一个内存泄漏的例子

我不是女神ヾ 2022-05-08 02:42 220阅读 0赞

### 内存泄漏 ###

看一个例子:

class Stack { private Object[] elements; private int size = 0; private static final int DEFAULT_INITIAL_CAPACITY = 16; public Stack() { elements = new Object[DEFAULT_INITIAL_CAPACITY]; } public void push(Object e) { ensureCapacity(); elements[size++] = e; } public Object pop() { if (size == 0) throw new EmptyStackException(); return elements[--size]; } public void ensureCapacity() { if (elements.length == size) elements = Arrays.copyOf(elements, 2 * size + 1); } }

这段代码存在一个内存泄漏问题:如果一个栈先增长,再收缩,那么从栈中弹出的元素不会被当做垃圾回收;即使栈的程序不在引用这些对象,他们也不会被回收.  
这是因为,栈内部维护着这些对象的过期引用;过期引用是指永远不再会被解除的引用.  
内存泄漏的来源可能:

*  自己管理内存的类,如Stack;
 *  缓存(使用weakHashMap或由后台线程定时清理)
 *  监听器和其他回调  
    Stack正是由于自己管理内存的特性使它容易遭受内存泄漏的影响;

修复:一旦对象引用已经过期,只需清理这些引用即可;对于上面的例子而言,一旦元素从栈中弹出,指向它的引用就过期了;pop方法修改如下:

public Object pop() { if (size == 0) throw new EmptyStackException(); Object result = elements[--size]; element[size]=null//消除过期引用 return result ; }

### weakHashMap ###

import java.util.HashMap; import java.util.Map; import java.util.Map.Entry; import java.util.WeakHashMap; public class WeakHashMapTester { static Map<String, String> wMap = new WeakHashMap<String, String>(); static Map<String, String> map = new HashMap<String, String>(); static { String ref1 = new String("obejct1"); String ref2 = new String("obejct2"); wMap.put(ref1, "chaheObject1"); map.put(ref2, "chaheObject2"); } public static void TestWeahHashMap() { System.out.println("WeakHashMap GC之前"); for (Entry<String, String> str : wMap.entrySet()) { System.out.println(str); } System.gc(); System.out.println("WeakHashMap GC之后"); for (Entry<String, String> str : wMap.entrySet()) { System.out.println(str); } } public static void TestHashMap() { System.out.println("HashMap GC之前"); for (Entry<String, String> str : map.entrySet()) { System.out.println(str); } System.gc(); System.out.println("HashMap GC之后"); for (Entry<String, String> str : map.entrySet()) { System.out.println(str); } } public static void main(String[] args) { TestWeahHashMap(); System.out.println("---------"); TestHashMap(); } }

结果:

WeakHashMap GC之前 obejct1=chaheObject1 WeakHashMap GC之后 --------- HashMap GC之前 obejct2=chaheObject2 HashMap GC之后 obejct2=chaheObject2

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更多:[https://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java/6471947\#6471947][https_stackoverflow.com_questions_6470651_creating-a-memory-leak-with-java_6471947_6471947]  
Effective java

[https_stackoverflow.com_questions_6470651_creating-a-memory-leak-with-java_6471947_6471947]: https://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java/6471947#6471947