话不多说上代码！其中main函数是模仿多线程访问单例，打印对象的hashCode值来查看多线程环境下拿到的实例是不是同一个。
package com.kali.jvm.singleton;
public class V1Singleton {
private static final V1Singleton INSTANCE = new V1Singleton();
private V1Singleton(){}
public static V1Singleton getInstance(){
return INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 10000; i++) {
new Thread(() ->{
System.out.println(getInstance().hashCode());
}).start();
}
}
}
这是最简单的单例模式。
以下是DCL单例模式形成的过程： 一、多线程情况下加锁：package com.kali.jvm.singleton;
public class V3Singleton {
private static V3Singleton SINGLETON_INSTANCE;
private V3Singleton(){}
public static synchronized V3Singleton getInstance(){
if(SINGLETON_INSTANCE == null){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
SINGLETON_INSTANCE = new V3Singleton();
}
return SINGLETON_INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 10000; i++) {
new Thread(() ->{
System.out.println(getInstance().hashCode());
}).start();
}
}
}
二、为了提高效率缩小锁的粒度：
package com.kali.jvm.singleton;
public class V3Singleton {
private static V3Singleton SINGLETON_INSTANCE;
private V3Singleton(){}
public static V3Singleton getInstance(){
if(SINGLETON_INSTANCE == null){
synchronized (V3Singleton.class){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
SINGLETON_INSTANCE = new V3Singleton();
}
}
return SINGLETON_INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 10000; i++) {
new Thread(() ->{
System.out.println(getInstance().hashCode());
}).start();
}
}
}
在JAVA中多线程并发环境下会出错的数据被称为竞态条件。
三、上面缩小锁粒度后的程序在并发情况下还是会出现问题，当一个线程判断SINGLETON_INSTANCE为null时，恰好时间片到了，下一个线程来了之后判断为null，拿到了锁然后返回实例对象，释放锁，这时候第一个线程继续执行，因为已经判断了SINGLETON_INSTANCE为null，然后拿锁执行，又产生了一个实例。保证不了单例。所以加了双重所检查机制。package com.kali.jvm.singleton;
public class V2Singleton {
private static V2Singleton SINGLETON_INSTANCE;
private V2Singleton(){}
public static V2Singleton getInstance(){
if(SINGLETON_INSTANCE == null){
synchronized (V2Singleton.class){
if(SINGLETON_INSTANCE == null){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
SINGLETON_INSTANCE = new V2Singleton();
}
}
}
return SINGLETON_INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 10000; i++) {
new Thread(() ->{
System.out.println(getInstance().hashCode());
}).start();
}
}
}
