如何使PDF阅读器Android应用程序从资产文件夹中获取PDF？

Question

我想：

package com.example.dell.pdfreader; 

import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.Matrix; 
import android.graphics.Rect; 
import android.graphics.pdf.PdfRenderer; 
import android.net.Uri; 
import android.os.ParcelFileDescriptor; 
import android.support.v7.app.AlertDialog; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.widget.Button; 
import android.widget.ImageView; 
import android.widget.Toast; 

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 { 

    private ImageView img; 
    private int currentpage = 0; 
    private Button next, previous; 
    File file; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     next = (Button) findViewById(R.id.next); 
     previous = (Button) findViewById(R.id.previous); 
     // file=getFile(); 


     next.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       currentpage++; 
       render(file); 
      } 
     }); 


     previous.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       currentpage--; 
       render(file); 
      } 
     }); 
    } 

    private void render(File file) { 

     try { 

      img = (ImageView) findViewById(R.id.image); 

      int width = img.getWidth(); 
      int height = img.getHeight(); 

      Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_4444); 





      PdfRenderer renderer = new PdfRenderer(ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY)); 

      if (currentpage < 0) { 
       currentpage = 0; 

      } else if (currentpage > renderer.getPageCount()) { 


       currentpage = renderer.getPageCount() - 1; 

       Matrix matrix = img.getImageMatrix(); 

       Rect rect = new Rect(0, 0, width, height); 

       renderer.openPage(currentpage).render(bitmap, rect, matrix, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY); 

       img.setImageMatrix(matrix); 
       img.setImageBitmap(bitmap); 
       img.invalidate(); 

      } 


     } catch (Exception e) { 

      e.printStackTrace(); 


     } 
    } 

    private File getFile() { 
     File file=new File(this.getExternalFilesDir(null),"Example.pdf");; 
     byte buffer[]=null; 
     OutputStream output = null; 


     InputStream input = null; 
     try { 
      input = this.getAssets().open("Example.pdf"); 

      buffer = new byte[input.available()]; 
      input.read(buffer); 
      output = new FileOutputStream(file); 
      output.write(buffer); 
     } 
     catch(FileNotFoundException e){ 
      e.printStackTrace(); 
      Toast.makeText(this,"File not Found",Toast.LENGTH_LONG).show(); 
      return null; 
     } 
     catch (IOException e) { 
      e.printStackTrace(); 
      Toast.makeText(this,"Failed to open",Toast.LENGTH_LONG).show(); 
      return null; 
     } 

    return file; 
    } 
} 

上面生成以下错误代码：

> java.lang.NullPointerException: Attempt to invoke virtual method 
> 'java.lang.String java.io.File.getPath()' on a null object reference 
> 03-18 15:42:16.245 6826-6826/com.example.dell.pdfreader W/System.err: 
> at 
> android.os.ParcelFileDescriptor.openInternal(ParcelFileDescriptor.java:269) 
> 03-18 15:42:16.245 6826-6826/com.example.dell.pdfreader W/System.err: 
> at android.os.ParcelFileDescriptor.open(ParcelFileDescriptor.java:200) 
> 03-18 15:42:16.246 6826-6826/com.example.dell.pdfreader W/System.err: 
> at 
> com.example.dell.pdfreader.MainActivity.render(MainActivity.java:76) 
> 03-18 15:42:16.246 6826-6826/com.example.dell.pdfreader W/System.err: 
> at 
> com.example.dell.pdfreader.MainActivity.access$100(MainActivity.java:26) 
> 03-18 15:42:16.246 6826-6826/com.example.dell.pdfreader W/System.err: 
> at 
> com.example.dell.pdfreader.MainActivity$1.onClick(MainActivity.java:47) 
> 03-18 15:42:16.246 6826-6826/com.example.dell.pdfreader W/System.err: 
> at android.view.View.performClick(View.java:5265) 03-18 15:42:16.246 
> 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> android.view.View$PerformClick.run(View.java:21534) 03-18 15:42:16.246 
> 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> android.os.Handler.handleCallback(Handler.java:815) 03-18 15:42:16.247 
> 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> android.os.Handler.dispatchMessage(Handler.java:104) 03-18 
> 15:42:16.247 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> android.os.Looper.loop(Looper.java:207) 03-18 15:42:16.247 
> 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> android.app.ActivityThread.main(ActivityThread.java:5728) 03-18 
> 15:42:16.247 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> java.lang.reflect.Method.invoke(Native Method) 03-18 15:42:16.247 
> 6826-6826/com.example.dell.pdfreader W/System.err:  at 
> com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 
> 03-18 15:42:16.247 6826-6826/com.example.dell.pdfreader W/System.err: 
> at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679) 

Answer 1

首先，在评论中指出，file是null，因为你永远不设置它的值。

二，对于资产中的PDF，没有文件。在AssetManager上使用openFd()以获得该资产上的AssetFileDescriptor，然后在AssetFileDescriptor上拨打getParcelFileDescriptor()以获得ParcelFileDescriptor。

三，PdfRenderer不处理任意PDF文件。它在那里用于打印预览，仅此而已。最有可能的是，您将需要使用something else。