BEA Weblogic Interview Preparation Guide

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BEA WebLogic is a J2EE application server and also an HTTP web server by BEA Systems of San Jose, California, for Unix, Linux, Microsoft Windows, and other platforms. WebLogic supports Oracle, DB2, Microsoft SQL Server, and other JDBC-compliant databases. WebLogic Server supports WS-Security and is compliant with J2EE 1.3.
BEA WebLogic Server is part of the BEA WebLogic Platform™. The other parts of WebLogic Platform are:
► Portal, which includes Commerce Server and Personalization Server (which is built on a BEA-produced Rete rules engine),
► WebLogic Integration,
► WebLogic Workshop, an IDE for Java, and
► JRockit, a JVM for Intel CPUs.

WebLogic Server includes .NET interoperability and supports the following native integration capabilities:
► Native enterprise-grade JMS messaging
► J2EE Connector Architecture
► WebLogic/Tuxedo Connector
► COM+ Connectivity
► CORBA connectivity
► IBM WebSphere MQ connectivity

BEA WebLogic Server Process Edition also includes Business Process Management and Data Mapping functionality.
WebLogic supports security policies managed by Security Administrators. The BEA WebLogic Server Security Model includes:
► Separate application business logic from security code
► Complete scope of security coverage for all J2EE and non-J2EE components

The EJB Specification prohibits this behavior, and the weblogic.ejbc compiler checks for this behavior and prohibits any polymorphic type of response from a create() or find() method.
The reason the create() and find() methods are not polymorphic is similar to the reason constructors are not polymorphic in Java. The derived classes generally do not know or cannot initialize the base class properly.

No. Within an unsigned applet, you cannot load native libraries over the wire, access the local file system, or connect to any host except the host from which you loaded the applet. The applet security manager enforces these restrictions on applets as protection against applets being able to do unsavory things to unsuspecting users.
If you are trying to use jDriver for Oracle from an applet, then you are violating the first restriction. Your applet will fail when it attempts to load the native (non-Java layer) library that allows jDriver for Oracle to make calls into the non-Java Oracle client libraries. If you look at the exception that is generated, you will see that your applet fails in java.lang.System.loadLibrary, because the security manager determined that you were attempting to load a local library and halted the applet.
You can, however, use the WebLogic JTS or Pool driver for JDBC connectivity in applets. When you use one of these WebLogic multitier JDBC drivers, you need one copy of WebLogic jDriver for Oracle (or any other two-tier JDBC driver) for the connection between the WebLogic Server and the DBMS.

I tried to run two of the applets in the examples directory of the distribution. I installed the WebLogic classes on my local machine (NT server) and on another machine (a Windows 95 client). I am not using any browsers, just trying to run the applets with Appletviewer. The applets work fine when I run Appletviewer from the NT server, but do not work at all from the Windows 95 client.


There are two possible problems: Either the CODEBASE tag is not properly set in the applet HTML file, or the class files are not properly loaded on the HTTP server.
The applet works on the NT server because you installed the WebLogic distribution on your NT server. Even if the applet cannot successfully load the necessary classes from the HTTP server, it does find them in your local CLASSPATH. But when you try to run it from the Windows 95 client, the applet must load the classes over the wire from the HTTP server, and if you haven't installed them correctly, it will fail.

Clients that connect to a WebLogic Server cluster and look up a clustered object obtain a replica-aware stub for the object. This stub contains the list of available server instances that host implementations of the object. The stub also contains the load balancing logic for distributing the load among its host servers.

Your JDBC code may throw the following exception:
"The coordinator has rolled back the transaction.
No further JDBC access is allowed within this transaction."

The WebLogic JTS JDBC driver throws this exception when the current JDBC connection transaction rolls back prior to or during the JDBC call. This exception indicates that the transaction in which the JDBC connection was participating was rolled back at some point prior to or during the JDBC call.
The rollback may have happened in an earlier EJB invoke that was part of the transaction, or the rollback may have occurred because the transaction timed out. In either case, the transaction will be rolled back, the connection returned to the pool and the database resources released. In order to proceed, the JTS JDBC connection must be closed and reopened in a new transaction.

a. Close any open resources, like database connections
b. All non-transient, non-serializable fields(except some special types) should be set to null.
c. All transient fields should be set to null
d. Make all database connection reference fields transient
e. All primitive type fields should be set to null



Choices A, B and D are correct. When a bean is about to be passivated, its ejbPassivate() method is invoked, alerting the bean instance that it is about to enter the Passivated state. At this time, the bean instance should close any open resources and set all non transient, non serializable fields to null. This will prevent problems from occurring when the bean is serialized. Transient fields will simply be ignored.Serializable fields will be saved.Open resources such as sockets or JDBC connections must be closed whenever the bean is passivated. In stateful session beans, open resources will not be maintained for the life of the bean instance. When a stateful session bean is passivated, any open resource can cause problems with the activation mechanism.
A bean's conversational state may consist of only primitive values, objects that are serializable, and the following special types-SessionContext, EJBHome, EJBObject, UserTransaction and Context (only when it references the JNDI ENC) . The types in this list (and their subtypes) are handled specially by the passivation mechanism. They don't need to be serializable; they will be maintained through passivation and restored automatically to the bean instance when it is activated

Setting up your CLASSPATH correctly depends on what you are trying to do. The most common tasks are described below:
* Starting WebLogic Server. See Setting the Classpath Option in the Starting and Stopping WebLogic Servers section of the Administration Guide. In addition, your WebLogic distribution includes shell scripts that you can use to start the server. These scripts, which are located in the domain directories under the config directory of your WebLogic Server distribution, automatically set up the CLASSPATH variable in the shell before starting the server.

For instance, I am deploying an EJB and a resource adapter (.rar), the EJB has no dependencies on the .rar because the EJB is writing to the common client interface (CCI). The EJB client application has sends/marshals as parameter classes that are defined in the .rar. For some reason the EJB's class loader hierarchy cannot find the definition of this .rar-specific class, even though the .rar is deploying successfully. I receive the following error on the EJB client:
java.rmi.UnmarshalException: error unmarshalling arguments; nested
exception
is:
java.lang.ClassNotFoundException:
com.mycompany.InteractionSpecImpl



When you pass an instance of com.myclientcompany.server.eai.InteractionSpecImpl as an argument to your EJB, the appServer needs to de-serialize (unmarshal) the object under the EJB context, and it needs the required class for unmarshalling, inside the ejb-jar(raTester.jar). So if you include the interactionspecimpl class in your ejb-jar file, then you do not need to include those classes in your server's classpath.

No, there's no way to allow a request to wait for a pool connection, and from the system point of view there should not be. Each requests that waits for a connection ties up one of the fixed number of execute threads in the server, which could otherwise be running another server task. Too many waiting requests could tie up all of the execute threads and freeze the server.

Each instance of MS SQL Server must be listening on a different port. So, you can use the port number in the properties that you pass to the getConnection() method or, in case of connection pools, you can specify the port property in the following properties:
server=machineName
port=instancePort

To find the port number where each MS SQL Server instance is running, run the server network utility (in the Microsoft SQL Server program group), select the server instance, select TCP/IP, and click the properties button.

You may receive the following error message when using your JDBC driver for Oracle: "The ordinal 40 could not be loaded in the dynamic link library OCIW32.dll." This problem is caused by an out-of-date version of OCIW32.DLL in your system directory. Some programs install this file in the system directory in order to run. If you remove this file from the system directory you should no longer receive this error.

What transaction isolation levels does the WebLogic jDriver for Oracle support?
Your servlet application may use Oracle Thin Drivers to access a database that includes BLOB fields. If you install and try to use WebLogic jDriver for Oracle and the same code fails and produces an exception similar to the following:

com.roguewave.jdbtools.v2_0.LoginFailureException:
TRANSACTION_READ_UNCOMMITTED isolation level not allowed
The Stack Trace:
com.roguewave.jdbtools.v2_0.LoginFailureException:
TRANSACTION_READ_UNCOMMITTED isolation level not allowed
at
com.roguewave.jdbtools.v2_0.jdbc.JDBCServer.createConnection
(JDBCServer.java :46)
at com.roguewave.jdbtools.v2_0.ConnectionPool.getConnection_
(ConnectionPool.jav a:412)
at com.roguewave.jdbtools.v2_0.ConnectionPool.getConnection
(ConnectionPool.java :109)


Setting the Isolation_level to 1 in the code that calls the RogueWave JDBCServer class works with the Oracle thin driver but fails with WebLogic jDriver for Oracle.

WebLogic jDriver for Oracle supports the following transaction isolation levels:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

According to the Oracle documentation, the Oracle DBMS only supports these two isolation levels. Unlike other JDBC drivers, WebLogic's drivers throw an exception if you try to use an isolation level that is unsupported. Some drivers silently ignore attempts to set an unsupported isolation level. WebLogic suggests testing whether the Oracle thin driver is not just ignoring settings for unsupported isolation events.

WebLogic jDriver for Oracle always returns a Java object that preserves the precision of the data retrieved. WebLogic jDriver for Oracle returns the following from the getObject() method:

* For columns of types NUMBER(n) and NUMBER(m,n): a Double is returned if the defined precision of the column can be represented by a Double; otherwise BigDecimal is returned.

* For columns of type NUMBER: Because there is no explicit precision, the Java type to return is determined based on the actual value in each row, and this may vary from row to row. An Integer is returned if the value has a zero-valued fractional component and the value can be represented by an integer.

For example, 1.0000 will be an integer. A long is returned for a value such as 123456789123.00000. If a value has a non-zero fractional component, a Double is returned if the precision of the value can be represented by a Double; otherwise a BigDecimal is returned.

Here is what we use that works:
CallableStatement cstmt = conn.prepareCall("Begin procName;
END;");
cstmt.execute();

where procName is the name of an Oracle stored procedure. This is standard Oracle SQL syntax that works with any Oracle DBMS. You might also use the following syntax:

CallableStatement cstmt = conn.prepareCall("{call procName};");
cstmt.execute();

This code, which conforms to the Java Extended SQL spec, will work with any DBMS, not just Oracle.

To find out what codesets you currently have available in Oracle, execute the following SQL query from SQLPlus at the command line:

SQL> SELECT value FROM v$nls_valid_values WHERE parameter='CHARACTERSET';

The response lists of all codesets currently installed on your system. This listing will look something like the following shortened list:
VALUE
---------------
US7ASCII
WE8DEC
WE8HP
US8PC437
WE8EBCDIC37
WE8EBCDIC500
WE8EBCDIC285
...

If you want to constrain the value in the query to a specific codeset you are searching for, you might use a SQL query like the following:
SQL> SELECT value FROM v$nls_valid_values
WHERE parameter='CHARACTERSET' and VALUE='AL24UTFFSS';

This would produce the following response if the codeset is installed:

VALUE
-------------------
AL24UTFFSS

You can use Oracle's installation tools to install additional codesets. Contact Oracle for more information.

This error occurs when you have not set the ORACLE_HOME environment variable properly — a common mistake. In order to use WebLogic jDriver for Oracle, the Oracle client software needs to be installed and ORACLE_HOME must be set.
You may also see this error message if you try to use WebLogic jDriver for Oracle's internationalization capabilities with a language/codeset combination that is not installed on your system. If you get the ORA-12705 error with the correct error text, then either you have set NLS_LANG improperly, or you do not have the right codesets installed on your system.

When you are testing your Oracle database connections under UNIX, you can run SQL*PLUS and can successfully ping the database using utils.dbping. However, when you use the multitier utils.t3dbping utility, you receive an ORA-12154 error message.

First, make sure that your ORACLE_HOME environment variable is correctly set to point to your Oracle installation. This variable must be set in the environment where the WebLogic server is running.

In the C-shell issue the following command:
$ setenv ORACLE_HOME path

where path is the path to your Oracle installation.
In the Bourne shell, issue the following commands:
$ ORACLE_HOME=path
$ export ORACLE_HOME


where path is the path to your Oracle installation. When you ping your database using the two-tier utils.dbping utility, the JDBC driver loads the database client library and establishes the connection to the database. When you use the multitier utils.t3dbping utility, the WebLogic Server loads a two-tier driver and uses it to establish a database connection. In both cases, the same method is used to connect to the database. SQL*PLUS works because it doesn't require ORACLE_HOME to find the client libraries.

If you are still experiencing problems, try this:
1. Open a command shell.
2. Run the two-tier version of utils.dbping in this shell.
3. Start WebLogic in this shell from the command line:
$ java -ms32m -mx32m weblogic.server
4. Open a second command shell.
5. Run the utils.t3dbping in the second shell against the server running in the first command shell.
If this procedure doesn't work, please send the output from these commands to WebLogic technical support.

This is an Oracle issue. According to Oracle's documentation, dynamic cursors can remain open from run to run in a session and are not closeable when a procedure closes. To work around this issue, you can increase the number of open cursors allowed in the database or you can reset the connection pool (close and reopen database connections in the connection pool).
To reset the connection pool, you can untarget and retarget the connection pool using the Administration Console. You can also use the reset() method through the JMX API or the RESET_POOL command on the WebLogic Server command line interface.

No, this is not supported.
* Write your own interfaces using JNI.
* Setup a Servlet that your C/C++ client calls to generate a JMS message. You should spawn multiple threads in C++ and use multiple posts to pass messages via http.

The correct way to set up security for JMS is to go to the console, select ACLs in the tree view, then create some access control lists.
1. Set the ACL name which should be weblogic.jms.queue.QUEUENAME or weblogic.jms.topic.TOPICNAME.
2. Select Create.
3. Enter the New Permission of send or receive.
4. Select Create.
5. Enter a comma separated list of users or groups.
6. Select Grant Permission.
7. Select "saved to the realm implementation" to save your changes.
8. Select Yes.
This will update the fileRealm.properties file with lines that look like the following:
acl.send.weblogic.jms.queue.TestQueue1=user1
acl.receive.weblogic.jms.queue.TestQueue1=user1
If you don't have an ACL for a queue or topic, security is wide open.
There are also ACL's for accessing the JNDI context; the JNDI context is a requirement for initially accessing JMS. See the JNDI documentation.

In WLS 5.1 createTopic() or createQueue() creates the destination permanently in the database if it doesn't already exist, but does not modify the weblogic.properties file.
According to the JavaSoft JMS specification version 1.0.2 regarding createQueue() and createTopic(), they are not for creating destinations dynamically. They are used to retrieve the destination referenced by using a string name instead of using JNDI lookup. The destination has to be in your config.xml file first. This change is documented in WLS 6.0 since it behaves differently than the previous release. You can use the WLS JMS helper class (weblogic.jms.extensions.JMSHelper) or the console to create destinations at the run time (note that there was a bug in 6.0 that caused a problem when the server restarted; this is fixed in Service Pack 1). These mechanisms create the destination and also modify the configuration file.
For more information on the JMSHelper classes, see the subsection called Creating Destinations Dynamically in Programming WebLogic JMS.

The following program creates a Topic.

import java.io.*;
import java.util.Hashtable;
import javax.jms.*;
import javax.naming.*;
import weblogic.jms.extensions.JMSHelper;


class t {
public final static String
JNDI_FACTORY="weblogic.jndi.WLInitialContextFactory";
public final static String JMS_SERVER_NAME="TestJMSServer";
public final static String DEST_JNDI_PREFIX="javax.destination.";


static public void main(String [] args) throws Exception {
try {
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY, JNDI_FACTORY);
env.put(Context.PROVIDER_URL, "t3://localhost:7001");
Context ctx = new InitialContext(env);


String topicName = "JMSHelperTestQueue01";
String topicJNDI = DEST_JNDI_PREFIX + topicName;
System.out.println("topic name=" + topicName + ", jndi=" +
topicJNDI);
JMSHelper.createPermanentTopicAsync(ctx, JMS_SERVER_NAME,
topicName,
topicJNDI);
} catch (JMSException e) {
e.printStackTrace();
}
}
}

You must create a template on every JMSServer where you want to be able to create temporary destinations. You can specify multiple JMSServer entries to support TemporaryTemplate and the system will load balance among those JMS servers to setup the temporary destination. See How do I start WLS and configure JMS? for a description about how to configure JMS. The resulting template definition looks something like the following:

<JMSTemplate Name="MyTemplate"/>

The JMSServer is defined something like:

<JMSServer Name="MyJMSServer" TemporaryTemplate="MyTemplate" Targets="MyServer" >

After the template name, you can set any queue/topic attribute you want in the template (not including a JNDI name or topic multicast settings). The template is at the outer most level; that is, it should not be nested in your <JMSServer>.

Temporary destinations can only be consumed by the creating connection. Using topics, you create your temporary topic and subscribe to that temporary topic. If you want someone to publish to that temporary topic, you need to tell that someone what your topic is. You can send them a message and include your temporary topic in the JMSReplyTo field. The creator of the TemporaryTopic and the subscriber must be one in the same.

import javax.jms.TopicSession;
TemporaryTopic myTopic = mySession.createTemporaryTopic();
TopicSubscriber = mySession.createSubscriber(myTopic);

Temporary topics do not get names and cannot be subscribed to by other connections. When you create a temporary topic, the JMS provider returns a javax.jms.Topic. You then need to advertise that topic to other parties (those who want to publish to the topic), putting it in your JMSReplyTo field so that they can respond. In general, no one else can subscribe to the topic. You advertise the topic any way you want. Topics are serializable (or, in our case, externalizable), which allows you to pass them around in RMI calls, through a file, binding it to a name in JNDI, etc. In short, create the topic at the subscriber side and advertise so that others can publish. You can get multiple subscribers on the same connection and get concurrent processing using multiple sessions.

The JMS database can be any database that is accessible through a JDBC driver. WebLogic supports and provides JDBC drivers for the following databases:
* Cloudscape
* Informix
* Microsoft SQL (MSSQL) Server (Versions 6.5 and 7)
* Oracle (Version 8.1.6)
* Sybase (Version 12)

The Multicast TTL setting for a cluster in the WebLogic Admin console sets which of the following values?
a. Maximum time taken for multicast messages to reach their final destination
b. The number of routers a multicast message can pass through before the packet can be discarded
c. The multicast address to be used by the messages sent from the cluster
d. Minimum time taken for broadcasting a multicast message from the cluster


Choice B is correct. The Multicast TTL(TTL-Time to Live) setting specifies the number of routers a multicast message can pass through before the packet can be discarded. To configure the multicast TTL for a cluster, you should change the Multicast TTL value in the WebLogic Server administration console. This sets the number of network hops a multicast message makes before the packet can be discarded.
If you choose to distribute a cluster over a WAN (or across multiple subnets), you must plan and configure your network topology to ensure that multicast messages are reliably transmitted to all servers in the cluster. One of the requirements to be met by the network is that the multicast Time To Live (TTL) value must be high enough to ensure that routers do not discard multicast packets before they reach their final destination.

Use the following guidelines:
1. Make sure the JMSServer you are using has a store configured. The JMSServer configuration entry in the config.xml file should contain a line of the form

Store="<YOUR-STORE-NAME>"

Note that if JMS boots without a store configured, it is assumed the customer did not want one, and persistent messages are silently downgraded to non-persistent (as specified for JMS 1.0.2).

2. Make sure you are not using "Message.setJMSDeliveryMode". This is overwritten, as it is a vendor-only method.

3. Make sure you are calling either:

QueueSender.send(msg, deliveryMode, ...)

-- or --

QueueSender.setDeliveryMode(deliveryMode)

-- or --

set the DefaultDeliveryMode mode on connection factory in the config.xml file to persistent (the QueueSender.setDeliver/send overrides this value). Similarly, for topics, you would set this via the TopicPublisher.

4. Make sure you don't have "DeliveryModeOverride" set to Non-Persistent on the Destination in the config.xml file.

5. If you are using pub/sub, only durable subscriptions persist messages. Non-durable subscriptions have no need to persist messages, as by definition they only exist for the life of the server.

6. If you are using JDBC, the JDBC tables, JMSSTATE and JMSSTORE, are created automatically when the JMS server boots. The DDL files used to create the tables are stored in weblogic.jar in weblogic/jms/ddl. The example configuration below shows a JDBC store for Oracle (client version 8.1.7 or later is needed to run with WLS 6.1 on JDK 1.3). To manually create the tables (also deleting any existing tables), run java utils.Schema as described in the previous question.

See the question, "How do I start WLS and configure JMS?" for a description of how to configure JMS.

Here is a sample config.xml file resulting from configuring JMS. It should look similar to yours. If you want JMS to use a file store instead of a database, just change JDBCStore to FileStore in the JMSServer section.

<Server Name="myserver"
ListenPort="7001" DefaultProtocol="t3"
ThreadPoolSize="8" >
</Server>
<Security Realm="defaultRealm"
GuestDisabled="false" />
<Realm Name="defaultRealm"
FileRealm="defaultFileRealm" />
<FileRealm Name="defaultFileRealm"
/>
<JMSServer Name="TestJMSServer"
TemporaryTemplate="TestTemplate1"
Targets="myserver" Store="JDBCStore">
<JMSQueue Name="TestQueue1"
JNDIName="jms.queue.TestQueue1"
Template="TestTemplate1"
/>
</JMSServer>
<JMSTemplate Name="TestTemplate1"
/>
<JMSFileStore Name="FileStore"
Directory="myfilestore"
JMSServer="TestJMSServer"
/>
<JMSJDBCStore Name="JDBCStore"
ConnectionPool="testpool2"
JMSServer="TestJMSServer"
/>
<JDBCConnectionPool Name="testpool2"
Targets="myserver"
URL="jdbc:weblogic:oracle"
DriverName="weblogic.jdbc.oci.Driver"
InitialCapacity="0"
MaxCapacity="1"
CapacityIncrement="1"
Properties="user=SCOTT;password=tiger;server=bay816"
/>
</Domain>

The following is a sample class that sends
a Topic message on construction:

import javax.naming.*;
import javax.jms.*;
import java.util.Hashtable;

public class t

{
public final static String DESTINATION="jms.topic.TestTopic1";

private TopicConnectionFactory connectionFactory;
private TopicConnection connection;
private TopicSession session;
private TopicPublisher producer;
private TextMessage message;
private Topic destination;

public t()
{
try {
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY,
"weblogic.jndi.WLInitialContextFactory");
env.put(Context.PROVIDER_URL, "t3://localhost:7001");
InitialContext ctx = new InitialContext(env);
destination = (Topic) ctx.lookup(DESTINATION);
connectionFactory = (TopicConnectionFactory)
ctx.lookup("javax.jms.TopicConnectionFactory");
connection = (TopicConnection)
connectionFactory.createTopicConnection();
session = (TopicSession) connection.createTopicSession(false,
Session.AUTO_ACKNOWLEDGE);
producer = (TopicPublisher) session.createPublisher(destination);
producer.setDeliveryMode(DeliveryMode.PERSISTENT);
message = (TextMessage) session.createTextMessage();
message.setText("hello world");
producer.publish(message);
} catch (Exception e) {
}
}
}