監聽當前對象被創建了幾個..我們可以用一個static來確定個數.(此處說明下為什麼要用static:一個類中靜態變量只會被初始化一次,不管你實例化多少個類,裡面始終只有一個).
下面是使用計數來跟蹤仍舊訪問著共享對象的對象數量:
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 跟蹤所創建的shared的實例的數量,還可以為id提供數值.counter的類型是long而不是int,這樣可以防止溢出.id是final類型的,我們不希望它在生命周期中改變.
在將一個共享對象附到類上時,必須記住調用addRef(),但是dispose()方法將跟蹤引用數,並決定何時執行清理.使用這種技巧需要加倍細心.
看下運行結果:
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
