Context parameters can not be anything other tham String. If we want to set init parameter to be sth else, we need to add a listener which would convert the String value to the desired value.
coz we can not put the code in Servlet as the servlet would have got the parameter value as String only by then.
This is done by implementing "ServletContextListener" interface which has two methods - contextInitialized(ServletContextEvent) and contextDestroyed(ServletContextEvent)
Difference between Attribute and Parameter
Attribute :
1. Type : Application / context , Request , Session
2. Method to set : setAttribute(String name , Object value)
3. Return type : Object
4. Method to get : getAttribute(String name)
Parameter :
1. Type : Application /context , Request , Servlet init parameters
2. Method : These can be set only in Deployment descriptor (web.xml)
3 . Return type : String
4. Method to get : getInitParameter(String name)
Context scope isnt thread safe. We can make them thread safe by using : synchronized(getServletContext())
Session persists across multiple requests from same client.
Its also not thread safe. We can makr it thread safe as we did to Context : synchronized(session)
Only requesr attributes and local variables are thread safe. Instance variables arnt.
we generally do not declare non final instance variables in Servlets.
We also have a "SingleThreadModel" interface, which can be used to make servlet thread safe. But this interface should better not be used as it effects performance a lot.
RequestDispatcher has two methods : forward(request , response) and include(request , response)
include() sends the request to do sth and it then comes back to the sender.
Difference b/w RequestDispatcher from context and from request
In the one from context, you can not specify relative path. u must specify absolute path.
October 27, 2009
October 25, 2009
Servlets and JSPs
GET requests can be bookmarked but POST can not be.
GET is idempotent(Its used for getting things from the server, its not supposed to change anything ) , POST is not
addHeader() and setHeader() methods of response : Both will add a header and value to response if the header is not already present. The difference between these two occurs when the header is already there.
setHeader() overwrites the existing value
addHeader() adds an additional value
We can not do a send redirect after writing to the response. If we try to do so, it will throw an llegal state exception
Difference b/w redirect and a request dispatch : redirect makes the client do the work and sendRedirect makes sth on the server do the work.
we can not use Servlet init param unless the servlet is initialised.
sendredirect is called on response and request dispatcher is called on request
response.sendRedirect() and request.getRequestDispatcher() and then dispatcher.forward()
Also, in case of request dispatcher, the URL is not changed, so the user does not get to know about that.
GET is idempotent(Its used for getting things from the server, its not supposed to change anything ) , POST is not
addHeader() and setHeader() methods of response : Both will add a header and value to response if the header is not already present. The difference between these two occurs when the header is already there.
setHeader() overwrites the existing value
addHeader() adds an additional value
We can not do a send redirect after writing to the response. If we try to do so, it will throw an llegal state exception
Difference b/w redirect and a request dispatch : redirect makes the client do the work and sendRedirect makes sth on the server do the work.
we can not use Servlet init param unless the servlet is initialised.
sendredirect is called on response and request dispatcher is called on request
response.sendRedirect() and request.getRequestDispatcher() and then dispatcher.forward()
Also, in case of request dispatcher, the URL is not changed, so the user does not get to know about that.
Hibernate related
Advantages :
1. Productivity : Persistence related code is very tough to write. Hibernate eliminates much of the grunt work.So we can utilize the effort we could have used in writing hand coded persistence logic.
2. Maintainability : In hand coded persistence, change in one of the object or relationship domain leads to change in other also. But hibernate provides us a buffer between the two midels, insultaing each model from minor changes in the other. Otherwise, most of the times, change needs to be made at the java side, which is thus prevented by using hibernate.
3. Performance : We can also optimize the existing hibernate code as per our requirements.Also, since hibernate provides automated persistence, it increases developer performance.
4. Vendor Independence : An ORM abstracts your application away from the underlying SQL databases and SQL dialect. If the tool supports a number of different databases, then this confers a certain level of portability on your application.
Persistence allows an object to outlice the process that created it. The state of the object may be stored to disk and an object with same state can be recreated.
problems with ORM that hibernate addresses:
1. Problem of Granularity : (Fine grained and coarse grained objects.)
2. Problem of identity : In java , identity can be defined by equals method or memory location and in DB, it is represented by primary key. Therefore,Identity should be something that should have no meaning to user.
3. Problem of association
4. Problem of subtyping
5. P of Object graph navigation.
for details refer page 10-15 chapter 1 of hibernate in action.
1. Productivity : Persistence related code is very tough to write. Hibernate eliminates much of the grunt work.So we can utilize the effort we could have used in writing hand coded persistence logic.
2. Maintainability : In hand coded persistence, change in one of the object or relationship domain leads to change in other also. But hibernate provides us a buffer between the two midels, insultaing each model from minor changes in the other. Otherwise, most of the times, change needs to be made at the java side, which is thus prevented by using hibernate.
3. Performance : We can also optimize the existing hibernate code as per our requirements.Also, since hibernate provides automated persistence, it increases developer performance.
4. Vendor Independence : An ORM abstracts your application away from the underlying SQL databases and SQL dialect. If the tool supports a number of different databases, then this confers a certain level of portability on your application.
Persistence allows an object to outlice the process that created it. The state of the object may be stored to disk and an object with same state can be recreated.
problems with ORM that hibernate addresses:
1. Problem of Granularity : (Fine grained and coarse grained objects.)
2. Problem of identity : In java , identity can be defined by equals method or memory location and in DB, it is represented by primary key. Therefore,Identity should be something that should have no meaning to user.
3. Problem of association
4. Problem of subtyping
5. P of Object graph navigation.
for details refer page 10-15 chapter 1 of hibernate in action.
Difference : Web Server and Application Server
A fully compliant Application Server must have both a Web Container and an EJB Container (plus other things including a JNDI and JMS implementation). Tomcat is just a web container. It is still compliant with portions of J2EE spec that address the web ocntainer.
JBoss is an application server
JBoss is an application server
October 24, 2009
Fine Grained Vs Coarse Grained
Coarse-grained: A few ojects hold a lot of related data. Example: A single "Account" object holds the customer name, address, account balance, opening date, last change date, etc.
Fine-grained: More objects each holding less data. Example: An Account object holds balance, a Customer object holds name and address, a AccountOpenings object holds opening date, etc. There are relationships defined between these objects.
Reference : http://www.theserverside.com/discussions/thread.tss?thread_id=5164
Fine-grained: More objects each holding less data. Example: An Account object holds balance, a Customer object holds name and address, a AccountOpenings object holds opening date, etc. There are relationships defined between these objects.
Reference : http://www.theserverside.com/discussions/thread.tss?thread_id=5164
October 11, 2009
Computer Networking questions
1. What do you need to know about addresses?
You probably know what an IP address is: a number that identifies that device on the network. But what else do you need to know? IP addresses are made up of 32 bits (IPv4 addresses, that is). We normally think of an IP address as something like 1.1.1.1, but really this can be translated into eight binary bits (see Binary-to-Decimal Conversion for more information). Each set of binary bits can represent only the numbers zero through 255. That is why your IP addresses can range only from 0.0.0.0 to 255.255.255.255.
By the way, the IP address 255.255.255.255 is called the "all ones" network because in binary it is represented by 32 numeral ones (1s). The all ones address is used to send a packet to all devices on all networks (as long as it isn't stopped by a router first).
Traditionally, IP addresses were broken up into classes, but those classes aren't used much any more unless you are taking a certification exam. We will learn more about classes below.
Most importantly, IP addresses must be unique on your network. If two devices have the exact same IP address, you have an IP address conflict. When that happens, either device or both devices will not work on the network. Commonly, DHCP is used to dynamically allocate IP addresses in hopes of preventing address duplication and easing the administrative burden of static IP addressing.
Q : What is a subnet mask?
A subnet mask is what tells your computer (or other network device) what portion of the IP address is used to represent your network and what part is used to represent hosts (other computers) on your network. For example, if you have an IP address of 1.1.1.1 and a subnet mask of 255.255.255.0, the 255s mask off the first three 1s. If you did the logical "AND" (the calculation your computer does -- see Binary-to-Decimal Conversion for more information), you would find out that the network ID for this network is 1.1.1.0. Where the 0 is located, you could fill in hosts numbered 1 to 254. For example, the first host on your network is 1.1.1.1 and the last host is 1.1.1.254.
Of special note when looking at the number of hosts in a network is this: The first IP address in a network is the network address and the last IP address is always the broadcast address. That's why I couldn't use IP address 1.1.1.0 and IP address 1.1.1.255. These are special, reserved addresses, but some computers will allow you to use the network address as a real computer address.
"Subnetting" is breaking up a single network into smaller networks. To do this, you add more bits (more numbers) to the subnet mask. Traditionally, we are used to seeing subnet masks that look like 255.0.0.0, 255.255.0.0, or 255.255.255.0. However, a subnet mask might also look like 255.255.128.0 or 255.255.255.224. In both of these cases, it is obvious that the network has been subnetted to break a single network into smaller networks.
3. What is the difference between "classful" and "classless" IP addressing?
When the concept of IP addressing was first thought up, it was decided that IP addresses would be put into classes. These classes are:
ClassIP address rangeDefault subnet mask
A 1.0.0.0 to 127.255.255.255 255.0.0.0
B 128.0.0.0 to 191.255.255.255 255.255.0.0
C 192.0.0.0 to 223.255.255.255 255.255.255.0
Today, these default subnet masks aren't much used except as a point of reference and trivia. For example, if I said that your IP address was 192.168.1.1 but didn't tell you the subnet mask, it would be safe to assume that your subnet mask is 255.255.255.0 because that IP address falls into the Class C range. This is also important when you take some certification tests.
In real life, an IP address today could have any legal subnet mask. For example, you may have an IP address of 1.1.1.1 with a subnet mask of 255.255.255.240. Or you may have an IP address of 192.192.192.192 with a subnet mask of 255.0.0.0. Sometimes, people will say things like "I need an entire Class C block of addresses." This just means that they want 254 contiguous and usable IP addresses.
The term "classful" means that the IP address or software is assuming that IP addresses fall into these classes and uses the default subnet mask shown. If a routing protocol, like RIP, is classful, it has trouble with the IP addresses that don't use the default subnet masks.
On the other hand, a "classless" routing protocol, like RIP version 2, doesn't assume that IP addresses have their default subnet masks. Today, you should assume that all network devices are classless unless you find that they are not (like routing protocols RIP or IGRP, or a very old computer operating system).
4. What is a default gateway?
Contrary to popular belief, a default gateway is not a required piece of IP address configuration on any computer. However, if you want to access devices outside of your local network (such as devices on the Internet), a default gateway is required.
A default gateway is where a computer sends requests to IP addresses that are not on its local network. How does the computer know what is and what is not on its local network? As discussed above, the subnet mask is what the computer uses to know what is and what is not on its local network. Say, for example, your IP address is 1.1.1.1 and your subnet mask is 255.255.255.0, and you make a Web request to 1.1.2.1. Because of your subnet mask, your local area network is the 1.1.1.0 network. Meaning anything that is 1.1.1.1 through 254 is on your local network. Because you are requesting 1.1.2.1, which is not on your local network, that packet would be sent to your default gateway.
5. What are private IP addresses?
The private IP address space is defined by RFC1918. In this RFC, it says that no public (take that as "no Internet") devices will use or recognize the following IP addresses:
Your IP address may be the same on your PC as someone else's if you have a private IP address. These ranges of IP addresses are available for anyone to use on their own internal (private) network. There is no need to keep them unique. I can have IP address 192.168.1.1 on my home network and so can everyone else in the world! When I go to make a request to the Internet, however, that private IP address must be converted into a public IP address or else the Internet router I make the request to will just throw my request away (because I have a private IP address). Network Address Translation (NAT) is what performs this public-to-private translation (see RFC1631 and RFC2663 for more information on NAT).
Private IP addresses are there to reduce the need for more public IP addresses. An unintentional consequence is that they provide a tiny bit of security.
So, if I am trying to FTP to your computer on the Internet and you tell me that your IP address is 192.168.3.3, I will tell you "No, I need your public IP address, not your private IP address."
Link : http://searchnetworking.techtarget.com/tip/0,289483,sid7_gci1220092,00.html
You probably know what an IP address is: a number that identifies that device on the network. But what else do you need to know? IP addresses are made up of 32 bits (IPv4 addresses, that is). We normally think of an IP address as something like 1.1.1.1, but really this can be translated into eight binary bits (see Binary-to-Decimal Conversion for more information). Each set of binary bits can represent only the numbers zero through 255. That is why your IP addresses can range only from 0.0.0.0 to 255.255.255.255.
By the way, the IP address 255.255.255.255 is called the "all ones" network because in binary it is represented by 32 numeral ones (1s). The all ones address is used to send a packet to all devices on all networks (as long as it isn't stopped by a router first).
Traditionally, IP addresses were broken up into classes, but those classes aren't used much any more unless you are taking a certification exam. We will learn more about classes below.
Most importantly, IP addresses must be unique on your network. If two devices have the exact same IP address, you have an IP address conflict. When that happens, either device or both devices will not work on the network. Commonly, DHCP is used to dynamically allocate IP addresses in hopes of preventing address duplication and easing the administrative burden of static IP addressing.
Q : What is a subnet mask?
A subnet mask is what tells your computer (or other network device) what portion of the IP address is used to represent your network and what part is used to represent hosts (other computers) on your network. For example, if you have an IP address of 1.1.1.1 and a subnet mask of 255.255.255.0, the 255s mask off the first three 1s. If you did the logical "AND" (the calculation your computer does -- see Binary-to-Decimal Conversion for more information), you would find out that the network ID for this network is 1.1.1.0. Where the 0 is located, you could fill in hosts numbered 1 to 254. For example, the first host on your network is 1.1.1.1 and the last host is 1.1.1.254.
Of special note when looking at the number of hosts in a network is this: The first IP address in a network is the network address and the last IP address is always the broadcast address. That's why I couldn't use IP address 1.1.1.0 and IP address 1.1.1.255. These are special, reserved addresses, but some computers will allow you to use the network address as a real computer address.
"Subnetting" is breaking up a single network into smaller networks. To do this, you add more bits (more numbers) to the subnet mask. Traditionally, we are used to seeing subnet masks that look like 255.0.0.0, 255.255.0.0, or 255.255.255.0. However, a subnet mask might also look like 255.255.128.0 or 255.255.255.224. In both of these cases, it is obvious that the network has been subnetted to break a single network into smaller networks.
3. What is the difference between "classful" and "classless" IP addressing?
When the concept of IP addressing was first thought up, it was decided that IP addresses would be put into classes. These classes are:
ClassIP address rangeDefault subnet mask
A 1.0.0.0 to 127.255.255.255 255.0.0.0
B 128.0.0.0 to 191.255.255.255 255.255.0.0
C 192.0.0.0 to 223.255.255.255 255.255.255.0
Today, these default subnet masks aren't much used except as a point of reference and trivia. For example, if I said that your IP address was 192.168.1.1 but didn't tell you the subnet mask, it would be safe to assume that your subnet mask is 255.255.255.0 because that IP address falls into the Class C range. This is also important when you take some certification tests.
In real life, an IP address today could have any legal subnet mask. For example, you may have an IP address of 1.1.1.1 with a subnet mask of 255.255.255.240. Or you may have an IP address of 192.192.192.192 with a subnet mask of 255.0.0.0. Sometimes, people will say things like "I need an entire Class C block of addresses." This just means that they want 254 contiguous and usable IP addresses.
The term "classful" means that the IP address or software is assuming that IP addresses fall into these classes and uses the default subnet mask shown. If a routing protocol, like RIP, is classful, it has trouble with the IP addresses that don't use the default subnet masks.
On the other hand, a "classless" routing protocol, like RIP version 2, doesn't assume that IP addresses have their default subnet masks. Today, you should assume that all network devices are classless unless you find that they are not (like routing protocols RIP or IGRP, or a very old computer operating system).
4. What is a default gateway?
Contrary to popular belief, a default gateway is not a required piece of IP address configuration on any computer. However, if you want to access devices outside of your local network (such as devices on the Internet), a default gateway is required.
A default gateway is where a computer sends requests to IP addresses that are not on its local network. How does the computer know what is and what is not on its local network? As discussed above, the subnet mask is what the computer uses to know what is and what is not on its local network. Say, for example, your IP address is 1.1.1.1 and your subnet mask is 255.255.255.0, and you make a Web request to 1.1.2.1. Because of your subnet mask, your local area network is the 1.1.1.0 network. Meaning anything that is 1.1.1.1 through 254 is on your local network. Because you are requesting 1.1.2.1, which is not on your local network, that packet would be sent to your default gateway.
5. What are private IP addresses?
The private IP address space is defined by RFC1918. In this RFC, it says that no public (take that as "no Internet") devices will use or recognize the following IP addresses:
Your IP address may be the same on your PC as someone else's if you have a private IP address. These ranges of IP addresses are available for anyone to use on their own internal (private) network. There is no need to keep them unique. I can have IP address 192.168.1.1 on my home network and so can everyone else in the world! When I go to make a request to the Internet, however, that private IP address must be converted into a public IP address or else the Internet router I make the request to will just throw my request away (because I have a private IP address). Network Address Translation (NAT) is what performs this public-to-private translation (see RFC1631 and RFC2663 for more information on NAT).
Private IP addresses are there to reduce the need for more public IP addresses. An unintentional consequence is that they provide a tiny bit of security.
So, if I am trying to FTP to your computer on the Internet and you tell me that your IP address is 192.168.3.3, I will tell you "No, I need your public IP address, not your private IP address."
Link : http://searchnetworking.techtarget.com/tip/0,289483,sid7_gci1220092,00.html
October 10, 2009
Why Strings are immutable
The basic reason is that we use String pool for String objects.So if we have a number of objects that have same value, all the reference variables would be pointing to the same value in the pool. So if string were nt immutable and somebody chaged that value, the value of all the objects pointing to that object would change, which is undesired.
You might wonder why Strings are immutable in first place. There are two very compelling reasons for it:
1. Immutable basic types makes things easier. If you pass a String to a function you can be sure that its value won't change.
2. Security. With mutable Strings one could bypass security checks by changing the value right after the check. (Same thing as the first point, really.)
Detailed Description :
http://kaioa.com/node/59
You might wonder why Strings are immutable in first place. There are two very compelling reasons for it:
1. Immutable basic types makes things easier. If you pass a String to a function you can be sure that its value won't change.
2. Security. With mutable Strings one could bypass security checks by changing the value right after the check. (Same thing as the first point, really.)
Detailed Description :
http://kaioa.com/node/59
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