如何修复从android打印文档的空指针异常

Question

经过长时间的搜索，我从here 找到了一些关于 android 打印的解释，我在下面做了一些代码，但它显示了空指针异常。请帮助我如何修复打印文档的空指针异常 这是我使用打印 API 所做的代码： 我的主要活动：

package com.example.customprint;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.print.PrintManager;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

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


        printing_document = (Button)findViewById(R.id.button1);
        printing_document.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                //start_printing();
                PrintManager printmanager = (PrintManager)MainActivity.this.getSystemService(Context.PRINT_SERVICE);
                String name = "MedeQuip";
                //printmanager.print(name, new MyPrintDocumentAdapater(this), null);                
                printmanager.print(name, new MyPrintDocumentAdapater(), null);


            }
        });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

打印适配器：

package com.example.customprint;

import java.io.FileOutputStream;
import java.io.IOException;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.pdf.PdfDocument;
import android.graphics.pdf.PdfDocument.PageInfo;
import android.os.Bundle;
import android.os.CancellationSignal;
import android.os.ParcelFileDescriptor;
import android.print.PageRange;
import android.print.PrintAttributes;
import android.print.PrintDocumentAdapter;
import android.print.PrintDocumentInfo;
import android.print.pdf.PrintedPdfDocument;

public class MyPrintDocumentAdapater extends PrintDocumentAdapter {


    PdfDocument pdfdocument;
    Context context;
    int pagewidth;
    int pageheight;
    int tota_pages=4;



    @Override
    public void onLayout(PrintAttributes oldAttributes,
            PrintAttributes newAttributes,
            CancellationSignal cancellationSignal,
            LayoutResultCallback callback, Bundle extras) {




        pdfdocument = new PrintedPdfDocument(context, newAttributes);
        pageheight = newAttributes.getMediaSize().getHeightMils()/1000*72;
        pagewidth =  newAttributes.getMediaSize().getWidthMils()/1000*72;


        if(cancellationSignal.isCanceled()){
            callback.onLayoutCancelled();
            return;
        }

        if(tota_pages>0){
            PrintDocumentInfo.Builder information = new PrintDocumentInfo
                                            .Builder("Lakshmansundeep")
                                            .setContentType(PrintDocumentInfo.CONTENT_TYPE_DOCUMENT)
                                            .setPageCount(tota_pages);

            PrintDocumentInfo information_build = information.build();
            callback.onLayoutFinished(information_build, true);

        }
        else{
            callback.onLayoutFailed("Error with page details");
        }
    }





        @Override
    public void onWrite(PageRange[] pageRanges, 
            ParcelFileDescriptor destination,
            CancellationSignal cancellationSignal,
            WriteResultCallback callback) {
        // TODO Auto-generated method stub

        for(int i=0;i<tota_pages;i++){
            if(pageInRange(pageRanges,i)){
                PageInfo newPage = new PageInfo.Builder(pagewidth, pageheight, i).create();
                PdfDocument.Page page = pdfdocument.startPage(newPage);

                if(cancellationSignal.isCanceled()){
                    callback.onWriteCancelled();
                    pdfdocument.close();
                    pdfdocument = null;
                    return;
                }
                drawPage(page,i);
                pdfdocument.finishPage(page);

            }

            try{

                pdfdocument.writeTo(new FileOutputStream(destination.getFileDescriptor()));

            }catch(IOException e){
                callback.onWriteFailed(e.toString());
            }
            finally{

                pdfdocument.close();
                pdfdocument = null;
            }


            callback.onWriteFinished(pageRanges);

        }




    }





        private void drawPage(PdfDocument.Page page, int pageNumber) {
            Canvas canvas = page.getCanvas();
            pageNumber++;
            int titleBaseLine = 72;
            int Leftmargin = 54;

            Paint paint = new Paint();
            paint.setColor(Color.BLACK);
            paint.setTextSize(25);
            canvas.drawText("This is a sample text print"+pageNumber,Leftmargin, titleBaseLine, paint);
            paint.setTextSize(15);
            canvas.drawText("This is a printing some Text from the android application by lakshmansundeep",Leftmargin,titleBaseLine+35,paint);

            if(pageNumber%2 == 0)
                    paint.setColor(Color.RED);
            else
                    paint.setColor(Color.GREEN);
            PageInfo pageInfo = page.getInfo();
            canvas.drawCircle(pageInfo.getPageWidth()/2, pageInfo.getPageHeight()/2, 150, paint);
        }





        private boolean pageInRange(PageRange[] pageRanges, int page) {
            for(int i=0;i<pageRanges.length;i++){
                if((page>=pageRanges[i].getEnd()))
                    return true;
            }
            return false;
        }
}

这是我的日志：

01-02 08:07:13.810: E/AndroidRuntime(13986): FATAL EXCEPTION: main
01-02 08:07:13.810: E/AndroidRuntime(13986): Process: com.example.customprint, PID: 13986
01-02 08:07:13.810: E/AndroidRuntime(13986): java.lang.NullPointerException
01-02 08:07:13.810: E/AndroidRuntime(13986):    at com.example.customprint.MyPrintDocumentAdapater.onWrite(MyPrintDocumentAdapater.java:80)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at android.print.PrintManager$PrintDocumentAdapterDelegate$MyHandler.handleMessage(PrintManager.java:812)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at android.os.Handler.dispatchMessage(Handler.java:102)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at android.os.Looper.loop(Looper.java:136)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at android.app.ActivityThread.main(ActivityThread.java:5017)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at java.lang.reflect.Method.invokeNative(Native Method)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at java.lang.reflect.Method.invoke(Method.java:515)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-02 08:07:13.810: E/AndroidRuntime(13986):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

根据贴出的代码，第 80 行（发生 NPE 的地方）是：

PdfDocument.Page page = pdfdocument.startPage(newPage);

pdfdocument 是在 onLayout() 中实例化的，因此这表明 onWrite() 在之前没有任何 onLayout() 调用的情况下被调用，从而导致了问题。

---

我的建议：阅读 https://developer.android.com/training/printing/custom-docs.html 特别是 onWrite 示例。请注意，每个页面都有一个循环然后将内容写入PDF（并且“pdfdocument”设置为null），您没有在代码中这样做。