监听当前对象被创建了几个..我们可以用一个static来确定个数.(此处说明下为什么要用static:一个类中静态变量只会被初始化一次,不管你实例化多少个类,里面始终只有一个).
下面是使用计数来跟踪仍旧访问着共享对象的对象数量:
package com.glacier.demo;
class Shared {
private int refcount = 0;
private long counter = 0;
private final long id = counter++;
public Shared() {
System.out.println("Creating " + this);
}
public void addRef() {
refcount++;
}
protected void dispose() {
if (refcount == 0) {
System.out.println("Disposing " + this);
}
}
public String toString() {
return "Shared " + id;
}
}
class Composing {
private Shared shared;
private static long counter = 0;
private final long id = counter++;
public Composing(Shared shared) {
System.out.println("Creating " + this);
this.shared = shared;
this.shared.addRef();
}
protected void dispose() {
System.out.println("disposing " + this);
shared.dispose();
}
public String toString() {
return "Compsing " + id;
}
}
public class Demo {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Shared shared = new Shared();
Composing[] composing = { new Composing(shared), new Composing(shared),
new Composing(shared), new Composing(shared),
new Composing(shared) };
for (Composing c : composing) {
c.dispose();
}
}
}
static long counter 跟踪所创建的shared的实例的数量,还可以为id提供数值.counter的类型是long而不是int,这样可以防止溢出.id是final类型的,我们不希望它在生命周期中改变.
在将一个共享对象附到类上时,必须记住调用addRef(),但是dispose()方法将跟踪引用数,并决定何时执行清理.使用这种技巧需要加倍细心.
看下运行结果:
Creating Shared 0
Creating Compsing 0
Creating Compsing 1
Creating Compsing 2
Creating Compsing 3
Creating Compsing 4
disposing Compsing 0
disposing Compsing 1
disposing Compsing 2
disposing Compsing 3
disposing Compsing 4