Thread issues with Itext PDF

Question

Threads are getting stuck waiting for lock while using Itext Pdf to merge pdfs in the application, can someone help on cause and fix?

java.lang.Thread.run(Thread.java:722)

        "[STUCK] ExecuteThread: '4' for queue: 'weblogic.kernel.Default (self-tuning)'" RUNNABLE

            com.itextpdf.text.pdf.ByteBuffer.append(ByteBuffer.java:225)

            com.itextpdf.text.pdf.ByteBuffer.append(ByteBuffer.java:236)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serObject(PdfSmartCopy.java:165)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serArray(PdfSmartCopy.java:185)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serObject(PdfSmartCopy.java:156)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serDic(PdfSmartCopy.java:176)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serObject(PdfSmartCopy.java:153)

.......

com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serDic(PdfSmartCopy.java:176)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.serObject(PdfSmartCopy.java:146)

            com.itextpdf.text.pdf.PdfSmartCopy$ByteStore.<init>(PdfSmartCopy.java:198)

            com.itextpdf.text.pdf.PdfSmartCopy.copyIndirect(PdfSmartCopy.java:93)