mapped to the java.sql.Array type.
17) How can I manage special characters (for example: " _
' % ) when I execute an INSERT query? If I don't filter the
quoting marks or the apostrophe, for example, the SQL
string will cause an error.
The characters "%" and "_" have special meaning in SQL LIKE clauses (to match zero or
more characters, or exactly one character, respectively). In order to interpret them literally,
they can be preceded with a special escape character in strings, e.g. "". In order to specify
the escape character used to quote these characters, include the following syntax on the
end of the query:
{escape '
escape-character'}
For example, the query
SELECT NAME FROM IDENTIFIERS WHERE ID LIKE '_%' {escape ''}
finds identifier names that begin with an underbar.
18) What is SQLJ and why would I want to use it instead of
JDBC?
SQL/J is a technology, originally developed by Oracle Corporation, that enables you to
embed SQL statements in Java. The purpose of the SQLJ API is to simplify the development
requirements of the JDBC API while doing the same thing. Some major databases (Oracle,
Sybase) support SQLJ, but others do not. Currently, SQLJ has not been accepted as a
standard, so if you have to learn one of the two technologies, I recommend JDBC.
19) How do I insert an image file (or other raw data) into
a database?
All raw data types (including binary documents or images) should be read and uploaded to
the database as an array of bytes, byte[]. Originating from a binary file,
1. Read all data from the file using a FileInputStream.
2. Create a byte array from the read data.
3. Use method setBytes(int index, byte[] data); of java.sql.PreparedStatement to
upload the data.
20) How can I pool my database connections so I don't
have to keep reconnecting to the database?
• you gets a reference to the pool
• you gets a free connection from the pool
• you performs your different tasks
• you frees the connection to the pool
Since your application retrieves a pooled connection, you don't consume your time to
connect / disconnect from your data source.
21) Will a call to PreparedStatement.executeQuery()
always close the ResultSet from the previous
executeQuery()?
A ResultSet is automatically closed by the Statement that generated it when that Statement
is closed, re-executed, or is used to retrieve the next result from a sequence of multiple
results.
22) How do I upload SQL3 BLOB & CLOB data to a
database?
Although one may simply extract BLOB & CLOB data from the database using the methods
of the java.sql.CLOB and java.sql.BLOB, one must upload the data as normal java
datatypes. The example below inserts a BLOB in the form of a byte[] and a CLOB in the
form of a String into the database
Inserting SQL3 type data [BLOB & CLOB]
private void runInsert() {
try {
// Log
this.log("Inserting values ... ");
// Open a new Statement
PreparedStatement stmnt = conn.prepareStatement(
"insert Lobtest (image, name) values (?, ?)");
// Create a timestamp to measure the insert time
Date before = new java.util.Date();
for(int i = 0; i < 500; i++) {
// Set parameters
stmnt.setBytes(1, blobData);
stmnt.setString(2, "i: " + i + ";" + clobData);
// Perform insert
int rowsAffected = stmnt.executeUpdate();
}
// Get another timestamp to complete the time measurement
Date after = new java.util.Date();
this.log(" ... Done!");
log("Total run time: " + (
after.getTime() - before.getTime()));
// Close database resources
stmnt.close();
} catch(SQLException ex) {
this.log("Hmm... " + ex);
}
}
23) What is the difference between client and server
database cursors?
What you see on the client side is the current row of the cursor which called a Result
(ODBC) or ResultSet (JDBC). The cursor is a server-side entity only and remains on the
server side.
24) Are prepared statements faster because they are
compiled? if so, where and when are they compiled?
Prepared Statements aren't actually compiled, but they are bound by the JDBC driver.
Depending on the driver, Prepared Statements can be a lot faster - if you re-use them.
Some drivers bind the columns you request in the SQL statement. When you execute
Connection.prepareStatement(), all the columns bindings take place, so the binding
overhead does not occur each time you run the Prepared Statement. For additional
information on Prepared Statement performance and binding see
IBM's website.
JDBC Performance Tips on
25) Is it possible to connect to multiple databases
simultaneously? Can one extract/update data from
multiple databases with a single statement?
In general, subject, as usual, to the capabilities of the specific driver implementation, one
can connect to multiple databases at the same time. At least one driver ( and probably
others ) will also handle commits across multiple connections. Obviously one should check
the driver documentation rather than assuming these capabilities.
As to the second part of the question, one needs special middleware to deal with multiple
databases in a single statement or to effectively treat them as one database. DRDA (
Distributed Relational Database Architecture -- I, at least, make it rhyme with "Gerta" ) is
probably most commonly used to accomplish this.
Oracle
product called DataJoiner that make multiple databases appear as one to your application.
No doubt there are other products available.
DRDA.
has a product called Oracle Transparent Gateway for IBM DRDA and IBM has aXOpen also has papers available regarding
26) Why do I get an UnsupportedOperationException?
JDBC 2.0, introduced with the 1.2 version of Java, added several capabilities to JDBC.
Instead of completely invalidating all the older JDBC 1.x drivers, when you try to perform a
2.0 task with a 1.x driver, an UnsupportedOperationException will be thrown. You need to
update your driver if you wish to use the new capabilities.
27) What advantage is there to using prepared statements
if I am using connection pooling or closing the connection
frequently to avoid resource/connection/cursor
limitations?
The ability to choose the 'best' efficiency ( or evaluate tradeoffs, if you prefer, ) is, at times,
the most important piece of a mature developer's skillset. This is YAA ( Yet Another Area, )
where that maxim applies. Apparently there is an effort to allow prepared statements to
work 'better' with connection pools in JDBC 3.0, but for now, one loses most of the original
benefit of prepared statements when the connection is closed. A prepared statement
obviously fits best when a statement differing only in variable criteria is executed over and
over without closing the statement.
However, depending on the DB engine, the SQL may be cached and reused even for a
different prepared statement and most of the work is done by the DB engine rather than the
driver. In addition, prepared statements deal with data conversions that can be error prone
in straight ahead, built on the fly SQL; handling quotes and dates in a manner transparent
to the developer, for example.
28) What is JDBC, anyhow?
JDBC is Java's means of dynamically accessing tabular data, and primarily data in relational
databases, in a generic manner, normally using standard SQL statements.
29) Can I reuse a Statement or must I create a new one
for each query?
When using a JDBC compliant driver, you can use the same Statement for any number of
queries. However, some older drivers did not always "respect the spec." Also note that a
Statement SHOULD automatically close the current ResultSet before executing a new query,
so be sure you are done with it before re-querying using the same Statement.
30) What is a three-tier architecture?
A three-tier architecture is any system which enforces a general separation between the
following three parts:
1.
2.
3.
Client Tier or user interfaceMiddle Tier or business logicData Storage Tier
Applied to web applications and distributed programming, the three logical tiers usually
correspond to the physical separation between three types of devices or hosts:
31) What separates one tier from another in the context of
n-tiered architecture?
It depends on the application.
In a web application, for example, where tier 1 is a web-server, it may communicate with a
tier 2 Application Server using RMI over IIOP, and subsequently tier 2 may communicate
with tier 3 (data storage) using JDBC, etc.
Each of these tiers may be on separate physical machines or they may share the same box.
The important thing is the functionality at each tier.
• Tier 1 - Presentation - should be concerned mainly with display of user interfaces
and/or data to the client browser or client system.
• Tier 2 - Application - should be concerned with business logic
Tier 3+ - Storage/Enterprise Systems - should be focused on data persistence and/or
communication with other Enterprise Systems.
32) What areas should I focus on for the best performance
in a JDBC application?
These are few points to consider:
• Use a connection pool mechanism whenever possible.
• Use prepared statements. These can be beneficial, for example with DB specific
escaping, even when used only once.
• Use stored procedures when they can be created in a standard manner. Do watch
out for DB specific SP definitions that can cause migration headaches.
• Even though the jdbc promotes portability, true portability comes from NOT
depending on any database specific data types, functions and so on.
• Select only required columns rather than using select * from Tablexyz.
• Always close Statement and ResultSet objects as soon as possible.
• Write modular classes to handle database interaction specifics.
• Work with DatabaseMetaData to get information about database functionality.
• Softcode database specific parameters with, for example, properties files.
• Always catch AND handle database warnings and exceptions. Be sure to check for
additional pending exceptions.
• Test your code with debug statements to determine the time it takes to execute
your query and so on to help in tuning your code. Also use query plan functionality
if available.
• Use proper ( and a single standard if possible ) formats, especially for dates.
• Use proper data types for specific kind of data. For example, store birthdate as a
date type rather than, say, varchar.
33) How can I insert multiple rows into a database in a
single transaction?
//turn off the implicit commit
Connection.setAutoCommit(false);
//..your insert/update/delete goes here
Connection.Commit();
a new transaction is implicitly started.
34) How do I convert a java.sql.Timestamp to a
java.util.Date?
While Timesteamp extends Date, it stores the fractional part of the time within itself instead
of within the Date superclass. If you need the partial seconds, you have to add them back
in.
Date date = new Date(ts.getTime() + (ts.getNanos() / 1000000 ));
35) What is SQL?
SQL is a standardized language used to create, manipulate, examine, and manage relational
databases.
36) Is Class.forName(Drivername) the only way to load a
driver? Can I instantiate the Driver and use the object of
the driver?
Yes, you can use the driver directly. Create an instance of the driver and use the connect
method from the
due to the expected standard behavior of drivers when the class is loaded.
Driver interface. Note that there may actually be two instances created,
37) What's new in JDBC 3.0?
Probably the new features of most interest are:
• Savepoint support
• Reuse of prepared statements by connection pools
• Retrieval of auto-generated keys
• Ability to have multiple open ResultSet objects
• Ability to make internal updates to the data in Blob and Clob objects
• Ability to Update columns containing BLOB, CLOB, ARRAY and REF types
• Both java.sql and javax.sql ( JDBC 2.0 Optional Package ) are expected to be
included with J2SE 1.4.
38) Why do I get the message "No Suitable Driver"?
Often the answer is given that the correct driver is not loaded. This may be the case, but
more typically, the JDBC database URL passed is not properly constructed. When a
Connection request is issued, the DriverManager asks each loaded driver if it understands
the URL sent. If no driver responds that it understands the URL, then the "No Suitable
Driver" message is returned.
39) When I create multiple Statements on my Connection,
only the current Statement appears to be executed. What's
the problem?
All JDBC objects are required to be threadsafe. Some drivers, unfortunately, implement this
requirement by processing Statements serially. This means that additional Statements are
not executed until the preceding Statement is completed.
40) Can a single thread open up mutliple connections
simultaneously for the same database and for same table?
The general answer to this is yes. If that were not true, connection pools, for example,
would not be possible. As always, however, this is completely dependent on the JDBC
driver.
You can find out the theoretical maximum number of active Connections that your driver
can obtain via the DatabaseMetaData.getMaxConnections method.
41) Can I ensure that my app has the latest data?
Typically an application retrieves multiple rows of data, providing a snapshot at an instant of
time. Before a particular row is operated upon, the actual data may have been modified by
another program. When it is essential that the most recent data is provided, a JDBC 2.0
driver provides the ResultSet.refreshRow method.
42) What does normalization mean for java.sql.Date and
java.sql.Time?
These classes are thin wrappers extending java.util.Date, which has both date and time
components. java.sql.Date should carry only date information and a normalized instance
has the time information set to zeros. java.sql.Time should carry only time information and
a normalized instance has the date set to the Java epoch ( January 1, 1970 ) and the
milliseconds portion set to zero.
43) What's the best way, in terms of performance, to do
multiple insert/update statements, a PreparedStatement
or Batch Updates?
Because PreparedStatement objects are precompiled, their execution can be faster than
that of Statement objects. Consequently, an SQL statement that is executed many times is
often created as a PreparedStatement object to increase efficiency.
A CallableStatement object provides a way to call stored procedures in a standard manner
for all DBMSes. Their execution can be faster than that of PreparedStatement object.
Batch updates are used when you want to execute multiple statements together. Actually,
there is no conflict here. While it depends on the driver/DBMS engine as to whether or not
you will get an actual performance benefit from batch updates, Statement,
PreparedStatement, and CallableStatement can all execute the addBatch() method.
44) What is JDO?
JDO provides for the transparent persistence of data in a data store agnostic manner,
supporting object, hierarchical, as well as relational stores.
45) What is the difference between setMaxRows(int) and
SetFetchSize(int)? Can either reduce processing time?
setFetchSize(int) defines the number of rows that will be read from the database when the
ResultSet needs more rows. The method in the java.sql.Statement interface will set the
'default' value for all the ResultSet derived from that Statement; the method in the
java.sql.ResultSet interface will override that value for a specific ResultSet. Since database
fetches can be expensive in a networked environment, fetch size has an impact on
performance.
setMaxRows(int) sets the limit of the maximum nuber of rows in a ResultSet object. If this
limit is exceeded, the excess rows are "silently dropped". That's all the API says, so the
setMaxRows method may not help performance at all other than to decrease memory
usage. A value of 0 (default) means no limit.
46) What is DML?
DML is an abbreviation for Data Manipulation Language. This portion of the SQL standard is
concerned with manipulating the data in a database as opposed to the structure of a
database. The core verbs for DML are SELECT, INSERT, DELETE, UPDATE, COMMIT and
ROLLBACK.
47) What is DDL?
DDL is an abbreviation for Data Definition Language. This portion of the SQL standard is
concerned with the creation, deletion and modification of database objects like tables,
indexes and views. The core verbs for DDL are CREATE, ALTER and DROP. While most
DBMS engines allow DDL to be used dynamically ( and available to JDBC ), it is often not
supported in transactions.
48) How can I get information about foreign keys used in a
table?
DatabaseMetaData.getImportedKeys() returns a ResultSet with data about foreign key
columns, tables, sequence and update and delete rules.
49) How do I disallow NULL values in a table?
Null capability is a column integrity constraint, normally aplied at table creation time. Note
that some databases won't allow the constraint to be applied after table creation. Most
databases allow a default value for the column as well. The following SQL statement
displays the NOT NULL constraint:
CREATE TABLE CoffeeTable (
Type VARCHAR(25) NOT NULL,
Pounds INTEGER NOT NULL,
Price NUMERIC(5, 2) NOT NULL
)
50) What isolation level is used by the DBMS when
inserting, updating and selecting rows from a database?
The answer depends on both your code and the DBMS. If the program does not explicitly set
the isolation level, the DBMS default is used. You can determine the default using
DatabaseMetaData.getDefaultTransactionIsolation() and the level for the current Connection
with Connection.getTransactionIsolation(). If the default is not appropriate for your
transaction, change it with Connection.setTransactionIsolation(int level).
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