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Minimizing memory Leaks in your java j2ee applications | Techartifact

Enough of ADF.Lets talk about memory leak in java today in our application.How we can reduce it.

Java’s memory managmement (i.e Garbage collectionGarbage collection -how it works) prevents lost references and dangling references but it is still possibilities to create memory leaks in other ways. If the application run with memory leaks for long duration ,you will get the error java.lang.OutOfMemoryError.

In java ,typically the memory leak occur when an object of a longer lifecycle has references to the objects of a short life cycle. This prevents the
object with short life cycle being garbage collected. The developer must remember to remove the references to the short lived object from the long-lived objects.Objects with same life cycle do not cause any problem because the garbage collector is smart enough to deal with the circular references.

-> Java collection class like HashTable, ArrayList etc maintain references to other objects.So having a long life cycle ArrayList pointing to many . . short-lifecyle objects can cause memory leaks.

-> Commonly used singleton Design pattern can cause memory leaks.Singleton typically have a long lifecycle. If a singleton has an arrayList or a HashTable then there is potential for memory leaks.

-> Java Programming language includes a finalize method that allows an object to free system resources ,in other words to clean up after itself. However using finalize doesn’t guarantee that a class will clean up resources expediently. A better approach for cleaning up resources involves the finally method and an explicit close statement.so freeing up the valuable resources in the finalize method or try {} block instead of finally() block can cause memory leaks.

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JSP Life cycle

Life cycle of a JSP page consists of two phases, translation phase and execution phase. Every JSP is a Servlet, a JSP page is translated and compiled into servlet and the resulting servlet handles the request, So life cycle of a JSP page largely depends on the Servlet API.

JSP engine does the following 7 phases.

• Page translation: page is parsed, and a java file which is a servlet is created.
• Page compilation: page is compiled into a class file
• Page loading : This class file is loaded.
• Create an instance : Instance of servlet is created
• jspInit() method is called
• jspService is called to handle service calls
• jspDestroy is called to destroy it when the servlet is not required.

Translation phase

During the translation phase, JSP page is translated into a servlet class. The entire static markup in the JSP page is converted into the code that writes the data to response stream. If you look at the source of the generated Servlet class you will find calls to the out.write() which write data to ServletOutputStream.

If you have following HTML code into your JSP page
JSP life cycle tutorial
It will generate code like
out.write(“JSP life cycle tutorial”)

During the translation phase JSP elements are treated as follows:
• JSP directives controls the behavior of the resultant servlet.
• Scripting elements results into the equivalent Java code.
• Custom tags are converted into the code that calls method on tag handlers.

JSP Page Compilation:

The generated java servlet file is compiled into a java servlet class.
Note: The translation of a JSP source page into its implementation class can happen at any time between initial deployment of the JSP page into the JSP container and the receipt and processing of a client request for the target JSP page.


Class Loading:

The java servlet class that was compiled from the JSP source is loaded into the container

Execution phase

JSP life cycle’s execution phase is almost similar to that of the Servlet life cycle, because ultimately it’s a servlet which is being executed. The Servlet class generated at the end of the translation phase represents the contract between container and the JSP page. According to the JSP specification the servlet class must implement the HttpJspPage interface which defines the life cycle methods.
JSP life cycle includes three methods jspInit(), _jspService() and jspDestroy()

Initialization:

jspInit() method is called immediately after the instance was created. It is called only once during JSP life cycle.
_jspService() execution:
This method is called for every request of this JSP during its life cycle. This is where it serves the purpose of creation. Oops! it has to pass through all the above steps to reach this phase. It passes the request and the response objects. _jspService() cannot be overridden.
jspDestroy() execution:
This method is called when this JSP is destroyed. With this call the servlet serves its purpose and submits itself to heaven (garbage collection). This is the end of jsp life cycle.
jspInit(), _jspService() and jspDestroy() are called the life cycle methods of the JSP.

The HttpJspPage Interface

The javax.servlet.jsp.HttpJspPage contains three methods

Public void jspInit()

This method is invoked when the JSP page is initialized. This method is similar to init() method in servlet. If you want to provide initialization for a JSP page, you define this method in declaration part of the JSP page. But most of the time you will not need to define this method.

public void _jspService
void _jspService(HttpServletRequest request, HttpServletResponse response)ThrowsIOException, ServletException
This method represents the body of the JSP page and invoked at each client request. This method is similar to service() method in servlet.
Note: You should never provide implementation _jspService() method as web container automatically generates this method based on the content of the JSP page

Public void jspDestroy()

This method is invoked just before the JSP page is destroyed by the web container. This method is similar to destroy() method in servlet. If you want to provide some cleanup code, you can define this method in declaration part of the JSP page.

Understanding the Servlet life cycle with an example

This example explains how to execute code at JSP initialization phase and destroy phase of JSP Life Cycle.
It is a simple request counter that displays how many time the page has been called or how many requests the page has received. The page declares a counter variable of type integer, the counter is set to zero when the page is initialized. Each time the page receives a request, the value of counter variable is incremented by one and displayed to user. During each life cycle stage, A message is logged to server log.
lifecycle.jsp

	<%! 
	int counter;
	public void jspInit() {
		counter = 0;
		log("The lifecycle jsp has been initialized");
	}		
	%>	
 
<html>
	<head>
		<title>JSP Life Cycle Example</title>	
	</head>
	<body>
		<%
		log("The lifecycle jsp has received a request");
		counter++;
		%>		
		<h2>JSP Life cycle : Request counter</h2>
		<p>This page has been called <%=counter %> times </p>
	</body>
</html>
	<%!
		public void jspDestroy() {
		log("The lifecycle jsp is being destroyed");
	}
	%>

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Struts Interview Questions

What is Struts?
Struts is a web page development framework and an open source software that helps developers build web applications quickly and easily. Struts combines Java Servlets, Java Server Pages, custom tags, and message resources into a unified framework. It is a cooperative, synergistic platform, suitable for development teams, independent developers, and everyone between.


How is the MVC design pattern used in Struts framework?
In the MVC design pattern, application flow is mediated by a central Controller. The Controller delegates requests to an appropriate handler. The handlers are tied to a Model, and each handler acts as an adapter between the request and the Model. The Model represents, or encapsulates, an application’s business logic or state. Control is usually then forwarded back through the Controller to the appropriate View. The forwarding can be determined by consulting a set of mappings, usually loaded from a database or configuration file. This provides a loose coupling between the View and Model, which can make an application significantly easier to create and maintain.
Controller–Servlet controller which supplied by Struts itself; View — what you can see on the screen, a JSP page and presentation components; Model — System state and a business logic JavaBeans.
Who makes the Struts?
Struts is hosted by the Apache Software Foundation(ASF) as part of its Jakarta project, like Tomcat, Ant and Velocity.

Why it called Struts?
Because the designers want to remind us of the invisible underpinnings that hold up our houses, buildings, bridges, and ourselves when we are on stilts. This excellent description of Struts reflect the role the Struts plays in developing web applications.
Do we need to pay the Struts if being used in commercial purpose?
No. Struts is available for commercial use at no charge under the Apache Software License. You can also integrate the Struts components into your own framework just as if they were written in house without any red tape, fees, or other hassles.

What are the core classes of Struts?
Action, ActionForm, ActionServlet, ActionMapping, ActionForward are basic classes of Structs.
What is the design role played by Struts?
The role played by Structs is controller in Model/View/Controller(MVC) style. The View is played by JSP and Model is played by JDBC or generic data source classes. The Struts controller is a set of programmable components that allow developers to define exactly how the application interacts with the user.

How Struts control data flow?
Struts implements the MVC/Layers pattern through the use of ActionForwards and ActionMappings to keep control-flow decisions out of presentation layer.
What configuration files are used in Struts?

ApplicationResources.properties
struts-config.xml
These two files are used to bridge the gap between the Controller and the Model.
What helpers in the form of JSP pages are provided in Struts framework?
–struts-html.tld
–struts-bean.tld
–struts-logic.tld

Is Struts efficient?
The Struts is not only thread-safe but thread-dependent(instantiates each Action once and allows other requests to be threaded through the original object.
ActionForm beans minimize subclass code and shorten subclass hierarchies
The Struts tag libraries provide general-purpose functionality
The Struts components are reusable by the application
The Struts localization strategies reduce the need for redundant JSPs
The Struts is designed with an open architecture–subclass available
The Struts is lightweight (5 core packages, 5 tag libraries)
The Struts is open source and well documented (code to be examined easily)
The Struts is model neutral

How you will enable front-end validation based on the xml in validation.xml?
The tag to allow front-end validation based on the xml in validation.xml. For example the code: generates the client side java script for the form logonForm as defined in the validation.xml file. The when added in the jsp file generates the client site validation script.

What is ActionServlet?
The class org.apache.struts.action.ActionServlet is the called the ActionServlet. In the the Jakarta Struts Framework this class plays the role of controller. All the requests to the server goes through the controller. Controller is responsible for handling all the requests.

How you will make available any Message Resources Definitions file to the Struts Framework Environment?
Message Resources Definitions file are simple .properties files and these files contains the messages that can be used in the struts project. Message Resources Definitions files can be added to the struts-config.xml file through tag. Example:

What is Action Class?
The Action Class is part of the Model and is a wrapper around the business logic. The purpose of Action Class is to translate the HttpServletRequest to the business logic. To use the Action, we need to Subclass and overwrite the execute() method. In the Action Class all the database/business processing are done. It is advisable to perform all the database related stuffs in the Action Class. The ActionServlet (commad) passes the parameterized class to Action Form using the execute() method. The return type of the execute method is ActionForward which is used by the Struts Framework to forward the request to the file as per the value of the returned ActionForward object.

Write code of any Action Class?
Here is the code of Action Class that returns the ActionForward object.
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.struts.action.Action;
import org.apache.struts.action.ActionForm;
import org.apache.struts.action.ActionForward;
import org.apache.struts.action.ActionMapping;

public class TestAction extends Action
{
public ActionForward execute(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response) throws Exception
{
return mapping.findForward(\”testAction\”);
}
}

What is ActionForm?
An ActionForm is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the ActionForm object is automatically populated on the server side with data entered from a form on the client side.

What is Struts Validator Framework?
Struts Framework provides the functionality to validate the form data. It can be use to validate the data on the users browser as well as on the server side. Struts Framework emits the java scripts and it can be used validate the form data on the client browser. Server side validation of form can be accomplished by sub classing your From Bean with DynaValidatorForm class. The Validator framework was developed by David Winterfeldt as third-party add-on to Struts. Now the Validator framework is a part of Jakarta Commons project and it can be used with or without Struts. The Validator framework comes integrated with the Struts Framework and can be used without doing any extra settings.

Give the Details of XML files used in Validator Framework?
The Validator Framework uses two XML configuration files validator-rules.xml and validation.xml. The validator-rules.xml defines the standard validation routines, these are reusable and used in validation.xml. to define the form specific validations. The validation.xml defines the validations applied to a form bean. How you will display validation fail errors on jsp page? – The following tag displays all the errors:

Why do we need Struts?
Java technologies give developers a serious boost when creating and maintaining applications to meet the demands of today’s public Web sites and enterprise intranets. Struts combines Java Servlets, Java ServerPages, custom tags, and message resources into a unified framework. The end result is a cooperative, synergistic platform, suitable for development teams, independent developers, and everyone in between.

How does Struts work?
Java Servlets are designed to handle requests made by Web browsers. Java ServerPages are designed to create dynamic Web pages that can turn billboard sites into live applications. Struts uses a special Servlet as a switchboard to route requests from Web browsers to the appropriate ServerPage. This makes Web applications much easier to design, create, and maintain.
Is Struts compatible with other Java technologies?
Yes. Struts is committed to supporting industry standards. Struts acts as an integrator of Java technologies so that they can be used in the “real world”.

Who wrote Struts?

There are several active committers to the Struts project, working cooperatively from around the globe. Dozens of individual developers and committers contributed to the Struts 1.x codebase. All interested Java developers are invited to contribute to the project. Struts is a Apache Software Foundation project, with the mission to “provide secure, enterprise-grade server solutions based on the Java Platform that are developed in an open and cooperative fashion”.
Struts was created by Craig R. McClanahan and donated to The Apache Software Foundation in May 2000. Craig was the primary developer of both Struts 1.x and Tomcat 4. Tomcat 4 was the basis for the official reference implementation for a servlet 2.3 and JSP 1.2 container.
Craig’s current focus is as architect of the Sun Java Studio Creator (formerly Project Rave). Craig also serves as the Specification Lead for JavaServer Faces (JSR-127), and is the Web Layer Architect for the Java2 Enterprise Edition (J2EE) platform as a whole

What is the term action mapping?
Struts provides a number of tag libraries that helps to create view components easily.
These tag libraries are:
a) Bean Tags: Bean Tags are used to access the beans and their properties.
b) HTML Tags: HTML Tags provides tags for creating the view components like forms, buttons, etc..
c) Logic Tags: Logic Tags provides presentation logics that eliminate the need for script lets.
d) Nested Tags: Nested Tags helps to work with the nested context

Can I setup Apache Struts to use multiple configuration files?
Yes Struts can use multiple configuration files. Here is the configuration example:

banking
org.apache.struts.action.ActionServlet

config /WEB-INF/struts-config.xml,
/WEB-INF/struts-authentication.xml,
/WEB-INF/struts-help.xml
1

What are the difference between and ?
: This tag is used to output locale-specific text (from the properties files) from a MessageResources bundle.

: This tag is used to output property values from a bean. is a commonly used tag which enables the programmers to easily present the data

What is LookupDispatchAction?
An abstract Action that dispatches to the subclass mapped execute method. This is useful in cases where an HTML form has multiple submit buttons with the same name. The button name is specified by the parameter property of the corresponding ActionMapping. (Ref. http://struts.apache.org/1.2.7/api/org/apache/struts/actions/LookupDispatchAction.html

How you will handle exceptions in Struts?
In Struts you can handle the exceptions in two ways:
a) Declarative Exception Handling: You can either define global exception handling tags in your struts-config.xml or define the exception handling tags within .. tag.
Example:

b) Programmatic Exception Handling: Here you can use try{}catch{} block to handle the exception.

Why ActionServlet is Singleton in struts framework?
org.apache.struts.action.ActionServlet in the web.xml configuration file must defined as
ActionServlet provided by the struts Framework is a built-in servlet. ActionServlet nothing but a controller.
It has the following inbuilt design patterns.
1 singleton designpatteren.
2.FrontController
3.ApplicationController
4.MVC
5.Data Transfer object/value object
6.Inversion of control/Dependency injection
ActionServlet take the multiple request but it creates single instance for that requests by using singleton design pattern.
Few servers likes weblogic violates single instance multiple threads principle of servlet programming.That means these servers that will create more than once object for a class in special situation. see here
To avoid this type of situations & to follow servlet specifications, struts framework software developers have made built-in servlet called ActionServlet.
Note:
1. According to MVC principles, there should be only one servlet acting as controller and this servlet should have only one object.
2. Both the approaches have advantages & disadvantages

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