只能生成一个实例的类是实现了Singleton(单例)模式的类。以下为C#实现单例模式的方式
方式一只使用于单线程环境
// 把构造函数设为私有函数以禁止他人创建实例
// 定义一个静态的实例，在需要的时候创建该实例
// 在Singleton的静态属性Instance中，只有在instance为null的时候才创建一个实例以避免重复创建
// 把构造函数定义为私有函数
public final class Singleton1 {
private Singleton1() {
}
private static Singleton1 _instance = null;
public static Singleton1 getInstance() {
if (_instance == null)
_instance = new Singleton1();
return _instance;
}
public static void main(String[] args) {
Singleton1 s1 = Singleton1.getInstance();
Singleton1 s2 = Singleton1.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
方式二 加同步锁
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public final class Singleton2 {
private Singleton2(){}
static Lock lock = new ReentrantLock();
private static Singleton2 _instance = null;
public static Singleton2 getInstance(){
lock.lock();
try {
if(_instance == null)
_instance = new Singleton2();
} finally {
lock.unlock();
}
return _instance;
}
public static void main(String[] args) {
Singleton2 s1 = Singleton2.getInstance();
Singleton2 s2 = Singleton2.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
可行的解法 加同步锁前后两次判断实例是否已存在
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public final class Singleton3 {
private Singleton3(){}
static Lock lock = new ReentrantLock();
private static Singleton3 _instance = null;
public static Singleton3 getInstance(){
if(_instance == null){
lock.lock();
try{
if(_instance == null)
_instance = new Singleton3();
}finally {
lock.unlock();
}
}
return _instance;
}
public static void main(String[] args) {
Singleton3 s1 = Singleton3.getInstance();
Singleton3 s2 = Singleton3.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
推荐的解法一利用静态构造函数
public final class Singleton4 {
private Singleton4(){}
private static Singleton4 _instance = new Singleton4();
public static Singleton4 getInstance()
{
return _instance;
}
public static void main(String[] args) {
Singleton4 s1 = Singleton4.getInstance();
Singleton4 s2 = Singleton4.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
推荐的解法二 实现按需创建实例
public class Singleton5 {
private Singleton5(){}
public static Singleton5 getInstance(){
return Nested.instance;
}
private static class Nested{
private Nested(){}
public static final Singleton5 instance = new Singleton5();
}
public static void main(String[] args) {
Singleton5 s1 = Singleton5.getInstance();
Singleton5 s2 = Singleton5.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
扩展
定义一个表示总统的类型President可以从该类型继承出FrenchPresident和AmericanPresident等类型。这些派生类型都只能产生一个实例
public class President {
private String _name;
public President(){}
public String getName(){
return _name;
}
public void setName(String name){
_name = name;
}
public static void main(String[] args) {
}
}
public class FrenchPresident extends President{
private FrenchPresident(){}
public static FrenchPresident getInstance()
{
return Nested.instance;
}
private static class Nested{
private Nested(){}
public static final FrenchPresident instance = new FrenchPresident();
}
public static void main(String[] args) {
FrenchPresident s1 = FrenchPresident.getInstance();
FrenchPresident s2 = FrenchPresident.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
public class AmericanPresident {
private AmericanPresident() {
}
public static AmericanPresident getInstance() {
return Nested.instance;
}
private static class Nested {
private Nested() {
}
public static final AmericanPresident instance = new AmericanPresident();
}
public static void main(String[] args) {
AmericanPresident s1 = AmericanPresident.getInstance();
AmericanPresident s2 = AmericanPresident.getInstance();
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
}
}
：http://www.linuxidc.com/Linux/2017-10/147514.htm
