Tuesday, February 21, 2012

What is Race Condition in multithreading – 2 Examples in Java

Race condition in Java is a type of concurrency bug or issue which is introduced in your program because  parallel execution of your program by multiple threads at same time, Since Java is a multi-threaded programming language hence risk of Race condition is higher in Java which demands clear understanding of what causes a race condition and how to avoid that. Anyway Race conditions are just one of hazards or risk presented by  use of multi-threading in Java just like deadlock in Java. Race conditions occurs when two thread operate on same object without proper synchronization and there operation interleaves on each other. Classical example of Race condition is incrementing a counter since increment is not an atomic operation and can be further divided into three steps like read, update and write. if two threads tries to increment count at same time and if they read same value because of interleaving of read operation of one thread to update operation of another thread, one count will be lost when one thread overwrite increment done by other thread. atomic operations are not subject to race conditions because those operation cannot be interleaved. This is also a popular multi-threading interview questions during core java interviews. In this article we will see how to find race condition in Java and  two sample code patterns which often causes race conditions in Java.

How to find Race Conditions in Java

Race condition in Java with Example tutorialFinding Race conditions in any language is most difficult job and Java is no different, though since readability of Java code is very good and synchronized constructs are well defined heaps to find race conditions by code review. finding race conditions by unit testing is not reliable due to random nature of race conditions. since race conditions surfaces only some time your unit test may passed without facing any race condition. only sure shot way to find race condition is reviewing code manually or using code review tools which can alert you on potential race conditions based on code pattern and use of synchronization in Java. I solely rely on code review and yet to find a suitable tool for exposing race condition in java.

Code Example of Race Condition in Java

Based on my experience in Java synchronization and where we use synchronized keyword I found that two code patterns namely "check and act" and "read modify write" can suffer race condition if not synchronized properly. both cases rely on natural assumption that a single line of code will be atomic and execute in one shot which is wrong e.g. ++ is not atomic.

"Check and Act" race condition pattern
classical example of "check and act" race condition in Java is getInstance() method of Singleton Class, remember that was one questions which we have discussed on 10 Interview questions on Singleton pattern in Java as "How to write thread-safe Singleton in Java". getInstace() method first check for whether instance is null and than initialized the instance and return to caller. Whole purpose of Singleton is that getInstance should always return same instance of Singleton. if you call getInstance() method from two thread simultaneously its possible that while one thread is initializing singleton after null check, another thread sees value of _instance reference variable as null (quite possible in java) especially if your object takes longer time to initialize and enters into critical section which eventually results in getInstance() returning two separate instance of Singleton. This may not happen always because a fraction of delay may result in value of _instance updated in main memory. here is a code example

public Singleton getInstance(){
if(_instance == null){   //race condition if two threads sees _instance= null
_instance = new Singleton();
}
}

an easy way to fix "check and act" race conditions is to synchronized keyword and enforce locking which will make this operation atomic and guarantees that block or method will only be executed by one thread and result of operation will be visible to all threads once synchronized blocks completed or thread exited form synchronized block.

read-modify-update race conditions
This is another code pattern in Java which cause race condition, classical example is the non thread safe counter we discussed in how to write thread safe class in Java. this is also a very popular multi-threading question where they ask you to find bugs on concurrent code. read-modify-update pattern also comes due to improper synchronization of non-atomic operations or combination of two individual atomic operations which is not atomic together e.g. put if absent scenario. consider below code

if(!hashtable.contains(key)){
hashtable.put(key,value);
}

here we only insert object into hashtable if its not already there. point is both contains() and put() are atomic but still this code can result in race condition since both operation together is not atomic. consider thread T1 checks for conditions and goes inside if block now CPU is switched from T1 to thread T2 which also checks condition and goes inside if block. now we have two thread inside if block which result in either T1 overwriting T2 value or vice-versa based on which thread has CPU for execution. In order to fix this race condition in Java you need to wrap this code inside synchronized block which makes them atomic together because no thread can go inside synchronized block if one thread is already there.

These are just some of examples of race conditions in Java, there will be numerous based on your business logic and code. best approach to find Race conditions is code review but its hard because thinking concurrently is not natural and we still assume code to run sequentially. Problem can become worse if JVM reorders code in absent of proper synchronization to gain performance benefit and this usually happens on production under heavily load, which is worst. I also suggest doing load testing in production like environment which many time helps to expose race conditions in java. Please share if you have faced any race condition in java projects.

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2 comments :

Sidhu said...

I have a question regarding the below example of not being thread safe .
All the operations on hashtable are syncronized right ? then where is the question of mutli threads acting on this datastructure at a time ?

if(!hashtable.contains(key)){
hashtable.put(key,value);
}

Anonymous said...

On these lines below:
1. if (! hashtable.contains(key)){
2. hashtable.put(key,value);
3. }

Say that two threads A and B both run line 1 before the two of them reach line 2. This is possible even though the Java hashtable is thread-safe. In this case, both threads will also run line 2 at any order depending on how the system runs at that time and one put will over-write the value of the other put.

In this case, the whole if block should be synchronized to solve the race-condition, like below:

synchronized {
if( ! hashtable.contains(key)){
hashtable.put(key,value);
}
}

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