java - 使用 SAXParser 从对象返回 Null - NullPointerException

Question

我正在使用 SAXParser 解析 XML 并尝试获取特定节点的值，但我的代码返回 null。我不知道我做错了什么。这是我的代码。任何帮助将不胜感激。

import java.io.IOException;
import java.io.StringReader;

import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.Attributes;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class SAXParserUsage extends DefaultHandler {

    SAXParserFactory factory;
    SAXParser parser;
    DefaultHandler handler;
    private String elementName;
    private String authorizationCode;

    public String getAuthCodeResponse(String message) throws SAXException, IOException, ParserConfigurationException {
    factory = SAXParserFactory.newInstance();
    handler = new SAXParserUsage();
    parser = factory.newSAXParser();
    parser.parse(new InputSource(new StringReader(message)), handler);
    return authorizationCode;
    }

    public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
    elementName = qName;
    }

    public void characters(char[] text, int start, int length) throws SAXException {
    if (elementName.equals("code")) {
        String code = new String(text, start, length);
        System.out.println("setting code: " + code);
        authorizationCode = code;
    }

    }

    public void endElement(String arg0, String arg1, String arg2) throws SAXException {
    }

    public static void main(String[] args) throws SAXException, IOException, ParserConfigurationException {
    SAXParserUsage usage = new SAXParserUsage();
    String code = usage.getAuthCodeResponse(getSampleString());
    System.out.println("code is: " + code);
    }

    private static String getSampleString() {
    String sample = "<washCode><getCode><code>12345</code></getCode></washCode>";
    return sample;
    }

}

Answer 1

public String getAuthCodeResponse(String message) throws SAXException, IOException,     ParserConfigurationException {
factory = SAXParserFactory.newInstance();
handler = new SAXParserUsage(); // <--- You create local instance here
parser = factory.newSAXParser();
parser.parse(new InputSource(new StringReader(message)), handler);
// ^^ parser writes to local instance here but ...
return authorizationCode;
//    you return authCode field of the instance that this method is invoked on, 
//    which has not been changed by this method. 
//    So if it was null before, it still is null.
}

所以你必须return handler.authorizationCode或parser.parse(..., this);