单例模式
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());
- }
- }
- 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());
- }
- }
来源: http://www.bubuko.com/infodetail-2309854.html