Java单例模式的9种实现方式

Java单例模式的9种实现方式

一.饿汉式

public class Singleton { 
    private static Singleton instance = new Singleton(); private Singleton (){ 
   } public static Singleton getInstance() { 
    return instance; } } 

JDK中，java.lang.Runtime就是典型的使用饿汉式的单例模式

二.懒汉式，线程不安全

public class Singleton { 
    private static Singleton instance; private Singleton (){ 
   } public static Singleton getInstance() { 
    if (instance == null) { 
    instance = new Singleton(); } return instance; } } 

三.懒汉式，线程安全

public class Singleton { 
    private static Singleton instance; private Singleton (){ 
   } public static synchronized Singleton getInstance() { 
    if (instance == null) { 
    instance = new Singleton(); } return instance; } } 

四.双检锁/双重校验锁（DCL，即 double-checked locking）

JDK 版本：JDK1.5 起

public class Singleton { 
    private volatile static Singleton singleton; private Singleton (){ 
   } public static Singleton getSingleton() { 
    if (singleton == null) { 
    synchronized (Singleton.class) { 
    if (singleton == null) { 
    singleton = new Singleton(); } } } return singleton; } } 

五.静态内部类

public class Singleton { 
    private static class SingletonHolder { 
    private static final Singleton INSTANCE = new Singleton(); } private Singleton (){ 
   } public static final Singleton getInstance() { 
    return SingletonHolder.INSTANCE; } } 

六.枚举

JDK 版本：JDK1.5 起

public enum Singleton { 
    INSTANCE; public void whateverMethod() { 
    } } 

七.双检锁/双重校验锁（DCL，即 double-checked locking）(加volatile)

JDK 版本：JDK1.5 起

public class Singleton { 
    # 使用volatile设置内存屏障，确保线程访问资源可见性。 private volatile static Singleton singleton; private Singleton (){ 
   } public static Singleton getSingleton() { 
    if (singleton == null) { 
    synchronized (Singleton.class) { 
    if (singleton == null) { 
    singleton = new Singleton(); } } } return singleton; } } 

八.使用ThreadLocal实现单例

public class Singleton { 
    // 用空间换时间 private static final ThreadLocal<Singleton> tl = new ThreadLocal<Singleton>(); @Override protected Singleton initialValue() { 
    return new Singleton(); } private Singleton() { 
   } public static Singleton getInstance() { 
    return tl.get(); } } 

九.使用CAS锁实现单例

private static final AtomicReference<Singleton> INSTANCE = new AtomicReference<Singleton>(); private Singleton() { 
    } public static final Singleton getInstance() { 
    for (; ; ) { 
    Singleton current = INSTANCE.get(); if (current != null) { 
    return current; } current = new Singleton(); if (INSTANCE.compareAndSet(null, current)) { 
    return current; } } } 

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