Remove some useless codes in agent-core.

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/context/Injectable.java

 package org.skywalking.apm.agent.core.context;
 
 /**
- * The <code>Injectable</code> represents a callback
+ * The <code>Injectable</code> represents a provider, which gives the reference of {@link ContextCarrier} and peer
+ * for the agent core, for cross-process propagation.
  *
  * @author wusheng
  */

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/context/tag/AbstractTag.java

@@ -2,16 +2,6 @@ package org.skywalking.apm.agent.core.context.tag;
 
 import org.skywalking.apm.agent.core.context.trace.AbstractSpan;
 
-/**
- * This is the abstract tag.
- * All span's tags inherit from {@link AbstractTag},
- * which provide an easy way to
- * {@link Span#setTag(String, String)} ,
- * {@link Span#setTag(String, Integer)}
- * {@link Span#setTag(String, boolean)} ,
- * <p>
- * Created by wusheng on 2017/2/17.
- */
 public abstract class AbstractTag<T> {
     /**
      * The key of this Tag.

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/context/tag/Tags.java

@@ -22,21 +22,6 @@ public final class Tags {
      */
     public static final StringTag STATUS_CODE = new StringTag("status_code");
 
-    /**
-     * SPAN_KIND hints at the relationship between spans, e.g. client/server.
-     */
-    public static final StringTag SPAN_KIND = new StringTag("span.kind");
-
-    /**
-     * A constant for setting the span kind to indicate that it represents a server span.
-     */
-    public static final String SPAN_KIND_SERVER = "server";
-
-    /**
-     * A constant for setting the span kind to indicate that it represents a client span.
-     */
-    public static final String SPAN_KIND_CLIENT = "client";
-
     /**
      * SPAN_LAYER represents the kind of span.
      * <p>

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/dictionary/DictionaryUtil.java

@@ -5,14 +5,10 @@ package org.skywalking.apm.agent.core.dictionary;
  */
 public class DictionaryUtil {
     public static int nullValue() {
-        return -1;
+        return 0;
     }
 
-    public static boolean isNull(int value) {
-        return value == nullValue();
-    }
-
-    public static boolean isNull(String text) {
-        return text == null;
+    public static boolean isNull(int id) {
+        return id == nullValue();
     }
 }

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/dictionary/DictionaryValueHolder.java

-package org.skywalking.apm.agent.core.dictionary;
-
-/**
- * @author wusheng
- */
-public class DictionaryValueHolder {
-    private int value = DictionaryUtil.nullValue();
-
-    public int getValue() {
-        return value;
-    }
-
-    public void setValue(int value) {
-        this.value = value;
-    }
-}