Android 下载网络图片 简单Demo

现在Android下载网络图片的开源框架有很多，而且使用起来也非常方便，只要一两句代码就能解决问题，但是我们仿佛并不懂其里面的实现机制。

private String path = "http://7xlwlf.com1.z0.glb.clouddn.com/%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg";

1.首先需要先获得一个连接，通过url.openConnection()方法来获得，设置这个连接的属性（比如请求的方式，连接超时，读取超时等）

url = new URL(path);
								connection = (HttpURLConnection) url
										.openConnection();
								connection.setConnectTimeout(5000);
								connection.setRequestMethod("GET");
								connection.setReadTimeout(5000);

2.请求成功可以获得所请求的图片资源的一个流，将这个流缓存到手机本地的文件中

int code = connection.getResponseCode();
								if (code == 200) {
									InputStream is = connection
											.getInputStream();
									File file = new File(getCacheDir(),
											"%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg");
									FileOutputStream out = new FileOutputStream(
											file);
									int len;
									byte[] data = new byte[1024];
									while ((len = is.read(data)) != -1) {
										out.write(data, 0, len);
									}

3.使用Handler机制来刷新UI

（1）Handler机制的简单理解

1）消息队列（MessageQueue）:存储从Handler对象发送过来的消息

2）轮询器（Looper）:不断从消息队列里去消息，发送给Handler对象处理

3）消息处理器（Handler）:处理从Looper对象传送过来的消息

有些同学可能有点疑惑：Handler对象既是发送消息的，又是处理消息，为什么还要去MessageQueue转一下然后通过Looper对象来取直接处理了不久得了？我们可以设想一个场景：假如你是一个很有钱的老板，有很多人找你借钱，作为大老板的你当然不能当面答复他们，所以就让他们先去你的小秘那边登记一下，然后你一个一个在考虑到底借不借钱给某人。

(2)Handler对象的定义(对各类消息的处理)

private Handler mhandler = new Handler() {
		@Override
		public void handleMessage(Message msg) {
			super.handleMessage(msg);
			switch (msg.what) {
			case 1:
				Bitmap bmp = (Bitmap) msg.obj;
				iv.setImageBitmap(bmp);
				break;
			default:
				break;
			}
		}
	};

（3）Handler对象向消息队列发送消息

Message msg = new Message();
									msg.what = 1;
									msg.obj = bmp;
									mhandler.sendMessage(msg);

4.是否有缓存判断，如果本地有缓存，直接加载本地文件（这里直接用的是File文件，当然Android提供了非常好用LruCache）

File file = new File(getCacheDir(),
						"%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg");
				if (file.exists()) {
					Bitmap bmp = BitmapFactory.decodeFile(file
							.getAbsolutePath());
					iv.setImageBitmap(bmp);
}

5.完整代码

1）XML文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/btn_load"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="下载图片" />

    <ImageView
        android:id="@+id/iv"
        android:layout_width="200dp"
        android:layout_height="200dp" />

</LinearLayout>

2）java代码

package com.example.androidpictureloaddemo;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;
import java.security.PublicKey;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
import android.os.Build;

public class MainActivity extends Activity {
	private ImageView iv;
	private Button btn;
	private String path = "http://7xlwlf.com1.z0.glb.clouddn.com/%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg";
	private Handler mhandler = new Handler() {
		@Override
		public void handleMessage(Message msg) {
			super.handleMessage(msg);
			switch (msg.what) {
			case 1:
				Bitmap bmp = (Bitmap) msg.obj;
				iv.setImageBitmap(bmp);
				break;
			default:
				break;
			}
		}
	};

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		iv = (ImageView) findViewById(R.id.iv);
		btn = (Button) findViewById(R.id.btn_load);
		btn.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View arg0) {

				File file = new File(getCacheDir(),
						"%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg");
				if (file.exists()) {
					Bitmap bmp = BitmapFactory.decodeFile(file
							.getAbsolutePath());
					iv.setImageBitmap(bmp);
				} else {
					new Thread() {
						@Override
						public void run() {
							URL url = null;
							HttpURLConnection connection = null;
							try {
								url = new URL(path);
								connection = (HttpURLConnection) url
										.openConnection();
								connection.setConnectTimeout(5000);
								connection.setRequestMethod("GET");
								connection.setReadTimeout(5000);
								int code = connection.getResponseCode();
								if (code == 200) {
									InputStream is = connection
											.getInputStream();
									File file = new File(getCacheDir(),
											"%40%2Ftt%2Fupload%2Fimages%2F(null)%2F2015%2F12%2F11%2F1449818979-1.jpg");
									FileOutputStream out = new FileOutputStream(
											file);
									int len;
									byte[] data = new byte[1024];
									while ((len = is.read(data)) != -1) {
										out.write(data, 0, len);
									}
									Bitmap bmp = BitmapFactory.decodeFile(file
											.getAbsolutePath());
									Message msg = new Message();
									msg.what = 1;
									msg.obj = bmp;
									mhandler.sendMessage(msg);
								}
							} catch (Exception e) {
								// TODO Auto-generated catch block
								e.printStackTrace();
							}

						}
					}.start();
				}

			}
		});
	}
}