Programming WebLogic JNDI

      

WebLogic JNDI

The following sections describe programming with WebLogic JNDI:

 

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. The client can specify standard JNDI environment properties that identify a particular WebLogic Server deployment and related connection properties for logging in to WebLogic Server.

To interact with WebLogic Server, a Java client must be able to get an object reference for a remote object and invoke operations on the object. To accomplish this, the client application code must perform the following procedure:

  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.

  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.

Before you can use JNDI to access an object in a WebLogic Server environment, load the object into the WebLogic Server JNDI tree.

 

Setting Up JNDI Environment Properties for the InitialContext

The first task that must be performed by any Java client application is to create 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:

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.

Listing 3-1 shows how to obtain a Context using the properties Context.INITIAL_CONTEXT_FACTORY and Context.PROVIDER_URL. Listing 3-1 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

    }

  }

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, always set the java.naming.factory.initial property to weblogic.jndi.WLInitialContextFactory. This setting identifies the factory that actually creates the Context.

 

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:

The WebLogic Environment object provides the following defaults:

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();

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.

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

Listing 3-2 illustrates using a JNDI Environment object to create a security context. Listing 3-2 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

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, 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.

    When the weblogic.jndi.enableDefaultUser flag is enabled, 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

Issuing a close() call is usually as described in JNDI Contexts and Threads. However, the following is an exception to the expected behavior that occur when the weblogic.jndi.enableDefaultUser flag is enabled:

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 3-3 shows how to retrieve an EJB named ServiceBean. Listing 3-3 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. RMI client applications get object references to other RMI objects from an initial named object. Both 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 3-4 contains a code fragment that obtains a remote object and then invokes a method on it. Listing 3-4 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, Oracle recommends that the client close the Context in order to release resources and avoid memory leaks. Oracle 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 3-5, the client closes the context, releasing the resource being used. Listing 3-5 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 Java EE 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.

 

Using the Relationship of RMI and JNDI to Enable WebLogic Clusters

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 Java EE 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:

 

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. You can use a third-party solution with WebLogic Server to provide distributed caches.

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, specify the REPLICATE_BINDINGS property when creating the context that binds the custom object to the namespace. Listing 3-6 illustrates the use of the REPLICATE_BINDINGS property. Listing 3-6 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, 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.

 

Data Caching Design Pattern

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.

For services with high-availability requirements, you can configure automatic migration of an RMI object to another server. For more information about automatic migration, see Whole Server Migration in Using Clusters.

 

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, 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 Using Clusters.

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 3-7 shows how a client uses the cluster's naming service. Listing 3-7 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” in Using Clusters.

In Listing 3-7, 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.

When you use a DNS name which maps to multiple servers, WebLogic Server relies 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 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 Understanding WebLogic Server Clustering.

 

Using JNDI from Within Java EE Components

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

Java EE 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 Java EE 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.

 

Setting Up Foreign JNDI

Foreign JNDI is an API that allows you to access objects on a remote JNDI tree without having to connect directly to the remote tree.

It enables you to make links to a JNDI tree on another server or provider including, but not limited to, WebLogic Server, or a JNDI tree in a java program. Once you have configured Foreign JNDI, you can use an object that is somewhere else with the same ease that you would use an object bound in your WebLogic server instance.

To configure Foreign JNDI, create a ForeignJNDIProvider with an address of the remote JNDI provider whose objects you want to use, and create a username and password to access those objects. Then you can create ForeignJNDILinks and ForeignJNDIObjects that set up a relationship between a name in the local JNDI tree to the object in the remote tree.

For more information on how to configure Foreign JNDI, see “ Create a foreign JNDI provider“ in the Administration Console Online Help.