Metadata is simply data about data. It defines the structure and meaning of data objects. Examples of metadata include database schema, the meaning of object attributes and methods in a Java class, annotations in a Java class, XML DTDs and Schemas, and UML models.
Wednesday, 20 June 2012
What does volatile do?
Java volatile keyword cannot be used with method or class and it can only be used with variables.
This is probably best explained by comparing the effects that volatile and synchronized have on a method. volatile is a field modifier, while synchronized modifies code blocks and methods. So we can specify three variations of a simple accessor using those two keywords:
On the other hand, geti2() effectively accesses the value of i2 from "main" memory. A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Effectively, a variable declared volatile must have it's data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Of course, it is likely that volatile variables have a higher access and update overhead than "plain" variables, since the reason threads can have their own copy of data is for better efficiency.
Well if volatile already synchronizes data across threads, what is synchronized for? Well there are two differences. Firstly synchronized obtains and releases locks on monitors which can force only one thread at a time to execute a code block, if both threads use the same monitor (effectively the same object lock). That's the fairly well known aspect to synchronized. But synchronized also synchronizes memory. In fact synchronized synchronizes the whole of thread memory with "main" memory. So executing geti3() does the following:
1) The thread acquires the lock on the monitor for object this (assuming the monitor is unlocked, otherwise the thread waits until the monitor is unlocked).
2) The thread memory flushes all its variables, i.e. it has all of its variables effectively read from "main" memory (JVMs can use dirty sets to optimize this so that only "dirty" variables are flushed, but conceptually this is the same. See section 17.9 of the Java language specification).
3) The code block is executed (in this case setting the return value to the current value of i3, which may have just been reset from "main" memory).
4) (Any changes to variables would normally now be written out to "main" memory, but for geti3() we have no changes.)
5) The thread releases the lock on the monitor for object this.
So where volatile only synchronizes the value of one variable between thread memory and "main" memory, synchronized synchronizes the value of all variables between thread memory and "main" memory, and locks and releases a monitor to boot. Clearly synchronized is likely to have more overhead than volatile.
This is probably best explained by comparing the effects that volatile and synchronized have on a method. volatile is a field modifier, while synchronized modifies code blocks and methods. So we can specify three variations of a simple accessor using those two keywords:
int i1;
volatile int i2;
int i3;
int geti1() {return i1;}
int geti2() {return i2;}
synchronized int geti3() {return i3;}
geti1() accesses the value currently stored in i1 in the current thread. Threads can have local copies of variables, and the data does not have to be the same as the data held in other threads. In particular, another thread may have updated i1 in it's thread, but the value in the current thread could be different from that updated value. In fact Java has the idea of a "main" memory, and this is the memory that holds the current "correct" value for variables. Threads can have their own copy of data for variables, and the thread copy can be different from the "main" memory. So in fact, it is possible for the "main" memory to have a value of 1 for i1, for thread1 to have a value of 2 for i1 and for thread2 to have a value of 3 for i1 if thread1 and thread2 have both updated i1 but those updated value has not yet been propagated to "main" memory or other threads.On the other hand, geti2() effectively accesses the value of i2 from "main" memory. A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Effectively, a variable declared volatile must have it's data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Of course, it is likely that volatile variables have a higher access and update overhead than "plain" variables, since the reason threads can have their own copy of data is for better efficiency.
Well if volatile already synchronizes data across threads, what is synchronized for? Well there are two differences. Firstly synchronized obtains and releases locks on monitors which can force only one thread at a time to execute a code block, if both threads use the same monitor (effectively the same object lock). That's the fairly well known aspect to synchronized. But synchronized also synchronizes memory. In fact synchronized synchronizes the whole of thread memory with "main" memory. So executing geti3() does the following:
1) The thread acquires the lock on the monitor for object this (assuming the monitor is unlocked, otherwise the thread waits until the monitor is unlocked).
2) The thread memory flushes all its variables, i.e. it has all of its variables effectively read from "main" memory (JVMs can use dirty sets to optimize this so that only "dirty" variables are flushed, but conceptually this is the same. See section 17.9 of the Java language specification).
3) The code block is executed (in this case setting the return value to the current value of i3, which may have just been reset from "main" memory).
4) (Any changes to variables would normally now be written out to "main" memory, but for geti3() we have no changes.)
5) The thread releases the lock on the monitor for object this.
So where volatile only synchronizes the value of one variable between thread memory and "main" memory, synchronized synchronizes the value of all variables between thread memory and "main" memory, and locks and releases a monitor to boot. Clearly synchronized is likely to have more overhead than volatile.
Monday, 18 June 2012
Program for printing numbers from 1 to 100, which contains digit 5?
public class PrintNumbers
{
public static void main(String[] args) {
for (int i = 0; i <= 100; i++) {
if (isContainGivenNo(i, 5)) {
System.out.println(i);
}
}
}
private static boolean isContainGivenNo(int no, int containNo) {
while (no > 9) {
int remainder = no % 10;
if (remainder == containNo) {
return true;
}
no /= 10;
}
return no == containNo;
}
}
(or)
public class PrintNumbers
{
public static void main(String[] args){
for(int i = 1; i < 100; i++){
String s = ""+i;
if (s.contains("5")){
System.out.println(i);
}
}
}
}
Output : 5 15 25 35 45 51 52 53 54 55 56 57 58 59 65 75 85 95
Sunday, 17 June 2012
What is the difference between thread and threadlocal?
Thread represents the flow of control, i.e. the code that is running as a thread. It can be started, interrupted, waited for, assigned a priority, etc.
ThreadLocal represents a piece of memory attached to the thread. It's a single object that maintains a different value for each thread that uses it. When a thread calls Set on a ThreadLocal, it will get back the same value later, even if other threads have also called set on the same ThreadLocal in the intervening time. Each will get back its own value.
You could achieve the same result with a Map<Thread, ?>, where the current thread is always the key, but this is neater and already checked to be thread-safe.
ThreadLocal represents a piece of memory attached to the thread. It's a single object that maintains a different value for each thread that uses it. When a thread calls Set on a ThreadLocal, it will get back the same value later, even if other threads have also called set on the same ThreadLocal in the intervening time. Each will get back its own value.
You could achieve the same result with a Map<Thread, ?>, where the current thread is always the key, but this is neater and already checked to be thread-safe.
Friday, 8 June 2012
Wednesday, 23 May 2012
Difference between Joint Point and Point Cut?Please explain with example or real life terminology?
When you go out to a restaurant, you look at a menu and see several options to choose from. You can order one or more of any of the items on the menu. But until you actually order them, they are just "opportunities to dine". Once you place the order and the waiter brings it to your table, it's a meal.
Join points are the options on the menu and pointcuts are the items you select. A joinpoint is an opportunity within code for you to apply an aspect...just an opportunity. Once you take that opportunity and select one or more joinpoints and apply an aspect to them, you've got a pointcut.
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