為什麼BitmapFactory.decodeByteArray()傳回null問題 activity_main.xml布局裡面一個按鈕跟一個imageView <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:id="@+id/button"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="從網絡上下載下傳一張圖檔" />
<ImageView
android:id="@+id/imageView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="fitXY"/>
</LinearLayout>
MainActivity.java代碼如下 import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.util.StringBuilderPrinter;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
public class MainActivity extends AppCompatActivity {
private Button bt;
private ImageView img;
private ProgressDialog progressDialog;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bt = (Button) findViewById(R.id.button);
img = (ImageView) findViewById(R.id.imageView);
progressDialog = new ProgressDialog(this);
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
progressDialog.setTitle("提示資訊");
progressDialog.setMessage("正在下載下傳");
progressDialog.setCancelable(false);
bt.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new MyAscyTask().execute();
}
});
}
public class MyAscyTask extends AsyncTask<String, Integer, byte[]> {
@Override
protected byte[] doInBackground(String... params) {
InputStream ism = null;
HttpURLConnection con = null;
BufferedReader read = null;
try {
URL url = new URL("http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg");
con = (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
con.setReadTimeout();
con.setConnectTimeout();
ism = con.getInputStream();
read = new BufferedReader(new InputStreamReader(ism));
StringBuffer sb = new StringBuffer();
String line = null;
while ((line = read.readLine()) != null) {
sb.append(line);
}
return sb.toString().getBytes();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (ism != null) {
ism.close();
ism = null;
}
if (read != null) {
read.close();
read = null;
}
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
progressDialog.show();
}
@Override
protected void onPostExecute(byte[] bytes) {
super.onPostExecute(bytes);
Log.i("TAG", "onPostExecute: "+bytes.length+"==="+bytes[]);
Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, , bytes.length);
img.setImageBitmap(bitmap);
progressDialog.dismiss();
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
}
}
運作結果: 記得運作時權重限,因為通路網絡,不會從網絡得到圖檔,因為BitmapFactory.decodeByteArray方法傳回空 android 運用AsyncTask 擷取圖檔并顯示 修改MainActivity.java import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.util.StringBuilderPrinter;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
import java.io.BufferedReader;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
public class MainActivity extends AppCompatActivity {
private Button bt;
private ImageView img;
private ProgressDialog progressDialog;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bt = (Button) findViewById(R.id.button);
img = (ImageView) findViewById(R.id.imageView);
progressDialog = new ProgressDialog(this);
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
progressDialog.setTitle("提示資訊");
progressDialog.setMessage("正在下載下傳");
progressDialog.setCancelable(false);
bt.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new MyAscyTask().execute();
}
});
}
public class MyAscyTask extends AsyncTask<String, Integer, byte[]> {
@Override
protected byte[] doInBackground(String... params) {
InputStream ism = null;
HttpURLConnection con = null;
ByteArrayOutputStream out=new ByteArrayOutputStream();
try {
URL url = new URL("http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg");
con = (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
con.setReadTimeout();
con.setConnectTimeout();
ism = con.getInputStream();
int size;
while ((size=ism.read())!=-) {
out.write(size);
}
return out.toByteArray();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (ism != null) {
ism.close();
ism = null;
}
if (out!= null) {
out.close();
out=null;
}
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
progressDialog.show();
}
@Override
protected void onPostExecute(byte[] bytes) {
super.onPostExecute(bytes);
Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, , bytes.length);
img.setImageBitmap(bitmap);
progressDialog.dismiss();
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
}
}
運作結果: android 運用AsyncTask 擷取圖檔并顯示 總結:源碼decodeByteArray方法的參數的說明 /**
* Decode an immutable bitmap from the specified byte array.
*
* @param data byte array of compressed image data
* @param offset offset into imageData for where the decoder should begin
* parsing.
* @param length the number of bytes, beginning at offset, to parse
* @return The decoded bitmap, or null if the image could not be decoded.
*/
public static Bitmap decodeByteArray(byte[] data, int offset, int length) {
return decodeByteArray(data, offset, length, null);
}
再次修改MainActivity,實作邊下載下傳有進度條顯示 import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.util.StringBuilderPrinter;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;
import java.io.BufferedReader;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
public class MainActivity extends AppCompatActivity {
private Button bt;
private ImageView img;
private ProgressDialog progressDialog;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bt = (Button) findViewById(R.id.button);
img = (ImageView) findViewById(R.id.imageView);
progressDialog = new ProgressDialog(this);
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
progressDialog.setTitle("提示資訊");
progressDialog.setMessage("正在下載下傳");
progressDialog.setCancelable(false);
bt.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new MyAscyTask().execute();
}
});
}
public class MyAscyTask extends AsyncTask<String, Integer, byte[]> {
@Override
protected byte[] doInBackground(String... params) {
InputStream ism = null;
HttpURLConnection con = null;
ByteArrayOutputStream out=new ByteArrayOutputStream();
try {
URL url = new URL("http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg");
con = (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
con.setReadTimeout();
con.setConnectTimeout();
ism = con.getInputStream();
byte[] data = new byte[];
int size;
long file_length=con.getContentLength();
long total_length=;
int i=;
while ((size=ism.read(data))!=-) {
out.write(data,,size);
total_length+=size;
int progress=(int)((total_length/(float)file_length) * );
Log.i("TAG", "doInBackground: "+total_length+"==="+file_length+
"==="+progress);
Thread.sleep();
publishProgress(progress);
}
return out.toByteArray();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
try {
if (ism != null) {
ism.close();
ism = null;
}
if (out!= null) {
out.close();
out=null;
}
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
progressDialog.show();
}
@Override
protected void onPostExecute(byte[] bytes) {
super.onPostExecute(bytes);
Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, , bytes.length);
img.setImageBitmap(bitmap);
progressDialog.dismiss();
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
Log.i("TAG", "onProgressUpdate:....... "+values[]);
progressDialog.setProgress(values[]);
}
}
}