生成pdf文档时如何解决java.lang.OutOfMemoryError：Java堆空间？

Question

My system throws exception: "java.lang.OutOfMemoryError: Java heap space", when it processed a huge file. I realized that StringWriter.toString() cause the double size in heap, so it could cause the issue. How can I optimize block of following code to avoid Out Of Memory.

public byte[] generateFromFo(final StringWriter foString) {
        try {
            StringReader foReader = new StringReader(foString.toString());
            ByteArrayOutputStream pdfWriter = new ByteArrayOutputStream();
            Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, fopFactory.newFOUserAgent(),pdfWriter);
            TRANSFORMER_FACTORY.newTransformer().transform(new StreamSource(foReader), new SAXResult(fop.getDefaultHandler()));
            LOG.debug("Completed rendering PDF output!");
            return pdfWriter.toByteArray();
        } catch (Exception e) {
            LOG.error("Error while generating PDF from FO",e);
            throw new AuditReportExportServiceException(AuditErrorCode.INTERNAL_ERROR,"Could not generate PDF from XSL-FO");
        }
    }

Answer 1

Using an InputStream of bytes may reduce the memory for foString by upto a factor 2 (char = 2 bytes).

A ByteArrayOutputStream resizes during its filling, so adding an estimated need speeds things up, and might prevent a resizing too much.

        InputStream foReader = new ByteArrayInputStream(
                foString.toString().getBytes(StandardCharsets.UTF_8);
        foString.close();
        final int initialCapacity = 160 * 1024;
        ByteArrayOutputStream pdfWriter = new ByteArrayOutputStream(initialCapacity);
        Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, fopFactory.newFOUserAgent(),
            pdfWriter);
        TRANSFORMER_FACTORY.newTransformer().transform(new StreamSource(foReader),
            new SAXResult(fop.getDefaultHandler()));

The best would be to change the API:

public void generateFromFo(final String foString, OutputStream pdfOut) { ... }

This might make the ByteArrayOutputStream superfluous, and you might immediately stream to a file, URL, or whatever.

The document itself and the generated PDF also has issues:

• image sizes (but remember the higher resolution of prints)
• some images can be nicely vectorized
• repeated images like in a page header, should be stored once
• fonts should ideally be the standard fonts, second best embedded subsets (of used chars)
• XML might be suboptimal, very repetitive

Answer 2

Broadly, you have two main options:

1. Increase the memory available to your process. The -Xmx option to Java will set this config. You could pass eg -Xmx8G to ask for 8GB of memory on a 64 bit system, if you have that much. Docs are here: http://docs.oracle.com/javase/7/docs/technotes/tools/windows/java.html#nonstandard

2. Change your code to "stream" the data through in smaller chunks, rather than trying to assemble the whole file into a byte[] in memory, as you have done here. You could change the output of your transformer to a FileOutputStream rather than a ByteArrayOutputStream and return a File rather than a byte[] in the code shown? Or, depending on what you do with the output of this method, you could return an InputStream and allow the consumer to receive the file data in a streaming fashion?

   You may also need to change things so that the input to this method is consumed in a streaming fashion. How to do that depends on the details of how StringWriter foString was created. You may need to "pipe" an OutputStream into an InputStream to make this work, see https://docs.oracle.com/javase/7/docs/api/java/io/PipedInputStream.html

1 is simpler. 2 is probably better here.