1.懶漢式 線程不安全,當(dāng)有多個(gè)線程并行調(diào)用getInstance()的時(shí)候，就會(huì)創(chuàng)建多個(gè)實(shí)例。也就是說(shuō)在多線程下不能正常工作。
public class Singleton{
private static Singleton instance;
private Singleton(){}
public static Singleton getInstance(){
if(instance == null){
instance = new Singleton();
? ? }
return instance;
}
}
2.懶漢式 線程安全,雖然做到了線程安全，并且解決了多實(shí)例的問(wèn)題，但是它并不高效。因?yàn)樵谌魏螘r(shí)候只能有一個(gè)線程調(diào)用getInstance()方法。但是同步操作只需要在第一次調(diào)用時(shí)才被需要，即第一次創(chuàng)建單例實(shí)例對(duì)象時(shí)。這就引出了雙重檢驗(yàn)鎖。
public class Singleton{
private static Singleton instance;
private Singleton(){}
public synchronized static Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return instance;
}
}
3.雙重檢驗(yàn)鎖
public static Singleton getSingleton() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance ;
}
>改進(jìn)代碼, instance 變量聲明成 volatile
public class Singleton {
private volatile static Singleton instance; //聲明成 volatile
private Singleton (){}
public static Singleton getSingleton() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
4.餓漢式,在第一次加載類到內(nèi)存中時(shí)就會(huì)初始化，所以創(chuàng)建實(shí)例本身是線程安全的。
public class Singleton {
//類加載的時(shí)候就初始化
private static final Singleton instance =? new Singleton();
private Singleton(){}
public static Singleton getInstance() {
return instance;
}
}
5.靜態(tài)內(nèi)部類.
public class Singleton {
private static class SingletonHolder{
private static final INCTANCE = new Singleton();
}
private Singleton(){}
public static final Singleton getInstance(){
return SingletonHolder.INSTANCE;
}
}
6.枚舉
public enum EasySingleton {
INSTANCE;
}
