我们知道从Android2.*后google不推荐使用软引用了， 因为google优化了gc回收机制， 每次gc时不管内存是否充足都会释放软引用。 google推荐使用LruCache.Java替代软引用， 而LruCache内部维护个LinkedList， 实际上就是当内存不足时删掉最远使用的对象。
在分析fresco内存相关的源码时， 可以到有个类叫OOMSoftReferecne， 使用3个SoftReference指向同一个对象。 看最后一句翻译过来就是“只有在OOM时才会释放这个引用”TLDR: It's a reference that's cleared if and only if we otherwise would have encountered an OOM.翻译过来就是“只有在OOM时才会释放这个引用”， 多个软引用放到一起相当于强引用了。
/**
* To eliminate the possibility of some of our objects causing an OutOfMemoryError when they are
* not used, we reference them via SoftReferences.
* What is a SoftReference?
** A Soft Reference is a reference that is cleared when its referent is not strongly reachable and* there is memory pressure. SoftReferences as implemented by Dalvik blindly treat every second* SoftReference as a WeakReference every time a garbage collection happens, - i.e. clear it unless* there is something else referring to it:*dalvik*art* It will however clear every SoftReference if we don't have enough memory to satisfy an* allocation after a garbage collection.** This means that as long as one of the soft references stays alive, they all stay alive. If we* have two SoftReferences next to each other on the heap, both pointing to the same object, then* we are guaranteed that neither will be cleared until we otherwise would have thrown an* OutOfMemoryError.Since we can't strictly guarantee the location of objects on the heap, we use
* 3 just to be on the safe side.* TLDR: It's a reference that's cleared if and only if we otherwise would have encountered an OOM.*/public classOOMSoftReference { SoftReferencesoftRef1; SoftReferencesoftRef2; SoftReferencesoftRef3;publicOOMSoftReference() {softRef1=null;softRef2=null;softRef3=null; }public voidset(@NonnullThardReference) {softRef1=newSoftReference(hardReference);softRef2=newSoftReference(hardReference);softRef3=newSoftReference(hardReference); }@NullablepublicTget() {return(softRef1==null?null:softRef1.get()); }public voidclear() {if(softRef1!=null) {softRef1.clear();softRef1=null; }if(softRef2!=null) {softRef2.clear();softRef2=null; }if(softRef3!=null) {softRef3.clear();softRef3=null; } }}
为了验证上述说法， 我写个简单的demo。
public classMainActivityextendsAppCompatActivity { SoftReference softReference1;
SoftReference softReference2;
SoftReference softReference3;
TextView tvTest;
@Override
protected voidonCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);
Bitmap bmp = Bitmap.createBitmap(2048,2048, Bitmap.Config.ARGB_8888); Log.d("brycegao","bitmap size:"+ bmp.getByteCount());
softReference1=newSoftReference(bmp);/
/softReference2 = new SoftReference(bmp);
//softReference3 = new SoftReference(bmp);
tvTest= (TextView) findViewById(R.id.tv_test);
tvTest.setOnClickListener(newView.OnClickListener() {
@Override
public void onClick(View v) {
System.gc(); //强制内存回收
Object obj =softReference1.get(); Log.d("brycegao","softreference:"+ obj); }
}); }}
09-03 09:21:15.371 6530-6530/com.example.brycegao.myapplication D/brycegao: bitmap size:16777216
09-03 09:21:15.502 6530-7062/com.example.brycegao.myapplication V/RenderScript: 0x9ffde000 Launching thread(s), CPUs 4
09-03 09:21:24.872 6530-6530/com.example.brycegao.myapplication D/brycegao: softreference:null
从日志可以看出只有一个软饮用指向bitmap时， gc时会回收这块内存的。
放开softReference2,softReference3的注释， 即使用3个软引用再次测试：
09-03 09:37:19.621 23036-23036/com.example.brycegao.myapplication D/brycegao: bitmap size:16777216
09-03 09:37:30.462 23036-23036/com.example.brycegao.myapplication D/brycegao: softreference:Android.graphics.Bitmap@3067335
果然！！！ 软饮用指向的大图片没被回收， 以后在项目管理内存时可以使用这种做法， 即多个软饮用指向同一个对象
http://blog.csdn.net/brycegao321/article/details/52421293
