add extractors chain

webmagic-core/src/main/java/us/codecraft/webmagic/model/ExtractBy2.java

+package us.codecraft.webmagic.model;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * 定义类或者字段的抽取规则。<br>
+ * @author code4crafter@gmail.com <br>
+ * @date: 13-8-1 <br>
+ * Time: 下午8:40 <br>
+ */
+@Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
+@Target({ElementType.FIELD})
+public @interface ExtractBy2 {
+
+    String value();
+
+    public enum Type {XPath2, XPath, Regex, Css}
+
+    Type type() default Type.XPath2;
+
+}

webmagic-core/src/main/java/us/codecraft/webmagic/model/ExtractBy3.java

+package us.codecraft.webmagic.model;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * 定义类或者字段的抽取规则。<br>
+ * @author code4crafter@gmail.com <br>
+ * @date: 13-8-1 <br>
+ * Time: 下午8:40 <br>
+ */
+@Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
+@Target({ElementType.FIELD})
+public @interface ExtractBy3 {
+
+    String value();
+
+    public enum Type {XPath2, XPath, Regex, Css}
+
+    Type type() default Type.XPath2;
+
+}

webmagic-core/src/main/java/us/codecraft/webmagic/model/Extractor.java

@@ -9,7 +9,7 @@ import us.codecraft.webmagic.selector.Selector;
  */
 class Extractor {
 
-    protected final Selector selector;
+    protected Selector selector;
 
     protected final Source source;
 
@@ -37,4 +37,12 @@ class Extractor {
     boolean isNotNull() {
         return notNull;
     }
+
+    boolean isMulti() {
+        return multi;
+    }
+
+    void setSelector(Selector selector) {
+        this.selector = selector;
+    }
 }