BEA Weblogic Interview Questions and Answers

1. What is BEA Weblogic?

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

2. Why is there no polymorphic-type response from a create() or find() method?

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.

3. Can I use a native two-tier driver for a browser applet?

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.

4. I tried to run two of the applets in the examples directory of the distribution?

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.

5. How do stubs work in a WebLogic Server cluster?

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.

6. Why did my JDBC code throw a rollback SQLException?

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.

7. Which of the following are recommended practices to be performed in the ejbPassivate() method of a stateful session bean?

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

8. How do I set up my CLASSPATH?

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.

9. When deploying a resource adapter (.rar) to WebLogic Server, are its classes placed in the WebLogic classpath?

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.

10. Can I enable requests to a JDBC connection pool for a database connection to wait until a connection is available?

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.

11. How do I connect to an SQL Server instance that is running on a machine with multiple instances of SQL Server 2000?

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.

12. What causes an OCIW32.dll error?

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.

13. What type of object is returned by ResultSet.getObject()?

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.

14. How do I call Oracle stored procedures that take no parameters?

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.

15. How do I learn what codesets are available in 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.

16. Why do I get an error while trying to retrieve the text for ORA-12705?

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.

17. How do I prevent errors when running t3dbping?

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.

18. Why am I getting an ORA-01000: maximum open cursors exceeded error, even though I closed all ResultSet, Statement, and Connection objects?

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.

19. Are there C/C++ interfaces to WLS JMS?

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.

20. How do I configure JMS security?

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.

21. Why does JMSSession.createTopic or JMSSession.createQueue fail to create a destination in WLS JMS 6.1 (it worked in 5.1)?

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();
}
}
}

22. How do I use a temporary destination?

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:

The JMSServer is defined something like:

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.

23. Which types of JDBC databases does WebLogic JMS support?

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)

24. The Multicast TTL setting for a cluster in the WebLogic Admin console sets which of the following values?

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.

25. How do I use persistence?

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.

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) {
}
}
}
26. How do the WLS JMS 6.1 server/destination message maximum and threshold values work?

The byte and message maximum values are quotas – not flow control. Message quotas prevent a WebLogic JMS server from filling up with messages and possibly running out of memory, causing unexpected results. When you reach your quota, JMS prevents further sends with a ResourceAllocationException (rather than blocking). You can set quotas on individual destinations or on a server as a whole.

The thresholds are also not flow control – though they would be better suited to that application than the quotas. The thresholds are simply settings that when exceeded cause a message to be logged to the console to let you know that you are falling behind.

Note that the messages maximum setting on a connection factory is not a quota. This specifies the maximum numbers of outstanding messages that can exist after they have been pushed from the server but before an asynchronous consumer has seen them; it defaults to a value of 10.

27. Does WebLogic JMS support clustering?

WebLogic JMS supports cluster-wide, transparent access to destinations from any server in the cluster. A system administrator can establish cluster-wide, transparent access to destinations from any server in the cluster by configuring multiple connection factories and using targets to assign them to WebLogic Servers. Each connection factory can be deployed on multiple WebLogic Servers.

The application uses the Java Naming and Directory Interface (JNDI) to look up a connection factory and create a connection to establish communication with a JMS server. Each JMS server handles requests for a set of destinations. Requests for destinations not handled by a JMS server are forwarded to the appropriate server.

You can configure multiple JMS servers on the various nodes in the cluster as long as you give them different names. You can assign destinations to the various JMS servers.

One problem to be aware of is the propagation delay in replicating entries in JNDI. If you have an MDB deployed on one node but reference a destination on another node, the deployment may fail with a javax.naming.NamingException exception. The problem occurs because the server is not synced up to the JNDI from the remote server (JMS server) yet, so the JNDI lookup of destination as part of MDB deployment will fail. One workaround is for each MDB to reference a local destination. Another approach is deploy the MDBs after the server boots (plus a delay for JNDI propagation). To get around losing messages before the MDB is deployed, use durable subscribers. This problem is fixed for MDBs in WLS 6.1, where the MDB will be deployed and reconnection will be retried until the destination is available. Note that this is still a problem for EJBs in general that try to reference a non-local JMS destination.

28. Which of the following statements are true regarding the identity of two EJBs?

a. Two stateful session beans are identical if their data attributes are identical.
b. Two stateful session beans are identical if their session contexts are equal.
c. Two stateless session beans are identical if they are of the same type.
d. Two stateless session beans are identical if their session contexts are equal.
e. Two entity beans are identical if they have same primary key but different home interface.
f. Two entity beans are identical if they have different primary key but same home interface.

B and C are correct.

Since the stateful session beans maintain the conversational state of the clients, they are identical when their session contexts are equal. Two stateful session beans may have identical data attributes, but if the session contexts are different they are not identical. Thus choice A is incorrect and B is correct. Since stateless beans do not retain the conversational state, they are considered identical if they are of the same type. Thus choice C is correct.

If two entity objects have the same home interface and primary key, they are considered identical. The EJB specification does not mention object equality based on the = = operator. Also, if you compare two object references using the Java API, Object.equals(Object obj), the result is unspecified. The only way to compare object equality is through the isIdentical (EJBObject) API. Thus choice E and F are incorrect.

29. In the WebLogic server, if stateless session bean instances are getting frequently created and removed, performance can improved by setting a high value for which of the following?

a. max-beans-in-free-pool
b. max-beans-in-cache
c. max-beans-in-memory
d. max-stateless-beans-in-cache

Choice A is correct.

WebLogic Server maintains a free pool of EJBs for every stateless session bean class. The max-beans-in-free-pool element defines the size of this pool. By default, max-beans-in-free-pool has no limit; the maximum number of beans in the free pool is limited only by the available memory.

When EJBs are created, the session bean instance is created and given an identity. When the client removes a bean, the bean instance is placed in the free pool. When you create a subsequent bean, you can avoid object allocation by reusing the previous instance that is in the free pool. So the max-beans-in-free-pool element can improve performance if EJBs are frequently created and removed. Keeping this parameter too high uses extra memory and keeping it too low causes unnecessary object creation.

WebLogic Server allows you to configure the number of active beans that are present in the EJB cache (the in-memory space where beans exist). The max-beans-in-cache element specifies the maximum number of objects of this class that are allowed in memory. When max-bean-in-cache is reached, WebLogic Server passivates some EJBs that have not been recently used by a client. Choices C and D are not valid properties.

30. What happens if acknowledge() is called within a transaction?

As per the JMS specification, when you are in a transaction, the acknowledgeMode is ignored. If acknowledge() is called within a transaction, it is ignored.

31. How should I use sorted queues?

Destination keys are used to define the sort order for a specific destination. Destination keys can be message header or property fields. For a list of valid message header and property fields, refer to the Programming WebLogic JMS.
Queues can be sorted in ascending or descending order based on the destination key. A destination is considered to be first-in-first-out if a destination key is defined as ascending for the JMSMessageID message header field, and last-in-first-out if defined as descending. The key defined for the JMSMessageID header field, if specified, must be the last key defined in the list of keys.

You can define multiple destination keys to sort a destination.

32. The MaxPostTimeSecs attribute set in the Administration console under Servers or virtual hosts section corresponds to which of the following?

a. The amount of time that WebLogic Server waits between receiving chunks of data in an HTTP POST.
b. The total amount of time that WebLogic Server spends receiving HTTP POST data.
c. The time spent by WebLogic server to post data to other servers in the cluster.
d. The number of bytes of data received in a POST from a single request.

Choice B is correct.

Web servers may face denial-of-service attacks, which is usually carried out by sending huge amounts of data in an HTTP POST method. You can set three attributes in WebLogic Server that help prevent this type of attack. These attributes are set in the console, under Servers or virtual hosts. You can limit the amount of time that WebLogic Server waits between receiving chunks of data in an HTTP POST by setting the attribute PostTimeoutSecs.

The MaxPostTimeSecs attribute limits the total amount of time that WebLogic Server spends receiving post data. If this limit is triggered, a PostTimeoutException is thrown and a message is sent to the server log. MaxPostSize attribute limits the number of bytes of data received in a POST from a single request. If this limit is triggered, a MaxPostSizeExceeded exception is thrown and a message is sent to the server log.

33. How do I get a thread dump to help track down a problem?

Ways to get a thread dump:

* Try running this from the command line (after running the setEnv script in /bea/wlserver6.1/config/mydomain/):

java weblogic.Admin -url t3://localhost:7001 THREAD_DUMP

* On Windows, from the console window, enter Ctrl+Break.
* On UNIX, signal the server using kill -3.

34. Why do I get an exception when trying to find a connection factory?

The exception is usually something like java.io.InvalidClassException or java.lang.NoClassDefFoundError.
Make sure weblogic.jar is in the CLASSPATH of the client. Also make sure you have the correct Java run-time jar files included (i.e., you might need rt.jar).

35. What is the NO_ACKNOWLEDGE acknowledge mode used for?

The NO_ACKNOWLEDGE acknowledge mode indicates that received messages do not need to be specifically acknowledged which improves performance, but risks that messages are lost. This mode is supported for applications that do not require the quality of service provided by session acknowledge and that do not want to incur the associated overhead. v Messages sent to a NO_ACKNOWLEDGE session are immediately deleted from the server. Messages received in this mode are not recovered and, as a result, messages may be lost and/or duplicate message may be delivered if an initial attempt to deliver a message fails.

Note: You should avoid using this mode if your application cannot handle lost or duplicate messages. Duplicate messages may be sent if an initial attempt to deliver a message fails.

In addition, we do not recommend that this acknowledge mode be used with persistent messaging, as it implies a quality of service that may be too low for persistent messaging to be useful.

36. When should I use server session pools and connection consumers?

WebLogic JMS implements an optional JMS facility for defining a server-managed pool of server sessions. This facility enables an application to process messages concurrently. A ConnectionConsumer object uses a server session to process received messages. If message traffic is heavy, the connection consumer can load each server session with multiple messages to minimize thread context switching. Multiple connection consumers can share server sessions in a server session pool.

To learn how to use the connection consumers within an application, see the section Processing Messages Concurrently in Programming WebLogic JMS, or the javax.jms.ConnectionConsumer javadoc.

Note: Server session pools can also be implemented using Message Driven Beans. Using MDBs is preferable to using server session pools – see the answer to the question, “How do server session pools and Message Driven Beans compare?” For information on using message driven beans to implement server session pools, see Programming WebLogic Enterprise JavaBeans.

37. How do I publish an XML message?

Follow these steps:

1. Generate XML from the DOM document tree.
2. Serialize the generated DOM document to a StringWriter.
3. Call toString on the StringWriter and pass it into message.setText.
4. Publish the message.

38. Is it possible to send or receive a message from within a message listener?

Yes. You can send to or receive from any queue or topic from within in a message listener.

If it’s not an MDB, you can use the same Connection or Session that the onMessage() is part of to do this. When you create your message listener, you pass in a session in your constructor. Then you have access to the session in your onMessage method and you would be able to make synchronous, not asynchronous, calls from within the onMessage method. Do not use another Session that is servicing another onMessage() because that would multi-thread that Session and Sessions don’t support multi-threading.

When things are done non-transactionally, there can be duplicates or lost messages (assuming your onMessage() code is attempting to forward messages):

1. If you call acknowledge after the publish() and the acknowledge fails for whatever reason (network/server failure), then you will see the message again and will end up publishing twice (possible duplicate semantics). You can try to keep track of sequence numbers to detect duplicates but this is not easy.

2. If you call acknowledge before the publish(), you get at-most-once semantics. If the publish() fails, you don’t know if the failure occurred before or after the message reached the server.

If you want exactly once, transactional semantics using onMessage, you must use transactional MDBs. The onMessage() for a transactional MDB starts the transaction, includes the WebLogic Server JMS message received within that transaction and the publish() would also be in the same transaction. The following code sends a response to each message that it receives. It creates the connection, etc. in the ejbCreate method so that it doesn’t need to create it every time onMessage is called. The QueueSender is anonymous (null Queue) since we don’t know to whom we will have to reply. The ejbRemove method cleans up by closing the connection. This same approach can be used to create a receiver, subscriber or publisher.

import javax.ejb.CreateException;
import javax.ejb.EJBContext;
import javax.naming.*;
import javax.naming.directory.*;
import java.util.Hashtable;
import javax.ejb.MessageDrivenBean;
import javax.ejb.MessageDrivenContext;
import javax.jms.*;

public class MDB
implements MessageDrivenBean, MessageListener {
public static final String WLSqcf =
“javax.jms.QueueConnectionFactory”;
public static final String WLSqname =
“jms.queue.TestQueue1”;
public static final String WLSurl =
“t3://localhost:7001”;
public static final String WLSJNDIfactory =
“weblogic.jndi.WLInitialContextFactory”;
private MessageDrivenContext context;
private QueueSession session;
private QueueConnection connection = null;
private QueueConnectionFactory factory;
private InitialContext ctx;
private QueueSender QueueSender;

// Required – public constructor with no argument
public MDB() {}

// Required – ejbActivate
public void ejbActivate() {}
// Required – ejbRemove
public void ejbRemove() {
context = null;
if (connection != null) {
try {
connection.close();
} catch(Exception e) {}
connection = null;
}
}

// Required – ejbPassivate
public void ejbPassivate() {}

public void setMessageDrivenContext(
MessageDrivenContext mycontext) {
context = mycontext;
}

// Required – ejbCreate() with no arguments
public void ejbCreate () throws CreateException {
try {
// Get the initial context
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY, WLSJNDIfactory);
env.put(Context.PROVIDER_URL, WLSurl);
env.put(Context.REFERRAL, “throw”);
ctx = new InitialContext(env);

factory = (QueueConnectionFactory)ctx.lookup(WLSqcf);

// Create a QueueConnection, QueueSession, QueueSender
connection = factory.createQueueConnection();
session = connection.createQueueSession(false,
Session.AUTO_ACKNOWLEDGE);
queueSender = session.createSender(null);
connection.start();
} catch (Exception e) {
throw(new CreateException(e.toString()));
}
}

// Implementation of MessageListener
// Throws no exceptions
public void onMessage(Message msg) {
try {
System.out.println(“MDB: ” +
((TextMessage)msg).getText());
msg.clearBody();
((TextMessage)msg).setText(“reply message”);
queueSender.send((Queue)msg.getJMSReplyTo(), msg);
}
catch(Exception e) { // Catch any exception
e.printStackTrace();
}
}
}

This approach creates a connection per EJB/MDB instance, so you might want to create a producer pool that is shared by the EJB instances. This is done by writing a class that populates a static pool with producers (see the next question for a sample producer pool). The onMessage call grabs a producer when needed. Since Sessions must be single threaded, make sure there is only one producer per session within the producer pool.

39. What are pending messages in the console?

Pending means the message could have been:

* sent in a transaction but not committed.
* received and not acknowledged.
* received and not committed.
* subject to a redelivery delay (as of WebLogic Server 6.1).
* subject to a delivery time (as of WebLogic Server 6.1).

A rolled back message remains pending until the transaction actually rolls back. Rolling it back multiple times does not cause double counting, nor does an exception that set a transaction as rollbackOnly followed by an actual rollback.

Current implies messages that are not pending.

Total implies total since server last started. The byte counts only consider the payload of messages which includes the properties and the body but not the header.

40. Is it better to have more or fewer sessions for a given number of subscribers?

Using N sessions for N subscribers gives you concurrency up to N simultaneous threads of execution provided you have as many threads to work with. Each Session gets its own thread as long as there are enough threads available. Otherwise, the sessions serially reuse the available threads.

One session for N subscribers serializes all subscribers through that one session. If the load is heavy they may not be able to keep up without the extra threads.

If you are using CLIENT_ACKNOWLEDGE, N sessions gives you N separate message streams that can be individually recovered. Having one session crosses the streams giving you less control.

41. A client invokes a method on a stateful session bean instance deployed in the WebLogic Server. While the method execution is in progress another method call arrives on the server. What will be the result?

a. RemoteException is thrown if the value of concurrency-strategy property is false
b. EJBException is thrown if the value of concurrency-strategy property is false
c. The EJB container blocks the concurrent method call and allows it to proceed when the previous call has completed if the value of allow-concurrent-calls is true
d. In all cases, RemoteException is thrown

Choice C is correct.

By default, simultaneous access to a stateful session EJB results in a RemoteException. However, you can set the allow-concurrent-calls option in the WebLogic EJB deployment descriptor to specify that a stateful session bean instance will allow concurrent method calls. This access restriction on stateful session EJBs applies whether the EJB client is remote or internal to WebLogic Server. By default, allows-concurrent-calls is false. However, when this value is set to true, the EJB container blocks the concurrent method call and allows it to proceed when the previous call has completed.

The concurrency-strategy element determines ejbLoad() and ejbStore() behavior for entity EJB instances.

42. What is the standard way to create threads, do initialization, etc. within the application server?

Threads should generally not be created by the user directly is because things may not work correctly. User-created threads do not have some of the thread-local variables pre-set by WebLogic when it creates it’s own execute threads, the associated transaction context, or the environment such as the proper class loader. The WebLogic-specific way of doing this is with a startup class or using the WebLogic Time Services.

The portable way to do this is to define a load-on-startup servlet, doing the initialization in the init() method and the cleanup in the destroy() method. The servlet itself does nothing. This approach also allows for undeploy/redeploy of the application without restarting the server, including proper cleanup/initialization each time. It also providers more dynamic management of the dependent classes without restarting the server.

43. What should an XPATH selector look like?

The following is an example of an XPATH selector. Pay careful attention to the use of double and single quotes.

String selector =
“JMS_BEA_SELECT(‘xpath’, ‘/recipient/transport/text()’) =
’email’”;
tsubscriber = tsession.createSubscriber(topic, selector, false);
JMS_BEA_SELECT is a built-in function in WebLogic Server JMS SQL syntax. You put it in your selector string when you create a consumer. Note the use of single quotes around xpath, the XML tab, and the string value.

44. How do I put a message back on the queue for processing?

The following are several approaches:

* Use a transacted session, then rollback the session so the message will go back to the queue.
* Use Session.CLIENT_ACKNOWLEDGE when creating a session, then recover the session so the message will go back to the queue.
* Use a JTA transaction, then rollback the transaction so the message will go back to the queue.

45. What can I do when I get java.lang.OutOfMemoryError because producers are faster than consumers?

Quotas can be used to help this situation. Your sender will then receive ResourceAllocationExceptions and the server will stay up. WLS 6.X does not support paging of messages out of memory.

As of WLS 6.1 SP02 or later, you can use the Message Paging feature, which can free up valuable virtual memory during peak message load periods by swapping out messages from virtual memory to persistent storage when message loads reach a specified threshold.

46. How do I debug WebLogic Server using Visual Cafe 4.1?

You can install VisualCafe Enterprise Edition 4.1 and attach it to the server, pretty much as it worked for 3.X.

The following are steps for debugging using VC 4.1. You may change the directory names as necessary.

1. Install it under D:\VisualCafeEE. No special options are needed.
2. Install the license under C:\Program Files\Common Files\WebGain Shared.
3. Start ddservices by selecting Start -> Programs -> WebGain Studio Professional ->Visual Cafe Enterprise Edition 4.1 -> Distributed Debugging Services -> Start DD Services (Java2 – 1.3)
4. Start WebLogic Server using debugvm.exe instead of java.exe.
cd D:\bea\wlserver6.1\config\mydomain
setEnv
edit startWebLogic.cmd
change “%JAVA_HOME%\bin\java” -hotspot -ms64m -mx64m to \visualcafeEE\jdk13\bin\debugvm.exe
5. Run startWebLogic. It prints out some debugging information.
6. Run VisualCafe – Start -> Programs -> WebGain Studio Professional -> Visual Cafe Enterprise Edition 4.1 -> Visual Cafe Enterprise Edition 4.1
7. From the File menu, select Attach to Process. If everything is working correctly, you should see your machine name.
8. Click the + sign to expand the tree and select your running WebLogic Server.

47. How does concurrency work for message-driven beans?

The way concurrency is achieved for Queues is by spawning one JMSSession per MDB instance in the pool. Since JMSSessions are processed in parallel by JMS, concurrency is obtained naturally this way and JMS takes care of delivering the message to, at most, one listener. If an MDB is deployed to multiple servers in a cluster, JMSSessions are created for each MDB instance on each server and load balancing will be done across them.

For Topics in WebLogic JMS 6.1, there is one JMSSession per bean instance in the pool. Because of the way Topics work, the session, and thus every bean instance, receives a copy of each message published on that Topic. (There was also a problem that caused parallel processing not to work correctly. This has been fixed for WLS 6.0 Service Pack 1.) Within a single server, one topic consumer is used to pass out messages to multiple threads to get the concurrency while producing only a single copy of each message. You can configure multiple MDBs to listen on the same topic and each MDB will receive a copy of every message. When using multiple servers, each server gets its own consumer and therefore its own copy of each message. It is not currently possible to share a consumer across multiple servers. If you want a message to be processed by exactly one MDB, use a queue.

One customer had an example where topic MDBs are needed in which there will be multiple implementations of the MDBs listening on the same topic. In other words, more than one MDB with a different implementation may be subscribing to the same topic. The client has no advanced way of knowing how many different kinds of MDBs may be listening on the same topic, but it is possible for there to be more than one listener, therefore topics, not queues. For each kind of MDB listening on the topic, the message is delivered exactly once (i.e., the message will be delivered exactly once to an instance in each named MDB pool listening on the topic).

48. Can an MDB be a message producer or both a producer and consumer?

Yes. You have no JMS context inside the MDB so you will need to establish a connection, session and producer yourself. One option is to do this every time you come into the onMessage routine for the MDB. This is sufficient if the message rate is relatively low.

The second option is to establish the necessary objects in ejbActivate(). Note that the objects are not serializable so they can’t be passivated for a stateful session bean or an entity bean. When the EJB deactivates, you need to close the associated objects. The third option is that you could build up a JMS connection/sender session pool within a startup class complete with your own synchronization and blocking to get a connection. There is an example in the answer to the question “Is it possible to send or receive a message from within a message listener?”

49. How do I use JTA transactions within an MDB?

In the ejb-jar.xml file, define the transaction type as Container and the trans-attribute as Required, as in the following example:

<ejb-jar>
<enterprise-beans>
<message-driven>
<ejb-name>MDB</ejb-name>
<ejb-class>MDB</ejb-class>
<transaction-type>Container</transaction-type>
<message-driven-destination>
<destination-type>javax.jms.Queue</destination-type>
</message-driven-destination>
</message-driven>
</enterprise-beans>
<assembly-descriptor>
<container-transaction>
<method>
<ejb-name>MDB</ejb-name>
<method-name>*</method-name>
</method>
<trans-attribute>
Required
</trans-attribute>
</container-transaction>
</assembly-descriptor>
</ejb-jar>

To rollback a transaction, you can either use the WebLogic extension TXHelper or you can use the MDB context as in the following code examples:

UserTransaction ut =
weblogic.transaction.TXHelper.getUserTransaction();
ut.setRollbackOnly();
or
private MessageDrivenContext context;
public void setMessageDrivenContext(
MessageDrivenContext mycontext) {
context = mycontext;
}
public void onMessage(Message msg) {
try { // some logic
}
catch(Exception e) {
System.out.println(“MDB doing rollback”);
context.setRollbackOnly();
}

50. Can the messaging bridge handle two-phase or global transactions between separate WebLogic Server domains or between different releases?

Yes, as long as the communication is between source and target WebLogic domains that are both running release 6.1 SP3 or later, and the bridge is configured to use the Exactly-once quality of service.

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