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 Driver interface. Note that there may actually be two instances created, due to the expected standard behavior of drivers when the class is loaded.
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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