从网络下载的图片加缓存功能

从网络中下载的图片放到本地，然后在用bitmap获取本地的图片，通过消息队列发送到主线程去执行，

最后一步我们要判断一下，如果这个文件存在，直接从本地来读取，如果不存在就从网络中获取。

package com.example.getimg;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;

import android.os.Bundle;
import android.os.Message;
import android.view.View;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;

import java.net.URL;

public class MyActivity extends Activity {
    /**
     * Called when the activity is first created.
     */
    private ImageView img;
    android.os.Handler handler = new android.os.Handler(){
       //用handlemessage来更新ui线程
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case 0:

                    img.setImageBitmap((Bitmap) msg.obj);
                    break;
                case 1:
                    Toast.makeText(MyActivity.this,"没有响应",Toast.LENGTH_SHORT).show();
                    break;
            }

        }
    };
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        img = (ImageView) findViewById(R.id.img);

        findViewById(R.id.btn).setOnClickListener(new View.OnClickListener() {
            final String path = "http://192.168.21.1:8080/ok/point_one.png";
            final File file = new File(getCacheDir(),getFeilName(path));
            @Override
            public void onClick(View view) {
                //判断缓存是否存在，因为这个是在主线程里，可以直接设置
                if(file.exists()){
                    Toast.makeText(MyActivity.this,"从缓存读取",Toast.LENGTH_SHORT).show();
                    Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
                    img.setImageBitmap(bitmap);
                }
                else{
                    System.out.print("从网络中读取");
                    //创建子线程来链接网络，并且使用get方法下载图片
                    Thread thread = new Thread(){
                        @Override
                        public void run() {
                            {

                                //uri
                                try {
                                    URL url = new URL(path);
                                    //获得链接
                                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                                    //设置参数
                                    conn.setRequestMethod("GET");
                                    conn.setConnectTimeout(5000);
                                    conn.setReadTimeout(5000);
                                    //正式链接
                                    conn.connect();
                                    if(conn.getResponseCode()==200){
                                        InputStream is = conn.getInputStream();
                                        //把InputStream通过bitmap转化成图片
                                        //   Bitmap bitmap = BitmapFactory.decodeStream(is);

                                       //把从网络中读取的图片放到缓存中
                                        FileOutputStream fos = new FileOutputStream(file);
                                        byte[] b = new byte[1024];
                                        int len = 0;
                                        while((len=is.read(b))!=-1){
                                            fos.write(b,0,len);
                                        }
                                        //从本地读取bitmap
                                        Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
                                        //创建消息对象
                                        Message msg = new Message();
                                        msg.what = 0;
                                        //传递bitmap所需要的数据,如果是多个数据可以传递数组啊！
                                        msg.obj = bitmap;
                                        //给消息队列发送消息
                                        handler.sendMessage(msg);

                                    }else{
                                        //一般不会new，这样会节省内存
                                        Message msg = handler.obtainMessage();
                                        //传递参数为了判断是谁发送的message
                                        msg.what = 1;
                                        handler.sendMessage(msg);

                                    }

                                } catch (Exception e) {
                                    e.printStackTrace();
                                }
                            }
                        }

                    };
                    thread.start();

                }

            }
        });
    }

    private String getFeilName(String path){
        int index = path.lastIndexOf("/");
        return path.substring(index + 1);
    }
}