从零开始编写图片加载库（四）之图片缓存MemoryCache

图片加载通过内存加载是最快的，然而手机的内存并不是取之不尽的，所以在开发过程中还需要注意就是内存的使用问题，本节将不涉及内存使用和优化问题，本节只介绍图片下载后从内存中加载。

第一步：编写MemoryCache类

package cn.sundroid.cache;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import android.graphics.Bitmap;

public class WeakMemoryCache {

/**保存Bitmap的对象*/
private final Map<String, Reference<Bitmap>> softMap = Collections
.synchronizedMap(new HashMap<String, Reference<Bitmap>>());

/**
* 根据键获得Bitmap
* @param key 查询需要的键
* @return
*/
public Bitmap get(String key) {
if (key == null) {
throw new NullPointerException("key == null");
}
Bitmap result = null;
Reference<Bitmap> reference = softMap.get(key);
if (reference != null) {
result = reference.get();
}
return result;
}

/**
*
* 保存Bitmap到内存
* @param key 键
* @param value 值
* @return
*/
public boolean put(String key, Bitmap value) {
if (key == null || value == null) {
throw new NullPointerException("key == null || value == null");
}
softMap.put(key, new WeakReference<Bitmap>(value));
return false;

}

/**
* 根据key从内存中移除Bitmap对象
* @param key
* @return
*/
public Bitmap remove(String key) {
Reference<Bitmap> bmpRef = softMap.remove(key);
return bmpRef == null ? null : bmpRef.get();
}

/**
* 清空操作
*/
public void clear() {
softMap.clear();
}

}

第二步：编写实现，下载图片后在handler里面接收更新UI通知，从cache里面取图片。

package cn.sundroid.loader;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import cn.sundroid.cache.WeakMemoryCache;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.widget.ImageView;

public class MainActivity extends Activity {

private ImageView image;
private final String TAG = "ImageLoader";

private WeakMemoryCache cache = new WeakMemoryCache();
private final String IMAGE_CACHE_KEY = "image_cache_key";
private Handler handler = new Handler() {
public void handleMessage(android.os.Message msg) {
Bitmap bitmap = cache.get(IMAGE_CACHE_KEY);
Log.e(TAG, "get bitmap from memory cache");
image.setImageBitmap(bitmap);
};
};

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
image = (ImageView) findViewById(R.id.image);
new Thread(new Runnable() {

@Override
public void run() {
try {
//网络连接
HttpURLConnection connection = (HttpURLConnection) new URL(
"http://pic5.nipic.com/20091228/2588536_142951087553_2.jpg")
.openConnection();
connection.setConnectTimeout(3 * 1000);
connection.setReadTimeout(20 * 1000);
//读取文件流
InputStream is = connection.getInputStream();
//通过BitmapFactory解码二进制字节流成Bitmap
Bitmap bitmap = BitmapFactory.decodeStream(is);
Log.e(TAG, "put bitmap into memory cache");
cache.put(IMAGE_CACHE_KEY, bitmap);
//发送一个消息给Handler用于更新消息
handler.sendEmptyMessage(0x10);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}

}
}).start();

}

}

源代码下载==>