单例模式我们很熟悉，而且网上也有很多文章，不过有很多文章都是不对的，所以今天来总结一下，如果大家想去看原理，推荐大家去看Java并发编程的艺术这本书，这个里面会有很多原理的东西
版本1
private static Singleton single = null;
//不对，会重复创建
public static Singleton getInstance() {
if (single == null) {
single = new Singleton();
}
return single;
}
版本2
//对，但是非常影响性能，不推荐
public static synchronized Singleton getInstance2() {
if (single == null) {
single = new Singleton();
}
return single;
}
版本3
//不对，会重复创建，具体分析可以去看书
public static Singleton getInstance3() {
if (single == null) {
synchronized (Singleton.class) {
if (single == null) {
single = new Singleton();
}
}
}
return single;
}
版本4
//正确，即时加载
private static Singleton instance5 = new Singleton();
public static Singleton getInstance5() {
return instance5;
}
版本5
//正确，用了volatile关键字
private static volatile Singleton instance;
public static Singleton getInstance4() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
版本6
//正确，延时加载
private static class InstanceHolder {
public static Singleton instance = new Singleton();
}
public static Singleton getInstance6() {
return InstanceHolder.instance;
}
版本7
//正确，推荐使用
创建一个枚举类
public enum SingleEunm {
INSTANCE;
private Singleton instance;
SingleEunm() {
instance = new Singleton();
}
public Singleton getInstance() {
return instance;
}
}
然后用SingleEunm.INSTANCE.getInstance();进行调用
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