Transcript Document 555783

How can we create deadlock
condition on our servlet?
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Ans: one simple way to call doPost()
method inside doGet() and
doGet()method inside doPost() it will
create deadlock situation for a servlet.
Classes and Interfaces
For initializing a servlet can we use
constructor in place of init ().
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No, we can not use constructor for initializing a servlet because for
initialization we need an object of servletConfig using this object we
get all the parameter which are defined in deployment descriptor for
initializing a servlet and in servlet class we have only default
constructor according to older version of java so if we want to pass a
Config object we don’t have parameterized constructor and apart
from this servlet is loaded and initialized by container so it is the job
of container to call the method according to servlet specification they
have lifecycle method so init() method is called firstly.
Classes and Interfaces
Suppose you have a class which you serialized it and
stored in persistence and later modified that class to
add a new field. What will happen if you deserialize the
object already serialized?
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in this case Java Serialization API
will throw java.io.InvalidClassException
Classes and Interfaces
Can we transfer a Serialized
object vie network?
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Yes you can transfer a Serialized object
via network because java serialized object
remains in form of bytes which can be
transmitter via network. You can also
store serialized object in Disk or database
as Blob.
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Classes and Interfaces
While serializing you want some of the
members not to serialize? How do you
achieve it?
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It is sometime also asked as what is the
use of transient variable,
Classes and Interfaces
Can you Customize Serialization process
or can you override default Serialization
process in Java?
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The answer is yes you can. We all know that for
serializing an
object ObjectOutputStream.writeObject (saveThisobject)
is invoked and for reading
object ObjectInputStream.readObject() is invoked but
there is one more thing which Java Virtual
Machine provides you is to define these two method in
your class. If you define these two methods in your
class then JVM will invoke these two methods instead of
applying default serialization mechanism
Classes and Interfaces
How can we refresh
servlet on client?
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On client side we can use Meta http
refresh
Classes and Interfaces
How many methods Serializable has? If
no method then what is the purpose of
Serializable interface?
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Serializable interface exists in java.io package
and forms core of java serialization mechanism.
It doesn't have any method and also
called Marker Interface in Java. When your class
implements java.io.Serializable interface it
becomes Serializable in Java and gives compiler
an indication that use Java Serialization
mechanism to serialize this object.
Classes and Interfaces
“Minimize Accessibility of Classes
and Members” how to do that?
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Standard advice for information
hiding/encapsulation
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Decouples modules
Allows isolated development and maintenance
Java has an access control mechanism to
accomplish this goal
Item 13
Classes and Interfaces
Make Each Class or Member
as Inaccessible as Possible
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Standard list of accessibility levels
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Huge difference between 2nd and 3rd
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private
package-private (aka package friendly)
protected
public
package-private: part of implementation
protected: part of public API
Classes and Interfaces
In Public Classes, Use Accessors,
Not Public Fields
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Avoid code such as:
class Point { public double x; public double y; }
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No possibility of encapsulation
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Also, public mutable fields are not thread safe
Use get/set methods instead:
public double getX() { return x; }
public void setX(double x) { this.x = x}
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Advice holds for immutable fields as well
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Limits possible ways for class to evolve
Item 14:
Classes and Interfaces
Example: Questionable Use of
Immutable Public Fields
public final class Time {
private static final int HOURS_PER_DAY = 24;
private static final int MINUTES_PER_HOUR = 60;
public final int hour;
public final int minute;
// Not possible to change rep for class
// But we can check invariants, since fields are final
public Time ( int hour, int minute ) {
if (hour < 0 || hour >= HOURS_PER_DAY)
throw new IllegalArgumentException(“Hour: “ + hour);
if (minute < 0 || minute >= MINUTES_PER_HOUR)
throw new IllegalArgumentException(“Minute: “ + minute);
this.hour = hour;
this.minute = minute;
}
// Remainder omitted
}
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Classes and Interfaces
Minimize Mutability , why?
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Reasons to make a class immutable:
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Easier to design
Easier to implement
Easier to use
Less prone to error
More secure
Easier to share
Thread safe
Item 15:
Classes and Interfaces
Mutability
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Mutable Objects: When you have a
reference to an instance of an object, the
contents of that instance can be altered
Immutable Objects: When you have a
reference to an instance of an object, the
contents of that instance cannot be
altered
Classes and Interfaces
Mutability
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Final classes
A final class cannot be subclassed. This is done for reasons of
security and efficiency.
Accordingly, many of the Java standard library classes are final, for
example java.lang.System and java.lang.String. All methods in a
final class are implicitly final.
Example:
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public final class MyFinalClass {...}
public class ThisIsWrong extends MyFinalClass {...} // forbidden
Restricted subclasses are often referred to as "soft final" classes.
Classes and Interfaces
Mutability
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Final methods
A final method cannot be overridden by subclasses.
This is used to prevent unexpected behavior from a
subclass altering a method that may be crucial to the
function or consistency of the class.
Example:
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public class MyClass { public void myMethod() {...}
public final void myFinalMethod() {...} }
public class AnotherClass extends MyClass { public void
myMethod() {...} // Ok
public final void myFinalMethod() {...} // forbidden }
Classes and Interfaces
Mutability
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Final variables
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A final variable can only be initialized once, either
via an initializer or an assignment statement.
It does not need to be initialized at the point of
declaration: this is called a "blank final" variable.
A blank final instance variable of a class must be
definitely assigned at the end of every constructor of
the class in which it is declared;
Classes and Interfaces
Example: Class Complex
public final class Complex {
private final double re;
private final double im;
public Complex (double re, double im) { this.re = re; this.im = im;}
// Accessors with no corresponding mutators
public double realPart()
{ return re; }
public double imaginaryPart() { return im; }
// Note standard “producer” pattern
public Complex add (Complex c) {
return new Complex (re + c.re, im + c.im);
}
// similar producers for subtract, multiply, divide
// implementations of equals() and hashCode() use Double class
}
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Classes and Interfaces
Inheritance is Forever
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A commitment to allow inheritance is part
of public API
If you provide a poor interface, you (and
all of the subclasses) are stuck with it.
You cannot change the interface in
subsequent releases.
TEST for inheritance
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How? By writing subclasses
Classes and Interfaces
Constructors Must Not Invoke
Overridable Methods
// Problem – constructor invokes overridden m()
public class Super {
public Super() { m();}
public void m() {…};
}
public class Sub extends Super {
private final Date date;
public Sub() {date = new Date();}
public void m() { // access date variable}
}
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Classes and Interfaces
What Is the Problem?
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Consider the code
Sub s = new Sub();
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The first thing that happens in Sub() constructor
is a call to constructor in Super()
The call to m() in Super() is overridden
But date variable is not yet initialized!
Further, initialization in Super m() never
happens!
Yuk!
Classes and Interfaces
What are the common mechanisms
used for session tracking?
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Cookies
SSL sessions
URL- rewriting
Classes and Interfaces
What is the difference between Difference
between doGet() and doPost()?
A doGet() method is limited with 2k of data to be sent, and doPost() method
doesn't have this limitation. A request string for doGet() looks like the following:
http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN
doPost() method call doesn't need a long text tail after a servlet name in a
request. All parameters are stored in a request itself, not in a request string, and
it's impossible to guess the data transmitted to a servlet only looking at a request
string.
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Classes and Interfaces
What are the different
kinds of enterprise beans?
Different kind of enterprise beans are
Stateless session bean, Stateful
session bean, Entity bean,
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Classes and Interfaces
What is Session Bean?
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A session bean is a non-persistent object that
implements some business logic running on the server.
One way to think of a session object is as a logical
extension of the client program that runs on the server.
Session beans are used to manage the interactions of
entity and other session beans, access resources, and
generally perform tasks on behalf of the client.
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Classes and Interfaces
What is software
architecture of EJB?
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session and Entity EJBs consist of 4 and 5 parts respectively:
1. A remote interface (a client interacts with it),
2. A home interface (used for creating objects and for declaring
business methods),
3. A bean object (an object, which actually performs business logic
and EJB-specific operations).
4. A deployment descriptor (an XML file containing all information
required for maintaining the EJB) or a set of deployment descriptors
(if you are using some container-specific features).
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5.A Primary Key class - is only Entity bean specific.
Classes and Interfaces
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Classes and Interfaces
What is the role of Remote
Interface in RMI?
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Remote interfaces are defined by extending
,an interface called Remote provided in the
java.rmi package. The methods must throw
RemoteException. But application specific
exceptions may also be thrown.
Classes and Interfaces
An applet may not create frames (instances of
java.awt.Frame class).
• True
• False
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False
While applets must themselves reside
within the browser, they may create other
frames.
Classes and Interfaces
An applet is an instance of the Panel class in
package jawa.awt.
• True
• False
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True
Applets are small programs, meant not to
run on their own but be embedded inside
other applications (such as a web
browser). All applets are derived from the
class java.applet.Applet, which in turn is
derived from java.awt.Panel.
Classes and Interfaces
The class file of an applet specified using the CODE parameter in
an <APPLET> tag must reside in the same directory as the calling
HTML page.
• True
• False
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False
The CODE parameter can specify
complete or relative path of the location
of the class file
Classes and Interfaces
Applets loaded over the internet have unrestricted
access to the system resources of the computer where
they are being executed.
• True
• False
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False
Due to security reasons, applets run under
a restricted environment and cannot
perform many operations, such as reading
or writing to files on the computer where
they are being executed.
Classes and Interfaces
An applet may be able to access some methods of
another applet running on the same page.
• True
• False
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True
Only public methods can be accessed in
such a fashion.
Classes and Interfaces
If getParameter method of an instance of java.applet.Applet class
returns null, then an exception of type a NullPointerException is
thrown.
• True
• False
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False
A NullPointerException is thrown only
when a null value is used in a case where
an object is required. For instance, trying
to execute the equals method on a String
object when the object is null.
Classes and Interfaces
One of the popular uses of applets involves making
connections to the host they came from.
• True
• False
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True
An example of this feature can be
implementation of a chat functionality.
Classes and Interfaces
Applets loaded from the same computer where they are executing
have the same restrictions as applets loaded from the network.
• True
• False
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False
Applets loaded and executing locally have
none of the restrictions faced by applets
that get loaded from the network.
Classes and Interfaces
The methods init, start, stop and destroy are the four
major events in the life of an applet.
• True
• False
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True
These methods initialize, start the
execution, stop the execution, and
perform the final cleanup respectively. Not
each of these methods may be overridden
in every applet.
Classes and Interfaces
Applets are not allowed to have contsructors
due to security reasons.
• True
• False
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False
Applets may have constructors, but
usually don't as they are not guaranteed
to have access to the environment until
their init method has been called by the
environment.
Classes and Interfaces
Exam
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Multiple choice and true/false questions
cover:
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Basics of the language
Topics that we studied
Effective Java Items
There will be a cover page to record your
choice; please use it since I will not look
at the internal pages
Classes and Interfaces