<LinearLayout 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"
  android:orientation="vertical"
  tools:context=".MainActivity" >

  <ImageView
    android:id="@+id/iv"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_weight="1" />

  <EditText
    android:id="@+id/et_path"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="请输入图片路径"
    android:src="@drawable/
    android:maxLines="1" />  

<Button  android:layout_width="fill_parent"  android:layout_height="wrap_content"  android:onClick="click"    android:text="浏览" />

</LinearLayout>

package cn.edu.bzu.imageviewdemo;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
public class MainActivity extends AppCompatActivity {

  protected static final int CHANGE_UI = 1;
  protected static final int ERROR = 2;
  private EditText et_path;
  private ImageView iv;
  // 主线程创建消息处理器
  private Handler handler = new Handler(){
    public void handleMessage(android.os.Message msg) {
      if(msg.what == CHANGE_UI){
        Bitmap bitmap = (Bitmap) msg.obj;
        iv.setImageBitmap(bitmap);
      }else if(msg.what == ERROR){
        Toast.makeText(MainActivity.this, "显示图片错误", 0).show();
      }
    };
  };
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    et_path = (EditText) findViewById(R.id.et_path);
    iv = (ImageView) findViewById(R.id.iv);
  }
  public void click(View view) {
    final String path = et_path.getText().toString().trim();
    if (TextUtils.isEmpty(path)) {
      Toast.makeText(this, "图片路径不能为空", Toast.LENGTH_SHORT).show();
    } else {
      //子线程请求网络,Android4.0以后访问网络不能放在主线程中
      new Thread() {
        public void run() {
          // 连接服务器 get 请求 获取图片.
          try {
            URL url = new URL(path);    //创建URL对象
            // 根据url 发送 http的请求.
            HttpURLConnection conn = (HttpURLConnection) url
                .openConnection();
            // 设置请求的方式
            conn.setRequestMethod("GET");
            //设置超时时间
            conn.setConnectTimeout(5000);
            // 得到服务器返回的响应码
            int code = conn.getResponseCode();
            //请求网络成功后返回码是200
            if (code == 200) {
              //获取输入流
              InputStream is = conn.getInputStream();
              //将流转换成Bitmap对象
              Bitmap bitmap = BitmapFactory.decodeStream(is);
              //iv.setImageBitmap(bitmap);
              //TODO: 告诉主线程一个消息:帮我更改界面。内容:bitmap
              Message msg = new Message();
              msg.what = CHANGE_UI;
              msg.obj = bitmap;
              handler.sendMessage(msg);
            } else {
              //返回码不是200 请求服务器失败
              Message msg = new Message();
              msg.what = ERROR;
              handler.sendMessage(msg);
            }
          } catch (Exception e) {
            e.printStackTrace();
            Message msg = new Message();
            msg.what = ERROR;
            handler.sendMessage(msg);
          }
        };
      }.start();
    }
  }

}