HBASE-20571 JMXJsonServlet generates invalid JSON if it has NaN in metrics

- CacheStats won't generate NaN metrics.
- JSONBean class will serialize special floating point values as
  "NaN", "Infinity" or "-Infinity"

Signed-off-by: Andrew Purtell <apurtell@apache.org>

hbase-common/src/main/java/org/apache/hadoop/hbase/util/JSONBean.java

@@ -310,7 +310,11 @@ public class JSONBean {
         jg.writeEndArray();
       } else if(value instanceof Number) {
         Number n = (Number)value;
+        if (Double.isFinite(n.doubleValue())) {
           jg.writeNumber(n.toString());
+        } else {
+          jg.writeString(n.toString());
+        }
       } else if(value instanceof Boolean) {
         Boolean b = (Boolean)value;
         jg.writeBoolean(b);

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java

@@ -388,23 +388,53 @@ public class CacheStats {
   }
 
   public double getHitRatio() {
-    return ((double) getHitCount() / (double) getRequestCount());
+    double requestCount = getRequestCount();
+
+    if (requestCount == 0) {
+      return 0;
+    }
+
+    return getHitCount() / requestCount;
   }
 
   public double getHitCachingRatio() {
-    return ((double) getHitCachingCount() / (double) getRequestCachingCount());
+    double requestCachingCount = getRequestCachingCount();
+
+    if (requestCachingCount == 0) {
+      return 0;
+    }
+
+    return getHitCachingCount() / requestCachingCount;
   }
 
   public double getMissRatio() {
-    return ((double) getMissCount() / (double) getRequestCount());
+    double requestCount = getRequestCount();
+
+    if (requestCount == 0) {
+      return 0;
+    }
+
+    return getMissCount() / requestCount;
   }
 
   public double getMissCachingRatio() {
-    return ((double) getMissCachingCount() / (double) getRequestCachingCount());
+    double requestCachingCount = getRequestCachingCount();
+
+    if (requestCachingCount == 0) {
+      return 0;
+    }
+
+    return getMissCachingCount() / requestCachingCount;
   }
 
   public double evictedPerEviction() {
-    return ((double) getEvictedCount() / (double) getEvictionCount());
+    double evictionCount = getEvictionCount();
+
+    if (evictionCount == 0) {
+      return 0;
+    }
+
+    return getEvictedCount() / evictionCount;
   }
 
   public long getFailedInserts() {