Preventing concurrent access to shared variables by threads

When writing multi-threaded applications, many programmers (myself included) often cut corners, and leave their applications and applets vulnerable to thread conflicts. When two or more threads access the same data concurrently, there exists the possibility (and Murphy's law holding, the probability) that two threads will access or modify the same data at the same time. Don't be fooled into thinking that such problems won't occur on single-threaded processors. While accessing some data (performing a read), your thread may be suspended, and another thread scheduled. It writes its data, which is then overwritten when the first thread makes its changes.

Such problems are not just limited to multi-threaded applications or applets. If you write Java APIs, or JavaBeans, then your code may not be thread-safe. Even if you never write a single application that uses threads, people that use your code WILL. For the sanity of others, if not yourself, you should always take precautions to prevent concurrent access to shared data.

How can this problem be solved? The simplest method is to make your variables private (but you do that already,  right?) and to use synchronized accessor methods. Accessor methods allow access to private member variables, but in a controlled manner. Take the following accessor methods, which provide a safe way to change the value of a counter.

public class MyCounter

{

private int count = 0; // count starts at zero

public synchronized void setCount(int amount)

{ 

count = amount;

}

public synchronized int getCount()

{

return count;

}

}

Capitalization errors

This is one of the most frequent errors that we all make. It's so simple to do, and sometimes one can look at an uncapitalized variable or method and still not spot the problem. I myself have often been puzzled by these errors, because I recognize that the method or variable does exist, but don't spot the lack of capitalization.

While there's no silver bullet for detecting this error, you can easily train yourself to make less of them. There's a very simple trick you can learn :-

    all methods and member variables in the Java API begin with lowercase letters

    all methods and member variables use capitalization where a new word begins e.g - getDoubleValue()

If you use this pattern for all of your member variables and classes, and then make a conscious effort to get it right, you can gradually reduce the number of mistakes you'll make. It may take a while, but it can save some serious head scratching in the future.