Using PDFViewer in ViewPager and swiping away before PDF loads causes NPE crash

Author: ZhuindenCreated Jan 25, 2026Updated Jan 25, 2026

Fatal Exception: java.lang.NullPointerException
Attempt to invoke virtual method 'boolean java.lang.Thread.isAlive()' on a null object reference
com.github.barteksc.pdfviewer.PDFView.loadComplete (PDFView.java:756)
com.github.barteksc.pdfviewer.DecodingAsyncTask.onPostExecute (DecodingAsyncTask.java:80)
com.github.barteksc.pdfviewer.DecodingAsyncTask.onPostExecute (DecodingAsyncTask.java:27)
android.os.AsyncTask.finish (AsyncTask.java:771)

com.android.internal.os.ZygoteInit.main (ZygoteInit.java:961)

I'm a little behind on schedule so I don't have a few months to wait for a fix, so this is how I fixed it

java

    /** Called when the PDF is loaded */
    void loadComplete(PdfFile pdfFile) {
        state = State.LOADED;

        this.pdfFile = pdfFile;

        if (renderingHandlerThread == null || !renderingHandlerThread.isAlive()) {
            if(renderingHandlerThread == null) {
                renderingHandlerThread = new HandlerThread("PDF renderer");
            }
            renderingHandlerThread.start();
        }

The real problem though is that onDetachedFromWindow kills the RenderingThread and sets it to null, but there is no onAttachedToWindow counterpart. If you swipe away in a ViewPager before the PDF loads, then the render thread will be destroyed. In fact, I'm guessing just swiping away swiping back swiping away swiping back would also crash as the RenderThread is destroyed but never re-created

So I also added this

java

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        if (renderingHandlerThread == null) {
             renderingHandlerThread = new HandlerThread("PDF renderer");
        }
    }

This fixes the crash entirely 100% of the time. It was causing a crash for 8% of users. :)

Source: DImuthuUpe/AndroidPdfViewer

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