Servlet Life Cycle

Servlet Life Cycle

• When the servlet is first created, its init method is invoked, so init is where you put one-time setup code. 
• After this, each user request results in a thread that calls the service method of the previously created instance. 
• Multiple concurrent requests normally result in multiple threads calling service simultaneously, although your servlet can implement a special interface (SingleThreadModel) that stipulates that only a single thread is permitted to run at any one time. 
• The service method then calls doGet, doPost, or another doXxx method, depending on the type of HTTP request it received. 
• Finally, if the server decides to unload a servlet, it first calls the servlet’s destroy method. 

init() method:

• It is called 1 time only.
• It is called when the servlet is first created, and not called again for each user request.
• The init() method simply creates or loads some data that will be used throughout the life of the servlet.
• There are two types…
• General Initialization
• Parameter Initialization

service()

• It is called N times.
• The service method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc., as appropriate. 
• A GET request results from a normal request for a URL or from an HTML form that has no METHOD specified. 
• A POST request results from an HTML form that specifically lists POST as the METHOD. 
• Other HTTP requests (Put, Delete, Options…) are generated only by custom clients.

Service methods:

• It is called N times based upon user request. 
• There are Get, Post, Delete, Put, Options, Trace request.
• Figure:
  
• 99% times you have to take care Get and Post, only.
• To handle request such asGet – doGet, Post – doPost, Put – doPut, Options – doOptions, Trace – doTrace methods, respectively.

Destroy():

• The destroy() method is called by container before removing a servlet instance from service. 
• It is called 1 time only.
• The destory() will be called :
• when the container shuts down or the application shuts down;
• when the container decides that there is a shortage of memory;
• when this servlet hasn't got a request in a long time.

Summary:
The init Method (1 Time)

• It is used for one-time initializations

The Service Method (N Times)

• Each time the server receives a request for a servlet, the server seed a new thread and calls service.
• The doGet, doPost and doXXX Methods (N Times)
• The service method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc., as appropriate.

The destroy Method (1 Time)

• To remove previously loaded instance.