单例模式五种写法：懒汉、饿汉、双重检验锁、静态内部类、枚举。

public class singletonTest {

 /*  懒汉式   多线程下不安全 */ 

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

  

/*   懒汉式   多线程下安全性较高 */ 

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

  

   /* 恶汉式*/

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

   /*双重检验锁*/

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

  /* 双重检验锁优化   因为编译器中存在指令重排序的问题   注意java5之前无效*/

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

   /*静态类部类*/

private singletonTest(){}
private static class getSingletonTestHolder{
private static singletonTest instatce=new singletonTest();
}
public static final singletonTest getInstance(){
return getSingletonTestHolder.instatce;
}

  

  /*枚举*/

public enum EasySingleton{
INSTANCE;
}

 

}

这么多，你都看到这里了，果然是真爱啊！！！！！