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The Quartz Architecture
The Quartz framework Scheduler is the engine that manages the runtime environment of Quartz. To schedule a task, you need to queue the task in the schedulers' queue. Quartz then allots a thread from its ThreadPool to execute the different tasks in the queue.
Because Quartz is a multi-threaded environment with a pre-defined set of threads configured at startup, it can manage independent tasks by multi-tasking them. When the Scheduler starts up, it initializes the already defined number of threads and waits for a task to be triggered. At the specified time, the Scheduler assigns a thread to the relevant task and waits. When the task is completed, the thread is returned to the ThreadPool. Therefore during design, it's important to take care that there are enough threads available for the Scheduler to run the tasks on time. If there is no thread available when a task is ready to execute, the scheduler will pause until one becomes available. Initializing too many threads can cause degradation of system performance and contention issues.
To use Quartz, the first thing you need to do is to create a Quartz job that defines a task to schedule. Create a Quartz job by defining a Java class and implementing the job interface. You then override the execute() method of the job Interface and add logic to be executed when the job is run. The logic can be to check for files on a server or delete temporary data from a database or send an email. Next, define a trigger parameter to determine when the job needs to be run and how often. Then, register the Trigger and the job with the scheduler to ensure that the Scheduler can trigger the job at the appropriate time. The Scheduler does not allow you to register a job directly—this needs to be a done through a JobDetail class, which allows you to categorize and group similar jobs. The JobDetail class also allows you to add a job and define properties like the job's name, the job's Job Group, and the job's Java class. If no Job group is specified, the Scheduler puts the job into a default Job group.
The easiest way to get a Scheduler working is to use the default Scheduler shipped with Quartz. Call the static getDefaultScheduler() method of the StdSchedulerFactory class to instantiate the default Scheduler. Now the Scheduler is ready to accept Jobs and Triggers.
Quartz's architecture intentionally separates the trigger from the job so you can make jobs reusable. Multiple jobs can be triggered from a single trigger and multiple triggers can invoke the same job. This facilitates easy codemaintenance. Moreover, Quartz provides simple, built-in triggers to do regularly used actions. JobGroups allowsyou to include the same job in different Job Groups and allows for common actions in the JobGroup.
Job Stores
Quartz allows two types of JobStores: one memory-based and a database-based. The memory-based job store is suitable for a small number of concurrent jobs—information about thes jobs is stored in the RAM of the computer. This is the fastest way to access jobs in the Scheduler's queue. However, the flip side is that all these jobs are lost when the machine is restarted—along with their information.
For jobs that need some persistence information, a JDBC-based store is the best solution. Information about these jobs is stored in a database and is accessed by the Scheduler through JDBC.
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