java - 哪个实现更好 : Cache based on WeakHashMap or cache based on ThreadLocal?

Question

我很难在以下两种实现之间做出决定。我想为每个线程缓存 javax.xml.parsers.DocumentBuilder 对象。我主要关心的是运行时性能 - Hench 我很乐意尽可能避免 GC。内存不是问题。

我已经编写了两个 POC 实现，很乐意听取社区关于每个实现的优缺点。

谢谢大家的帮助。

选项 #1 - WeakHashMap

import java.io.IOException;
import java.io.StringReader;
import java.util.WeakHashMap;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.Document;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;


public class DocumentBuilder_WeakHashMap {
    private static final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    private static final WeakHashMap<Thread, DocumentBuilder> CACHE = new WeakHashMap<Thread, DocumentBuilder>();

    public static Document documentFromXMLString(String xml) throws SAXException, IOException, ParserConfigurationException {
        DocumentBuilder builder = CACHE.get(Thread.currentThread());
        if(builder == null) {
            builder = factory.newDocumentBuilder();
            CACHE.put(Thread.currentThread(), builder);
        }

        return builder.parse(new InputSource(new StringReader(xml)));
    }

}

选项#2 - ThreadLocal

import java.io.IOException;
import java.io.StringReader;
import java.lang.ref.WeakReference;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.Document;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;


public class DocumentBuilder_ThreadLocal {
    private static final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    private static final ThreadLocal<WeakReference<DocumentBuilder>> CACHE = 
        new ThreadLocal<WeakReference<DocumentBuilder>>() {
            @Override 
            protected WeakReference<DocumentBuilder> initialValue() {
                try {
                    return new WeakReference<DocumentBuilder>(factory.newDocumentBuilder());
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
        };

    public static Document documentFromXMLString(String xml) throws ParserConfigurationException, SAXException, IOException {
        WeakReference<DocumentBuilder> builderWeakReference = CACHE.get();
        DocumentBuilder builder = builderWeakReference.get();

        if(builder == null) {
            builder = factory.newDocumentBuilder();
            CACHE.set(new WeakReference<DocumentBuilder>(builder));
        }

        return builder.parse(new InputSource(new StringReader(xml)));
    }
}

它们都做同样的事情(将 documentFromXMLString() 暴露给外界)那么你会使用哪一个？

谢谢，格言。

Answer 1

只要您不使用弱引用而是直接使用 ThreadLocal<DocumentBuilder>，ThreadLocal 解决方案会更好。 .访问 ThreadLocal 值的速度更快，因为线程直接引用包含所有 ThreadLocal 值的数组，并且它只需计算该数组中的索引即可进行查找。查看ThreadLocal source看看为什么索引计算很快(int index = hash & values.mask;)