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一.为什么Bitmap三级缓存？
没有缓存的弊端 ：费流量, 加载速度慢
加入缓存的优点： 省流量，支持离线浏览
二.原理

从内存获取图片, 如果存在, 则显示; 如果不存在, 则从SD卡中获取图片
从SD卡中获取图片, 如果文件中存在, 显示, 并且添加到内存中; 否则开启网络下载图片
从网络下载图片, 如果下载成功, 则添加到缓存中, 存入SD卡, 显示图片
三.代码
(1)添加读写SD卡的权限和网络权限



// //Lrucache存储工具类
public class LruUtils {
private LruCache<String,Bitmap> lruCache;
private long max=Runtime.getRuntime().maxMemory();
public LruUtils(){
lruCache=new LruCache<String,Bitmap>((int)max/8){

        @Override
        protected int sizeOf(String key, Bitmap value) {
            return value.getByteCount();
        }
    };
}
public Bitmap getBitmap(String key){
    return lruCache.get(key);
}
public void setBitmap(String key,Bitmap bitmap){
    lruCache.put(key,bitmap);
}
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}
//SD卡工具类
public class SDUtils {

public static void setBitmap(String name, Bitmap bitmap) {

    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
        File file = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
        File file1 = new File(file, name);

        try {

            bitmap.compress(Bitmap.CompressFormat.JPEG, 100, new FileOutputStream(file1));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}
public static Bitmap getBitmap(String name){

    if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
        File file=Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
        File file1=new File(file,name);

        return BitmapFactory.decodeFile(file1.getAbsolutePath());

    }
    return null;
}

}
//网络

import android.app.AlertDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.ExecutionException;

public class NewUtils {
public static Bitmap getBitmap(String url) throws ExecutionException, InterruptedException {
return new MyTask().execute(url).get();
}
static class MyTask extends AsyncTask<String,Void,Bitmap>{
@Override
protected Bitmap doInBackground(String… strings) {
String imageUrl = strings[0];
HttpURLConnection conn = null;
try {
URL url = new URL(imageUrl);
conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(5000);
conn.setConnectTimeout(5000);
conn.setRequestMethod(“GET”);
if (conn.getResponseCode() == 200) {
InputStream is = conn.getInputStream();
Bitmap bitmap = BitmapFactory.decodeStream(is);
return bitmap;
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if (conn != null) {
conn.disconnect();
}
}
return null;
}
}

}
//使用三个工具类完成Bitmap的三级缓存
package com.example.administrator.myapplication;

import android.graphics.Bitmap;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import java.util.concurrent.ExecutionException;

public class MainActivity extends AppCompatActivity {
private ImageView imageView;
Button button;
private LruUtils lruUtils= new LruUtils();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button=findViewById(R.id.button);
imageView=findViewById(R.id.imageview);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Bitmap bitmap=lruUtils.getBitmap(“czn”);
if (bitmap!=null){
imageView.setImageBitmap(bitmap);
Toast.makeText(MainActivity.this, “图片存内存”, Toast.LENGTH_SHORT).show();
}else{
bitmap=SDUtils.getBitmap(“czn.jpg”);
if (bitmap!=null){
imageView.setImageBitmap(bitmap);
Toast.makeText(MainActivity.this, “图片存SD卡”, Toast.LENGTH_SHORT).show();
lruUtils.setBitmap(“czn”,bitmap);
}else{
try {
bitmap=NewUtils.getBitmap(“http://pic1.win4000.com/wallpaper/e/50d80458e1373.jpg”);
if (bitmap!=null){
imageView.setImageBitmap(bitmap);
Toast.makeText(MainActivity.this, “图片存网络”, Toast.LENGTH_SHORT).show();
SDUtils.setBitmap(“czn.jpg”,bitmap);
lruUtils.setBitmap(“czn”,bitmap);
}else{
Toast.makeText(MainActivity.this, “没有找到”, Toast.LENGTH_SHORT).show();
}

                    } catch (ExecutionException e) {
                        e.printStackTrace();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                }
            }
        }
    });
}

}
Bitmap二次采样


import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.concurrent.ExecutionException;

public class Main2Activity extends AppCompatActivity {
Button bt;
ImageView imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
bt=findViewById(R.id.bt);
imageView=findViewById(R.id.mimage);
bt.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
try {
Bitmap bitmap = new MyTask().execute(“https://cdn.duitang.com/uploads/item/201211/24/20121124230042_Bfhim.jpeg”).get();
imageView.setImageBitmap(bitmap);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}

        }

    });
}
class MyTask extends AsyncTask<String,Object,Bitmap>{

    @Override
    protected Bitmap doInBackground(String... strings) {
        try {
            URL url = new URL(strings[0]);
            HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
            if(urlConnection.getResponseCode()==200){
                InputStream inputStream = urlConnection.getInputStream();
                //将inputStream流存储起来
                ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
                byte[] bytes = new byte[1024];
                int len=0;
                while((len=inputStream.read(bytes))!=-1){
                    byteArrayOutputStream.write(bytes,0,len);
                }
                //桶：网络的图片都放在数组里面了
                byte[] data = byteArrayOutputStream.toByteArray();
                //TODO 1：第一次采样：只采边框 计算压缩比例
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inJustDecodeBounds=true;//设置只采边框
                BitmapFactory.decodeByteArray(data,0,data.length,options);//采样
                int outWidth = options.outWidth;//获得原图的宽
                int outHeight = options.outHeight;//获得原图的高
                //计算缩放比例
                int size=1;
                while(outWidth/size>100||outHeight/size>100){
                    size*=2;
                }
                //TODO 2:第二次采样：按照比例才像素
                options.inJustDecodeBounds=false;//设置只采边框为fasle
                options.inSampleSize=size;//设置缩放比例
                Bitmap bitmap= BitmapFactory.decodeByteArray(data,0,data.length,options);//采样
                return  bitmap;
            }

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}
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