转：Java内存储器泄露的理解与解决

转：Java内存泄露的理解与解决

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Java内存管理机制

在C++语言中，如果需要动态分配一块内存，程序员需要负责这块内存的整个生命周期。从申请分配、到使用、再到最后的释放。这样的过程非常灵活，但是却十分繁琐，程序员很容易由于疏忽而忘记释放内存，从而导致内存的泄露。Java语言对内存管理做了自己的优化，这就是垃圾回收机制。Java的几乎所有内存对象都是在堆内存上分配（基本数据类型除外），然后由GC（garbage?collection）负责自动回收不再使用的内存。

????上面是Java内存管理机制的基本情况。但是如果仅仅理解到这里，我们在实际的项目开发中仍然会遇到内存泄漏的问题。也许有人表示怀疑，既然Java的垃圾回收机制能够自动的回收内存，怎么还会出现内存泄漏的情况呢？这个问题，我们需要知道GC在什么时候回收内存对象，什么样的内存对象会被GC认为是“不再使用”的。

????Java中对内存对象的访问，使用的是引用的方式。在Java代码中我们维护一个内存对象的引用变量，通过这个引用变量的值，我们可以访问到对应的内存地址中的内存对象空间。在Java程序中，这个引用变量本身既可以存放堆内存中，又可以放在代码栈的内存中（与基本数据类型相同）。GC线程会从代码栈中的引用变量开始跟踪，从而判定哪些内存是正在使用的。如果GC线程通过这种方式，无法跟踪到某一块堆内存，那么GC就认为这块内存将不再使用了（因为代码中已经无法访问这块内存了）。

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通过这种有向图的内存管理方式，当一个内存对象失去了所有的引用之后，GC就可以将其回收。反过来说，如果这个对象还存在引用，那么它将不会被GC回收，哪怕是Java虚拟机抛出OutOfMemoryError。

Java内存泄露

????一般来说内存泄漏有两种情况。一种情况如在C/C++语言中的，在堆中的分配的内存，在没有将其释放掉的时候，就将所有能访问这块内存的方式都删掉（如指针重新赋值）；另一种情况则是在内存对象明明已经不需要的时候，还仍然保留着这块内存和它的访问方式（引用）。第一种情况，在Java中已经由于垃圾回收机制的引入，得到了很好的解决。所以，Java中的内存泄漏，主要指的是第二种情况。

????可能光说概念太抽象了，大家可以看一下这样的例子：

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??3//:?SoftHashMap.java?
??4import?java.util.*;?
??5import?java.lang.ref.*;?
??6
??7import?android.util.Log;
??8
??9public?class?SoftHashMap?extends?AbstractMap?{?
?10??/**?The?internal?HashMap?that?will?hold?the?SoftReference.?*/?
?11??private?final?Map?hash?=?new?HashMap();?
?12??/**?The?number?of?"hard"?references?to?hold?internally.?*/?
?13??private?final?int?HARD_SIZE;?
?14??/**?The?FIFO?list?of?hard?references,?order?of?last?access.?*/?
?15??private?final?LinkedList?hardCache?=?new?LinkedList();?
?16??/**?Reference?queue?for?cleared?SoftReference?objects.?*/?
?17??private?ReferenceQueue?queue?=?new?ReferenceQueue();?
?18
?19??//Strong?Reference?number
?20??public?SoftHashMap()?{?this(100);?}?
?21??public?SoftHashMap(int?hardSize)?{?HARD_SIZE?=?hardSize;?}?
?22??
?23
?24??public?Object?get(Object?key)?{?
?25????Object?result?=?null;?
?26????//?We?get?the?SoftReference?represented?by?that?key?
?27????SoftReference?soft_ref?=?(SoftReference)hash.get(key);?
?28????if?(soft_ref?!=?null)?{?
?29??????//?From?the?SoftReference?we?get?the?value,?which?can?be?
?30??????//?null?if?it?was?not?in?the?map,?or?it?was?removed?in?
?31??????//?the?processQueue()?method?defined?below?
?32??????result?=?soft_ref.get();?
?33??????if?(result?==?null)?{?
?34????????//?If?the?value?has?been?garbage?collected,?remove?the?
?35????????//?entry?from?the?HashMap.?
?36????????hash.remove(key);?
?37??????}?else?{?
?38????????//?We?now?add?this?object?to?the?beginning?of?the?hard?
?39????????//?reference?queue.??One?reference?can?occur?more?than?
?40????????//?once,?because?lookups?of?the?FIFO?queue?are?slow,?so?
?41????????//?we?don't?want?to?search?through?it?each?time?to?remove?
?42????????//?duplicates.?
?43??????????//keep?recent?use?object?in?memory
?44????????hardCache.addFirst(result);?
?45????????if?(hardCache.size()?>?HARD_SIZE)?{?
?46??????????//?Remove?the?last?entry?if?list?longer?than?HARD_SIZE?
?47??????????hardCache.removeLast();?
?48????????}?
?49??????}?
?50????}?
?51????return?result;?
?52??}?
?53
?54??/**?We?define?our?own?subclass?of?SoftReference?which?contains?
?55???not?only?the?value?but?also?the?key?to?make?it?easier?to?find?
?56???the?entry?in?the?HashMap?after?it's?been?garbage?collected.?*/?
?57??private?static?class?SoftValue?extends?SoftReference?{?
?58????private?final?Object?key;?//?always?make?data?member?final?
?59????/**?Did?you?know?that?an?outer?class?can?access?private?data?
?60?????members?and?methods?of?an?inner?class???I?didn't?know?that!?
?61?????I?thought?it?was?only?the?inner?class?who?could?access?the?
?62?????outer?class's?private?information.??An?outer?class?can?also?
?63?????access?private?members?of?an?inner?class?inside?its?inner?
?64?????class.?*/?
?65????private?SoftValue(Object?k,?Object?key,?ReferenceQueue?q)?{?
?66??????super(k,?q);?
?67??????this.key?=?key;?
?68????}?
?69??}?
?70
?71??/**?Here?we?go?through?the?ReferenceQueue?and?remove?garbage?
?72???collected?SoftValue?objects?from?the?HashMap?by?looking?them?
?73???up?using?the?SoftValue.key?data?member.?*/?
?74??public?void?processQueue()?{?
?75????SoftValue?sv;?
?76????while?((sv?=?(SoftValue)queue.poll())?!=?null)?{?
?77????????if(sv.get()==?null){
?78????????????Log.e("processQueue",?"null");
?79????????}else{
?80????????????Log.e("processQueue",?"Not?null");
?81????????}
?82??????hash.remove(sv.key);?//?we?can?access?private?data!
?83??????Log.e("SoftHashMap",?"release?"?+?sv.key);
?84????}?
?85??}?
?86??/**?Here?we?put?the?key,?value?pair?into?the?HashMap?using?
?87???a?SoftValue?object.?*/?
?88??public?Object?put(Object?key,?Object?value)?{?
?89????processQueue();?//?throw?out?garbage?collected?values?first?
?90????Log.e("SoftHashMap",?"put?into?"?+?key);
?91????return?hash.put(key,?new?SoftValue(value,?key,?queue));?
?92??}?
?93??public?Object?remove(Object?key)?{?
?94????processQueue();?//?throw?out?garbage?collected?values?first?
?95????return?hash.remove(key);?
?96??}?
?97??public?void?clear()?{?
?98????hardCache.clear();?
?99????processQueue();?//?throw?out?garbage?collected?values?
100????hash.clear();?
101??}?
102??public?int?size()?{?
103????processQueue();?//?throw?out?garbage?collected?values?first?
104????return?hash.size();?
105??}?
106??public?Set?entrySet()?{?
107????//?no,?no,?you?may?NOT?do?that!!!?GRRR?
108????throw?new?UnsupportedOperationException();?
109??}?
110}?
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参考文献

Java?内存管理原理、内存泄漏实例及解决方案研究
Java中的强引用、软引用、弱引用和虚引用
垃圾回收与强引用，软引用，弱引用，幻引用的关系
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转自：http://www.blogjava.net/zh-weir/archive/2011/02/23/345007.html
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