GeoServer之kvpParser解析器&KvpRequestReader

参考1：https://blog.csdn.net/eternity_xyf/article/details/78531834

/**
 * Parses a key-value pair into a key-object pair.
 *
 * <p>This class is intended to be subclassed. Subclasses need declare the key in which they parse,
 * and the type of object they parse into.
 *
 * <p>Instances need to be declared in a spring context like the following:
 *
 * <pre>
 *         <code>
 *  &lt;bean id="myKvpParser" class="org.xzy.MyKvpParser"/&gt;
 *         </code>
 * </pre>
 *
 * Where <code>com.xzy.MyKvpParser</code> could be something like:
 *
 * <pre>
 *         <code>
 *  public class MyKvpParser extends KvpParser {
 *
 *     public MyKvpParser() {
 *        super( "MyKvp", MyObject.class )l
 *     }
 *
 *     public Object parse( String value ) {
 *        return new MyObject( value );
 *     }
 *  }
 *         </code>
 * </pre>
 *
 * <p><b>Operation Binding</b>
 *
 * <p>In the normal case, a kvp parser is engaged when a request specifies a name which matches the
 * name declared by the kvp parser. It is also possible to attach a kvp parser so that it only
 * engages on a particular operation. This is done by declaring the one or more of the following:
 *
 * <ul>
 *   <li>service
 *   <li>version
 *   <li>request
 * </ul>
 *
 * <p>When a kvp parser declares one or more of these properties, it will only be engaged if an
 * incoming request specicfies matching values of the properties.
 *
 * <p>The following bean declaration would create the above kvp parser so that it only engages when
 * the service is "MyService", and the request is "MyRequest".
 *
 * <pre>
 *         <code>
 *  &lt;bean id="myKvpParser" class="org.xzy.MyKvpParser"&gt;
 *    &lt;property name="service"&gt;MyService&lt;/property&gt;
 *    &lt;property name="request"&gt;MyRequest&lt;/property&gt;
 *  &lt;bean&gt;
 *         </code>
 * </pre>
 *
 * @author Justin Deoliveira, The Open Planning Project, jdeolive@openplans.org
 */

可以看出，这其实是一个虚函数。。

一个实现它的类FormatOptionsKvpParser extends KvpParser，主要是实现parse()方法。

@Override
public Object parse(String value) throws Exception {
    Map<String, Object> formatOptions = new CaseInsensitiveMap<>(new TreeMap<>());
    List<String> kvps = KvpUtils.escapedTokens(value, ';');
    List parsers = GeoServerExtensions.extensions(KvpParser.class, applicationContext);
    for (String kvp : kvps) {
        List<String> kv = KvpUtils.escapedTokens(kvp, ':', 2);
        String key = kv.get(0);
        String raw = kv.size() == 1 ? "true" : KvpUtils.unescape(kv.get(1));

        Object parsed = null;

        for (Object o : parsers) {
            KvpParser parser = (KvpParser) o;
            if (key.equalsIgnoreCase(parser.getKey())) {
                parsed = parser.parse(raw);
                if (parsed != null) {

                    break;
                }
            }
        }

        if (parsed == null) {
            if (LOGGER.isLoggable(Level.FINER))
                LOGGER.finer(
                        "Could not find kvp parser for: '" + key + "'. Storing as raw string.");
            parsed = raw;
        }

        formatOptions.put(key, parsed);
    }

    return formatOptions;
}

可以看出，这里调用GeoServerExtensions.extensions(KvpParser.class, applicationContext)注册KvpParser类到扩展中。这样做的好处是，下次就可以直接调用，不需要重新初始化了吗？（工厂模式的Registry）。那为什么不直接getbean。。

>>KvpRequestReader

findKvpRequestReader(Class<?> type)：寻找KvpRequestReader，可以接收任意类型的类为参数，这里以Message.class为例。

首先调用loadKvpRequestReaders，加载所有的KvpRequestReaders。。

public static Collection<KvpRequestReader> loadKvpRequestReaders() {
    Collection<KvpRequestReader> kvpReaders =
            GeoServerExtensions.extensions(KvpRequestReader.class);

    if (!(new HashSet<>(kvpReaders).size() == kvpReaders.size())) {
        String msg = "Two identical kvp readers found";
        throw new IllegalStateException(msg);
    }

    return kvpReaders;
}

当然在查找之前，肯定要先加载bean。。也就是applicationContext.xml，

URL url = getClass().getResource("applicationContext.xml");
FileSystemXmlApplicationContext context = new FileSystemXmlApplicationContext(url.toString())
Dispatcher dispatcher = (Dispatcher) context.getBean("dispatcher");

所以是怎么在extensions中通过KvpRequestReader找到MessageKvpRequestReader的？

public class MessageKvpRequestReader extends KvpRequestReader {
    public MessageKvpRequestReader() {
        super(Message.class);
    }

    @Override
    public Object createRequest() {
        return new Message();
    }
}

可以看出MessageKvpRequestReader是KvpRequestReader的子类。。

context.getBeanNamesForType(extensionPoint);