UI設計

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height
 
="match_parent"
    tools:context=".MainActivity"
    tools:layout_editor_absoluteY="81dp">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <EditText
            android:id="@+id/et_url"
 

            android:layout_width="match_parent"
            android:layout_height="wrap_content" />

        <Button
            android:id="@+id/bt_look"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="click"
            android:text
 
="檢視原始碼" />

       <ImageView
           android:id="@+id/iv_image"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content" />
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

直接載入網頁圖片

package com.example.administrator.logincase;

import android.Manifest;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static android.content.pm.PackageManager.PERMISSION_DENIED;
import static android.content.pm.PackageManager.PERMISSION_GRANTED;

public class MainActivity extends AppCompatActivity {
    static final int m_nError = 0;
    static final int m_nLookImage = 1;
    /*
     * 定義一個handler用於接收執行緒訊息
     * */
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case m_nError:
                    Toast.makeText(MainActivity.this, "http請求錯誤",
                            Toast.LENGTH_SHORT).show();
                    return;
                case m_nLookImage:
                    ImageView image=findViewById(R.id.iv_image);
                    Bitmap bitmap= (Bitmap) msg.obj;
                    image.setImageBitmap(bitmap);
                    return;
                default:
                    break;
            }
            super.handleMessage(msg);
        }
    };

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 0x11: {
                if (grantResults[0] == PERMISSION_GRANTED) {
                    Toast.makeText(this, "網路許可權請求成功",
                            Toast.LENGTH_SHORT).show();
                    return;
                } else {
                    Toast.makeText(this, "網路許可權請求失敗",
                            Toast.LENGTH_SHORT).show();
                    return;
                }
            }
            default:
                break;
        }
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (checkSelfPermission(Manifest.permission.INTERNET) == PERMISSION_DENIED) {
            requestPermissions(new String[]{Manifest.permission.INTERNET}, 0x11);
        }
    }

    public void click(View v) {
        /*
         * 網路的訪問必須線上程裡面實現
         * */
        new Thread() {
            @Override
            public void run() {
                EditText editText = findViewById(R.id.et_url);
                String strUrl = editText.getText().toString().trim();

                try {
                    URL url = new URL(strUrl);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setReadTimeout(5000);
                    if (conn.getResponseCode() != 200) {
                        Message msg = Message.obtain();
                        msg.what = m_nError;
                        handler.sendMessage(msg);
                        return;
                    }
                    InputStream in = new BufferedInputStream(conn.getInputStream());
                    Bitmap bitmap = BitmapFactory.decodeStream(in);

                    Message msg = Message.obtain();
                    msg.what = m_nLookImage;

                    msg.obj = bitmap;
                    handler.sendMessage(msg);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }

}

利用快取

生成到快取中的檔案可以在安卓應用中點選清除資料清楚

package com.example.administrator.logincase;

import android.Manifest;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static android.content.pm.PackageManager.PERMISSION_DENIED;
import static android.content.pm.PackageManager.PERMISSION_GRANTED;

public class MainActivity extends AppCompatActivity {
    static final int m_nError = 0;
    static final int m_nLookImage = 1;
    /*
     * 定義一個handler用於接收執行緒訊息
     * */
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case m_nError:
                    Toast.makeText(MainActivity.this, "http請求錯誤",
                            Toast.LENGTH_SHORT).show();
                    return;
                case m_nLookImage:
                    ImageView image = findViewById(R.id.iv_image);
                    Bitmap bitmap = (Bitmap) msg.obj;
                    image.setImageBitmap(bitmap);
                    return;
                default:
                    break;
            }
            super.handleMessage(msg);
        }
    };

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 0x11: {
                if (grantResults[0] == PERMISSION_GRANTED) {
                    Toast.makeText(this, "網路許可權請求成功",
                            Toast.LENGTH_SHORT).show();
                    return;
                } else {
                    Toast.makeText(this, "網路許可權請求失敗",
                            Toast.LENGTH_SHORT).show();
                    return;
                }
            }
            default:
                break;
        }
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (checkSelfPermission(Manifest.permission.INTERNET) == PERMISSION_DENIED) {
            requestPermissions(new String[]{Manifest.permission.INTERNET}, 0x11);
        }
    }

    public void click(View v) {

        new Thread() {
            @Override
            public void run() {
                EditText editText = findViewById(R.id.et_url);
                String strUrl = editText.getText().toString().trim();

                try {
                    /*
                     * 快取圖片，如果圖片已經存在了就不從網上下載了
                     * */
                    File file = new File(getCacheDir(), "test.jpg");
                    if (file.exists() && file.length() > 0) {
                        Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
                        Message msg = Message.obtain();
                        msg.what = m_nLookImage;
                        msg.obj = bitmap;
                        handler.sendMessage(msg);
                        return;
                    }
                    URL url = new URL(strUrl);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setReadTimeout(5000);
                    if (conn.getResponseCode() != 200) {
                        Message msg = Message.obtain();
                        msg.what = m_nError;
                        handler.sendMessage(msg);
                        return;
                    }
                    InputStream in = new BufferedInputStream(conn.getInputStream());
                    FileOutputStream fos = new FileOutputStream(file);
                    int len = -1;
                    byte[] buffer = new byte[1024];
                    while ((len = in.read(buffer)) != -1) {
                        fos.write(buffer, 0, len);
                    }
                    in.close();
                    fos.close();
                    Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());

                    Message msg = Message.obtain();
                    msg.what = m_nLookImage;

                    msg.obj = bitmap;
                    handler.sendMessage(msg);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }

}

使用runOnUiThread

package com.example.administrator.logincase;

import android.Manifest;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static android.content.pm.PackageManager.PERMISSION_DENIED;
import static android.content.pm.PackageManager.PERMISSION_GRANTED;

public class MainActivity extends AppCompatActivity {
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 0x11: {
                if (grantResults[0] == PERMISSION_GRANTED) {
                    Toast.makeText(this, "網路許可權請求成功",
                            Toast.LENGTH_SHORT).show();
                    return;
                } else {
                    Toast.makeText(this, "網路許可權請求失敗",
                            Toast.LENGTH_SHORT).show();
                    return;
                }
            }
            default:
                break;
        }
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (checkSelfPermission(Manifest.permission.INTERNET) == PERMISSION_DENIED) {
            requestPermissions(new String[]{Manifest.permission.INTERNET}, 0x11);
        }
    }

    public void click(View v) {

        new Thread() {
            @Override
            public void run() {
                EditText editText = findViewById(R.id.et_url);
                String strUrl = editText.getText().toString().trim();

                try {
                    /*
                     * 快取圖片，如果圖片已經存在了就不從網上下載了
                     * */
                    File file = new File(getCacheDir(), "test.jpg");
                    if (file.exists() && file.length() > 0) {
                        final Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
                        runOnUiThread(new Runnable() {
                            @Override
                            public void run() {
                                ImageView image = findViewById(R.id.iv_image);
                                image.setImageBitmap(bitmap);
                            }
                        });
                        return;
                    }
                    URL url = new URL(strUrl);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setReadTimeout(5000);
                    if (conn.getResponseCode() != 200) {
                        runOnUiThread(new Runnable() {
                            @Override
                            public void run() {
                                Toast.makeText(MainActivity.this, "網路許可權請求成功",
                                        Toast.LENGTH_SHORT).show();
                            }
                        });
                        return;
                    }
                    InputStream in = new BufferedInputStream(conn.getInputStream());
                    FileOutputStream fos = new FileOutputStream(file);
                    int len = -1;
                    byte[] buffer = new byte[1024];
                    while ((len = in.read(buffer)) != -1) {
                        fos.write(buffer, 0, len);
                    }
                    in.close();
                    fos.close();
                    final Bitmap bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            ImageView image = findViewById(R.id.iv_image);
                            image.setImageBitmap(bitmap);
                        }
                    });
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }

}

介面效果