Saturday 21 May 2011

Strong,Soft,Weak and Phantom References in Java

If you read any documentation on Java garbage collection, the term "strong/weak/soft/phantom reference"  crops up quite frequently. It's easy to guess what strong references are, but what the heck is a phantom/weak/soft reference?

I stumbled on a very informative post about each of the reference types at http://weblogs.java.net/blog/enicholas/archive/2006/05/understanding_w.html. It is an article that perhaps every Java programmer not familiar with this topic should read.  To quote the author of the article: "If you don't know what they are, how will you know when to use them?"

 

I am paraphrasing the main contents of the article below for posterity and for my own reference:

Strong references:


Just like it says on the tin, the vanilla object references that we are all used to. For example, in the following code snippet, someClassObj and someOtherObj are both strong references.

 
public class SomeClass
{
private SomeObject someOtherObject = new SomeObject();
...
...

public static void main(String[] args)
{
SomeClass someClassObj = new SomeClass();
...
...
}
}

As long as there is a strong reference to an object, it cannot be garbage collected.

 

Weak References


In certain situations, strong references can be a pain to manage and lead to nasty memory leaks. Imagine a global object cache where object references are stored for faster retrieval. The burden is on the programmer to manage the memory used by the cache. If objects that are no longer needed elsewhere are not removed from the cache manually, they will keep keep using memory unnecessarily. As far as the garbage collector is concerned, these objects are still alive because the cache is holding strong references to them.  Therefore, the programmer ends up doing extra work to manage the memory used by the program.

The above unfortunate situation can be made much better by offloading bulk of the work to the garbage collector itself. The way to achieve this is to store weak references in the cache. Weak references are a hint to the garbage collector that the memory occupied by those objects can be reclaimed if no other links to them are found during the link traversal phase of the collector.

 
WeakReference myWeakReference = new WeakReference(someClassObj);
objectRef = myWeakReference.get();

The get method of the weak reference object will return null if the object that it points to has been garbage collected. This takes care of the memory used by the unused object, but what about the WeakReference object itself? ReferenceQueue to the rescue!
ReferenceQueue refQueue = new ReferenceQueue();
WeakReference myWeakReference1 = new WeakReference(someClassObj1,refQueue);
WeakReference myWeakReference2 = new WeakReference(someClassObj2,refQueue);

If a reference to a ReferenceQueue object is passed to the WeakReference object when it is constructed, the garbage collector will automatically put the WeakReference object into the queue once the object is garbage collected. Now all we need to do is to periodically look through the reference queue and dispose of any useless weak refs.
Java provides a handy WeakHashMap class that automatically handles the cleanup when it's entries are garbage collected.

 

Soft References


Soft references are a less eager form of weak references. Generally, objects pointed to by soft references will stay in memory as long as there's enough memory to go around.

 

Phantom References


The main difference between a weak and a phantom reference is that the get method of a phantom reference will always return null.  The get method of a weak reference returning null does not necessarily mean that the pointed object is removed from the memory. The garbage collector is yet to call the finalizer on that object, and hence there is a slim chance that the object could resurrect itself by virtue of having a weird finalize method that creates a strong reference to it.

Objects pointed to be phantom references  have had their finalizers executed and are already physically removed from memory. Therefore, phantom refs only serve to indicate that a certain piece of memory has been reclaimed.

Monday 16 May 2011

The Python Challenge

The Python challenge is an online riddle inspired by notpron. The catch is that each level requires you to write some Python code (or Perl, Ruby etc. etc. if you are so inclined) to arrive at the solution. The hardest part is figuring out the cryptic clues. The Python bit is easy; so far, the longest piece of Python I have written to solve a riddle is about 6 lines. (Admittedly, I am still on level 6. But according to the forum posts, none of the challenges require a lot of code)

It's a very challenging but fun way to learn the intricacies of Python, regardless of whether you are a novice or a pro. Give it a try at http://www.pythonchallenge.com.

Sunday 15 May 2011

Javac type inference bug

Scenario: A generics based configuration reader that worked perfectly fine under Eclipse, suddenly started failing inside a Jenkins build. The cause was a compilation error: "type parameters of <T>T cannot be determined; no unique maximal instance exists for type variable T with upper bounds int,java.lang.Object".

According to this bug report, this is an almost 6 years old javac bug. Apprently Java 1.7 fixes it, but I haven't had a chance to verify it. Here's some sample code to reproduce it:


package com.lucidelectricdreams;

public class GenericsTest
{
@SuppressWarnings("unchecked")
public <T> T genericReturnTest(int typeToReturn)
{

switch(typeToReturn)
{
case 1: // integer
return (T)(new Integer(12));

case 2: // double
return (T)(new Double(56.2D));

default: // String
return (T)"test";
}
}


public static void main(String[] args)
{
GenericsTest gt = new GenericsTest();

int intRetVal = gt.genericReturnTest(1);
double doubleRetVal = gt.genericReturnTest(2);
String stringRetVal = gt.genericReturnTest(3);

System.out.println(intRetVal);
System.out.println(doubleRetVal);
System.out.println(stringRetVal);
}
}

 

Changing the main method as follows, gets rid of the compiler error:

public static void main(String[] args)
{
GenericsTest gt = new GenericsTest();

int intRetVal = gt.<Integer>genericReturnTest(1);
double doubleRetVal = gt.<Double>genericReturnTest(2);
String stringRetVal = gt.genericReturnTest(3);

System.out.println(intRetVal);
System.out.println(doubleRetVal);
System.out.println(stringRetVal);
}

Thursday 12 May 2011

Kill tasks in Windows through the command line

I received a frantic phone call from my Dad last night, who lives a few thousand miles away, across a few oceans. "My computer is not working. Something is wrong with my hard disk" - he said. "Fix it!". Although it was  flattering that my dad thinks that I could magically fix a bad hard drive from a few thousand miles away, it didn't sound quite right because he had just recently bought that machine. I will spare you my dear readers, of the painful 10 minutes that I spent talking in a very slow and calm voice to my dad to figure out what was really wrong. Eventually I managed to figure out that he was infected with the "Windows Recovery" virus. (http://www.bleepingcomputer.com/virus-removal/remove-windows-recovery).

Removing this infection is simple enough if you follow the excellent instructions from the bleepingcomputer guide above. However,  what can you do when you don't have physical access to the machine? Luckily the LogmeIn app I had installed sometime back was still running and accessible, so I could access the computer remotely. However, trying to download the rkill application to stop the virus was impossible because it was blocking all DNS requests out of the machine. The task manager was disabled by the virus as well, so pressing Ctrl+Alt+Del didn't work either. Restarting the computer in safe mode would cut off my remote access through LogMeIn. Asking my dad to press even a single key takes more than 5 minutes of explanations and several wrong attempts - so telling him what to do was not an option either.

The Solution:

tasklist


There is a little known command in Windows named tasklist, which does the same thing as the Linux ps command. Running the command on a command prompt will display a list of all running processes along with their PIDs. To kill any process, type tskill followed by the PID. For example, to kill PID 2476, type:

tskill 2476


Pretty simple, but very handy command for those sticky situations!