完善文档注释。

src/main/java/xyz/zhouxy/plusone/commons/collection/SafeConcurrentHashMap.java

@@ -22,9 +22,21 @@ import java.util.function.Function;
 
 import javax.annotation.concurrent.ThreadSafe;
 
+import xyz.zhouxy.plusone.commons.base.JRE;
 import xyz.zhouxy.plusone.commons.util.ConcurrentHashMapUtil;
 
-// TODO 添加文档注释
+/**
+ * SafeConcurrentHashMap
+ *
+ * <p>
+ * Java 8 的 {@link ConcurrentHashMap#computeIfAbsent(Object, Function)} 方法有 bug，
+ * 使用 Java 8 时，可使用这个类进行替换。
+ * 
+ * @author <a href="http://zhouxy.xyz:3000/ZhouXY108">ZhouXY</a>
+ * @since 1.0
+ * @see ConcurrentHashMap
+ * @see ConcurrentHashMapUtil#computeIfAbsentForJava8(ConcurrentHashMap, Object, Function)
+ */
 @ThreadSafe
 public class SafeConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
 
@@ -102,6 +114,8 @@ public class SafeConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
     /** {@inheritDoc} */
     @Override
     public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
-        return ConcurrentHashMapUtil.computeIfAbsentForJava8(this, key, mappingFunction);
+        return JRE.isJava8()
+                ? ConcurrentHashMapUtil.computeIfAbsentForJava8(this, key, mappingFunction)
+                : super.computeIfAbsent(key, mappingFunction);
     }
 }

src/main/java/xyz/zhouxy/plusone/commons/util/ConcurrentHashMapUtil.java

@@ -21,12 +21,28 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Function;
 
 import xyz.zhouxy.plusone.commons.base.JRE;
+import xyz.zhouxy.plusone.commons.collection.SafeConcurrentHashMap;
 
-public class ConcurrentHashMapUtil { // TODO 添加文档注释
+/**
+ * ConcurrentHashMapUtil
+ *
+ * <p>
+ * Java 8 的 {@link ConcurrentHashMap#computeIfAbsent(Object, Function)} 方法有 bug，
+ * 可使用这个工具类的 {@link computeIfAbsentForJava8} 进行替换。
+ *
+ * <p>
+ * <b>NOTE: 方法来自Dubbo，见：issues#2349</b>
+ * 
+ * @author <a href="http://zhouxy.xyz:3000/ZhouXY108">ZhouXY</a>
+ * @since 1.0
+ * @see ConcurrentHashMap
+ * @see SafeConcurrentHashMap
+ */
+public class ConcurrentHashMapUtil {
 
     public static <K, V> V computeIfAbsent(ConcurrentHashMap<K, V> map, final K key,
             final Function<? super K, ? extends V> mappingFunction) {
-
+        Objects.requireNonNull(map, "map");
         return JRE.isJava8()
                 ? computeIfAbsentForJava8(map, key, mappingFunction)
                 : map.computeIfAbsent(key, mappingFunction);
@@ -34,6 +50,7 @@ public class ConcurrentHashMapUtil { // TODO 添加文档注释
 
     public static <K, V> V computeIfAbsentForJava8(ConcurrentHashMap<K, V> map, final K key,
             final Function<? super K, ? extends V> mappingFunction) {
+        Objects.requireNonNull(key);
         Objects.requireNonNull(mappingFunction);
         V v = map.get(key);
         if (null == v) {