Saturday 4 June 2016

Simple PL/SQL code to throw NO_DATA_FOUND exception

Background

Good database question for beginners.Write a simple PL/SQL code snippet to throw NO_DATA_FOUND exception. You cannot raise this exception. Maybe try to understand how the candidate answers this. Simple code is as follows - 

DECLARE
   nowt VARCHAR(10);
BEGIN
   SELECT * INTO nowt FROM DUAL WHERE 1=0;
END;
/

and it should throw the exception - 

Error starting at line 8 in command:
DECLARE
   nowt VARCHAR(10);
BEGIN
   SELECT * INTO nowt FROM DUAL WHERE 1=0;
END;
Error report:
ORA-01403: no data found
ORA-06512: at line 4
01403. 00000 -  "no data found"
*Cause:    
*Action: 


Related Links

Simple program to create deadlock between two threads and it's fix

Background

This is one of the very basic Java multithreading question - to write a simple java program to demonstrate a deadlock. So in this post I will provide code to demonstrate that - 


Java code to create deadlock between two threads

Code is as follows - 

/**
 * 
 * @author athakur
 * Simple deadlock program
 */
public class Deadlock {
    
    public static void main(String args[]) {
        
        final Object resourceOne = "res1";
        final Object resourceTwo = "res2";
        
        new Thread(new Runnable() {
            
            @Override
            public void run() {
                
                synchronized(resourceOne) {
                    System.out.println(Thread.currentThread().getName() + " accquired resource 1");
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    synchronized(resourceTwo) {
                        System.out.println(Thread.currentThread().getName() + " accquired resource 2");
                    }
                }
            }
        }).start();
        
        new Thread(new Runnable() {
            
            @Override
            public void run() {
                synchronized(resourceTwo) {
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + " accquired resource 2");
                    synchronized(resourceOne) {
                        System.out.println(Thread.currentThread().getName() + " accquired resource 1");
                    }
                }
            }
        }).start();
        
    }

}


Copy above code in Deadloc.java file, compile and run it. You should see following output in console - 
Thread-0 accquired resource 1
Thread-1 accquired resource 2

And the program should hang.

Explanation

Thread 1 acquires lock over resource 1 and goes to sleep for 2 seconds. After sleep it would try to acquire lock on resource 2. When thread 1 was sleeping, thread 2 acquired lock over resource 2 and went to sleep for 2 seconds. After 2 seconds thread 2 would try to acquired lock on resource 1. Now it does not matter which thread wakes up. It will not be able to get lock on inner resource as other thread has acquired it and waiting for other to release. This will led to deadlock.

How to resolve this deadlock?

You can use Reentrant locks introduced in Java 1.5 to check if lock is available before locking it or you can do a timed lock (If lock is not available in x time move on). 

Another way to resolve this is to always have a fixed order to acquire lock. So both T1 and T2 threads will always lock resource 1 before trying to acquire lock on resource 2. This breaks the cycle that can lead to deadlock.

There is more more stupid way to prevent deadlock. When you are spawning new threads from main thread call join from main thread on all new threads before calling start() on next thread. Stupid? I know. It is same as single threaded application as you are executing threads sequentially. 

Couple of tips that may come handy -
  • don't use multiple threads (like Swing does, for example, by mandating that everything is done in the EDT)
  • don't hold several locks at once. If you do, always acquire the locks in the same order
  • don't execute foreign code while holding a lock
  • use interruptible locks



Related Links

t> UA-39527780-1 back to top