1、httpclient過時，

在Android studio中可以這樣設定再使用：

    useLibrary 'org.apache.http.legacy'

2、許可權：

    <!--許可權-->
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name
 
="android.permission.WRITE_EXTERNAL_STORAGE"/>

3、佈局

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id
 
="@+id/img_down"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher" />

    <Button
        android:id="@+id/btn_down"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below
 
="@+id/img_down"
        android:onClick="clickbutton"
        android:text="下載" />

</RelativeLayout>

4、檔案工具類

package com.example.administrator.downloadimage;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Environment;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * Created by Administrator on 2017/5/4.
 */

public class FileUtils {

    private static final String CACHE_DIR = Environment.getExternalStorageDirectory()
            + "/my_caches/images";// 快取資料夾

    /**
     * 判斷sdcard是否掛載
     *
     * @return
     */
    public static boolean isMounted() {
        String state = Environment.getExternalStorageState();
        return state.equals(Environment.MEDIA_MOUNTED);
    }

    /**
     * 獲取sdcard的根目錄
     *
     * @return
     */
    public static String getSDCARDDir() {
        return Environment.getExternalStorageDirectory().getAbsolutePath();
    }

    /**
     * 儲存圖片一
     *
     * @param url  ： 圖片的http網路地址
     * @param data
     * @throws IOException
     */
    public static void saveImage(String url, byte[] data) throws IOException {
        // 1. 判斷是否有sdcard
        if (!isMounted()) {
            return;
        }
        // 2. 判斷是否有快取的資料夾
        File dir = new File(CACHE_DIR);
        if (!dir.exists()) {
            dir.mkdirs();// 多層資料夾
        }
        // 3. 儲存圖片到sdcard
        File file = new File(dir, getFilename(url));
        FileOutputStream fos = new FileOutputStream(file);

        fos.write(data);
        fos.close();
    }

    /**
     * 儲存圖片二
     *
     * @param url    : 圖片的http網路地址
     * @param bitmap
     * @throws IOException
     */
    public static void saveImage(String url, Bitmap bitmap) throws IOException {
        // 1. 判斷是否有sdcard
        if (!isMounted()) {
            return;
        }
        // 2. 判斷是否有快取的資料夾
        File dir = new File(CACHE_DIR);
        if (!dir.exists()) {
            dir.mkdirs();
        }
        // 3. 儲存圖片到sdcard
        File file = new File(dir, getFilename(url));
        FileOutputStream fos = new FileOutputStream(file);

        bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos);
    }

    /**
     * 獲取圖片名
     *
     * @param url
     * @return
     */
    public static String getFilename(String url) {
        return url.substring(url.lastIndexOf("/") + 1);
    }

    /**
     * 讀圖片
     *
     * @param url
     * @return
     */
    public static Bitmap readImage(String url) {
        // 判斷是否有sdcard
        if (!isMounted()) {
            return null;
        }
        File file = new File(CACHE_DIR, getFilename(url));
        if (file.exists()) {
            // file->bitmap
            return BitmapFactory.decodeFile(file.getAbsolutePath());
        }
        return null;
    }

    /**
     * 清空快取目錄
     */
    public void clearCaches() {
        File dir = new File(CACHE_DIR);
        File[] allfiles = dir.listFiles();
        for (File file : allfiles) {
            file.delete();
        }
    }
}

5、載入資料並儲存

利用httpclient請求，asynctask非同步，載入圖片並儲存在本地。

package com.example.administrator.downloadimage;

import android.app.ProgressDialog;
import android.content.Context;
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.ImageView;

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

import java.io.IOException;

public class MainActivity extends AppCompatActivity {

    private ImageView img_down;
    private String url = "http://b.hiphotos.baidu.com/image/pic/item/d1160924ab18972bf5f68cc8e0cd7b899f510ae7.jpg";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        img_down = (ImageView) this.findViewById(R.id.img_down);
    }

    public void clickbutton(View view) {
        Bitmap bitmap = FileUtils.readImage(url);
        if (bitmap != null) {
            img_down.setImageBitmap(bitmap);
        } else {
            // 傳送非同步請求
            new MyAsyncTask(this).execute(url);
        }
    }

    class MyAsyncTask extends AsyncTask<String, Void, byte[]> {

        private ProgressDialog pDialog;

        public MyAsyncTask(Context context) {
            pDialog = new ProgressDialog(context);
            pDialog.setIcon(R.mipmap.ic_launcher);
            pDialog.setTitle("提示");
            pDialog.setMessage("The data is loading...");
        }

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pDialog.show();
        }

        @Override
        protected byte[] doInBackground(String... params) {
            try {
                HttpClient client = new DefaultHttpClient();
                HttpGet get = new HttpGet(params[0]);
                HttpResponse response = client.execute(get);

                if (response.getStatusLine().getStatusCode() == 200) {
                    byte[] datas = EntityUtils.toByteArray(response.getEntity());
                    // 快取工具類：儲存圖片
                    FileUtils.saveImage(params[0], datas);// url:params[0]
                    return datas;
                }

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

        @Override
        protected void onPostExecute(byte[] result) {
            super.onPostExecute(result);
            if (result != null) {
                Bitmap bitmap = BitmapFactory.decodeByteArray(result, 0, result.length);
                img_down.setImageBitmap(bitmap);
            }
            pDialog.dismiss();
        }
    }
}

注意：

 //儲存圖片後傳送廣播通知更新資料庫
        Uri uri = Uri.fromFile(file);
        sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE, uri));