设计模式之详解七种单例模式

单例模式比较

1.饿汉式单例模式

    影响内存,在初始化的时候就会创建实例,不能做到按需生成

2.懒汉式

    线程不安全,多个线程能够同时判断实例为空并创建多个实例

3.懒汉式加锁

    线程安全,但是多线程效率低,加锁太耗时间

4.懒汉式变种

    和饿汉式差不多,在生成类的同时生成实例

5.使用静态内部类 (推荐)

    当调用getInstance方法时,会调用内部类的构造方法,其他人无法使用静态内部类,因此可以按需创建!

6.使用双重校验锁 (推荐)

    需要知道一个锁的弊端,当第一个线程获得锁并创建实例后,第二个线程仍然会获得锁,导致效率降低

    因此,只在第一次试图创建实例的时候需要加锁!

    方法复杂,容易出错建议第五种~!

    

package o2_singleton;public class Test02 {/** * 单例模式，饿汉式，线程安全 */public static class Singleton {private final static Singleton INSTANCE = new Singleton();private Singleton() {}public static Singleton getInstance() {return INSTANCE;}}/** * 单例模式，懒汉式，线程不安全 */public static class Singleton2 {private static Singleton2 instance = null;private Singleton2() {}public static Singleton2 getInstance() {if (instance == null) {instance = new Singleton2();}return instance;}}/** * 单例模式，懒汉式，线程安全，多线程环境下效率不高 */public static class Singleton3 {private static Singleton3 instance = null;private Singleton3() {}public static synchronized Singleton3 getInstance() {if (instance == null) {instance = new Singleton3();}return instance;}}/** * 单例模式，懒汉式，变种，线程安全 */public static class Singleton4 {private static Singleton4 instance = null;static {instance = new Singleton4();}private Singleton4() {}public static Singleton4 getInstance() {return instance;}}/** * 单例模式，使用静态内部类，线程安全【推荐】 */public static class Singleton5 {private final static class SingletonHolder {private static final Singleton5 INSTANCE = new Singleton5();}private Singleton5() {}public static Singleton5 getInstance() {return SingletonHolder.INSTANCE;}}/** * 静态内部类，使用枚举方式，线程安全【推荐】 */public enum Singleton6 {INSTANCE;public void whateverMethod() {}}/** * 静态内部类，使用双重校验锁，线程安全【推荐】 */public static class Singleton7 {private volatile static Singleton7 instance = null;private Singleton7() {}public static Singleton7 getInstance() {if (instance == null) {synchronized (Singleton7.class) {if (instance == null) {instance = new Singleton7();}}}return instance;}}public static void main(String[] args) {System.out.println(Singleton.getInstance() == Singleton.getInstance());System.out.println(Singleton2.getInstance() == Singleton2.getInstance());System.out.println(Singleton3.getInstance() == Singleton3.getInstance());System.out.println(Singleton4.getInstance() == Singleton4.getInstance());System.out.println(Singleton5.getInstance() == Singleton5.getInstance());System.out.println(Singleton6.INSTANCE == Singleton6.INSTANCE);System.out.println(Singleton7.getInstance() == Singleton7.getInstance());}}