Java - Race condition in Java multithreading

http://javarevisited.blogspot.de/2012/02/what-is-race-condition-in.html

Because sometimes I can't open this page, so I copy the content here.

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 riskpresented by use of multi-threading in
Java just like deadlock in Java.
Race conditions occurs when two threadoperate on same object without proper synchronization and there operation interleaves on each other.
Classicalexample of Race conditionis incrementing a counter since increment
is not an atomic operation and can be further divided intothree steps like read, update and write. if two
threads tries to increment count at same time and if theyread
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 notsubject
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 raceconditions in Java.

How to find Race Conditions in Java

Finding 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.