为什么使用单例模式? 我认为是由于某些对象的创建比较耗时,对内存的消耗又比较大回收成本过高,这时候如果允许 不断的创建对象代价势必太高,如果使用单例让内存中始终保持一个对象那么显然更划算
反例:
①. 同时执行1000个线程测试
public class Singleton3 {
private static Singleton3 INSTANCE;
private Singleton3(){}
public static Singleton3 getInstance(){
if(INSTANCE == null){
newInstance();
}
return INSTANCE;
}
private static void newInstance(){
INSTANCE = new Singleton3();
}
//test
public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
final CyclicBarrier cb = new CyclicBarrier(1000);
for(int i = 0; i <1000;i++){
Thread t = new Thread(new Runnable() {
@Override
public void run() {
try {
cb.await();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(Singleton3.getInstance().hashCode());
}
});
t.start();
}
}
测试结果通过notepad++计数发现有几个不一致的发现有不一致的hashcode
11009893 --不一致
31038155
31038155
31038155
31038155
31038155
31038155
31038155
31038155
31038155
31038155
31038155
31038155
26417394 --不一致
31038155
31038155
31038155
...
省略
②.加锁的方式:
public static Singleton3 getInstance(){
if (INSTANCE == null) {
try {
// 模拟一个耗时操作
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
synchronized (Singleton3.class) {
newInstance();
}
}
return INSTANCE;
}
//结果显而易见
18000767
10320474
9913454
2027657
10262576
15487566
15487566
28392674
15487566
6268557
6268557
25920236
....略
③.双重检查加锁(即做两次判断):这种方式虽然可行,但是使用锁毕竟会加重程序不建议使用
if (INSTANCE == null) {
try {
// 模拟一个耗时操作
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
synchronized (Singleton3.class) {
newInstance();
}
}
return INSTANCE;
}
private static void newInstance(){
if(INSTANCE == null){
INSTANCE = new Singleton3();
}
}
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使用如下几种方式:
①.静态块的方式:只会第一次创建对象的方式进来
private static final Singleton2 INSTANCE;
static{
System.out.println("static.");
INSTANCE = new Singleton2();
}
private Singleton2(){}
public static Singleton2 getInstance(){
return INSTANCE;
}
②.类似于①
private static final Singleton INSTANCE = new Singleton();
private Singleton(){
System.out.println("construct.");
}
public static Singleton getInstance(){
return INSTANCE;
}
③.枚举的方式(必须是单元素):
public enum Singleton4 {
INSTANCE;
public int get(){
try {
// 模拟耗时
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return Singleton4.class.hashCode();
}
}
④.静态内部类:略
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