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

@@ -41,7 +41,7 @@ public class CacheStats {
 
   /** The number of getBlock requests that were cache hits from primary replica */
   private final Counter primaryHitCount = new Counter();
-  
+
   /**
    * The number of getBlock requests that were cache hits, but only from
    * requests that were set to use the block cache.  This is because all reads
@@ -387,23 +387,53 @@ public class CacheStats {
   }
 
   public double getHitRatio() {
-    return ((float)getHitCount()/(float)getRequestCount());
+    double requestCount = getRequestCount();
+
+    if (requestCount == 0) {
+      return 0;
+    }
+
+    return getHitCount() / requestCount;
   }
 
   public double getHitCachingRatio() {
-    return ((float)getHitCachingCount()/(float)getRequestCachingCount());
+    double requestCachingCount = getRequestCachingCount();
+
+    if (requestCachingCount == 0) {
+      return 0;
+    }
+
+    return getHitCachingCount() / requestCachingCount;
   }
 
   public double getMissRatio() {
-    return ((float)getMissCount()/(float)getRequestCount());
+    double requestCount = getRequestCount();
+
+    if (requestCount == 0) {
+      return 0;
+    }
+
+    return getMissCount() / requestCount;
   }
 
   public double getMissCachingRatio() {
-    return ((float)getMissCachingCount()/(float)getRequestCachingCount());
+    double requestCachingCount = getRequestCachingCount();
+
+    if (requestCachingCount == 0) {
+      return 0;
+    }
+
+    return getMissCachingCount() / requestCachingCount;
   }
 
   public double evictedPerEviction() {
-    return ((float)getEvictedCount()/(float)getEvictionCount());
+    double evictionCount = getEvictionCount();
+
+    if (evictionCount == 0) {
+      return 0;
+    }
+
+    return getEvictedCount() / evictionCount;
   }
 
   public long getFailedInserts() {

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

@@ -317,7 +317,12 @@ public class JSONBean {
         jg.writeEndArray();
       } else if(value instanceof Number) {
         Number n = (Number)value;
-        jg.writeNumber(n.toString());
+        double doubleValue = n.doubleValue();
+        if (Double.isNaN(doubleValue) || Double.isInfinite(doubleValue)) {
+          jg.writeString(n.toString());
+        } else {
+          jg.writeNumber(n.toString());
+        }
       } else if(value instanceof Boolean) {
         Boolean b = (Boolean)value;
         jg.writeBoolean(b);