This chapter provides an overview of the Timer and Work Manager API and demonstrates how to implement it within an application in WebLogic Server 12.1.3.
This chapter includes the following sections:
The Timer and Work Manager API is defined in a specification created jointly by Oracle and IBM. This API enables concurrent programming of EJBs and Servlets within a Java EE application. This API is often referred to as CommonJ.
The CommonJ API contains the following components:
Timer API
The Timer API allows applications to schedule and receive timer notification callbacks for a specific listener defined within an application. Timers allow you to schedule and perform work at specific times or intervals. See Timer API Overview.
You implement this API by importing the commonj.timer
package.
Work Manager API
The Work Manager API allows an application to prioritize work within an EJB or servlet. Applications can programmatically execute multiple work items within a container. See Work Manager API.
You implement this API by importing the commonj.work
package.
In addition to the CommonJ Work Manager API, WebLogic Server includes server-level Work Managers that provide prioritization and thread management. These can be configured globally or for a specific module in an application.
Although commonj.timer
and commonj.work
are part of the same API, each provides different functionality. Which one you implement depends on the specific needs of your application. The CommonJ Timer API is ideal for scheduling work at specific intervals; for example, when you know that a certain job should run at a specific time. The CommonJ Work API is ideal for handling work based on priority. For example, you may not be able to predict exactly when a specific job will occur, but when it does you want it to be given a higher (or lower) priority.
The following sections describe the CommonJ APIs in detail.
The Timer API consist of three interfaces:
TimerManager
TimerListener
Timer
The TimerManager
interface provides the framework for creating and using timers within a managed environment. The TimerListener
receives timer notifications. The TimerManager.schedule
method is used to schedule the TimerListener
to run at a specific time or interval.
For a detailed description of how to implement Timers, see Using the Timer API.
The TimerManager
interface provides the general scheduling framework within an application. A managed environment can support multiple TimerManager
instances. Within an application you can have multiple instances of a TimerManager
.
A TimerManager
is configured during deployment by means of deployment descriptors. The TimerManager
definition may also contain additional implementation-specific configuration information.
Once a TimerManager
is defined in a deployment descriptor, instances of it can be accessed using a JNDI lookup in the local Java environment. Each invocation of the JNDI lookup()
on a TimerManager
returns a new logical instance of a TimerManager
.
The TimerManager
interface is thread-safe.
For more information about using JNDI, see Developing JNDI Applications for Oracle WebLogic Server.
This section explains the steps required for using the Timer API within an application.
Before deploying your application, ensure that you have created a deployment descriptor that contains a resource reference for the Timer Manager.
This allows the TimerManager
to be accessed using JNDI. For more information about JNDI lookup, see Developing JNDI Applications for Oracle WebLogic Server.
To implement the Timer API, complete the following steps:
Import the commonj.timers.*
packages.
Create an InitialContext
that allows the TimerManager
to be looked up in JNDI. For example:
InitialContext inctxt = new InitialContext();
See Developing JNDI Applications for Oracle WebLogic Server for more information about JNDI lookup.
Create a new TimerManager
based on the JNDI lookup of the TimerManager
. For example:
TimerManager mgr = (TimerManager)ctx.lookup('java:comp/env/timer/MyTimer');
In this statement, the result of the JNDI lookup is cast to a TimerManager
.
Implement a TimerListener
to receive timer notifications. For example:
TimerListener listener = new StockQuoteTimerListener('abc', 'example');
Invoke the TimerManager.schedule
method. For example:
mgr.schedule(listener, 0, 1000*60)
The schedule
method returns a Timer
object.
Implement the timerExpired
method. For example:
public void timerExpired(Timer timer) { //Business logic is executed //in this method }
Implementing the Timer API for cluster-wide timers has additional requirements, described in Life Cycle of Timers.
package examples.servlets; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.naming.InitialContext; import javax.naming.NamingException; import commonj.timers.*; /** * TimerServlet demonstrates a simple use of commonj timers */ public class TimerServlet extends HttpServlet { /** * A very simple implementation of the service method, * which schedules a commonj timer. */ public void service(HttpServletRequest req, HttpServletResponse res) throws IOException { res.setContentType("text/html"); PrintWriter out = res.getWriter(); try { InitialContext ic = new InitialContext(); TimerManager tm = (TimerManager)ic.lookup ("java:comp/env/tm/default"); // Execute timer every 10 seconds starting immediately tm.schedule (new MyTimerListener(), 0, 10*1000); out.println("<h4>Timer scheduled!</h4>"); } catch (NamingException ne) { ne.printStackTrace(); out.println("<h4>Timer schedule failed!</h4>"); } } private static class MyTimerListener implements TimerListener { public void timerExpired(Timer timer) { System.out.println("timer expired called on " + timer); // some useful work here ... // let's just cancel the timer System.out.println("cancelling timer ..."); timer.cancel(); } } }
This section explains how to use the Job Scheduler functionality. The Job Scheduler allows you to implement the commonj.timer
API within a clustered environment.
The Job Scheduler is essentially an implementation of the commonj.timer
API package that can be used within a cluster. In this context, a job is defined as a commonj.timers.TimerListener
instance that is submitted to the Job Scheduler for execution.
This section includes the following topics:
When you implement the commonj.timer
API within an application, you can configure two possible life cycles for a timer:
Local timer
A local timer is scheduled within a single server JVM and is handled within this JVM throughout its life cycle. The timer continues running as long as the JVM is running and fails when the JVM exits. The application is responsible for rescheduling the timer after server startup.
This is the basic implementation of the commonj.timers
package.
Cluster-wide timer
A cluster-wide timer is aware of the other JVMs containing each server within the cluster and is therefore able to perform load balancing and failover. The life cycle of a cluster-wide timer is not bound to the server that created it, but continues to function throughout the life cycle of the cluster. If at least one cluster member is alive, the timer continues to function. This functionality is referred to as the Job Scheduler.
Implementing the Timer API for a Job Scheduler has the following requirements in addition to those listed in Implementing the Timer API:
The Timer Listener class must be serializable.
The Timer Listener class must be present in the server system classpath.
The minimum time for recurring execution of a timer is 30 seconds because Job Schedulers pick up timers for execution every 30 seconds.
Each timer has its own advantages and disadvantages. Local timers can process jobs with much smaller time intervals between jobs. Due to the persistence requirements within a cluster, Job Schedulers cannot handle jobs with as much precision. On the other hand, Job Schedulers are better suited for tasks that must be performed even if the initial server that created the task has failed.
This section describes the basic procedure for implementing Job Schedulers within an application and for configuring your WebLogic Server environment to utilize them. The following topics are included:
To maintain persistence and make timers cluster-aware, Job Schedulers require a database connection. The Job Scheduler functionality supports the same databases that are supported by server migration.
For convenience, you can use the same database used for session persistence, server migration, and so on. For example, see "Configure server migration in a cluster" in the Oracle WebLogic Server Administration Console Online Help for information about how to create or select a data source for server migration.
In the database, you must create a table named WEBLOGIC_TIMERS
. Schemas for creating this table are in the following location:
WL_HOME/server/db/dbname/scheduler.ddl
In the preceding path, dbname
represents the name of the database.
After you create a table with the required schema, you must define a data source that is referenced from within the cluster configuration. Job Scheduler functionality is available only if a valid data source is defined in the ClusterMBean.DataSourceForJobScheduler
attribute. For information about how to use the WebLogic Server Administration Console to configure this attribute, see "Configure a data source for a job scheduler" in the Oracle WebLogic Server Administration Console Online Help.
The following config.xml
excerpt shows how this is defined:
<domain> ... <cluster> <name>Cluster-0</name> <multicast-address>239.192.0.0</multicast-address> <multicast-port>7466</multicast-port> <data-source-for-job-scheduler>JDBC Data Source-0</data-source-for-job-scheduler> </cluster> ... <jdbc-system-resource> <name>JDBC Data Source-0</name> <target>myserver,server-0</target> <descriptor-file-name>jdbc/JDBC_Data_Source-0-3407-jdbc.xml</descriptor-file-name> </jdbc-system-resource> </domain>
Leasing must be enabled for Job Schedulers. You can use either high-availability database leasing or non-database consensus leasing. When using high-availability database leasing, you must create the leasing table in the database.
Schemas for creating this table are in the following location:
WL_HOME/server/db/dbname/leasing.ddl
In the preceding path, dbname
represents the name of the database.
For more information, see "Leasing" in Administering Clusters for Oracle WebLogic Server.
The procedure for performing JNDI lookup within a clustered timer is different from that used in the general commonj.timer
API. The following code snippet shows how to cast a JNDI lookup to a TimerManager
.
InitialContext ic = new InitialContext(); commonj.timers.TimerManager jobScheduler =(common.timers.TimerManager)ic.lookup ("weblogic.JobScheduler"); commonj.timers.TimerListener timerListener = new MySerializableTimerListener(); jobScheduler.schedule(timerListener, 0, 30*1000); // execute this job every 30 seconds
You can cancel jobs programmatically or by using the WebLogic Server Administration Console.
To cancel a job programmatically, invoke the cancel
method of the job's corresponding JobRuntimeMBean. You can access a JobRuntimeMBean using either of the following ways:
Invoke JobSchedulerRuntimeMBean.getJob(id)
with the ID of a scheduled job. To get the ID, invoke the JobScheduler.schedule
method to return a Timer object, then use the Timer's toString
method to return the ID.
Invoke JobSchedulerRuntimeMBean.getExecutedJobs()
to return an array of JobRunTimes
for all jobs that have been executed at least once.
You cannot invoke the cancel
method to cancel a scheduled job that has not executed at least once.
For information about how to use the WebLogic Server Administration Console to cancel jobs, see "Cancel jobs" in Oracle WebLogic Server Administration Console Online Help.
The following debugging flags enable more verbose output:
-Dweblogic.debug.DebugSingletonServices=true -Dweblogic.JobScheduler=true
The following methods and interfaces in the commonj.timer
package are not supported in the Job Scheduler environment:
CancelTimerListener
interface
StopTimerListener
interface
The following methods of the TimerManager
interface:
suspend
resume
scheduleAtFixedRate
stop
waitForStop
waitForSuspend
The Work Manager API, commonj.work
, provides a set of interfaces that allows an application to execute multiple work items concurrently within a container.
Essentially this API provides a container-managed alternative to the java.lang.Thread
API. The latter should not be used within applications that are hosted in a managed Java EE environment. In such environments, the Work Manager API is a better choice because it allows the container to have full visibility and control over all executing threads.
Note:
The Work Manager API provides no failover or persistence mechanisms. If the Managed Server environment fails or is shut down, any current work is lost.This section summarizes the interfaces in the Work Manager API. For details about using these interfaces, see commonj.work
in the Java API Reference for Oracle WebLogic Server.
The Work Manager API contains the following interfaces:
WorkManager - Provides a set of scheduling methods that are used to schedule work for execution.
A WorkManager is defined by system administrators at the server level. A WorkManager
instance is obtained by performing a JNDI lookup. A managed environment can support multiple WorkManager
instances. You configure WorkManagers during deployment as resource-refs
. See Work Manager Deployment.
At the application level, each instance of WorkManager
returns a WorkItem
. For more information about implementing a WorkManager
within an application, see WorkManager
in the Java API Reference for Oracle WebLogic Server.
For information about JNDI, see Developing JNDI Applications for Oracle WebLogic Server.
Work - Allows you to run application code asynchronously. By creating a class that implements this interface, you can create blocks of code that can be scheduled to run at a specific time or at defined intervals. In other words, this is the "work" that is handled within the Work Manager API.
WorkItem - Determines the status of a completed Work
instance. A WorkItem
is returned by a WorkManager
after a Work
instance has been submitted to that WorkManager
.
For more information, see Work
in the Java API Reference for Oracle WebLogic Server.
WorkListener - Provides communication between the WorkManager
and the scheduled work defined within the Work
instance. WorkListener
is a callback interface.
You can use WorkListener
to determine the current status of the Work
item. For more information, see WorkListener
in the Java API Reference for Oracle WebLogic Server.
Note:
WorkListener
instances are always executed in the same JVM as the original thread used to schedule the Work
by means of the WorkManager
.WorkEvent - A WorkEvent
is sent to a WorkListener
as Work
is processed by a WorkManager
.
For more information, see WorkEvent
in the Java API Reference for Oracle WebLogic Server.
RemoteWorkItem - The RemoteWorkItem
interface is an extension of the WorkItem
interface that allows work to be executed remotely. This interface allows serializable work to be executed on any member of a cluster.
For more information, see RemoteWorkItem
in the Java API Reference for Oracle WebLogic Server.
Work Managers are defined at the server level by means of a resource-ref in the appropriate deployment descriptor. This can be web.xml
or ejb-jar.xml
, among others.
The following deployment descriptor snippet shows the configuration of a WorkManager
:
... <resource-ref> <res-ref-name>wm/MyWorkManager</res-ref-name> <res-type>commonj.work.WorkManager</res-type> <res-auth>Container</res-auth> <res-sharing-scope>Shareable</res-sharing-scope> </resource-ref> ...
Note:
The recommended prefix for the JNDI namespace forWorkManager
objects is java:comp/env/wm
.The following example shows using a CommonJ Work Manager within an HTTP servlet.
import java.io.IOException; import java.io.PrintWriter; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.naming.InitialContext; import javax.naming.NamingException; import weblogic.work.ExecuteThread; import commonj.work.WorkManager; import commonj.work.Work; import commonj.work.WorkException; public class HelloWorldServlet extends HttpServlet { public void service(HttpServletRequest req, HttpServletResponse res) throws IOException { res.setContentType("text/html"); PrintWriter out = res.getWriter(); try { InitialContext ic = new InitialContext(); System.out.println("## [servlet] executing in: " + ((ExecuteThread)Thread.currentThread()).getWorkManager() .getName()); WorkManager wm = (WorkManager)ic.lookup ("java:comp/env/foo-servlet"); System.out.println("## got Java EE work manager !!!!"); wm.schedule(new Work(){ public void run() { ExecuteThread th = (ExecuteThread) Thread.currentThread(); System.out.println("## [servlet] self-tuning workmanager: " + th.getWorkManager().getName()); } public void release() {} public boolean isDaemon() {return false;} }); } catch (NamingException ne) { ne.printStackTrace();} catch (WorkException e) { e.printStackTrace(); } out.println("<h4>Hello World!</h4>"); // Do not close the output stream - allow the servlet engine to close it // to enable better performance. System.out.println("finished execution");} }