单例模式几种常见写法

经典写法（线程不安全）

/** * @author hall_p * @date 2020-05-19 12:36 * @desc 经典单例，线程不安全；多线程下 会破坏掉单例； */ public class ClassicSingleton { private static ClassicSingleton classicSingleton; private ClassicSingleton(){ } public static ClassicSingleton getInstance(){ if(classicSingleton == null){ classicSingleton = new ClassicSingleton(); } return classicSingleton; } }

经典写法加锁

/** * @author hall_p * @date 2020-07-04 17:28 * @desc 线程安全 缓加载 但是效率低 */ public class SafeSingleton { private static SafeSingleton safeSingleton; private SafeSingleton() { } public static synchronized SafeSingleton getInstance(){ if(safeSingleton == null){ safeSingleton = new SafeSingleton(); } return safeSingleton; } }

双重检查

/** * @author hall_p * @date 2020-05-19 12:41 * @desc 线程安全的单例模式；改进classic 延迟初始化 */ public class ThreadSafeSingleton { private volatile static ThreadSafeSingleton threadSafeSingleton; private ThreadSafeSingleton(){ } public static ThreadSafeSingleton getInstance(){ if(threadSafeSingleton == null){ synchronized (ThreadSafeSingleton.class){ if(threadSafeSingleton == null){ threadSafeSingleton = new ThreadSafeSingleton(); } } } return threadSafeSingleton; } }

基于类的初始化

public class InstanceFactory { private static class InstanceHolder{ private static InstanceFactory instance = new InstanceFactory(); } private InstanceFactory(){ } public static InstanceFactory getInstance(){ return InstanceHolder.instance; } } /** * @author hall_p * @date 2020-05-19 12:38 * @desc 随类加载的单例模式；线程安全，获取速度快，但是加载慢； */ public class EagerlySingleton { private static EagerlySingleton eagerlySingleton = new EagerlySingleton(); private EagerlySingleton(){ } public static EagerlySingleton getInstance(){ return eagerlySingleton; } }

错误的双重检查

/** * @author * @date * @desc 双重检查锁定来实现延迟初始化的示例代码 */ public class DoubleCheckedLocking { private static DoubleCheckedLocking doubleCheckedLocking; private DoubleCheckedLocking(){} public static DoubleCheckedLocking getInstance(){ if(doubleCheckedLocking == null){ //1 synchronized (DoubleCheckedLocking.class){ //2 if(doubleCheckedLocking == null){//3 doubleCheckedLocking = new DoubleCheckedLocking(); } } } return doubleCheckedLocking; } //代码读取到doubleCheckedLocking instance不为null时，instance引用的对象有可能还没有完成初始化。 } 线程B在1处 线程A在1处 得到锁； 进入代码块 开始进行初始化 分配对象的内存空间 设置instance指向内存空间 线程B在1处执行，判断instance不为null；实际上此时对象还没有完成初始化；对instance的使用就会出现异常 线程A初始化对象 分配对象的内存空间 //1 初始化对象 //2 设置instance指向内存空间//3 2和3之间存在着重排序的可能性就是问题的根源。