Let's start by looking at some concurrency bugs. Here is a simple example of incrementing a counter:
public class Counter {
private int counter;
public int incrementAndGet() {
++counter;
return counter;
}This code is not thread-safe. If two threads are running using the same object concurrently, the sequence of counter values each gets is essentially unpredictable. The reason for this is the ++counter operation. This simple-looking statement is actually made up of three distinct operations, namely read the new value, modify the new value, and store the new value:
As shown in the preceding diagram, the thread executions are oblivious of each other, and hence interfere unknowingly, thereby introducing a lost update.
The following code illustrates the singleton design pattern. To create an instance of LazyInitializationis expensive in terms of time and memory. So, we take over object creation. The idea is to delay the creation till its first use, and then...