JDBC Interview Questions and Answers

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).

How can I determine the isolation levels supported by my DBMS?
Use DatabaseMetaData.supportsTransactionIsolationLevel(int level).

Connecting to a database through the Proxy I want to connect to remote database using a program that is running in the local network behind the proxy. Is that possible?
I assume that your proxy is set to accept http requests only on port 80. If you want to have a local class behind the proxy connect to the database for you, then you need a servlet/JSP to receive an HTTP request and use the local class to connect to the database and send the response back to the client.
You could also use RMI where your remote computer class that connects to the database acts as a remote server that talks RMI with the clients. if you implement this, then you will need to tunnel RMI through HTTP which is not that hard.
In summary, either have a servlet/JSP take HTTP requests, instantiate a class that handles database connections and send HTTP response back to the client or have the local class deployed as RMI server and send requests to it using RMI.

How do I receive a ResultSet from a stored procedure?
Stored procedures can return a result parameter, which can be a result set. For a discussion of standard JDBC syntax for dealing with result, IN, IN/OUT and OUT parameters, see Stored Procedures.

How can I write to the log used by DriverManager and JDBC drivers?
The simplest method is to use DriverManager.println(String message), which will write to the current log.

How can I get or redirect the log used by DriverManager and JDBC drivers?
As of JDBC 2.0, use DriverManager.getLogWriter() and DriverManager.setLogWriter(PrintWriter out). Prior to JDBC 2.0, the DriverManager methods getLogStream() and setLogStream(PrintStream out) were used. These are now deprecated.

What does it mean to "materialize" data?
This term generally refers to Array, Blob and Clob data which is referred to in the database via SQL locators "Materializing" the data means to return the actual data pointed to by the Locator.
For Arrays, use the various forms of getArray() and getResultSet().
For Blobs, use getBinaryStream() or getBytes(long pos, int length).
For Clobs, use getAsciiStream() or getCharacterStream().

Why do I have to reaccess the database for Array, Blob, and Clob data?
Most DBMS vendors have implemented these types via the SQL3 Locator type
Some rationales for using Locators rather than directly returning the data can be seen most clearly with the Blob type. By definition, a Blob is an arbitrary set of binary data. It could be anything; the DBMS has no knowledge of what the data represents. Notice that this effectively demolishes data independence, because applications must now be aware of what the Blob data actually represents. Let's assume an employee table that includes employee images as Blobs.
Say we have an inquiry program that presents multiple employees with department and identification information. To see all of the data for a specific employee, including the image, the summary row is selected and another screen appears. It is only at this pont that the application needs the specific image. It would be very wasteful and time consuming to bring down an entire employee page of images when only a few would ever be selected in a given run.
Now assume a general interactive SQL application. A query is issued against the employee table. Because the image is a Blob, the application has no idea what to do with the data, so why bring it down, killing performance along the way, in a long running operation?
Clearly this is not helpful in those applications that need the data everytime, but these and other considerations have made the most general sense to DBMS vendors.

What is an SQL Locator?
A Locator is an SQL3 data type that acts as a logical pointer to data that resides on a database server. Read "logical pointer" here as an identifier the DBMS can use to locate and manipulate the data. A Locator allows some manipulation of the data on the server. While the JDBC specification does not directly address Locators, JDBC drivers typically use Locators under the covers to handle Array, Blob, and Clob data types.

How do I set properties for a JDBC driver and where are the properties stored?
A JDBC driver may accept any number of properties to tune or optimize performance for the specific driver. There is no standard, other than user and password, for what these properties should be. Therefore, the developer is dependent on the driver documentation to automatically pass properties. For a standard dynamic method that can be used to solicit user input for properties, see What properties should I supply to a database driver in order to connect to a database?
In addition, a driver may specify its own method of accepting properties. Many do this via appending the property to the JDBC Database URL. However, a JDBC Compliant driver should implement the connect(String url, Properties info) method. This is generally invoked through DriverManager.getConnection(String url, Properties info).
The passed properties are ( probably ) stored in variables in the Driver instance. This, again, is up to the driver, but unless there is some sort of driver setup, which is unusual, only default values are remembered over multiple instantiations.

What is the JDBC syntax for using a literal or variable in a standard Statement?
First, it should be pointed out that PreparedStatement handles many issues for the developer and normally should be preferred over a standard Statement.
Otherwise, the JDBC syntax is really the same as SQL syntax. One problem that often affects newbies ( and others ) is that SQL, like many languages, requires quotes around character ( read "String" for Java ) values to distinguish from numerics. So the clause:
"WHERE myCol = " + myVal
is perfectly valid and works for numerics, but will fail when myVal is a String. Instead use:
"WHERE myCol = '" + myVal + "'"
if myVal equals "stringValue", the clause works out to:
WHERE myCol = 'stringValue'
You can still encounter problems when quotes are embedded in the value, which, again, a PreparedStatement will handle for you.

How do I check in my code whether a maximum limit of database connections have been reached?
Use DatabaseMetaData.getMaxConnections() and compare to the number of connections currently open. Note that a return value of zero can mean unlimited or, unfortunately, unknown. Of course, driverManager.getConnection() will throw an exception if a Connection can not be obtained.

Why do I get UnsatisfiedLinkError when I try to use my JDBC driver?
The first thing is to be sure that this does not occur when running non-JDBC apps. If so, there is a faulty JDK/JRE installation. If it happens only when using JDBC, then it's time to check the documentation that came with the driver or the driver/DBMS support. JDBC driver types 1 through 3 have some native code aspect and typically require some sort of client install. Along with the install, various environment variables and path or classpath settings must be in place. Because the requirements and installation procedures vary with the provider, there is no reasonable way to provide details here. A type 4 driver, on the other hand, is pure Java and should never exhibit this problem. The trade off is that a type 4 driver is usually slower.

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