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Programming WebLogic JNDI

# WebLogic JNDI

The following sections describe programming with WebLogic JNDI including:

- Using WebLogic JNDI to Connect a Java Client to a Single Server
- Setting Up JNDI Environment Properties for the InitialContext
- Using the Context to Look Up a Named Object
- Using a Named Object to Get an Object Reference
- Closing the Context
- Using WebLogic JNDI in a Clustered Environment
- Using JNDI from Within J2EE Components
- Viewing the JNDI Tree from Within the WebLogic Server Administration Console

# Using WebLogic JNDI to Connect a Java Client to a Single Server

The WebLogic Server JNDI Service Provider Interface (SPI) provides an `InitialContext` implementation that allows remote Java clients to connect to WebLogic Server. You can specify standard JNDI environment properties that identify a particular WebLogic Server deployment and related connection properties for logging in to WebLogic Server.

Before you can use JNDI to access an object in a WebLogic Server environment, you must load or bind, the object into the WebLogic Server JNDI tree. To load an object into the JNDI tree, do the following:

1. Choose a name under which you want the object to appear in the JNDI tree.
2. Enter that name in the JNDI Name attribute field when you create the object. When the object is loaded, JNDI provides a path to the object.

For instructions on viewing objects in the JNDI tree, see Viewing the JNDI Tree from Within the WebLogic Server Administration Console.

To interact with a WebLogic Server instance, a Java client must be able to get an object reference for a remote object which may be on the same server as the WebLogic Server instance, on another server, or in a different location, and invoke operations on the object. To accomplish this, you must perform the following procedure from within the client application:

1. Set up JNDI environment properties for the `InitialContext`.
2. Establish an `InitialContext` with WebLogic Server.
3. Use the Context to look up a named object in the WebLogic Server namespace (domain structure).
4. Use the named object to get a reference for the remote object and invoke operations on the remote object.
5. Close the context.

The following sections discuss JNDI client operations for connecting to a specific WebLogic Server. For information about using JNDI in a cluster of WebLogic Servers, see Using WebLogic JNDI from a Client in a Clustered Environment on page 2-15.

Note: Binding objects built with WebLogic Server 6.1 classes and deploying them to WebLogic Server 7.0 or higher is not supported and can result in failures during startup.

# Setting Up JNDI Environment Properties for the InitialContext

The first task you must perform in your Java client application is to set the environment properties. The `InitialContext` factory uses various properties to customize the `InitialContext` for a specific environment. You set these properties either by using a hashtable or the `set()` method of a WebLogic Environment object. These properties, which are specified name-to-value pairs, determine how the `WLInitialContextFactory` creates the Context.

The following properties are used to customize the `InitialContext`:

- `Context.PROVIDER_URL`— specifies the URL of the WebLogic Server that provides the name service. The default is `t3://localhost:7001`.

Note: For initial context requests over T3, using the default channel, `Context.PROVIDER_URL` may be the IP address of a load balancer.

- `Context.SECURITY_PRINCIPAL`—specifies the identity of the User (that is, a User defined in a WebLogic Server security realm) who will be accessing the JNDI tree, for authentication purposes. The property defaults to the `guest` User unless the thread has already been associated with a WebLogic Server User. For more information, see Associating a WebLogic User with a Security Context.
- `Context.SECURITY_CREDENTIALS`—specifies either the password for the User defined in the `Context.SECURITY_PRINCIPAL` property or an object that implements the `weblogic.security.acl.UserInfo` interface with the `Context.SECURITY_CREDENTIALS` property defined. If you pass a UserInfo object in this property, the `Context.PROVIDER_URL` property is ignored. The property defaults to the `guest` User unless the thread has already been associated with a User. For more information, see Associating a WebLogic User with a Security Context.

You can use the same properties on either a client or a server. If you define the properties on a server-side object, a local Context is used. If you define the properties on a client or another WebLogic Server, the Context delegates to a remote Context running on the WebLogic Server specified by the `Context.PROVIDER_URL` property. A remote object bound to the server will not be serviced by peerGone, and will not be reachable if the client should fail.

There are some properties that cannot be changed after the creation of the context. These properties include provider url, user credentials, and factories. AddToEnvironment can be used to change other properties after the creation of the context.

Additional WebLogic-specific properties are also available for controlling how objects are bound into the cluster-wide JNDI tree. Bindings may or may not be replicated across the JNDI tree of each server within the cluster due to the way these properties are set. Properties such as these are identified by constants in the `weblogic.jndi.WLContext` class. For more information about JNDI-related clustering issues, see Using WebLogic JNDI from a Client in a Clustered Environment.

## Creating a Context Using a Hashtable

You can create a Context with a hashtable in which you have specified the properties described in Setting Up JNDI Environment Properties for the InitialContext.

To do so, pass the hashtable to the constructor for `InitialContext`. The property `java.naming.factory.initial` is used to specify how the `InitialContext` is created. To use WebLogic JNDI, you must always set the `java.naming.factory.initial` property to weblogic.jndi.WLInitialContextFactory. This setting identifies the factory that actually creates the Context.

Listing 2-1 shows how to obtain a Context using the properties `Context.INITIAL_CONTEXT_FACTORY` and `Context.PROVIDER_URL`.

Listing 2-1 Obtaining a Context

>       Context ctx = null;
>       Hashtable ht = new Hashtable();
>       ht.put(Context.INITIAL_CONTEXT_FACTORY,
>              "weblogic.jndi.WLInitialContextFactory");
>       ht.put(Context.PROVIDER_URL,
>              "t3://localhost:7001");

>       try {
>         ctx = new InitialContext(ht);
>         // Use the context in your program
>       }
>       catch (NamingException e) {
>         // a failure occurred
>       }
>       finally {
>         try {ctx.close();}
>      catch (Exception e) {
>           // a failure occurred
>         }
>       }

## Creating a Context Using a WebLogic Environment Object

You can also create a Context by using a WebLogic environment object implemented by weblogic.jndi.environment. Although the environment object is WebLogic-specific, it offers the following advantages:

- A set of defaults which reduces the amount of code you need to write.
- Convenience `set()` methods that provide compile-time type-safety. The type-safety `set()` methods can save you time both writing and debugging code by throwing an exception if you attempt to use an incorrect data type..

The WebLogic Environment object provides the following defaults:

- If you do not specify an `InitialContext` factory, `WLInitialContextFactory` is used.
- If you do not specify a user and password in the `Context.SECURITY_PRINCIPAL` and `Context.CREDENTIALS` properties, the `guest` User and password are used unless the thread has already been associated with a user.
- If you do not specify a `Context.PROVIDER_URL` property, `t3://localhost:7001` is used.

If you want to create `InitialContext` with these defaults, write the following code:

>       Environment env = new Environment();
>       Context ctx = env.getInitialContext();

If you want to set only a WebLogic Server to a Distributed Name Service (DNS) name for client cluster access, write the following code:

>       Environment env = new Environment();
>       env.setProviderURL("t3://myweblogiccluster.com:7001");
>       Context ctx = env.getInitialContext();

Note: Every time you create a new JNDI environment object, you are creating a new security scope. This security scope ends with a `context.close()` method.

The `environment.getInitialContext()` method does not work correctly with the IIOP protocol.

Listing 2-2 illustrates using a JNDI Environment object to create a security context.

Listing 2-2 Creating a Security Context with a JNDI Environment Object

>     weblogic.jndi.Environment environment = new weblogic.jndi.Environment();
>     environment.setInitialContextFactory(
>       weblogic.jndi.Environment.DEFAULT_INITIAL_CONTEXT_FACTORY);
>     environment.setProviderURL("t3://bross:4441");
>     environment.setSecurityPrincipal("guest");
>     environment.setSecurityCrendentials("guest");
>     InitialContext ctx = environment.getInitialContext;

## Creating a Context from a Server-Side Object

You may also need to create a Context from an object (an Enterprise JavaBean (EJB) or Remote Method Invocation (RMI) object) that is instantiated in the Java Virtual Machine (JVM) of WebLogic Server. When using a server-side object, you do not need to specify the `Context.PROVIDER_URL` property. Usernames and passwords are required only if you want to sign in as a specific User.

To create a Context from within a server-side object, you first must create a new `InitialContext,` as follows:

>       Context ctx = new InitialContext();

You do not need to specify a factory or a provider URL. By default, the context is created as a Context and is connected to the local naming service.

## Associating a WebLogic User with a Security Context

See JNDI Contexts and Threads.

## JNDI Contexts and Threads

Note: BEA recommends using the Java Authentication and Authorization Service (JAAS) rather than JNDI to associate a User with a security context. For more information, see Programming WebLogic Security.

When you create a JNDI Context with a username and password, you associate a user with a thread. When the Context is created, the user is pushed onto the context stack associated with the thread. Before starting a new Context on the thread, you must close the first Context so that the first user is no longer associated with the thread. Otherwise, users are pushed down in the stack each time a new context created. This is not an efficient use of resources and may result in the incorrect user being returned by `ctx.lookup()``` calls. This scenario is illustrated by the following steps:

1. Create a Context (with username and credential) called `ctx1` for `user1`. In the process of creating the context, `user1` is associated with the thread and pushed onto the stack associated with the thread. The current user is now `user1`.
2. Create a second Context (with username and credential) called `ctx2` for `user2`. At this point, the thread has a stack of users associated with it. `User2` is at the top of the stack and `user1` is below it in the stack, so `user2` is used is the current user.
3. If you do a `ctx1.lookup("abc")` call, `user2` is used as the identity rather than `user1`, because `user2` is at the top of the stack. To get the expected result, which is to have `ctx1.lookup("abc")` call performed as `user1`, you need to do a `ctx2.close()``` call. The `ctx2.close()` call removes `user2` from the stack associated with the thread and so that a `ctx1.lookup("abc")` call now uses `user1` as expected.

Note: There are two situations where a `close()` call does not remove the current user from the stack and this can cause JNDI context problems. For information on how to avoid JNDI context problems, see How to Avoid Potential JNDI Context Problems.

### How to Avoid Potential JNDI Context Problems

While issuing a `close()` call usually behaves as described in JNDI Contexts and Threads. However, there are two exceptions to expected behavior:

- First Login
- Last Used

#### First Login

When using protocols other than IIOP, the first user is "sticky" in the sense that it becomes the default user when no other user is present. This scenario is described in the following steps:

1. Create a Context (with username and credential) called `ctx1` for `user1`. In the process of creating the context, `user1` is associated with the thread and stored in the stack, that is, the current identity is set to `user1`.
2. Do a `ctx1.close()` call.
3. Do a `ctx1.lookup()`call. The current identity is user1.
4. Create a Context (with username and credential) called `ctx2` for `user2`. In the process of creating the context, `user2` is associated with the thread and stored in the stack, that is, the current identity is set to `user2`.
5. Do a `ctx2.close()` call.
6. Do a `ctx2.lookup()`call. The current identity is user1.

Since `ctx1` was the first user, the current identity stays set to `user1` after step 4. Note that not only is `user1` the current user on this thread, it is the current user on all threads that do not have another identity defined. Thus, `user1` becomes the default user when no other user identity is present. This is not good practice as any subsequent logins that do not have a username and credential will be granted the identify of `user1` by default.

To work around this problem, implement one of the following options:

- Option 1: If the client has control of `main()`, implement the wrapper code shown in Listing 2-3 in the client code.

Listing 2-3 JNDI Context and Threads Wrapper Code

>     import java.security.PrivilegedAction;
>     import javax.security.auth.Subject;
>     import weblogic.security.Security;

>     public class client
>     {
>       public static void main(String[] args)
>       {
>       Security.runAs(new Subject(),
>         new PrivilegedAction() {
>          public Object run() {
>           //
>           //If implementing in client code, main() goes here.
>           //
>          return null;
>          }
>        });
>       }
>     }

- Option 2: If the client does not have control of `main()`, implement the wrapper code shown in Listing 2-3 on each thread's `run()` method.
- Option 3: Create a Context that logs in as a non-privileged user (a non-privileged user is a user that is not a member of any group). Be sure this is the first logon on the client. Immediately execute `ctx.close()` call to remove the non-privileged user from the thread's user stack. Because the non-privileged user is the first user to logon, it becomes the default user. Subsequently, any thread that has an empty user stack will have the identity of non-privileged user.

Note: If you choose to use Option 3, be advised of how non-privileged users relate to the `users` and `everyone` groups, which are configured by default in the security realm in this release of WebLogic Server. The `users` and `everyone` groups are convenience groups that allow you to apply global roles and security policies. By default, all WebLogic Server users, including non-privileged users, are members of the `everyone` group, but non-privileged users are not members of the `users` group.

#### Last Used

When using IIOP, an exception to expected behavior arises when there is one Context on the stack and that Context is removed by a `close()`. The identity of the last context removed from the stack determines the current identity of the user. This scenario is described in the following steps:

1. Create a Context (with username and credential) called `ctx1` for `user1`. In the process of creating the context, `user1` is associated with the thread and stored in the stack, that is, the current identity is set to `user1`.
2. Do a `ctx1.close()` call.
3. Do a `ctx1.lookup()`call. The current identity is user1.
4. Create a Context (with username and credential) called `ctx2` for `user2`. In the process of creating the context, `user2` is associated with the thread and stored in the stack, that is, the current identity is set to `user2`.
5. Do a `ctx2.close()` call.
6. Do a `ctx2.lookup()`call. The current identity is user2.

# Using the Context to Look Up a Named Object

The `lookup()` method on the Context is used to obtain named objects. The argument passed to the `lookup()` method is a string that contains the name of the desired object. Listing 2-4 shows how to retrieve an EJB named `ServiceBean`.

Listing 2-4 Looking Up a Named Object

>       try {
>         ServiceBean bean = (ServiceBean)ctx.lookup("ejb.serviceBean");
>       }catch (NameNotFoundException e) {
>         // binding does not exist
>       }catch (NamingException e) {
>         // a failure occurred
>       }

# Using a Named Object to Get an Object Reference

EJB client applications get object references to EJB remote objects from EJB Homes (servers on which EJBs are hosted). RMI client applications get object references to other RMI objects from an initial named object. Both EJB and RMI initial named remote objects are known to WebLogic Server as factories. A factory is any object that can return a reference to another object that is in the WebLogic namespace.

The client application invokes a method on a factory to obtain a reference to a remote object of a specific class. The client application then invokes methods on the remote object, passing any required arguments.

Listing 2-5 contains a code fragment that obtains a remote object and then invokes a method on it.

Listing 2-5 Using a Named Object to Get an Object Reference

>     ServiceBean bean = ServiceBean.Home.create("ejb.ServiceBean")

>     Servicebean.additem(66);

# Closing the Context

After clients finish working with a Context, BEA Systems recommends that the client close the Context in order to release resources and avoid memory leaks. BEA recommends that you use a `finally{}` block and wrap the `close()` method in a `try{}` block. If you attempt to close a context that was never instantiated because of an error, the Java client application throws an exception.

In Listing 2-6, the client closes the context, releasing the resource being used.

Listing 2-6 Closing the Context

>     try {
>     	ctx.close();
>     } catch () {
>     //a failure occurred
>     }

# Using WebLogic JNDI in a Clustered Environment

The intent of WebLogic JNDI is to provide a naming service for J2EE services, specifically EJB, RMI, and Java Messaging Service (JMS). Therefore, it is important to understand the implications of binding an object to the JNDI tree in a clustered environment.

The following sections discuss how WebLogic JNDI is implemented in a clustered environment and offer some approaches you can take to make your own objects available to JNDI clients.

## Making RMI Objects Available from a WebLogic Server Cluster

WebLogic RMI is the enabling technology that allows clients in one JVM to access EJBs and JMS services from a client in another JVM. RMI stubs marshal incoming calls from the client to the RMI object. To make J2EE services available to a client, WebLogic binds an RMI stub for a particular service into its JNDI tree under a particular name. The RMI stub is updated with the location of other instances of the RMI object as the instances are deployed to other servers in the cluster. If a server within the cluster fails, the RMI stubs in the other server's JNDI tree are updated to reflect the server failure.

When a client connects to a cluster, it is actually connecting to one of the WebLogic Servers in the cluster. Because the JNDI tree for this WebLogic Server contains the RMI stubs for all services offered by the other WebLogic Servers in the cluster in addition to its own services, the cluster appears to the client as one WebLogic Server hosting all of the cluster-wide services. When a new WebLogic Server joins a cluster, each WebLogic Server already in the cluster is responsible for sharing information about its own services to the new WebLogic Server. With the information collected from all the other servers in the cluster, the new server will create its own copy of the cluster-wide JNDI tree.

RMI stubs significantly affect how WebLogic JNDI is implemented in a clustered environment:

- RMI stubs are relatively small. This allows WebLogic JNDI to replicate stubs across all WebLogic Servers in a cluster with little overhead in terms of server-to-server cross-talk.
- RMI stubs serve as the mechanism for replication across a cluster. An instance of a RMI object is deployed to a single WebLogic Server, however, the stub is replicated across the cluster.

## Making Custom Objects Available to a WebLogic Server Cluster

When you bind a custom object (a non-RMI object) into a JNDI tree in a WebLogic Server cluster, the object is replicated across all the servers in the cluster. However, if the host server goes down, the custom object is removed from the cluster's JNDI tree. Custom objects are not replicated unless the custom object is bound again. You need to unbind and rebind a custom object every time you want to propagate changes made to the custom object. Therefore, WebLogic JNDI should not be used as a distributed object cache.

Suppose the custom object needs to be accessed only by EJBs that are deployed on only one WebLogic Server. Obviously it is unnecessary to replicate this custom object throughout all the WebLogic Servers in the cluster. In fact, you should avoid replicating the custom object in order to avoid any performance degradation due to unnecessary server-to-server communication. To create a binding that is not replicated across WebLogic Servers in a cluster, you must specify the `REPLICATE_BINDINGS` property when creating the context that binds the custom object to the namespace. Listing 2-7 illustrates the use of the `REPLICATE_BINDINGS` property.

Listing 2-7 Using the REPLICATE_BINDINGS Property

>       Hashtable ht = new Hashtable();
>       //turn off binding replication
>       ht.put(WLContext.REPLICATE_BINDINGS, "false");
>       try {
>         Context ctx = new InitialContext(ht);
>         //bind the object
>         ctx.bind("my_object", MyObect);
>       } catch (NamingException ne) {
>         //failure occured
>       }

When you are using this technique and you need to use the custom object, you must explicitly obtain an `InitialContext` for the WebLogic Server. If you connect to any other WebLogic Server in the cluster, the binding does not appear in the JNDI tree.

If you need a custom object accessible from any WebLogic Server in the cluster, deploy the custom object on each WebLogic Server in the cluster without replicating the JNDI bindings.

When using WebLogic JNDI to replicate bindings, the bound object will be handled as if it is owned by the host WebLogic Server. If the host WebLogic Server fails, the custom object is removed from all the JNDI trees of all WebLogic Servers in the cluster. This behavior can have an adverse effect on the availability of the custom object.

## Replicating a Custom Data Caching Object Across a Cluster

A common task in Web applications is to cache data used by multiple objects for a period of time to avoid the overhead associated with data computation or connecting to another service.

Suppose you have designed a custom data caching object that performs well on a single WebLogic Server and you would like to use this same object within a WebLogic cluster. If you bind the data caching object in the JNDI tree of one of the WebLogic Servers, WebLogic JNDI will, by default, copy the object to each of the other WebLogic Servers in the cluster. It is important to note that since this is not an RMI object, what you are binding into the JNDI tree (and copying to the other WebLogic Servers) is the object itself, not a stub that refers to a single instance of the object hosted on one of the WebLogic Servers. Do not assume from the fact that WebLogic Server copies a custom object between servers that custom objects can be used as a distributed cache. Remember the custom object is removed from the cluster if the WebLogic Server to which it was bound fails and changes to the customer object are not propagated through the cluster unless the object is unbound and rebound to the JNDI tree.

For the sake of performance and availability, it is often desirable to avoid using WebLogic JNDI's binding replication to copy large custom objects with high availability requirements to all of the WebLogic Servers in a cluster. As an alternative, you can deploy a separate instance of the custom object on each of the WebLogic Servers in the cluster. When binding the object to each WebLogic Server's JNDI tree, you should make sure to turn off binding replication as described in the Making Custom Objects Available to a WebLogic Server Cluster section. In this design pattern, each WebLogic Server has a copy of the custom object but you will avoid copying large amounts of data from server to server.

Regardless of which approach you use, each instance of the object should maintain its own logic for when it needs to refresh its cache independently of the other data cache objects in the cluster. For example, suppose a client accesses the data cache on one WebLogic Server. It is the first time the caching object has been accessed, so it computes or obtains the information and saves a copy of the information for future requests. Now suppose another client connects to the cluster to perform the same task as the first client only this time the connection is made to a different WebLogic Server in the cluster. If this the first time this particular data caching object has been accessed, it will need to compute the information regardless of whether other data caching objects in the cluster already have the information cached. Of course, for any future requests, this instance of the data cache object will be able to refer to the information it has saved.

## Exactly-Once-Per-Cluster Design Pattern

In some cases, it is desirable to have a service that appears only once in the cluster. This is accomplished by deploying the service on one machine only. For RMI objects, you can use the default behavior of WebLogic JNDI to replicate the binding (the RMI stub) and the single instance of your object will be accessible from all WebLogic Servers in the cluster. This is referred to as a pinned service. For non-RMI objects, make sure that you use the `REPLICATE_BINDINGS` property when binding the object to the namespace. In this case, you will need to explicitly connect to the host WebLogic Server to access the object. Alternatively, you can create an RMI object that is deployed on the same host WebLogic Server that can act as a proxy for your non-RMI object. The stub for the proxy can be replicated (using the default WebLogic JNDI behavior) allowing clients connected to any WebLogic Server in the cluster to access the non-RMI object via the RMI proxy.

This design pattern for an exactly-once-per-cluster service presents an additional challenge for services with high availability requirements. Since the failover feature of WebLogic Clusters relies on having multiple deployments of each clustered service, failover for an exactly-once-per-cluster service will not be available. For services that require high availability, it is suggested that you implement a hardware, High-Availability (HA) framework for the host WebLogic Server. The framework allows WebLogic Server to be restarted in the event of a failure with a minimal amount of disruption to availability of the service.

## Using WebLogic JNDI from a Client in a Clustered Environment

The JNDI binding for an object can appear in the JNDI tree for one WebLogic Server in the cluster, or it can be replicated to all the WebLogic Servers in the cluster. If the object of interest is bound in only one WebLogic Server, you must explicitly connect to the host WebLogic Server by setting the `Context.PROVIDER_URL` property to the host WebLogic Server's URL when creating the Initial Context, as described in Using WebLogic JNDI to Connect a Java Client to a Single Server.

In most cases, however, the object of interest is either a clustered service or a pinned service. As a result, a stub for the service is displayed in the JNDI tree for each WebLogic Server in the cluster. In this case, the client does not need to name a specific WebLogic Server to provide its naming service. In fact, it is best for the client to simply request that a WebLogic Cluster provide a naming service, in which case the context factory in WebLogic Server can choose whichever WebLogic Server in the cluster seems most appropriate for the client.

Currently, a naming service provider is chosen within WebLogic using a DNS name for the cluster that can be defined by the ClusterAddress attribute. This attribute defines the address to be used by clients to connect to a cluster. This address may be either a DNS host name that maps to multiple IP addresses or a comma separated list of single address host names or IP addresses. If network channels are configured, it is possible to set the cluster address on a per channel basis. See WebLogic Server Configuration Reference.

The context that is returned to a client of clustered services is, in general, implemented as a failover stub that can transparently change the naming service provider if a failure (such as a communication failure) with the selected WebLogic Server occurs.

Listing 2-8 shows how a client uses the cluster's naming service.

Listing 2-8 Using the Naming Service in a WebLogic Cluster

>       Hashtable ht = new Hashtable();
>       ht.put(Context.INITIAL_CONTEXT_FACTORY,
>              "weblogic.jndi.WLInitialContextFactory");
>       ht.put(Context.PROVIDER_URL, "t3://acmeCluster:7001");
>       try {
>         Context ctx = new InitialContext(ht);
>         // Do the client's work
>       }
>       catch (NamingException ne) {
>         // A failure occurred
>       }
>       finally {
>         try {ctx.close();}
>         catch (Exception e) {
>           // a failure occurred
>         }
>       }

The `hostname` specified as part of the provider URL is the DNS name for the cluster that can be defined by the `ClusterAddress setting in a Cluster stanza of the config.xml file`. ClusterAddress maps to the list of hosts providing naming service in this cluster. For more information, see "Understanding Cluster Configuration and Application Deployment" in Using WebLogic Server Clusters.

In Listing 2-8, the cluster name `acmeCluster` is used to connect to any of the WebLogic Servers in the cluster. The resulting Context is replicated so that it can fail over transparently to any WebLogic Server in the cluster.

An alternative method of specifying the initial point of contact with the WebLogic Cluster is to supply a comma-delimited list of DNS Server names or IP addresses.

- The following example specifies a list of WebLogic Servers using the same port:

>     ht.put(Context.PROVIDER_URL,"t3://acme1,acme2,acme3:7001");

All the WebLogic Servers listen on the port specified at the end of the URL.

- The following example specifies a list of WebLogic Servers using the different ports:

>     ht.put(Context.PROVIDER_URL,"t3://node1:7001,node2:7002,node3:7003");

When you use a DNS name which maps to multiple servers, WebLogic Server rilies on DNS for load balancing.

When you use a comma delimited list of DNS names for WebLogic Server nodes, failover is accomplished using the round-robin method, with the request going to a randomly chosen server until that server fails to respond, after which the request will go to the next server on the list. This will continue for each server that fails.

Once the client has gotten a context, no additional load balancing occurs unless there is a failure, in which case WebLogic Server will fail over to another node in the cluster.

A remote client will get the context from the first available server. A client that is local to a server in the cluster will never go to to a remote server for JNDI operations.

When you look up a stub, the first invocation of the stub will ordinarily go to the server from which you got the context. If the stub is clusterable, subsequent invocations will be load balanced based on the user defined load balancing policy.For additional information about JNDI and Clusters see "Introduction to WebLogic Server Clustering."

# Using JNDI from Within J2EE Components

Although it is possible for J2EE components to use the global environment directly, it is preferable to use the component environment. Each J2EE component within a J2EE application had its own component environment which is set up based on information contained in the component's deployment descriptors.

J2EE components are able to look up their component environments using the following code:

Context ctx = new InitailContext();

Context comp_env = (Context)ctx.lookup("java:comp/env");

Because you are working within a J2EE component, you do not need to set up the Hashtable or Environment objects to define the connection information.

This context is used in the same way as the global environment, however, the names you use are the ones defined in the deployment descriptor for your component. For example, if you have an ejb-ref in your deployment descriptor that looks like:

<ejb-ref>

...

<ejb-ref-name>ejb1</ejb-ref-name>

<ejb-ref-type>Session</ejb-ref-type>

<home>ejb1.EJB1Home</home>

<remote>ejb1.EJB1</remote>

...

</ejb-ref>

you would look up the name defined with the <ejb-ref-name> setting, which in this case is "ejb1."

Using the component environment rather than the global environment to set your JNDI name is advantageous because the name it refers to is resolved during deployment. This means that naming conflicts can be resolved without rewriting the code.

For additional information about setting up and using the component environment, see the J2EE Specification at http://java.sun.com/j2ee/j2ee-1_3-fr-spec.pdf.

# Viewing the JNDI Tree from Within the WebLogic Server Administration Console

You can monitor your deployments by viewing the JNDI tree from the WebLogic Server Administration Console. The JNDI tree will open in a separate browser window when you perform the following steps:

1. Click the Servers node to expand it and expose the names of the servers currently being administered through the console.
2. Click the name of the server whose JNDI tree you want to view.
3. Scroll down to the bottom of the Configuration pane, and
4. Click the "View JNDI Tree" link.

The JNDI tree will appear in a new browser window. You can click on an individual object name to view information about its bind name and hash code.