即使页面不包含前向引用并且 <page-sequence> 块非常小,如何避免 FOP 消耗越来越多的内存?
这是一个测试 java 程序,它使用手工制作的 FO 为 FOP 提供服务,它只是一遍又一遍地重复相同的非常基本的页面序列:
Fo2Pdf.java
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.io.PipedInputStream;
import javax.xml.transform.Result;
import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.sax.SAXResult;
import javax.xml.transform.stream.StreamSource;
import org.apache.fop.apps.FOUserAgent;
import org.apache.fop.apps.Fop;
import org.apache.fop.apps.FopFactory;
import org.apache.fop.apps.MimeConstants;
import org.xml.sax.helpers.DefaultHandler;
public class Fo2Pdf implements Runnable {
private PipedInputStream in;
public Fo2Pdf(PipedInputStream in) {
this.in = in;
}
@Override
public void run() {
// instantiate Fop factory
FopFactory fopFactory = FopFactory.newInstance();
fopFactory.setStrictValidation(false);
// Setup output
OutputStream out = null;
try {
out = new FileOutputStream("output.pdf");
} catch (FileNotFoundException e) {
e.printStackTrace();
}
try {
// Setup user agent
FOUserAgent userAgent = fopFactory.newFOUserAgent();
userAgent.setConserveMemoryPolicy(true);
Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, userAgent, out);
// Setup JAXP using identity transformer
TransformerFactory factory = TransformerFactory.newInstance();
Transformer transformer = factory.newTransformer();
// Setup input stream
Source src = new StreamSource(in);
// Resulting SAX events (the generated FO) must be piped through to FOP
DefaultHandler defaultHandler = (DefaultHandler) fop.getDefaultHandler();
Result res = new SAXResult(defaultHandler);
// Start FOP processing
transformer.transform(src, res);
} catch (Exception e) {
e.printStackTrace();
}
}
}
FeedFo.java
import java.io.IOException;
import java.io.PipedInputStream;
import java.io.PipedOutputStream;
public class FeedFo {
public static void main(String args[]) throws IOException, InterruptedException {
// instantiate and connect the pipes
PipedInputStream in = new PipedInputStream();
PipedOutputStream out = new PipedOutputStream(in);
// Fo2Pdf - instantiate and start consuming the stream
Fo2Pdf fo2Pdf = new Fo2Pdf(in);
Thread fo2PdfThread = new Thread(fo2Pdf, "Fo2Pdf");
fo2PdfThread.start();
/*
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
<fo:layout-master-set>
<fo:simple-page-master master-name="A4" page-width="210mm" page-height="297mm">
<fo:region-body/>
</fo:simple-page-master>
</fo:layout-master-set>
*/
out.write(("<fo:root xmlns:fo=\"http://www.w3.org/1999/XSL/Format\"><fo:layout-master-set>" +
"<fo:simple-page-master master-name=\"A4\" page-width=\"210mm\" page-height=\"297mm\">" +
"<fo:region-body/></fo:simple-page-master></fo:layout-master-set>").getBytes());
for(int i=0; i<100000000; i++) {
// sleep 3 seconds every 50000 page-sequences to make sure the consumer is faster than the producer
if(i % 50000 == 0) {
Thread.currentThread().sleep(3000);
}
/*
<fo:page-sequence xmlns:fo="http://www.w3.org/1999/XSL/Format" master-reference="A4">
<fo:flow flow-name="xsl-region-body">
<fo:block/>
</fo:flow>
</fo:page-sequence>
*/
out.write(("<fo:page-sequence xmlns:fo=\"http://www.w3.org/1999/XSL/Format\" master-reference=\"A4\"><fo:flow flow-name=\"xsl-region-body\"><fo:block/></fo:flow></fo:page-sequence>").getBytes());
}
out.write("</fo:root>".getBytes());
out.flush();
out.close();
fo2PdfThread.join();
System.out.println("Exit");
}
}
正如您所注意到的,一旦页面序列关闭,FOP 就会将 PDF 写入磁盘。这意味着页面(应该?)不被保存在内存中。但是,内存只是不断增长和增长。对于 256MB 的堆大小,生成停止在大约 150000 个页面序列处。
为什么会这样?