Java单例模式与延迟加载分析 Singleton & Lazy Initialized

冷不防 2022-07-14 04:15 128阅读 0赞

/\*  
 \* 单态类:使用内部类方式 根据Java规范，JVM将保证在使用到内部类时才去初始化其成员变量，并保证线程安全（推荐）  
 \*/  
public class Singleton\{  
    private static class InnerClass\{  
        private static Singleton singleton = new Singleton();  
    \}  
    public static Singleton getInstance()\{  
        return InnerClass.singleton;  
    \}  
\}  
  
/\*  
 \* 单态类：非线程安全，仅在单线程中能够保证唯一实例  
 \*/  
class SingletonSingleThread\{  
    private static SingletonSingleThread instance;  
    private SingletonSingleThread()\{\}  
    public static SingletonSingleThread getInstance()\{  
        if(instance == null)  
            instance = new SingletonSingleThread();  
        return instance;  
    \}  
\}  
  
/\*  
 \* 单态类一：线程安全。但非延迟初始化，可能造成资源浪费  
 \*/  
class SingletonOne\{  
    private static SingletonOne instance = new SingletonOne();  
    private SingletonOne()\{\}  
    public static SingletonOne getInstance()\{  
        return instance;  
    \}  
\}  
  
/\*  
 \* 单态类二：普通的单态类设计，但可能效率相对低  
 \*/  
class SingletonTwo\{  
    private static SingletonTwo instance;  
    private SingletonTwo()\{\}  
    public static synchronized SingletonTwo getInstance()\{  
        if(instance == null)  
            instance = new SingletonTwo();  
        return instance;  
    \}     
\}  
/\*  
 \* 单态类三：允许延迟初始化单态类，需要JSR133(但仍然可能存在问题)  
 \*/  
class SingletonThree\{  
    private static volatile SingletonThree instance;  
    private SingletonThree()\{\}  
    public static SingletonThree getInstance()\{  
        if(instance == null)\{  
            synchronized(SingletonThree.class)\{  
                if(instance == null)  
                    instance = new SingletonThree();  
            \}  
        \}  
        return instance;  
    \}  
\}