Singleton
Source：http://blog.wjin.org/posts/singleton.html
Declaration: this work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
Singleton
Introduction
The singleton pattern ensures a class has only one instance, and provides a global point of access to it.
Multithread
Singleton is the simplest design pattern among all patterns, however, we must be careful when dealing with multithread environment. Basically, we have following ways:
-
java synchronized method if performance is not critical to application
-
move to an early created instance rather than a lazily created one (jvm can guarantee thread-safe when load this class, however, it may fail when using multiple class loaders. Also it may waste memory if we do not use it later)
-
use double-checked locking to reduce use of synchronization in getInstance(), define unique instance as volatile. This method does not suit c++
-
static code block in java and and static constructor in C#
Code
Cpp
class Singleton
{
public:
static Singleton& getInstance() // reference
{
// Guaranteed to be destroyed. Instantiated on first use
static Singleton instance;
return instance;
}
void dump()
{
cout << "I am singleton pattern: " << this << endl;
}
private:
Singleton() {} // Constructor? (the {} brackets) are needed here.
// Dont forget to declare these two. You want to make sure they
// are unaccessable otherwise you may accidently get copies of
// your singleton appearing.
Singleton(const Singleton &) = delete; // Don't Implement
Singleton& operator=(const Singleton&) = delete; // Don't implement
};
int main()
{
Singleton::getInstance().dump();
Singleton::getInstance().dump();
Singleton::getInstance().dump();
return 0;
}
Java
// do not consider multithread
public class Singleton {
private static Singleton instance; // static
private Singleton() { // private
}
public static Singleton getInstance() { // static
if (instance == null) {
instance = new Singleton();
}
return instance;
}
public static void main(String[] args) {
Singleton obj1 = getInstance();
Singleton obj2 = getInstance();
if (obj1 != obj2) {
System.out.println("Wrong");
} else {
System.out.println("Right");
}
}
}
// synchronized method
public class Singleton {
private static Singleton instance; // static
private Singleton() { // private
}
public static synchronized Singleton getInstance() { // static
if (instance == null) {
instance = new Singleton();
}
return instance;
}
// ...
}
// double-checked locking
public class Singleton {
private volatile static Singleton instance; // volatile static
private Singleton() { // private
}
public static Singleton getInstance() { // static
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
// ...
}
// early create object
public class Singleton {
private final static Singleton instance = new Singleton(); // static
private Singleton() { // private
}
public static Singleton getInstance() { // static
return instance;
}
// ...
}
// static code block
public class Singleton {
private final static Singleton instance; // static
private Singleton() { // private
}
static {
instance = new Singleton();
}
public static Singleton getInstance() { // static
return instance;
}
// ...
}