Web container

Web container (also known as a Servlet container) is the component of a web server that interacts with the servlets. A web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights. A web container implements the web component contract of theJava EE architecture, specifying a runtime environment for web components that includes security, concurrency, lifecycle management,transaction, deployment, and other services

Java IO

Programs use byte streams to perform input and output of 8-bit bytes. All byte stream classes are descended from InputStream and OutputStream.

InputStream.read();

OutputStream.write(c)

both are abstract classes.

http://docs.oracle.com/javase/tutorial/essential/io/bytestreams.html

FileInputStream in = null;         FileOutputStream out = null;          try {             in = new FileInputStream("xanadu.txt");             out = new FileOutputStream("outagain.txt");

Closing a stream when it's no longer needed is very important — so important that CopyBytes uses afinally block to guarantee that both streams will be closed even if an error occurs. This practice helps avoid serious resource leaks. 

all other stream types are built on byte streams. 

--------------

All character stream classes are descended from Reader and Writer. 

Reader.read();

Writer.write(c)

Line-Oriented IO has to use bufferedReader and BufferedWriter

--

Above are unbuffered IO,This means each read or write request is handled directly by the underlying OS.

to reduce overhead, the Java platform implements buffered I/O streams. 

using wrapping approach

inputStream = new BufferedReader(new FileReader("xanadu.txt")); outputStream = new BufferedWriter(new FileWriter("characteroutput.txt"));
Decorator Design pattern!!!

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Objects of type Scanner are useful for breaking down formatted input into tokens and translating individual tokens according to their data type. 

 s = new Scanner(new BufferedReader(new FileReader("xanadu.txt")));              while (s.hasNext()) {                 System.out.println(s.next());             }
s.useDelimiter(",\\s*");
s.hasNextDouble();
---
Stream objects that implement formatting are instances of either PrintWriter, a character stream class, or PrintStream, a byte stream class. 
 System.out  is PrintStream 
 

Decorator Pattern

Attaches additional responsibilities to an object dynamically. Decorators provides a flexible alternative to subclassing for extending functionality.

http://javapapers.com/design-patterns/decorator-pattern/

(abstract class) compontent{

m1();

m2();

}

concreteComponent{

m1();

m2();

}

(abstract) Decorator extends component{

m1();

m2();

}

you can use the decorator to decorate concreteComponent to concreteNewComponent with something additional

Design pattern book 95~98

Beverage beverage = new Espresso();

Beverage beverage2 = new DarkRoast();

beverage2 = new Mocha(beverage2);

beverage2 = new Mocha(beverage2); // double mocha

beverage2 = new Whip(beverage2); // with whip cream

HOW PL/SQL GETS EXECUTED

http://comsci.liu.edu/~vasilaky/db2/lec1.htm

the PL/SQL engine on the server where it is compiled. The named PL/SQL
block is compiled only at the time of its creation, or if it has been
changed. The compilation process includes syntax checking, binding,
and p-code generation.

Oracle guarantees a read-consistent view
of the data. Until that point, all data that has been inserted or updated
will be held in memory and only be available to the current user. The
rows that have been changed will be locked by the current user and will
not be available for updating to other users until the locks have been re-leased.

Difference between web server and app server

http://www.javaworld.com/javaworld/javaqa/2002-08/01-qa-0823-appvswebserver.html

A Web server exclusively handles HTTP requests, whereas an application server serves business logic to application programs through any number of protocols.

table scan and indexes

http://msdn.microsoft.com/en-us/library/aa224773(v=sql.80).aspx

Servlet basic

SampleServlet extends HttpServlet{

init();

service(httprequest, httpresponse);

}

server engine call service() to handle request, return response.

when the servlet first loaded, init() is called once for initialization

after service() is done, servlet waits till next request

the server can decide to unload this servlet by calling destroy(), you want to

release all the shared resources that were created in init()

two packages

javax.servlet

javax.servlet.http

HttpServlet class methods

service()--serve all requests,defaut implementation is calling doGet, doPost,doDelete,doPut in response to http request

doHeader() defaut calling doGet(), chop off body, only return header

Thread safe Servlet

An application is thread safe if it always behaves predictablly regardless of the number of concurrent threads running in this

process space.

Synchronization in java is the process of locking a variable so that it can be read and modified by only a single thread at a time.

The synchronized keyword eliminates race conditions by locking variables for a specific duration

synchronized(this){

...

..

}

it is possible to synchronize the entire service() method rather than blocks of code, in this way, only on thread at a time

can execute the method. the more code is synchronized, the greater the performance penalty. To aoid this penalty,minimie the amount

of code.

a new servlet object is not instantiated for each new client request, Rather, a single servlet usually services all requests.

This behavior explains how instance variables can often be shared

Because every call to service(), doget(), doPost() is executed in its own thread, variables local to these methods are not shared between

requests and are automatically thread safe.

public class AServlet extends HttpServlet

{

//instance variable

String UserName; // not thread safe

}

public class BServlet extends HttpServlet

{

//instance variable

String userName; // not thread safe

public void service(){

String userName; //thread safe

}

or use SingleThreadModel interface

public class CServlet extends HttpServlet implements SingleThreadModel

{

//instance variable

String userName; // thread safe

}

If servlet implemnts the SinglethreadModel interface, the server guarantees that no more than on thread can execute the service,

doGet, doPost method at a time for a particular servlet instance. this gurantees thread safe.

Http redirects

An Http redirect is a set of instructions included in the header of an Http response that instruct the browser

to issue a new request to a new URL.

An HTTP redirect response uses an HTTP status code of either '301 Moved Permanently" or "302 Moved Temporarily" accompanied

by a Location header containing a valid URL.the browser receives an Http response with status code 301,302,it will

immediately request the resource at the URL designated by the Location Header

sample redirect response

HTTP/1.1 301 Moved Permanently

..

Location:http://www..../....html

sending a redirect with the servlet API

two ways:

service(HttpServletRequest request,HttpServletResponse response){

..

reponse.status(HttpServletResponse.SC_MOVED_PERMANENTLY);

response.setHeader("Location", someurl);

}

or

response.setRedirect(url); //this way only set 302, url has to be full qualified url