CORE JAVA /OOPS Q1: What are the advantages of OOPL?
Ans: Ans: Object Object orient oriented ed progra programmi mming ng langua languages ges direct directly ly repres represen entt the real real life objects. The features of OOPL as inhreitance, polymorphism, encapsulation makes it powerful. Q2: What do mean ! "o#!mor"h$s%m& $nher$tan'e& en'a"s%#at$on?
Ans: Polymorhisum: is a feature of OOPl that at run time depending upon the type of object the appropriate method is called. nheritance: is a feature of OOPL that represents the !is a! relationship between different objects"classes#. $ay in real life a manager is a employee. $o in OOPL manger class is inherited from the employee class. %ncapsulation: is a feature of OOPL that is used to hide the information. Q(: What do !o% mean ! stat$' methods?
Ans: &y using the static method there is no need creating an object of that class to use that method. 'e can directly call that method on that class. (or e)ample, say class A has static function f"#, then we can call f"# function as A.f"#. There is no need of creating an object of class A. Q): What do !o% mean ! v$rt%a# methods?
Ans: *irtual methods are used to use the polymorphism feature in +. $ay class A is inherited from class &. f we declare say function f"# as *irtual in class & and o*erride the same function in class A then at runtime appropriate method of the class will be called depending upon the type of the object. Q*: +$ven t,o ta#es St%dent-S.& 0ame& Co%rse and Leve#-S.& #eve# ,r$te the SQL statement to get the name and S. of the st%dent ,ho are ta$ng 'o%rse 3 ( and at freshman #eve#4
Ans: $%L%+T $tudent. name, $tudent. $(O/ $tudent, Le*el '0%% $tudent. $- 1 Le*el. $A2- Le*el. Le*el 1 !freshman! A2- $tudent. +ourse 1 34
Q5: What are the d$sadvantages of %s$ng threads?
Ans: -eadLock. Q1: Wr$te the Java 'ode to de'#are an! 'onstant -sa! grav$tat$ona# 'onstant and to get $ts va#%e
Ans: +lass A&+ 5 static final float 6A7TATO2AL8+O2$TA2T 6A7TATO2AL8+O2$TA2T 1 9.4 public *oid get+onstant"# 5 system.out.println"!6ra*itational8+onstant: system.out.println"!6ra*ita tional8+onstant: ! 6A7TATO2AL8+O2$TA2T#4 6A7TATO2AL8+O2$TA2T#4 ; ; Q2: What do !o% mean ! m%#t$"#e $nher$tan'e $n C66 ? Ans: /ultiple inheritance is a feature in + by which one class can be of different types. $ay class teachingAssistant is inherited from two classes say teacher and $tudent. Q(: Can !o% ,r$te Java 'ode for de'#arat$on of m%#t$"#e $nher$tan'e $n Java ?
Ans: +lass + e)tends A implements & 5 ; <. 'hat are the Objec Objectt and and +lass +lass classe classess used used for= for= The Object class is the highest>le*el class in the ?a*a class hierarchy. The +lass class is used to represent the classes and interfaces that are loaded by a ?a*a program. @. 'hat is $eria $erialiat liation ion and deseriali deserialiatio ation= n= $erialiation is the process of writing the state of an object to a byte stream. -eserialiation -eserialiation is the t he process of restoring these objects. <. 'hat 'hat inter interfa face ce must must an objec objectt impl implem emen entt befo before re it can can be writt written en to a stream as an object=
An object must implement the $erialiable or %)ternaliable interface before it can be written to a stream as an object. @. 'hat is the esource&undle class= The esource&undle class is used to store locale>specific resources that can be loaded by a program to tailor the programBs appearance to the particular locale in which it is being run. <. 'hat class allows you to read objects directly from a stream= The Objectnput$tream class supports the reading of objects from input streams. @. 0ow are this"# and super"# used with constructors= this"# is used to in*oke a constructor of the same class. super"# is used to in*oke a superclass constructor. 3. 0ow is it possible for two $tring objects with identical *alues not to be eCual under the 11 operator= The 11 operator compares two objects to determine if they are the same object in memory. t is possible for two $tring objects to ha*e the same *alue, but located indifferent areas of memory. <. 0ow does multithreading take place on a computer with a single +PD= The operating systemBs task scheduler allocates e)ecution time to multiple tasks. &y Cuickly switching between e)ecuting tasks, it creates the impression that tasks e)ecute seCuentially. @. 'hat restrictions are placed on method o*erloading= Two methods may not ha*e the same name and argument list but different return types. 3. 'hat restrictions are placed on method o*erriding= O*erridden methods must ha*e the same name, argument list, and return type. The o*erriding method may not limit the access of the method it o*errides. The o*erriding method may not throw any e)ceptions that may not be thrown by the o*erridden method.
<. 'hat happens when a thread cannot acCuire a lock on an object= f a thread attempts to e)ecute a synchronied method or synchronied statement and is unable to acCuire an objectBs lock, it enters the waiting state until the lock becomes a*ailable. @. 'hat is the difference between the eaderE'riter class hierarchy and the nput$treamEOutput$tream class hierarchy= The eaderE'riter class hierarchy is character>oriented, and the nput$treamEOutput$tream class hierarchy is byte>oriented. 3. 'hat classes of e)ceptions may be caught by a catch clause= A catch clause can catch any e)ception that may be assigned to the Throwable type. This includes the %rror and %)ception types. <. 'hat is the purpose of finaliation= The purpose of finaliation is to gi*e an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. <. 'hat is synchroniation and why is it important= 'ith respect to multithreading, synchroniation is the capability to control the access of multiple threads to shared resources. 'ithout synchroniation, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that objectBs *alue. This often causes dirty data and leads to significant errors. @. 'hat are synchronied methods and synchronied statements= $ynchronied methods are methods that are used to control access to an object. A thread only e)ecutes a synchronied method after it has acCuired the lock for the methodBs object or class. $ynchronied statements are similar to synchronied methods. A synchronied statement can only be e)ecuted after a thread has acCuired the lock for the object or class referenced in the synchronied statement.
<. 'hat is the serialiation= The serialiation is a kind of mechanism that makes a class or a bean persistence by ha*ing its properties or fields and state information sa*ed and restored to and from storage. @. 0ow to make a class or a bean serialiable= &y implementing either the ja*a.io.$erialiable interface, or the ja*a.io.%)ternaliable interface. As long as one class in a classBs inheritance hierarchy implements $erialiable or %)ternaliable, that class is serialiable. 3. 0ow many methods in the $erialiable interface= There is no method in the $erialiable interface. The $erialiable interface acts as a marker, telling the object serialiation tools that your class is serialiable. F. 0ow many methods in the %)ternaliable interface= There are two methods in the %)ternaliable interface. Gou ha*e to implement these two methods in order to make your class e)ternaliable. These two methods are read%)ternal"# and write%)ternal"#. H. 'hat is the difference between $erialialble and %)ternaliable interface= 'hen you use $erialiable interface, your class is serialied automatically by default. &ut you can o*erride writeObject"# and readObject"# two methods to control more comple) object serailiation process. 'hen you use %)ternaliable interface, you ha*e a complete control o*er your classBs serialiation process. I. 'hat is a transient *ariable= A transient *ariable is a *ariable that may not be serialied. f you donBt want some field not to be serialied, you can mark that field transient or static.
Conne't$ng to a ataase J7C Q%est$ons
<. 'hat is ?-&+= ?-&+ is a layer of abstraction that allows users to choose between databases. t allows you to change to a different database engine and to write to a single AP. ?-&+ allows you to write database applications in ?a*a without ha*ing to concern yourself with the underlying details of a particular database. @. 'hat are the two major components of ?-&+= One implementation interface for database manufacturers, the other implementation interface for application and applet writers. 3. 'hat is ?-&+ -ri*er interface= The ?-&+ -ri*er interface pro*ides *endor>specific implementations of the abstract classes pro*ided by the ?-&+ AP. %ach *endors dri*er must pro*ide implementations of the ja*a.sCl.+onnection,$tatement,Prepared$tatement, +allable$tatement, esult$et and -ri*er. F. 'hat are the common tasks of ?-&+= +reate an instance of a ?-&+ dri*er or load ?-&+ dri*ers through jdbc.dri*ers egister a dri*er $pecify a database Open a database connection $ubmit a Cuery ecei*e results H. 'hat packages are used by ?-&+= o
o o o o o
There are packages: ja*a.sCl.-ri*er, +onnection,$tatement, Prepared$tatement, +allable$tatement, esult$et, esult$et/eta-ata, -atabase/eta-ata. I. 'hat are the flow statements of ?-&+= A DL string >>Jget+onnection>>J-ri*er/anager>>J-ri*er>>J+onnection>> J$tatement>>Je)ecuteKuery>>Jesult$et.
. 'hat are the steps in*ol*ed in establishing a connection= This in*ol*es two steps: "<# loading the dri*er and "@# making the connection. . 0ow can you load the dri*ers= Loading the dri*er or dri*ers you want to use is *ery simple and in*ol*es just one line of code. f, for e)ample, you want to use the ?-&+>O-&+ &ridge dri*er, the following code will load it: %g. +lass.for2ame"!sun.jdbc.odbc.?dbcOdbc-ri*er!#4 Gour dri*er documentation will gi*e you the class name to use. (or instance, if the class name is jdbc.-ri*erMGN , you would load the dri*er with the following line of code: %.g. +lass.for2ame"!jdbc.-ri*erMGN!#4 9. 'hat +lass.for2ame will do while loading dri*ers= t is used to create an instance of a dri*er and register it with the -ri*er/anager. 'hen you ha*e loaded a dri*er, it is a*ailable for making a connection with a -&/$. <. 0ow can you make the connection= n establishing a connection is to ha*e the appropriate dri*er connect to the -&/$. The following line of code illustrates the general idea: %.g. $tring url 1 !jdbc:odbc:(red!4 +onnection con 1 -ri*er/anager.get+onnection"url, !(ernanda!, !?!#4 <<. 0ow can you create ?-&+ statements= A $tatement object is what sends your $KL statement to the -&/$. Gou simply create a $tatement object and then e)ecute it, supplying the appropriate e)ecute method with the $KL statement you want to send. (or a $%L%+T statement, the method to use is e)ecuteKuery. (or statements that create or modify tables, the method to use is e)ecuteDpdate. %.g. t takes an instance of an acti*e connection to create a $tatement object. n the following e)ample, we use our +onnection object con to create the $tatement object stmt :
$tatement stmt 1 con.create$tatement"#4 <@. 0ow can you retrie*e data from the esult$et= (irst ?-&+ returns results in a esult$et object, so we need to declare an instance of the class esult$et to hold our results. The following code demonstrates declaring the esult$et object rs. %.g. esult$et rs 1 stmt.e)ecuteKuery"!$%L%+T +O(82A/%, P+% (O/ +O((%%$!#4 $econd: $tring s 1 rs.get$tring"!+O(82A/%!#4 The method get$tring is in*oked on the esult$et object rs , so get$tring will retrie*e "get# the *alue stored in the column +O(82A/% in the current row of rs <3. 'hat are the different types of $tatements= <.$tatement "use create$tatement method# @. Prepared $tatement "Dse prepare$tatement method# and 3. +allable $tatement "Dse prepare+all#
while "warning Q1 null# 5 $ystem.out.println"!/essage: ! warning.get/essage"##4 $ystem.out.println"!$KL$tate: ! warning.get$KL$tate"##4 $ystem.out.print"!7endor error code: !#4 $ystem.out.println"warning.get%rror+ode"##4 warning 1 warning.get2e)t'arning"#4 ;
; <. 0ow to /ake Dpdates to Dpdatable esult $ets= Another new feature in the ?-&+ @. AP is the ability to update rows in a result set using methods in the ?a*a programming language rather than ha*ing to send an $KL command. &ut before you can take ad*antage of this capability, you need to create a esult$et object that is updatable. n order to do this, you supply the esult$et constant +O2+D8DP-ATA&L% to the create$tatement method. %.g. +onnection con 1 -ri*er/anager.get+onnection"!jdbc:my$ubprotocol:my$ub2ame!#4 $tatement stmt 1 con.create$tatement"esult$et.TGP%8$+OLL8$%2$T7%, esult$et.+O2+D8DP-ATA&L%#4 esult$et uprs 1 "!$%L%+T +O(82A/%, P+% (O/ +O((%%$!#4
Str$ngs 8A0L.0+
Constr%'t$ng a Str$ng
f you are constructing a string with se*eral appends, it may be more efficient to construct it using a $tring&uffer and then con*ert it to an immutable $tring object. $tring&uffer buf 1 new $tring&uffer"!nitial Te)t! #4 EE /odify int inde) 1 <4 buf.insert"inde), !abc! #4 buf.append"!def! #4 EE +on*ert to string $tring s 1 buf.to$tring"#4 6etting a $ubstring from a $tring int start 1 < 4 int end 1 F 4 $tring substr 1 !a$tring! .substring"start, end#4 EE $tr
Sear'h$ng a Str$ng
$tring string 1 !a$tring! 4 EE (irst occurrence. int inde) 1 string.inde)Of" B$B #4 EE < EE Last occurrence. inde) 1 string.lastnde)Of"BiB #4 EE F EE 2ot found.
inde) 1 string.lastnde)Of"BAB #4 EE ><
Re"#a'$ng Chara'ters $n a Str$ng
EE eplace all occurrences of BaB with BoB $tring new$tring 1 string .replace"BaB , BoB #4
Re"#a'$ng S%str$ngs $n a Str$ng
static $tring replace"$tring str, $tring pattern, $tring replace# 5 int s 1 4 int e 1 4 $tring&uffer result 1 new $tring&uffer"#4 while ""e 1 str.inde)Of"pattern, s## J1 # 5 result.append"str.substring"s, e##4 result.append"replace#4 s 1 epattern.length"#4 ; result.append"str.substring"s##4 return result.to$tring"#4 ; Convert$ng a Str$ng to 9""er or Lo,er Case // Convert to %""er 'ase $tring upper 1 string .toDpper+ase"#4
EE +on*ert to lower case $tring lower 1 string .toLower+ase"#4
Convert$ng a Str$ng to a 0%mer int i 1 nteger.parsent"!<@3! #4 long l 1 Long.parseLong"!<@3! #4 float f 1 (loat.parse(loat"!<@3.F! #4
double d 1 -ouble.parse-ouble"!<@3.Fe
7rea$ng a Str$ng $nto Words $tring a$tring 1 !word< word@ word3! 4 $tringTokenier parser 1 new $tringTokenier"a$tring#4 while "parser.has/oreTokens"## 5 process'ord "parser.ne)tToken"##4
JSP /Serv#ets
<. 'hat is the ser*let= $er*lets are modules that e)tend reCuestEresponse>oriented ser*ers, such as ?a*a>enabled web ser*ers. (or e)ample, a ser*let may be responsible for taking data in an 0T/L order>entry form and applying the business logic used to update a companyBs order database. @. 'hatBs the difference between ser*lets and applets= $er*lets are to ser*ers4 applets are to browsers. Dnlike applets, howe*er, ser*lets ha*e no graphical user interface. 3. 'hatBs the ad*antages using ser*lets than using +6= $er*lets pro*ide a way to generate dynamic documents that is both easier to write and faster to run. t is efficient, con*enient, powerful, portable, secure and ine)pensi*e. $er*lets also address the problem of doing ser*er> side programming with platform>specific APs: they are de*eloped with ?a*a $er*let AP, a standard ?a*a e)tension. F. 'hat are the uses of $er*lets= A ser*let can handle multiple reCuests concurrently, and can synchronie reCuests. This allows ser*lets to support systems such as on>line conferencing. $er*lets can forward reCuests to other ser*ers and ser*lets. Thus ser*lets can be used to balance load among se*eral ser*ers that mirror the same content, and to partition a single logical ser*ice o*er se*eral ser*ers, according to task type or organiational boundaries.
H. 'hatBs the $er*let nterface= The central abstraction in the $er*let AP is the $er*let interface. All ser*lets implement this interface, either directly or, more commonly, by e)tending a class that implements it such as 0ttp$er*let. $er*lets>>J6eneric $er*let>>J0ttp$er*let>>J/y$er*let. The $er*let interface declares, but does not implement, methods that manage the ser*let and its communications with clients. $er*let writers pro*ide some or all of these methods when de*eloping a ser*let. I. 'hen a ser*let accepts a call from a client, it recei*es two objects. 'hat are they= $er*elteCuest: which encapsulates the communication from the client to the ser*er. $er*letesponse: which encapsulates the communication from the ser*let back to the client. $er*leteCuest and $er*letesponse are interfaces defined by the ja*a).ser*let package. . 'hat information that the $er*leteCuest interface allows the ser*let access to= nformation such as the names of the parameters passed in by the client, the protocol "scheme# being used by the client, and the names of the remote host that made the reCuest and the ser*er that recei*ed it. The input stream, $er*letnput$tream.$er*lets use the input stream to get data from clients that use application protocols such as the 0TTP PO$T and PDT methods. . 'hat information that the $er*letesponse interface gi*es the ser*let methods for replying to the client= t Allows the ser*let to set the content length and //% type of the reply. Pro*ides an output stream, $er*letOutput$tream and a 'riter through which the ser*let can send the reply data. 9. f you want a ser*let to take the same action for both 6%T and PO$T reCuest, what should you do=
$imply ha*e do6et call doPost, or *ice *ersa. <. 'hat is the ser*let life cycle= %ach ser*let has the same life cycle: A ser*er loads and initialies the ser*let "init"## The ser*let handles ero or more client reCuests "ser*ice"## The ser*er remo*es the ser*let "destroy"## "some ser*ers do this step only when they shut down# <<. 'hich code line must be set before any of the lines that use the Print'riter= set+ontentType"# method must be set before transmitting the actual document. <@. 0ow 0TTP $er*let handles client reCuests= An 0TTP $er*let handles client reCuests through its ser*ice method. The ser*ice method supports standard 0TTP client reCuests by dispatching each reCuest to a method designed to handle that reCuest.
Q%est$on
0ow do you pre*ent the +reation of a $ession in a ?$P Page and why= ?$P
&y default, a ?$P page will automatically create a session for the reCuest if one does not e)ist. 0owe*er, sessions consume resources and if it is not necessary to maintain a session, one should not be created. (or e)ample, a marketing campaign may suggest the reader *isit a web page for more information. f it is anticipated that a lot of traffic will hit that page, you Ans,er may want to optimie the load on the machine by not creating useless sessions. The page directi*e is used to pre*ent a ?$P page from automatically creating a session: RS page session1!false!J
Q%est$on
s it possible to share an 0ttp$ession between a ?$P and %?&= 'hat happens when change a *alue in the 0ttp$ession from inside an %?&= %?&
Gou can pass the 0ttp$ession as parameter to an %?& method, only if all objects in session are serialiable.This has to be consider as !passed>by> *alue!, that means that itBs read>only in the %?&. f anything is altered from inside the %?&, it wonBt be reflected back to the 0ttp$ession of the $er*let +ontainer.The !pass>by>reference! can be used between %?&s emote nterfaces, as they are remote references. 'hile it $ possible to pass an 0ttp$ession as a parameter to an %?& object, it is considered to be !bad practice "<#! in terms of object oriented design. This is because Ans,er you are creating an unnecessary coupling between back>end objects "ejbs# and front>end objects "0ttp$ession#. +reate a higher>le*el of abstraction for your ejbBs api. ather than passing the whole, fat, 0ttp$ession "which carries with it a bunch of http semantics#, create a class that acts as a *alue object "or structure# that holds all the data you need to pass back and forth between front>endEback>end. +onsider the case where your ejb needs to support a non>http>based client. This higher le*el of abstraction will be fle)ible enough to support it. "<# +ore ?@%% design patterns "@<# ?$P
Q%est$on 0ow can implement a thread>safe ?$P page=
Gou can make your ?$Ps thread>safe by ha*ing them implement the Ans,er $ingleThread/odel interface. This is done by adding the directi*e RS page isThread$afe1!false! S J within your ?$P page. Q%est$on 0ow do include static files within a ?$P page=
?$P
$tatic resources should always be included using the ?$P include directi*e. This way, the inclusion is performed just once during the translation phase. The following e)ample shows the synta): -o note that Ans,er you should always supply a relati*e DL for the file attribute. Although you can also include static resources using the action, this is not ad*isable as the inclusion is then performed for each and e*ery reCuest. Q%est$on 'hat ?$P lifecycle methods can o*erride= Ans,er
?$P
Gou cannot o*erride the 8jsp$er*ice"# method within a ?$P page. Gou can howe*er, o*erride the jspnit"# and jsp-estroy"# methods within a ?$P page. jspnit"# can be useful for allocating resources like database connections, network connections, and so forth for the ?$P page. t is good programming practice to free any allocated resources within jsp-estroy"#. The jspnit"# and jsp-estroy"# methods are each e)ecuted just once
during the lifecycle of a ?$P page and are typically declared as ?$P declarations: RSQ public *oid jspnit"# 5 . . . ; SJ RSQ public . ; SJ
*oid
jsp-estroy"# .
5 .
Q%est$on 0ow do include static files within a ?$P page=
?$P
$tatic resources should always be included using the ?$P include directi*e. This way, the inclusion is performed just once during the translation phase. The following e)ample shows the synta): -o note that Ans,er you should always supply a relati*e DL for the file attribute. Although you can also include static resources using the action, this is not ad*isable as the inclusion is then performed for each and e*ery reCuest.
Q%est$on 0ow do mi) ?$P and $$ Uinclude=
Ans,er
?$P
f youBre just including raw 0T/L, use the Uinclude directi*e as usual inside your .jsp file. RQ>>Uinclude file1!data.inc!>>J &ut itBs a little trickier if you want the ser*er to e*aluate any ?$P code thatBs inside the included file. f your data.inc file contains jsp code you will ha*e to use RS *include1!data.inc! SJ The RQ>>Uinclude file1!data.inc!>>J is used for including non>?$P files.
Q%est$on +an a ?$P page process 0T/L (O/ data= Ans,er
?$P
Ges. 0owe*er, unlike ser*lets, you are not reCuired to implement 0TTP> protocol specific methods like do6et"# or doPost"# within your ?$P page.
Gou can obtain the data for the (O/ input elements *ia the reCuest implicit object within a scriptlet or e)pression as: RS $tring item 1 reCuest.getParameter"!item!#4 int how/any 1 new nteger"reCuest.getParameter"!units!##.int7alue"#4 SJ or RS1 reCuest.getParameter"!item!# SJ
Q%est$on 'hat ?$P lifecycle methods can o*erride=
?$P
Gou cannot o*erride the 8jsp$er*ice"# method within a ?$P page. Gou can howe*er, o*erride the jspnit"# and jsp-estroy"# methods within a ?$P page. jspnit"# can be useful for allocating resources like database connections, network connections, and so forth for the ?$P page. t is good programming practice to free any allocated resources within jsp-estroy"#. The jspnit"# and jsp-estroy"# methods are each e)ecuted just once during the lifecycle of a ?$P page and are typically declared as ?$P declarations: RSQ Ans,er public *oid jspnit"# 5 . . . ; SJ RSQ public . ; SJ
*oid
jsp-estroy"# .
5 .
Q%est$on 0ow do include static files within a ?$P page= Ans,er
?$P
$tatic resources should always be included using the ?$P include directi*e. This way, the inclusion is performed just once during the translation phase. The following e)ample shows the synta): RS include file1!copyright.html! SJ -o note that you should always supply a relati*e DL for the file
attribute. Although you can also include static resources using the action, this is not ad*isable as the inclusion is then performed for each and e*ery reCuest.
Q%est$on 0ow do perform browser redirection from a ?$P page=
?$P
Gou can use the response implicit object to redirect the browser to a different resource, as: response.sendedirect"!http:EEwww.foo.comEpathEerror.html!#4 Gou can also physically alter the Location 0TTP header attribute, as shown below: RS response.set$tatus"0ttp$er*letesponse.$+8/O7%-8P%/A2%2TLG#4 $tring newLocn 1 !EnewpathEinde).html!4 Ans,er response.set0eader"!Location!,newLocn#4 SJ Gou can also use the: Rjsp:forward page1!Enewpage.jsp! EJ Also note that you can only use this before any output has been sent to the client. bele*e this is the case with the response.sendedirect"# method as well. f you want to pass any paramateres then you can pass using Rjsp:forward page1!Eser*letElogin!J Rjsp:param name1!username! *alue1!jsmith! EJ REjsp:forwardJJ
Q%est$on +an a ?$P page instantiate a serialied bean=
?$P
2o problemQ The use&ean action specifies the bean2ame attribute, which can be used for indicating a serialied bean. (or e)ample: Rjsp:use&ean id1!shop! type1!shopping.+-! bean2ame1!+-! EJ Rjsp:getProperty name1!shop! property1!album! EJ Ans,er A couple of important points to note. Although you would ha*e to name your serialied file !filename.ser!, you only indicate !filename! as the *alue for the bean2ame attribute. Also, you will ha*e to place your serialied file within the '%&>2(VjspVbeans directory for it to be located by the ?$P engine.
Q%est$on
+an you make use of a $er*letOutput$tream object from within a ?$P page= ?$P
2o. Gou are supposed to make use of only a ?$P'riter object "gi*en to you in the form of the implicit object out# for replying to clients. A ?$P'riter can be *iewed as a buffered *ersion of the stream object Ans,er returned by response.get'riter"#, although from an implementational perspecti*e, it is not. A page author can always disable the default buffering for any page using a page directi*e as: RS page buffer1!none! SJ
Q%est$on
Ans,er
Q%est$on Ans,er
'hatBs a better approach for enabling thread>safe ser*lets and ?$Ps= $ingleThread/odel nterface or $ynchroniation= ?$P Although the $ingleThread/odel techniCue is easy to use, and works well for low *olume sites, it does not scale well. f you anticipate your users to increase in the future, you may be better off implementing e)plicit synchroniation for your shared data. The key howe*er, is to effecti*ely minimie the amount of code that is synchronied so that you take ma)imum ad*antage of multithreading. Also, note that $ingleThread/odel is pretty resource intensi*e from the ser*erBs perspecti*e. The most serious issue howe*er is when the number of concurrent reCuests e)haust the ser*let instance pool. n that case, all the unser*iced reCuests are Cueued until something becomes free > which results in poor performance. $ince the usage is non>deterministic, it may not help much e*en if you did add more memory and increased the sie of the instance pool.
+an stop ?$P e)ecution while in the midst of processing a reCuest= ?$P
Ges. Preempti*e termination of reCuest processing on an error condition is a good way to ma)imie the throughput of a high>*olume ?$P engine. The trick "asuming ?a*a is your scripting language# is to use the return statement when you want to terminate further processing. (or e)ample, consider: RS if "reCuest.getParameter"!foo!# Q1 null# 5 EE generate some html or update bean property ; else 5 EW output some error message or pro*ide redirection back to the input form after creating a memento bean updated with the B*alidB form elements that were input. this bean can now be used by the pre*ious form
to initialie the input elements that were *alid then, return from the body of the 8jsp$er*ice"# method to terminate further processing WE return4 ; SJ
Q%est$on s there a way to reference the !this! *ariable within a ?$P page= Ans,er
Ges, there is. Dnder ?$P <., the page implicit object is eCui*alent to !this!, and returns a reference to the ser*let generated by the ?$P page.
Q%est$on Ans,er
?$P
0ow do instantiate a bean whose constructor accepts parameters using the use&ean tag= ?$P +onsider the following bean: package bar4 public class (oo&ean 5 public (oo&ean"$omeObj arg# 5 ... ; EEgetters and setters here ; The only way you can instantiate this bean within your ?$P page is to use a scriptlet. (or e)ample, the following snippet creates the bean with session scope: Xl4S $omeObj ) 1 new $omeObj"...#4 bar.(oo&ean foobar 1 new (oo&ean")#4 session.put7alue"!foobar!,foobar#4 SJ Gou can now access this bean within any other page that is part of the same session as: Xl4S bar.(oo&ean foobar 1 session.get7alue"!foobar!#4 SJ To gi*e the bean !application scope!, you will ha*e to place it within the $er*let+onte)t as: Xl4S application.setAttribute"!foobar!,foobar#4 SJ To gi*e the bean !reCuest scope!, you will ha*e to place it within the reCuest object as: Xl4S
reCuest.setAttribute"!foobar!,foobar#4 SJ f you do not place the bean within the reCuest, session or application scope, the bean can be accessed only within the current ?$P page "page scope#. Once the bean is instantiated, it can be accessed in the usual way: jsp:getProperty name1!foobar! property1!someProperty!E jsp:setProperty name1!foobar! property1!someProperty! *alue1!some7alue!E
Q%est$on +an in*oke a ?$P error page from a ser*let= Ans,er
?$P
Ges, you can in*oke the ?$P error page and pass the e)ception object to it from within a ser*let. The trick is to create a reCuest dispatcher for the ?$P error page, and pass the e)ception object as a ja*a).ser*let.jsp.jsp%)ception reCuest attribute. 0owe*er, note that you can do this from only within controller ser*lets. f your ser*let opens an Output$tream or Print'riter, the ?$P engine will throw the following translation error: ja*a.lang.llegal$tate%)ception: +annot forward as Output$tream or 'riter has already been obtained The following code snippet demonstrates the in*ocation of a ?$P error page from within a controller ser*let: protected *oid send%rroredirect"0ttp$er*leteCuest reCuest, 0ttp$er*letesponse response, $tring errorPageDL, Throwable e# throws $er*let%)ception, O%)ception 5 reCuest.setAttribute "!ja*a).ser*let.jsp.jsp%)ception!, e#4 get$er*let+onfig"#.get$er*let+onte)t"#. geteCuest-ispatcher"errorPageDL#.forward"reCuest, response#4 ; public *oid doPost"0ttp$er*leteCuest reCuest, 0ttp$er*letesponse response# 5 try 5 EE do something ; catch "%)ception e)# 5 try 5 send%rroredirect"reCuest,response,!EjspE/y%rrorPage.jsp!,e)#4 ; catch "%)ception e# 5 e.print$tackTrace"#4 ;
; ; Q%est$on +an just abort processing a ?$P= Ans,er
$er*lets
Ges. &ecause your ?$P is just a ser*let method, you can just put "wheree*er necessary# a R S return4 S J
Q%est$on 0ow does ?$P handle run>time e)ceptions=
?$P
Gou can use the errorPage attribute of the page directi*e to ha*e uncaught run>time e)ceptions automatically forwarded to an error processing page. (or e)ample: RS page errorPage1!error.jsp! SJ redirects the browser to the ?$P page error.jsp if an uncaught e)ception is encountered during reCuest processing. 'ithin error.jsp, if you indicate Ans,er that it is an error>processing page, *ia the directi*e: RS page is%rrorPage1!true! SJ the Throwable object describing the e)ception may be accessed within the error page *ia the e)ception implicit object. 2ote: Gou must always use a relati*e DL as the *alue for the errorPage attribute.
Q%est$on
Ans,er
0ow do pre*ent the output of my ?$P or $er*let pages from being cached by the browser= ?$P Gou will need to set the appropriate 0TTP header attributes to pre*ent the dynamic content output by the ?$P page from being cached by the browser. ?ust e)ecute the following scriptlet at the beginning of your ?$P pages to pre*ent them from being cached at the browser. Gou need both the statements to take care of some of the older browser *ersions. RS response.set0eader"!+ache>+ontrol!,!no>store!#4 EE0TTP <.< response.set0eader"!Pragma!,!no>cache!#4 EE0TTP <. response.set-ate0eader "!%)pires!, #4 EEpre*ents caching at the pro)y ser*er SJ
Q%est$on 0ow do use comments within a ?$P page=
?$P
Gou can use !?$P>style! comments to selecti*ely block out code while debugging or simply to comment your scriptlets. ?$P comments are not *isible at the client. (or e)ample: RS>> the scriptlet is now commented out RS out.println"!0ello 'orld!#4 SJ >>SJ Gou can also use 0T/L>style comments anywhere within your ?$P page. These comments are *isible at the client. (or e)ample: Ans,er RQ>> "c# @F ja*agala)y.com >>J Of course, you can also use comments supported by your ?$P scripting language within your scriptlets. (or e)ample, assuming ?a*a is the scripting language, you can ha*e: RS EEsome comment EWW yet another comment WWE SJ
Q%est$on 0ow do use a scriptlet to initialie a newly instantiated bean=
?$P
A jsp:use&ean action may optionally ha*e a body. f the body is specified, its contents will be automatically in*oked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialie the newly instantiated bean, although you are not restricted to using those alone. The following e)ample shows the !today! property of the (oo bean initialied to the current date when it is instantiated. 2ote that here, we Ans,er make use of a ?$P e)pression within the jsp:setProperty action. Rjsp:use&ean id1!foo! class1!com.&ar.(oo! J Rjsp:setProperty name1!foo! property1!today! *alue1!R S1ja*a.te)t.-ate(ormat.get-atenstance"#.format"new ja*a.util.-ate"## SJ!E J RS>> scriptlets calling bean setter methods go here >>SJ REjsp:use&ean J
Q%est$on Ans,er
0ow can enable session tracking for ?$P pages if the browser has disabled cookies= ?$P 'e know that session tracking uses cookies by default to associate a session identifier with a uniCue user. f the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using DL rewriting. DL rewriting essentially includes the session - within the link itself as a nameE*alue pair. 0owe*er, for this to be effecti*e, you need to append the session - for each and e*ery link that is part of your ser*let response. Adding the session - to a link is greatly simplified by means of of a couple of methods: response.encodeDL"# associates a session - with a gi*en DL, and if you are using redirection, response.encodeedirectDL"# can be used by gi*ing the redirected DL as input. &oth encodeDL"# and encodeedirectedDL"# first determine whether cookies are supported by the browser4 if so, the input DL is returned unchanged since the session - will be persisted as a cookie. +onsider the following e)ample, in which two ?$P files, say hello<.jsp and
[email protected], interact with each other. &asically, we create a new session within hello<.jsp and place an object within this session. The user can then tra*erse to
[email protected] by clicking on the link present within the page.'ithin
[email protected], we simply e)tract the object that was earlier placed in the session and display its contents. 2otice that we in*oke the encodeDL"# within hello<.jsp on the link used to in*oke
[email protected] if cookies are disabled, the session - is automatically appended to the DL, allowing
[email protected] to still retrie*e the session object. Try this e)ample first with cookies enabled. Then disable cookie support, restart the brower, and try again. %ach time you should see the maintenance of the session across pages. -o note that to get this e)ample to work with cookies disabled at the browser, your ?$P engine has to support DL rewriting. hello<.jsp RS page session1!true! SJ RS nteger num 1 new nteger"<#4 session.put7alue"!num!,num#4 $tring url 1response.encodeDL"
[email protected]!#4 SJ Ra
[email protected] [email protected] RS page session1!true! SJ
RS nteger i1 "nteger #session.get7alue"!num!#4 out.println"!2um *alue in session is !i.int7alue"##4
Q%est$on 0ow can declare methods within my ?$P page=
Ans,er
?$P
Gou can declare methods for use within your ?$P page as declarations. The methods can then be in*oked within any other methods you declare, or within ?$P scriptlets and e)pressions. -o note that you do not ha*e direct access to any of the ?$P implicit objects like reCuest, response, session and so forth from within ?$P methods. 0owe*er, you should be able to pass any of the implicit ?$P *ariables as parameters to the methods you declare. (or e)ample: RSQ public $tring where(rom"0ttp$er*leteCuest reC# 5 0ttp$ession ses 1 reC.get$ession"#4 ... return reC.getemote0ost"#4 ; SJ RS out.print"!0i there, see that you are coming in from !#4 SJ RS1 where(rom"reCuest# SJ Another %)ample file<.jsp: RSpage contentType1!te)tEhtml!SJ RSQ public *oid test"?sp'riter writer# throws O%)ception5 writer.println"!0elloQ!#4 ; SJ
[email protected] RSinclude RhtmlJ RbodyJ RStest"out#4S REbodyJ REhtmlJ
file1!file<.jsp!SJ
J
Q%est$on
s there a way can set the inacti*ity lease period on a per>session basis= ?$P
Typically, a default inacti*ity lease period for all sessions is set within your ?$P engine admin screen or associated properties file. 0owe*er, if your ?$P engine supports the $er*let @.< AP, you can manage the inacti*ity lease period on a per>session basis. This is done by in*oking the 0ttp$ession.set/a)nacti*enter*al"# method, right after the session been created. (or e)ample: Ans,er has RS session.set/a)nacti*enter*al"3#4 SJ would reset the inacti*ity period for this session to H minutes. The inacti*ity inter*al is set in seconds.
Q%est$on 0ow can set a cookie and delete a cookie from within a ?$P page=
Ans,er
A cookie, mycookie, can be deleted using the following scriptlet: RS EEcreating a cookie +ookie mycookie 1 new +ookie"!a2ame!,!a7alue!#4 response.add+ookie"mycookie#4 EEdelete a cookie +ookie kill/y+ookie 1 new +ookie"!mycookie!, null#4 kill/y+ookie.set/a)Age"#4 kill/y+ookie.setPath"!E!#4 response.add+ookie"kill/y+ookie#4 SJ
Q%est$on 0ow does a ser*let communicate with a ?$P page= Ans,er
?$P
?$P
The following code snippet shows how a ser*let instantiates a bean and initialies it with (O/ data posted by a browser. The bean is then placed into the reCuest, and the call is then forwarded to the ?$P page, &ean<.jsp, by means of a reCuest dispatcher for downstream processing. public *oid doPost "0ttp$er*leteCuest reCuest, 0ttp$er*letesponse response# 5 try 5
go*i.(orm&ean f 1 new go*i.(orm&ean"#4 $tring id 1 reCuest.getParameter"!id!#4 f.set2ame"reCuest.getParameter"!name!##4 f.setAddr"reCuest.getParameter"!addr!##4 f.setAge"reCuest.getParameter"!age!##4 EEuse the id to compute EEadditional bean properties like info EEmaybe perform a db Cuery, etc. EE . . . f.setPersonaliationnfo"info#4 reCuest.setAttribute"!f&ean!,f#4 get$er*let+onfig"#.get$er*let+onte)t"#.geteCuest-ispatcher "!EjspE&ean<.jsp!#.forward"reCuest, response#4 ; catch "%)ception e)# 5 . . . ; ; The ?$P page &ean<.jsp can then process f&ean, after first e)tracting it from the default reCuest scope *ia the use&ean action. jsp:use&ean id1!f&ean! class1!go*i.(orm&ean! scope1!reCuest!E jsp:getProperty name1!f&ean! property1!name! E jsp:getProperty name1!f&ean! property1!addr! E jsp:getProperty name1!f&ean! property1!age! E jsp:getProperty name1!f&ean! property1!personaliationnfo! E
Q%est$on Ans,er
0ow do ha*e the ?$P>generated ser*let subclass my own custom ser*let class, instead of the default= ?$P One should be *ery careful when ha*ing ?$P pages e)tend custom ser*let classes as opposed to the default one generated by the ?$P engine. n doing so, you may lose out on any ad*anced optimiation that may be pro*ided by the ?$P engine. n any case, your new superclass has to fulfill the contract with the ?$P engine by: mplementing the 0ttp?spPage interface, if the protocol used is 0TTP, or implementing ?spPage otherwise %nsuring that all the methods in the $er*let interface are declared final Additionally, your ser*let superclass also needs to do the following: The ser*ice"# method has to in*oke the 8jsp$er*ice"# method The init"# method has to in*oke the jspnit"# method The destroy"# method has to in*oke jsp-estroy"#
f any of the abo*e conditions are not satisfied, the ?$P engine may throw a translation error. Once the superclass has been de*eloped, you can ha*e your ?$P e)tend it as follows: RS page e)tends1!package2ame.$er*let2ame! SR Q%est$on
Ans,er
Q%est$on
Ans,er
Q%est$on Ans,er
'hat is the difference between $er*let+onte)t and $er*let+onfig= $er*lets
&oth are interfaces. The ser*let engine implements the $er*let+onfig interface in order to pass configuration information to a ser*let. The ser*er passes an object that implements the $er*let+onfig interface to the ser*letBs init"# method. The $er*let+onte)t interface pro*ides information to ser*lets regarding the en*ironment in which they are running. t also pro*ides standard way for ser*lets to write e*ents to a log file.
'hat are the differences between 6%T and PO$T ser*ice methods= $er*lets
A 6%T reCuest is a reCuest to get a resource from the ser*er. +hoosing 6%T as the !method! will append all of the data to the DL and it will show up in the DL bar of your browser. The amount of information you can send back using a 6%T is restricted as DLs can only be <@F characters. A PO$T reCuest is a reCuest to post "to send# form data to a resource on the ser*er. A PO$T on the other hand will "typically# send the information through a socket back to the webser*er and it wonBt show up in the DL bar. Gou can send much more information to the ser*er this way > and itBs not restricted to te)tual data either. t is possible to send files and e*en binary data such as serialied ?a*a objectsQ
'hat is the difference between 6eneric$er*let and 0ttp$er*let= $er*lets
6eneric$er*let is for ser*lets that might not use 0TTP, like for instance (TP ser*ice.As of only 0ttp is implemented completely in 0ttp$er*let. The 6eneric$er*let has a ser*ice"# method that gets called when a client
reCuest is made. This means that it gets called by both incoming reCuests and the 0TTP reCuests are gi*en to the ser*let as they are
