线程安全的单例模式 的各种实现 总结

第一种：饿汉式

public class EagerSingleton {

private static EagerSingleton instance = new EagerSingleton();

private EagerSingleton(){};

public static EagerSingleton getInstance(){
return instance;
}
}

第二种：懒汉式同步整个getInstance

public class ThreadSafeSingelton{
//variables and constructors...
//多线程并发时效率低下
public static   synchronized   ThreadSafeSingelton getInstance(){

if(instance == null){
instance = new ThreadSafeSingelton();
}
retrun instance;
}
}

第三种：Doublle-Check Locking

public class DoublleCheckSingleton{
private volatile static DoublleCheckSingleton instance = null;

//constructors

public static DoublleCheckSingleton getInstance(){
if(instance = null){//check if it is created
synchronized ( DoublleCheckSingleton.class) {
//synchronize creation block
if(instance ==null) //double check if it is created
instance = new DoublleCheckSingleton();
}

}

retrun instance;
}
}

Volatile修饰的成员变量在每次被线程访问时，都强迫从共享内存中重读该成员变量的值。而且，当成员变量发生变化时，强迫线程将变化值回写到共享内存。这样在任何时刻，两个不同的线程总是看到某个成员变量的同一个值。

第四种：内部类 Initialization on demand holder

public class LazyLoadedSingleton{
private LazyLoadedSingleton (){
}

private static class LazyHolder { //holds the singleton class
private static final LazyLoadedSingleton singletonInstance = new LazyLoadedSingleton();
}

public static LazyLoadedSingleton getInstance() {
return LazyHolder.singletonInstance;
}
}

JLS保证 JVM加载 LazyHolder时不会出现并发问题。

****防止反序列化

public class SerialibleSingleton implements Serialible {
private static final long serialVersionUID = -609961712631545485L;
static SerialibleSingleton singleton = new SerialibleSingleton();

private SerialibleSingleton(){}

private Object readResole(){
return singleton;
}
}

第五种：枚举单例模式

public   enum   Sington{

INSTANCE;

public void singletonOpr(){

// some opertions

}

}