1

准备3个jar包httpcore-4.4.3.jar，android-async-http-1.4.8.jar，httpclient-4.3.6.jar
链接：https://pan.baidu.com/s/1CK1NPfHBtppF-lxR-RcUmQ
密码：f16a

2

MainActivity

package com.example.xeonrnc.download_demo;
 
import android.app.Activity;
import android.content.pm.PackageManager;
import android.os.Environment;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import com.loopj.android.http.AsyncHttpClient;
import com.loopj.android.http.FileAsyncHttpResponseHandler;
import org.apache.http.Header;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
 
public class MainActivity extends AppCompatActivity {
    //指定要下载的文件的url
    String url="http://192.168.43.173:8080/my_upload/pic/22.png";
    private static final int REQUEST_EXTERNAL_STORAGE = 1;
    private static String[] PERMISSIONS_STORAGE = {
            "android.permission.READ_EXTERNAL_STORAGE",
            "android.permission.WRITE_EXTERNAL_STORAGE"
    };
    public static void verifyStoragePermissions(Activity activity) {
 
        try {
            //检测是否有写的权限
            int permission = ActivityCompat.checkSelfPermission(activity,
                    "android.permission.WRITE_EXTERNAL_STORAGE");
            if (permission != PackageManager.PERMISSION_GRANTED) {
                // 没有写的权限，去申请写的权限，会弹出对话框
                ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn=(Button)findViewById(R.id.download);
        verifyStoragePermissions(this);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                uploadClick();
            }
        });
    }
 
 
 
    public void uploadClick(){
        AsyncHttpClient client=new AsyncHttpClient();
        client.get(url, new FileAsyncHttpResponseHandler(this) {
            @Override
            public void onFailure(int i, Header[] headers, Throwable throwable, File file) {
 
            }
 
            @Override
            public void onSuccess(int i, Header[] headers, File file) {
                System.out.println(file.getAbsolutePath());
                //将下载的文件保存到本地sdcard的picture文件夹，并重命名为abc.png
                File path=Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES+"/abc.png");
                try {
                    InputStream in=new FileInputStream(file);
                    OutputStream out=new FileOutputStream(path);
                    byte[] bytes=new byte[100];
                    int len=-1;
                    while((len=in.read(bytes))!=-1){
                        out.write(bytes,0,len);
                    out.flush();
                }
                out.close();
                in.close();
            }catch (FileNotFoundException e){
                e.printStackTrace();
            }catch (IOException e){
                e.printStackTrace();
                }
                System.out.println();
            }
        });
 
 
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.example.xeonrnc.download_demo">
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <!--<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"-->
        <!--tools:ignore="ProtectedPermissions" />-->
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        android:usesCleartextTraffic="true">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
 
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <uses-library android:name="org.apache.http.legacy" android:required="false" />
    </application>
 
</manifest>

activity_main.xml

<?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">
 
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <Button
            android:text="下载"
            android:id="@+id/download"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
    </LinearLayout>
 
</android.support.constraint.ConstraintLayout>