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JDBC Interview Questions and Answers



Can I set up a connection pool with multiple user IDs? The single ID we are forced to use causes problems when debugging the DBMS.
Since the Connection interface ( and the underlying DBMS ) requires a specific user and password, there's not much of a way around this in a pool. While you could create a different Connection for each user, most of the rationale for a pool would then be gone. Debugging is only one of several issues that arise when using pools.
However, for debugging, at least a couple of other methods come to mind. One is to log executed statements and times, which should allow you to backtrack to the user. Another method that also maintains a trail of modifications is to include user and timestamp as standard columns in your tables. In this last case, you would collect a separate user value in your program.

How can I protect my database password ? I'm writing a client-side java application that will access a database over the internet. I have concerns about the security of the database passwords. The client will have access in one way or another to the class files, where the connection string to the database, including user and password, is stored in as plain text. What can I do to protect my passwords?
This is a very common question.
Conclusion: JAD decompiles things easily and obfuscation would not help you. But you'd have the same problem with C/C++ because the connect string would still be visible in the executable.
SSL JDBC network drivers fix the password sniffing problem (in MySQL 4.0), but not the decompile problem. If you have a servlet container on the web server, I would go that route (see other discussion above) then you could at least keep people from reading/destroying your mysql database.
Make sure you use database security to limit that app user to the minimum tables that they need, then at least hackers will not be able to reconfigure your DBMS engine.
Aside from encryption issues over the internet, it seems to me that it is bad practice to embed user ID and password into program code. One could generally see the text even without decompilation in almost any language. This would be appropriate only to a read-only database meant to be open to the world. Normally one would either force the user to enter the information or keep it in a properties file.

Detecting Duplicate Keys I have a program that inserts rows in a table. My table has a column 'Name' that has a unique constraint. If the user attempts to insert a duplicate name into the table, I want to display an error message by processing the error code from the database. How can I capture this error code in a Java program?
A solution that is perfectly portable to all databases, is to execute a query for checking if that unique value is present before inserting the row. The big advantage is that you can handle your error message in a very simple way, and the obvious downside is that you are going to use more time for inserting the record, but since you're working on a PK field, performance should not be so bad.
You can also get this information in a portable way, and potentially avoid another database access, by capturing SQLState messages. Some databases get more specific than others, but the general code portion is 23 - "Constraint Violations". UDB2, for example, gives a specific such as 23505, while others will only give 23000.

What driver should I use for scalable Oracle JDBC applications?
Sun recommends using the thin ( type 4 ) driver.
* On single processor machines to avoid JNI overhead.
* On multiple processor machines, especially running Solaris, to avoid synchronization bottlenecks.

Can you scroll a result set returned from a stored procedure? I am returning a result set from a stored procedure with type SQLRPGLE but once I reach the end of the result set it does not allow repositioning. Is it possible to scroll this result set?
A CallableStatement is no different than other Statements in regard to whether related ResultSets are scrollable. You should create the CallableStatement using Connection.prepareCall(String sql, int resultSetType, int resultSetConcurrency).

How do I write Greek ( or other non-ASCII/8859-1 ) characters to a database?
From the standard JDBC perspective, there is no difference between ASCII/8859-1 characters and those above 255 ( hex FF ). The reason for that is that all Java characters are in Unicode ( unless you perform/request special encoding ). Implicit in that statement is the presumption that the data store can handle characters outside the hex FF range or interprets different character sets appropriately. That means either:

* The OS, application and database use the same code page and character set. For example, a Greek version of NT with the DBMS set to the default OS encoding.
* The DBMS has I18N support for Greek ( or other language ), regardless of OS encoding. This has been the most common for production quality databases, although support varies. Particular DBMSes may allow setting the encoding/code page/CCSID at the database, table or even column level. There is no particular standard for provided support or methods of setting the encoding. You have to check the DBMS documentation and set up the table properly.
* The DBMS has I18N support in the form of Unicode capability. This would handle any Unicode characters and therefore any language defined in the Unicode standard. Again, set up is proprietary.

How can I insert images into a Mysql database?
This code snippet shows the basics:

File file = new File(fPICTURE);
FileInputStream fis = new FileInputStream(file);
PreparedStatement ps =
ConrsIn.prepareStatement("insert into dbPICTURE values (?,?)");

// ***use as many ??? as you need to insert in the exact order***
ps.setString(1,file.getName());
ps.setBinaryStream(2,fis,(int)file.length());
ps.close();
fis.close();

Is possible to open a connection to a database with exclusive mode with JDBC?
I think you mean "lock a table in exclusive mode". You cannot open a connection with exclusive mode. Depending on your database engine, you can lock tables or rows in exclusive mode.
In Oracle you would create a statement st and run
st.execute("lock table mytable in exclusive mode");
Then when you are finished with the table, execute the commit to unlock the table. Mysql, Informix and SQLServer all have a slightly different syntax for this function, so you'll have to change it depending on your database. But they can all be done with execute().

What are the standard isolation levels defined by JDBC?
The values are defined in the class java.sql.Connection and are:
* TRANSACTION_NONE
* TRANSACTION_READ_COMMITTED
* TRANSACTION_READ_UNCOMMITTED
* TRANSACTION_REPEATABLE_READ
* TRANSACTION_SERIALIZABLE

Update fails without blank padding. Although a particular row is present in the database for a given key, executeUpdate() shows 0 rows updated and, in fact, the table is not updated. If I pad the Key with spaces for the column length (e.g. if the key column is 20 characters long, and key is msgID, length 6, I pad it with 14 spaces), the update then works!!! Is there any solution to this problem without padding?
In the SQL standard, CHAR is a fixed length data type. In many DBMSes ( but not all), that means that for a WHERE clause to match, every character must match, including size and trailing blanks. As Alessandro indicates, defining CHAR columns to be VARCHAR is the most general answer.

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