我需要使用 JDOM 来生成可能非常大的 XML 文件。我想知道除了已经在内存中的数据（主要是字符串）之外，JDOM 还需要多少额外的内存空间。我编写了一个简单的程序进行测试，结果发现开销大约是 XML 内容的两倍。
有谁知道为什么 JDOM 需要这么多额外的内存以及是否有办法优化它？JDOM 对象不应该只保留对现有字符串的引用吗？
这是我用来测试的程序：
public class TestJdomMemoryOverhead {
private static Runtime runtime = Runtime.getRuntime();
public static void gc() {
// Try to give the JVM some hints to run garbage collection
for (int i = 0; i < 5; i++) {
runtime.runFinalization();
runtime.gc();
Thread.currentThread().yield();
}
}
public static void generateXml(List<String> filenames) throws IOException {
// generate a simple XML file by these file names. It looks like:
// <?xml version="1.0" encoding="UTF-8"?>
// <files>
// <f n="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
// <f n="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
// <f n="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
// ....
// ....
// </files>
Element filesElem = new Element("files");
Document doc = new Document(filesElem);
for (String name : filenames) {
Element fileElem = new Element("f");
fileElem.setAttribute("n", name);
filesElem.addContent(fileElem);
}
gc();
System.out.println("After generating JDOM objects: " + (runtime.totalMemory() - runtime.freeMemory()) + " bytes");
XMLOutputter outputter = new XMLOutputter(Format.getPrettyFormat());
BufferedWriter writer = new BufferedWriter(new FileWriter("test.xml", false));
outputter.output(doc, writer);
writer.close();
gc();
System.out.println("After writing to XML file: " + (runtime.totalMemory() - runtime.freeMemory()) + " bytes");
}
public static void main(String[] cmdArgs) throws IOException {
List<String> filenames = new ArrayList<String>();
StringBuilder builder = new StringBuilder();
// 30 unicode chracters, repated 500,000 times. The memory to store
// these file name strings should be about 30MB.
builder.append("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
for (int i = 0; i < 500000; i++) {
filenames.add(builder.toString());
}
gc();
System.out.println("After generating file names: " + (runtime.totalMemory() - runtime.freeMemory()) + " bytes");
generateXml(filenames);
gc();
System.out.println("Get back to main: " + (runtime.totalMemory() - runtime.freeMemory()) + " bytes");
}
}
输出是：
After generating file names: 51941096 bytes
After generating JDOM objects: 125766824 bytes
After writing to XML file: 126036768 bytes
Get back to main: 51087440 bytes
如您所见，JDOM 对象使用了大约 70MB。