網上關于這個方面的文章也不少,基本的思路是線程+緩存來解決。下面提出一些優化:
1、采用線程池
2、記憶體緩存+檔案緩存
3、記憶體緩存中網上很多是采用softreference來防止堆溢出,這兒嚴格限制隻能使用最大jvm記憶體的1/4
4、對下載下傳的圖檔進行按比例縮放,以減少記憶體的消耗
具體的代碼裡面說明。先放上記憶體緩存類的代碼memorycache.java:
public class memorycache {
private static final string tag = "memorycache";
// 放入緩存時是個同步操作
// linkedhashmap構造方法的最後一個參數true代表這個map裡的元素将按照最近使用次數由少到多排列,即lru
// 這樣的好處是如果要将緩存中的元素替換,則先周遊出最近最少使用的元素來替換以提高效率
private map<string, bitmap> cache = collections
.synchronizedmap(new linkedhashmap<string, bitmap>(10, 1.5f, true));
// 緩存中圖檔所占用的位元組,初始0,将通過此變量嚴格控制緩存所占用的堆記憶體
private long size = 0;// current allocated size
// 緩存隻能占用的最大堆記憶體
private long limit = 1000000;// max memory in bytes
public memorycache() {
// use 25% of available heap size
setlimit(runtime.getruntime().maxmemory() / 4);
}
public void setlimit(long new_limit) {
limit = new_limit;
log.i(tag, "memorycache will use up to " + limit / 1024. / 1024. + "mb");
public bitmap get(string id) {
try {
if (!cache.containskey(id))
return null;
return cache.get(id);
} catch (nullpointerexception ex) {
return null;
}
public void put(string id, bitmap bitmap) {
if (cache.containskey(id))
size -= getsizeinbytes(cache.get(id));
cache.put(id, bitmap);
size += getsizeinbytes(bitmap);
checksize();
} catch (throwable th) {
th.printstacktrace();
/**
* 嚴格控制堆記憶體,如果超過将首先替換最近最少使用的那個圖檔緩存
*
*/
private void checksize() {
log.i(tag, "cache size=" + size + " length=" + cache.size());
if (size > limit) {
// 先周遊最近最少使用的元素
iterator<entry<string, bitmap>> iter = cache.entryset().iterator();
while (iter.hasnext()) {
entry<string, bitmap> entry = iter.next();
size -= getsizeinbytes(entry.getvalue());
iter.remove();
if (size <= limit)
break;
}
log.i(tag, "clean cache. new size " + cache.size());
public void clear() {
cache.clear();
* 圖檔占用的記憶體
* @param bitmap
* @return
long getsizeinbytes(bitmap bitmap) {
if (bitmap == null)
return 0;
return bitmap.getrowbytes() * bitmap.getheight();
}
也可以使用softreference,代碼會簡單很多,但是我推薦上面的方法。
private map<string, softreference<bitmap>> cache = collections
.synchronizedmap(new hashmap<string, softreference<bitmap>>());
if (!cache.containskey(id))
softreference<bitmap> ref = cache.get(id);
return ref.get();
cache.put(id, new softreference<bitmap>(bitmap));
下面是檔案緩存類的代碼filecache.java:
public class filecache {
private file cachedir;
public filecache(context context) {
// 如果有sd卡則在sd卡中建一個lazylist的目錄存放緩存的圖檔
// 沒有sd卡就放在系統的緩存目錄中
if (android.os.environment.getexternalstoragestate().equals(
android.os.environment.media_mounted))
cachedir = new file(
android.os.environment.getexternalstoragedirectory(),
"lazylist");
else
cachedir = context.getcachedir();
if (!cachedir.exists())
cachedir.mkdirs();
public file getfile(string url) {
// 将url的hashcode作為緩存的檔案名
string filename = string.valueof(url.hashcode());
// another possible solution
// string filename = urlencoder.encode(url);
file f = new file(cachedir, filename);
return f;
file[] files = cachedir.listfiles();
if (files == null)
return;
for (file f : files)
f.delete();
最後最重要的加載圖檔的類,imageloader.java:
public class imageloader {
memorycache memorycache = new memorycache();
filecache filecache;
private map<imageview, string> imageviews = collections
.synchronizedmap(new weakhashmap<imageview, string>());
// 線程池
executorservice executorservice;
public imageloader(context context) {
filecache = new filecache(context);
executorservice = executors.newfixedthreadpool(5);
// 當進入listview時預設的圖檔,可換成你自己的預設圖檔
final int stub_id = r.drawable.stub;
// 最主要的方法
public void displayimage(string url, imageview imageview) {
imageviews.put(imageview, url);
// 先從記憶體緩存中查找
bitmap bitmap = memorycache.get(url);
if (bitmap != null)
imageview.setimagebitmap(bitmap);
else {
// 若沒有的話則開啟新線程加載圖檔
queuephoto(url, imageview);
imageview.setimageresource(stub_id);
private void queuephoto(string url, imageview imageview) {
phototoload p = new phototoload(url, imageview);
executorservice.submit(new photosloader(p));
private bitmap getbitmap(string url) {
file f = filecache.getfile(url);
// 先從檔案緩存中查找是否有
bitmap b = decodefile(f);
if (b != null)
return b;
// 最後從指定的url中下載下傳圖檔
bitmap bitmap = null;
url imageurl = new url(url);
httpurlconnection conn = (httpurlconnection) imageurl
.openconnection();
conn.setconnecttimeout(30000);
conn.setreadtimeout(30000);
conn.setinstancefollowredirects(true);
inputstream is = conn.getinputstream();
outputstream os = new fileoutputstream(f);
copystream(is, os);
os.close();
bitmap = decodefile(f);
return bitmap;
} catch (exception ex) {
ex.printstacktrace();
// decode這個圖檔并且按比例縮放以減少記憶體消耗,虛拟機對每張圖檔的緩存大小也是有限制的
private bitmap decodefile(file f) {
// decode image size
bitmapfactory.options o = new bitmapfactory.options();
o.injustdecodebounds = true;
bitmapfactory.decodestream(new fileinputstream(f), null, o);
// find the correct scale value. it should be the power of 2.
final int required_size = 70;
int width_tmp = o.outwidth, height_tmp = o.outheight;
int scale = 1;
while (true) {
if (width_tmp / 2 < required_size
|| height_tmp / 2 < required_size)
width_tmp /= 2;
height_tmp /= 2;
scale *= 2;
// decode with insamplesize
bitmapfactory.options o2 = new bitmapfactory.options();
o2.insamplesize = scale;
return bitmapfactory.decodestream(new fileinputstream(f), null, o2);
} catch (filenotfoundexception e) {
return null;
// task for the queue
private class phototoload {
public string url;
public imageview imageview;
public phototoload(string u, imageview i) {
url = u;
imageview = i;
class photosloader implements runnable {
phototoload phototoload;
photosloader(phototoload phototoload) {
this.phototoload = phototoload;
@override
public void run() {
if (imageviewreused(phototoload))
return;
bitmap bmp = getbitmap(phototoload.url);
memorycache.put(phototoload.url, bmp);
bitmapdisplayer bd = new bitmapdisplayer(bmp, phototoload);
// 更新的操作放在ui線程中
activity a = (activity) phototoload.imageview.getcontext();
a.runonuithread(bd);
* 防止圖檔錯位
* @param phototoload
boolean imageviewreused(phototoload phototoload) {
string tag = imageviews.get(phototoload.imageview);
if (tag == null || !tag.equals(phototoload.url))
return true;
return false;
// 用于在ui線程中更新界面
class bitmapdisplayer implements runnable {
bitmap bitmap;
public bitmapdisplayer(bitmap b, phototoload p) {
bitmap = b;
phototoload = p;
if (bitmap != null)
phototoload.imageview.setimagebitmap(bitmap);
else
phototoload.imageview.setimageresource(stub_id);
public void clearcache() {
memorycache.clear();
filecache.clear();
public static void copystream(inputstream is, outputstream os) {
final int buffer_size = 1024;
byte[] bytes = new byte[buffer_size];
for (;;) {
int count = is.read(bytes, 0, buffer_size);
if (count == -1)
os.write(bytes, 0, count);
主要流程是先從記憶體緩存中查找,若沒有再開線程,從檔案緩存中查找都沒有則從指定的url中查找,并對bitmap進行處理,最後通過下面方法對ui進行更新操作。
a.runonuithread(...);
在你的程式中的基本用法:
imageloader imageloader=new imageloader(context);
...
imageloader.displayimage(url, imageview);
比如你的放在你的listview的adapter的getview()方法中,當然也适用于gridview。ok,先到這。