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package com.so;
/**
* 第2题 单例设计模式
* 设计一个类，只能生成该类的一个实例。
*
* @author qgl
* @date 2019/06/21
*/
public class SingletonPattern2 {}
/**
* 1.饿汉式：线程安全，耗费资源
*/
class HugerSingleton1 {
//该对象的引用不可修改
private static final HugerSingleton1 ourInstance = new HugerSingleton1();
public static HugerSingleton1 getInstance() {
return ourInstance;
}
private HugerSingleton1() {}
}
/**
* 2.饿汉式：在静态代码块实例对象
*/
class HugerSingleton2 {
private static HugerSingleton2 ourInstance;
static {
ourInstance = new HugerSingleton2();
}
public static HugerSingleton2 getInstance() {
return ourInstance;
}
private HugerSingleton2() {}
}
/**
* 3.懒汉式：非线程安全
*/
class Singleton1 {
private static Singleton1 ourInstance;
public static Singleton1 getInstance() {
if (null == ourInstance) {
ourInstance = new Singleton1();
}
return ourInstance;
}
private Singleton1() {}
}
/**
* 4.线程安全的懒汉式：给方法加锁
*/
class Singleton2 {
private static Singleton2 ourInstance;
public synchronized static Singleton2 getInstance() {
if (null == ourInstance) {
ourInstance = new Singleton2();
}
return ourInstance;
}
private Singleton2() {}
}
/**
* 5.线程安全的懒汉式：双重检查锁（同步代码块）
*/
class Singleton3 {
private static Singleton3 ourInstance;
public static Singleton3 getInstance() {
if (null == ourInstance) {
synchronized (Singleton3.class) {
if (null == ourInstance) {
ourInstance = new Singleton3();
}
}
}
return ourInstance;
}
private Singleton3() {}
}
/**
* 6.线程安全的懒汉式：静态内部类（推荐）
*/
class Singleton4 {
private static class SingletonHolder {
private static Singleton4 ourInstance = new Singleton4();
}
public static Singleton4 getInstance() {
return SingletonHolder.ourInstance;
}
private Singleton4() {
}
}
/**
* 7.线程安全的懒汉式：枚举
*/
enum Singleton5 {
INSTANCE;
public void whateverMethod() {
// do something
}
}